Thermostatic Radiator 

Valve Evaluation

Jordan Dentz and Eric Ansanelli 

Advanced Residential Integrated Energy Solutions 

Collaborative

January 2015
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iii

Thermostatic Radiator Valve Evaluation

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

Jordan Dentz and Eric Ansanelli

Advanced Residential Integrated Energy Solutions Collaborative

The Levy Partnership, Inc.

1776 Broadway, Suite 2205

New York, NY 10019

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40347-05

January 2015
iv

The work presented in this report does not represent 

performance of any product relative to regulated 

minimum efficiency requirements.

The laboratory and/or field sites used for this work are 

not certified rating test facilities. The conditions and 

methods under which products were characterized for 

this work differ from standard rating conditions, as 

described.

Because the methods and conditions differ, the reported 

results are not comparable to rated product performance 

and should only be used to estimate performance under 

the measured conditions.
v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

Acknowledgments ................................................................................................................................... viii

Executive Summary................................................................................................................................... ix

1 Introduction and Background ............................................................................................................. 1

1.1 Introduction .................................................................................................................................. 1

1.2 Background................................................................................................................................... 1

1.3 Thermostatic Radiator Valve Literature ....................................................................................... 4

1.4 Research Gaps .............................................................................................................................. 5

1.5 Relevance to Building America’s Goals....................................................................................... 8

1.6 Research Questions....................................................................................................................... 8

2 Methods and Analysis.......................................................................................................................... 9

2.1 Field Experiment .......................................................................................................................... 9

2.2 Field Data Analysis .................................................................................................................... 10

2.2.1 Field Data Quality and Processing ................................................................................ 10

2.2.2 Graphical Analysis ........................................................................................................ 11

2.2.3 Pre- and Post-Thermostatic Radiator Valve Installation Space Temperature 

Comparison.................................................................................................................... 14

2.2.4 Mild and Severe Heating Weather Space Temperature Comparison ............................ 15

2.3 Utility Bill Analysis for Entire Test Property............................................................................. 15

2.4 Building Energy Optimization Program Conceptual Payback Model........................................ 15

3 Results................................................................................................................................................. 16

3.1 Effect of Thermostatic Radiator Valves on Indoor Space Temperature..................................... 16

3.2 Thermostatic Radiator Valve Performance in Mild and Severe Heating Weather..................... 16

3.3 Effect of the Building-Wide Thermostatic Radiator Valve Installation on Space Heating 

Energy Use ................................................................................................................................. 17

3.4 Cost-Benefit Conceptual Model Results .................................................................................... 18

3.5 Resident and Superintendent Interviews .................................................................................... 19

4 Discussion........................................................................................................................................... 20

4.1 Influence of Steam System Configuration on Thermostatic Radiator Valve Performance ........ 20

4.1.1 Air Venting Imbalance .................................................................................................. 20

4.1.2 One-Pipe Convector Radiator Considerations............................................................... 20

4.1.3 Boiler Control Setpoints................................................................................................ 21

4.2 Existing Level and Character of Overheating............................................................................. 22

4.3 Resident Behavior and Perception.............................................................................................. 22

4.4 Ventilation System ..................................................................................................................... 23

4.5 Thermostatic Radiator Valve Considerations............................................................................. 23

4.6 Cost-Benefit Analysis................................................................................................................. 23

4.7 Impacts and Tradeoffs................................................................................................................ 24

5 Conclusions ........................................................................................................................................ 25

5.1 Research Questions..................................................................................................................... 25

5.2 Key Lesson: Monitor Space Temperatures and Balance Steam Distribution............................. 26

5.3 Resident Interaction.................................................................................................................... 26

5.4 Future Research .......................................................................................................................... 27

References................................................................................................................................................. 28

Appendix.................................................................................................................................................... 29
vi

List of Figures

Figure 1. TRV schematic (Figure courtesy of Danfoss, Inc.).................................................................. 3

Figure 2. TRV operation (Figure courtesy of Danfoss, Inc.)................................................................... 4

Figure 3. Typical convector with datalogger, thermocouples, and TRV installed ............................. 10

Figure 4. Typical data from apartment 1F master bedroom ................................................................. 11

Figure 5. Radiator temperatures in apartment 1F living room evidencing heating imbalance 

between pre- and post-TRV installation periods............................................................................. 12

Figure 6. Radiator temperatures indicating uneven heat distribution between small and master 

bedrooms in apartment 1F ................................................................................................................ 13

Figure 7. Window opening indicated by temperature drops, circled in green, in apartment 1F living 

room (top) and apartment 1A living room (bottom)........................................................................ 14

Figure 8. Heating fuel usage versus average outdoor temperature before and after building-wide 

TRV installation................................................................................................................................... 18

Unless otherwise indicated, all figures were created by the Advanced Residential Integrated 

Energy Solutions Collaborative.

List of Tables

Table 1. Summary of TRV Studies............................................................................................................. 6

Table 2. TRV Installed Cost Estimates...................................................................................................... 7

Table 3. Data Collection Equipment........................................................................................................ 10

Table 4. Cost-Benefit Financial Assumptions........................................................................................ 16

Table 5. Average Pre- and Post-TRV Space Temperatures .................................................................. 16

Table 6. Mild and Severe Heating Weather Space Temperatures (°F)................................................. 17

Table 7. Present Value of Fuel Savings Over Life of TRV..................................................................... 18

Table 8. TRV Impacts and Tradeoffs....................................................................................................... 24

Table 9. TRV Comments and Findings ................................................................................................... 25

Table 10. BEopt Modeling Assumptions ................................................................................................ 29

Unless otherwise indicated, all tables were created by the Advanced Residential Integrated 

Energy Solutions Collaborative.
vii

Definitions

BEoptTM Building Energy Optimization Software

NYSERDA New York State Energy Research and Development Authority

psi Pounds per Square Inch

TRV Thermostatic Radiator Valve
viii

Acknowledgments

The authors would like to express their appreciation to Steven Bluestone and Robert Zapisek of 

the Bluestone Organization for their cooperation in the use of their building as a test bed for this 

Building America project, and to Bill Boss of Danfoss for technical advice. We are also grateful 

to Michael Gestwick of the National Renewable Energy Laboratory for guidance and support of 

this work.
ix

Executive Summary

A large stock of multifamily buildings in the Northeast and Midwest uses hot water or steam for 

space heating. Typically, residents do not pay for heat directly (i.e., heating fuel serves a central 

plant and use is not submetered). Losses from these systems are typically high, and a significant 

number of apartments are overheated much of the time. This is often evidenced by open 

windows on winter days. Controls and distribution are often faulty, and improving them can be 

more cost effective than replacing boilers.

Thermostatic radiator valves (TRVs), which have been in use for many decades, are one 

potential strategy to combat this problem. They are commonly used in Europe and in other 

markets such as commercial buildings, but have not been widely accepted by the residential 

retrofit market in the northeastern United States. Anecdotal evidence suggests that heating 

systems engineers and contractors have a variety of opinions about their effectiveness, 

illustrating a lack of consensus on this potentially important energy efficiency measure. A review 

of the limited available literature revealed that, in one study, heating fuel savings as high as 15% 

was achieved through TRV retrofits.

In this project, the U.S. Department of Energy Building America team, Advanced Residential 

Integrated Energy Solutions, sought to better understand the current usage of TRVs by key 

market players in steam and hot water heating and to conduct limited experiments on the 

effectiveness of new and old TRVs as a means of controlling space temperatures and reducing 

heating fuel consumption. The project included a survey of industry professionals, a field 

experiment comparing old and new TRVs, and cost-benefit modeling analysis using BEoptTM

(Building Energy Optimization software). Radiator and apartment space temperature data were 

collected and analyzed for two similar apartment units in a building that underwent a one-pipe 

steam TRV retrofit. Space temperature comparisons were made across the pre- and post-TRV 

installation heating periods and between rooms equipped with old or new TRVs in an attempt to 

show the comparative effectiveness of each vintage of TRV. Analyses of the heating fuel utility 

bills before and after the building-wide TRV installation were conducted to quantify potential 

savings.

The results of the field experiment and utility bill analysis did not show energy savings at either 

the unit or the building-wide level. The results provided inconclusive answers to the original 

study questions but provided valuable insight into common steam system imbalance and resident 

behavior issues that are critical to address in conjunction with TRV retrofits. Specific issues 

identified included steam distribution imbalance, possible TRV sensor location issues, a 

persistent window-opening habit, and a failure to optimize the boiler control set points as part of 

the TRV retrofit. The lack of heating fuel savings underscored the need to include whole steam 

system commissioning alongside or as a prerequisite to TRVs. Failed air vents and uneven steam 

main venting are critical to address either in conjunction with or before a TRV installation. 

Monitoring existing space temperatures before a retrofit strategy is chosen would allow the 

consultant and building owner to better assess the potential benefits of a whole-building TRV 

retrofit, selective installation of TRVs in some units, or simply balancing the steam distribution 

venting.
x

Further gaps in the understanding of underlying processes that allow TRVs to function 

effectively were also identified and are presented as opportunities for future research.
1

1 Introduction and Background

1.1 Introduction

A large stock of multifamily buildings in the Northeast and Midwest uses hot water or steam for

space heating. Typically, residents do not pay for heat directly (i.e., heating-related energy use is 

not submetered). Losses from these systems are typically high because a significant number of 

apartments are overheated much of the time. This is often evidenced by open windows on cold 

winter days. Controls and distribution are often faulty, and improving them can be more cost

effective than replacing boilers.

Thermostatic radiator valves (TRVs), which have been in use for many decades, are one strategy 

to combat this problem. They are commonly used in Europe and in other markets such as 

commercial buildings, but have not been widely accepted by the residential retrofit market in the 

northeastern United States. The following observations were made at the Hydronic Heating 

Systems Expert Meeting (ARIES Collaborative, 2011), and illustrate a divergence of opinions 

about and experience with TRVs, including that TRVs are limited by their durability, that they 

can be effective if used correctly, and that psychological factors increase tenant comfort without 

significantly affecting space temperatures.

Other anecdotal evidence, including that collected over the course of this research study, 

suggests that heating systems engineers and contractors have a variety of opinions on the 

effectiveness of TRVs, illustrating a lack of consensus on this potentially important energy 

measure.

1.2 Background

Central hydronic and steam space heating systems typically work by generating heat in a central 

location (the boiler or boilers) and transmitting that heat to the various parts of the building

through pipes carrying hot water or steam. Often, multifamily buildings with these systems have 

one or a few heating zones with no or very limited ability to control the heat locally. There are 

three main types of such space heating distribution systems in the United States:

• One-pipe steam systems. In these systems, steam is generated in a boiler and then 

transmitted to different parts of the building through a network of pipes that gradually 

diminish in size as they become more distant from the boiler. The steam then enters 

radiators in the spaces, gives up the heat, condenses into the bottom of the radiator, then

drains down to the boiler through the same pipes that it came up in. Venting of steam 

lines is critical in one-pipe steam systems; only with properly sized and operating air 

vents on headers, risers, and radiators will steam reach all radiators at approximately the 

same time, which is a hallmark of a well-tuned system. One-pipe steam systems are 

commonly found in older buildings, especially those built before 1970.

• Two-pipe steam systems. In these heating systems, steam is transmitted through one 

pipe and the condensate (water) returns to the boiler through a second pipe connected to 

the back end of the heat emitter (radiator). Two-pipe systems rely on steam traps to 

prevent steam from passing through the radiator and exiting into the condensate return 

pipe. Both one-pipe and two-pipe steam systems are typically controlled by warm 

weather shut-off controls in which the boiler stops supplying heat to the building if the 
2

outdoor air temperature reaches a predetermined threshold. Outdoor reset controls cycle 

the steam supply as a function of outdoor temperature to deliver only the heat required 

given the current weather. Two-pipe steam systems are common in larger buildings built 

after 1970.

• Hot water systems. Hot water systems are also common in multifamily buildings. 

Boilers provide hot water to the radiators or other heat emitters and the return water is 

pumped back to the boiler to be reheated. Often, multifamily hot water systems have 

circuit setters or balancing valves to make one-time or periodic adjustments to balance 

the heat distribution. Hot water systems are also two-pipe systems. Most multifamily hot 

water boilers are controlled by outdoor reset control where the supply water temperature 

is controlled as a function of outdoor temperature.

Steam and hot water heating systems in multifamily buildings can overheat buildings. While 

there is no single definition of overheating, studies point to temperatures above the range of 70°–

72°F (Dentz, Henderson, & Varshney, 2013) (McNamara, 1995). Overheating is recognized as a 

problem (Urban Green Council, 2012) and proven out by data. In a recent project by the U.S. 

Department of Energy Building America team, Advanced Residential Integrated Energy 

Solutions (Dentz, Henderson, & Varshney, 2013), 18 multifamily residential buildings were 

investigated to quantify overheating. The data were obtained from the archives of companies that 

provide energy management systems to multifamily buildings. Energy management systems

typically include temperature sensors in apartments networked to a central controller that 

modulates the heating system. This study concluded that all the buildings were overheating at 

some point during the heating season. In 15 of the 18 buildings, average temperatures in all the 

apartments exceeded 70°F (ranging from 70.7°F to 87.4°F). In the remaining three buildings, 

average temperatures in 88% of apartments exceeded 70°F (ranging from 70.3°F to 85.2°F).

Efficient control of heating systems has a large influence on energy savings and on the thermal

comfort of residents. According to the U.S. Department of Energy, overheating can increase 

annual heating fuel usage by as much as 3% for each degree Fahrenheit above the desired space 

temperature set point (U.S. Department of Energy, 2013).

Building overheating can vary in general severity, in spatial distribution, and temporally. 

Generally, the building-wide, heating-season average space temperature can be some number of 

degrees Fahrenheit above the desired set point or the legally mandated minimum (this varies by 

municipality; e.g., 68°F during daytime heating season hours in New York City) (New York City 

Administrative Code §27-2029). Space temperatures can vary from apartment to apartment 

depending on the performance of the distribution system and the building envelope; space 

temperatures can also vary across time because of heating system cycles, programmed setbacks, 

end-user (building superintendent and/or occupants) manipulation, and weather severity. This 

variation affects the strategy chosen to remedy overheating issues. If overheating is consistent

throughout a building and across time, reducing heat at the central heating plant is a logical 

solution. However, if the space temperature varies widely between apartments and over time, the 

solution may involve heating plant control and distribution system measures.

Depending on the system type, there are a number of ways to balance the system and even out 

distribution, after which the central heat supply can be reduced. For one-pipe steam, venting is a
3

primary means (McNamara, 1995) (Jayne Choi, 2012). Two-pipe systems can often benefit from 

properly sized orifices and correcting faulty steam traps (Gifford, 2003). For hot water systems, 

adjusting the balancing valves can help (Trüschel, 2002). However, for many buildings this may 

not be sufficient, meaning that local controls will be required. One room-level strategy is using a 

local powered two-way zone valve and thermostat. This strategy provides a high level of control 

but requires low voltage power to the valve and is expensive (at least $350 per unit). TRVs are a 

potentially lower cost room-level strategy, particularly for certain applications.

TRVs do not require electric power. They consist of a valve and an operator as shown in Figure 

1. A fluid-filled capsule inside the TRV operator expands as room temperature increases. As it 

expands, it closes the valve. In the case of one-pipe steam, the TRV is connected to the air vent 

and blocks the release of air when it is in the closed position, preventing steam from entering the 

radiator (Figure 2). When the room temperature decreases, the capsule contracts; this allows air 

to escape the radiator and steam to take its place. In the case of two-pipe steam, the TRV is 

connected to the radiator inlet and, when in the closed position, prevents steam from entering the 

radiator body entirely.

Figure 1. TRV schematic (Figure courtesy of Danfoss, Inc.)
4

Figure 2. TRV operation (Figure courtesy of Danfoss, Inc.)

As the room temperature decreases, the capsule contracts, opening the TRV and allowing steam 

to enter the radiator. A knob on the operator allows an occupant to adjust the temperature at 

which heat is allowed into the heat emitter (Gifford, 2004). The small body of published 

literature on TRV performance is summarized below.

1.3 Thermostatic Radiator Valve Literature

Several studies on TRV performance conclude that they work well and can be used to control 

space temperatures (Peterson, 1985) (Trüschel, 2002). Peterson (1985) found that rebalancing a 

steam system can save as much as 15%–25% heating fuel and that TRVs can be used to reduce 

overheating in a zone. Trüschel (2002) ran simulations and conducted a field study on hydronic 

(hot water) heat systems and found that TRVs are most effective in low-flow systems, partly

because of increased radiator sensitivity to flow changes.

In a project funded by The New York State Energy Research and Development Authority

(NYSERDA), Michael McNamara (1995) evaluated the effectiveness of TRVs in eliminating 

overheating in apartments and in saving energy. A total of 224 TRVs were installed and 

monitored for energy consumption for 3 years (1991–1994) in eight privately owned multifamily 

buildings. All the buildings had one-pipe, low-pressure steam distribution systems. As a 

prerequisite to project participation, steam main and radiator air vents were inspected before the 

baseline data were collected. TRVs were installed with set points of 72°F in half the apartments 

in four buildings. The buildings with partial installations reduced their annual heating use an 

average of 9%. After full installation, the average annual heating use savings totaled 15%. 

Payback ranged from 1 to 5 years in those buildings where TRVs produced energy savings. The 
5

study recommended installing TRVs on one-pipe steam distribution systems in apartments that 

are heated higher than 72°F, those where the control system causes temperatures to fluctuate 

higher than 72°F, and in specific rooms in which the radiator has become oversized because

thermal-pane windows were installed.

Tahersima et al. (2010) derived a model for a heating, ventilation, and air-conditioning system 

consisting of a room and a hydronic radiator with a TRV with a step motor to adjust the valve 

opening. The modeling presented in the article sheds light on a regular and undesirable 

phenomenon that occurs in low-demand situations: when flow is very low in the radiator and the 

supply water temperature and the pressure drop across the valve are constant, room temperature 

frequently oscillates. The simulations show that the oscillations occur partly because of the high 

gain of the radiator in low-demand weather conditions and the subsequent low flows.

In another simulation study, Tahersima et al. (2011) derived a linear parameter-varying model of 

a radiator that was based on the operating flow rate, room temperature, and radiator 

specifications. They presented a solution to the partial differential equation of a radiator’s heat 

transfer dynamics. The simulations showed that the designed controller based on the proposed 

model performs well and is stable under full operating conditions.

Weker and Mineur (1980) presented a performance index for TRVs derived from standard 

equations that define the heating process in a system. The effect of outside temperature on room 

temperature is slight if controlled by a TRV with a high performance index. Their analysis shows 

how to optimize a TRV design for any sensor and actuator and for a given temperature control 

band. The analysis provides insight into the effects of water flow and temperature changes on 

pre-retrofit room temperature. This study also provides a theoretical and practical basis for 

comparing the energy-saving performance of TRVs on hot water systems.

Xu et al. (2008) developed a model for simulating the thermal and hydraulic behavior of space 

heating systems with radiators controlled by TRVs in multifamily buildings. The authors 

developed submodels for rooms, radiators, and TRVs and then integrated the submodels to 

obtain a full model for the building that takes into account heat transfer across walls and the selfadjusting characteristic of TRVs. They further tested the model by investigating two space 

heating systems in multifamily buildings and found a good agreement between measured and 

simulated values. The study showed that an approximate 12.4% reduction of heat consumption 

could be expected with TRVs.

1.4 Research Gaps

Despite being an old technology, the published literature on TRVs is limited; eight published 

studies were identified (Table 1). Four were purely modeling and four were published field 

evaluations.

Based on the literature and interviews, it is unclear how effective TRVs remain after many years 

in service. A number of interviewees believed that TRV performance deteriorates with time. No 

study was found to confirm or dispute this opinion.
6

Table 1. Summary of TRV Studies

Study Type Application Type of 

System Finding

(McNamara, 

1995) Experimental

Eliminating 

overheating in 

multifamily 

buildings

One-pipe 

steam

TRVs saved approximately 15% 

heating energy and payback was 

shorter than 5 years.

(Xu, Fu, & 

Di, 2008) Modeling Heating energy 

savings

Hydronic 

heating

If the set value of the TRVs were 

kept on 2–3, about 12.4% 

reduction of heat consumption 

could be expected. When an 

apartment stopped using the 

heating system during a heating 

season, the heat consumption of its 

neighboring apartments would

increase by about 6%–14%.

(Weker & 

Mineur, 

1980)

Theoretical 

and 

experimental

Optimize the 

design of a TRV

Hydronic 

heating

Presented performance index for 

TRVs.

(Tahersima 

et al. 2011) 

and 

(Tahersima 

et al. 2010)

Modeling

Stabilize 

oscillatory 

behavior of TRVs 

under low heat 

demands

Hydronic 

heating

A TRV controller is presented that

keeps it stable over the range of 

operating conditions.

(Peterson, 

1985) Experimental Reducing 

overheating

One-pipe 

steam

Rebalancing a steam system can 

save as much as 15%–25% heating 

fuel. TRVs can be used to reduce 

overheating in a zone.

(Trüschel, 

2002)

Modeling 

and field 

work

Reducing 

overheating

Hydronic 

heating

TRVs are most effective in lowflow systems because the radiators

are sensitive to flow changes.

A brief search was conducted of state and utility energy efficiency programs. Only one utility 

program, from Consolidated Edison of New York, was found to offer rebates for TRVs. To the 

authors’ knowledge, only one state program, NYSERDA’s Multifamily Performance Program,

encourages TRVs. According to NYSERDA, TRVs are used modestly in the program, being 

recommended (but not necessarily used) for about 10% of apartments, about 17,000 apartments 

since 2007 (Colgrove, 2012). Most buildings in the Multifamily Performance Program have 

steam or hot water heating. The Consolidated Edison program started off slowly but gained 

momentum, and in its second year installed 11,000 TRVs in 176 (mostly one-pipe steam)

buildings. Consolidated Edison offered a rebate of $115 per TRV (Madnick, 2014). Interestingly, 

approximately two-thirds of these installations were installed by a single contractor.
7

In conversations with engineers and contractors, it became apparent that, even among 

experienced professionals, there is a range of understanding and opinions regarding the 

effectiveness and applicability of TRVs. Nearly all consultants and contractors agreed that TRVs 

are effective heat-limiting devices in some applications; however, a number doubted that they 

worked for one-pipe steam systems. Some consultants had little experience or knowledge of 

TRVs and expressed a desire for more information. The dominant view among those with 

experience was that, because of their relatively high installed cost, TRVs are best used 

selectively in spaces that are reported to be overheated. Most agreed that other means of 

improving heat delivery, such as balancing, installing orifices for two-pipe steam, and using 

boiler controls should be the first strategies, be followed by TRVs where needed.

The perception of costs ranged widely (see Table 2). The consultants felt that TRVs were too 

expensive, even though their cost estimates were typically lower than actual supplier and 

contractor costs.

Table 2. TRV Installed Cost Estimates

Application Low High Median Actual

(based on supplier and contractor prices)

One-Pipe Steam $65 $300 $140 $110 do-it-yourself; $160–$225 contractor

Two-Pipe Steam $65 $315 $140 $240–$315

Hot Water $87.5 $315 $213 $250–$315 (not including cost of draining)

t )

There was a general consensus that TRVs are rarely seen on multifamily audits; that they are 

underutilized, and have a large untapped potential. Most interviewees also recognized that while 

TRVs are generally simple to install, there are numerous ways to install them incorrectly that can 

lead to subpar performance, and incorrect installation is commonplace. Mistakes include poorly 

positioned valves and sensors and a failure to ensure that the overall system is working with the 

TRVs (i.e., system pressure and cycle lengths are appropriate). Most agree that TRVs are very 

reliable and durable. However, some have concerns stemming from specific experiences such as 

failures attributed to failed seals (hypothesized to be due to repeated wetting and drying in steam 

systems) and others due to hypothetical concerns such as particulates fouling the valves or 

imprecise air temperature sensing.

Few respondents were familiar with range-limited TRVs, which might address concerns related 

to improper usage by tenants (i.e., turning the knob to maximum and continuing to open 

windows). Tenant misuse was a common theme, with anecdotal reports of vandalism or opening 

of windows; however, admittedly few or no data were available on this phenomenon prior to this 

study.

Of 12 consultants actively working on building heating systems, seven sometimes recommend 

TRVs and five do not or do so rarely. Five of the 16 non-manufacturer/supplier interviewees did 

not think TRVs would be effective on one-pipe steam systems. Two consultants volunteered the 
8

opinion that more data are needed on the interaction between TRVs and various boiler control 

systems.

1.5 Relevance to Building America’s Goals

This project addresses the following Building America Implementation Standing Technical 

Committee milestone:

“Deliver systems and strategies that achieve 10%-15% space conditioning energy savings by 

upgrading existing heating/cooling equipment for homes where [heating, ventilation, and air 

conditioning] HVAC change-out is not cost effective” (National Renewable Energy Laboratory, 

2012).

This project evaluates the cost and effectiveness of a technology that can improve the efficiency 

of existing steam and hot water heating systems. The literature indicates that up to 15% heating 

energy savings are possible (McNamara, 1995). 

1.6 Research Questions

This research addresses the following questions: 

1. What are common prevailing opinions and perceptions regarding TRVs in the building 

community and how does this affect their deployment?

2. How effective are new and old TRVs in controlling indoor temperature under various 

outdoor conditions?

3. How much space heating energy can be saved through TRV installations and how cost 

effective are TRV retrofits in low-rise multifamily buildings?
9

2 Methods and Analysis

The project consisted of the following components:

• Literature review of TRV research

• Evaluation of the market perception of TRVs through discussions with leading heating 

system engineers and contractors

• Monitoring indoor air and radiator temperatures in two apartments before and after TRVs 

were installed (one apartment received new TRVs and the other received TRVs that were 

at least 10 years old)

• Estimated energy savings via modeling based on the change in indoor air temperatures in 

the apartments after the TRVs were installed and actual savings based on utility bills 

from the test building (all other apartments in the building received new TRVs during the 

summer of 2013).

2.1 Field Experiment

Data loggers were installed in a multifamily building in Flushing, New York, to monitor space

and heat emitter temperatures in each room of two apartments (kitchen, bedroom, master 

bedroom, and living room). These apartments were located on the floor above the boiler room 

and had identical floor plans, with equal areas of exterior wall/glazing. The contractor who 

installed the TRVs throughout the rest of the building placed the TRV sensor components for 

each radiator outside the convector cabinet, several inches away on the exterior wall. This was 

not the manufacturer’s preferred location; the sensors should be positioned below the convector

and inside the convector cabinet to be protected from unintended impacts and wear. The sensors 

in the two test apartments, however, were installed in the suggested location. The existing 

radiator air vents were not replaced with new vents at the time of the TRV installation.

The building has a one-pipe steam system with convectors. The natural gas-fired (with 

interruptible oil service) boiler (Weil McLain model H1388S) is controlled with a Heat Timer 

MPC outdoor reset boiler controller. The existing control settings (reset curve “I” for both day 

and night throughout the heating season; boiler operating pressuretrol set to operate at 2–5 psi)

remained unchanged after the building-wide TRV installation was completed in the summer of 

2013. The TRV manufacturer recommended steam pressure no greater than 2 psi be used in 

conjunction with the TRVs.

The following measurements were collected during the 2013–2014 winter season:

• Surface temperature of each radiator with T-type surface mounted thermocouples 

mounted at three locations on each radiator (inlet, midpoint, and vent end)

• Indoor space temperatures in each of the four rooms in each apartment (consistent with 

the Building America indoor temperature measurement protocol (Metzger & Norton, 

2014))

• One T-type thermocouple near the window above each radiator, to determine if the 

window is opened.
10

The following equipment (Table 3) was used to perform the measurements described above.

Table 3. Data Collection Equipment

Measurement Equipment Frequency

Radiator Surface Temperatures U-12 datalogger with T-type thermocouples 1 min

Near-Window Temperature U-12 datalogger with T-type thermocouples 1 min

Indoor Air Temperature UX100-001 datalogger 5 min

The sensors were installed on October 8, 2013, before the start of the heating season. Data were 

collected throughout the first half of the winter. TRVs were installed in the two test apartments 

in January 13, 2014, and data collection continued through the end of the heating season. 

In the case of the older, previously used TRVs (apartment 1F), the valve bodies and seats had 

seen approximately 10 years of service in a prior installation. The operators installed with them 

had not been previously used but were also at least 10 years old and had experienced normal 

space temperature fluctuations while in storage. As the two likely points of failure are the valve 

seats (which may lose their tight seal) and the bellows elements (which are subject to frequent 

expansion and contraction), heavily used TRVs in one of the two monitored apartments were 

installed to determine whether the older equipment had suffered any loss of performance.

Figure 3. Typical convector with datalogger, thermocouples, and TRV installed

2.2 Field Data Analysis

2.2.1 Field Data Quality and Processing

Hour-interval outdoor temperature records from the nearby LaGuardia airport weather station 

were interpolated against the space temperature, radiator, and near-window temperature readings 

obtained from the test site. Some temperature readings, especially those taken from the radiator 
11

points, began to degrade over the course of the collection period. The frequent and extreme 

fluctuations in the temperature of the radiator surface may have caused the thermocouples to

malfunction. Radiator temperatures lower than 30°F and higher than 220°F were deemed 

inaccurate and removed from consideration, as were any near-window temperatures recorded as 

lower than 0°F or higher than 100°F. Space temperatures were recorded on separate sensors and 

did not show any errant readings.

2.2.2 Graphical Analysis

Individual data sets for each of the eight rooms in apartments 1F and 1A were graphed alongside 

outdoor temperature and analyzed visually. This revealed some of the data quality issues 

mentioned above as well as several aspects of heating system operation and resident behavior.

The sample graph below (Figure 4) shows typical temperature data from all six sensor channels 

over a 2-day period in January 2013 for the master bedroom in apartment 1F:

• The blue line at the bottom is outdoor temperature. The bottom of the trough on the righthand side is a result of the early January “polar vortex.” 

• Space temperature hovers at 65°–75°F for this time period. 

• The blue, red, and green lines indicate the temperatures of the radiator inlet, midpoint, 

and valve end, respectively. The inlet is cooler than the midpoint, likely because cooler 

condensate drains back through the inlet.

• From left to right, the figure shows that the boiler does not run during the relatively mild 

night of January 6, indicating that the boiler control nighttime setback feature is working. 

Regular hourly cycles occur over the course of both days, and the longer boiler cycle in 

the middle of the graph reflects the “morning boost” feature of the boiler control. 

• Longer boiler cycles are observable during the colder day of January 7.

• The abrupt drops in near-window temperature (purple line) indicate window opening 

events.

Figure 4. Typical data from apartment 1F master bedroom
12

Several graphs indicate an imbalance in steam distribution. Figure 5 shows that the living room 

radiator in apartment 1F was not filling with steam until after the TRV installation date of 

January 13. This trend held consistently for the entire pre- and post-TRV installation periods for 

that room. The cause may have been that the inlet valve was in the closed position, but more 

likely the air vent had an unintended obstruction that was alleviated when the new vent was 

installed as part of the TRV installation.

Figure 5. Radiator temperatures in apartment 1F living room evidencing heating imbalance 

between pre- and post-TRV installation periods 

Figure 6 overlays temperature data from the small and master bedrooms in apartment 1F for a 3-

day period that occurred post-TRV installation. The midpoint temperature of the small bedroom 

radiator (shown in pink) never exceeds 150°F, even though the master bedroom radiator 

temperature (shown in dark red) shows that the boiler is operating. This cannot be attributed to 

the function of the TRV in the small bedroom because its space temperature (indicated by the 

bright red line) remains mostly in the low 60°F range at the times when the radiator is not getting 

heat.
13

Figure 6. Radiator temperatures indicating uneven heat distribution between small and master 

bedrooms in apartment 1F

Window opening was fairly simple to observe graphically, but somewhat difficult to quantify. 

The two graphs in Figure 7 show that window opening events (circled in green) appear as sudden 

drops in near-window temperature, sometimes accompanied by a more erratic trend line. Also,

the near-window temperature was in some cases governed more by the radiator temperatures, 

and in other cases tracked fairly well with the outdoor temperature. A reasonable threshold for

the near-window temperature below which windows were considered to be open was 55°F 

(except for apartment 1F’s living room window before the TRV installation; because of cooler 

average window temperatures 45°F was deemed a better fit).
14

Figure 7. Window opening indicated by temperature drops, circled in green, in apartment 1F living 

room (top) and apartment 1A living room (bottom)

2.2.3 Pre- and Post-Thermostatic Radiator Valve Installation Space Temperature 

Comparison

As a measure of TRV efficacy, space temperature readings for each room in apartments 1A and 

1F were averaged and compared before and after the TRV installation date. To isolate the 

influence of the TRVs on space temperature from other variables, space temperature readings 

that corresponded with window opening events and boiler cycle “off” time were excluded from 

these averages. Window opening was defined as above; radiator temperatures lower than 100°F 

were judged to represent boiler cycle “off” time. The resulting space temperature averages are 

given in Table 5 in Section 3.
15

2.2.4 Mild and Severe Heating Weather Space Temperature Comparison

Recorded indoor space temperatures for each of the eight rooms were also grouped along two 

outdoor temperature bands: mild heating weather and severe heating weather. These bands 

included all times in the post-TRV installation data collection period when the outdoor 

temperature was 35°–50°F and 10°–25°F, respectively. The average and standard deviation were 

calculated for each group and are shown in Table 6 in Section 3. 

2.3 Utility Bill Analysis for Entire Test Property

Natural gas consumption records for the property were obtained from the utility company for a 

27-month period (April 4, 2012 to July 1, 2014), capturing full heating seasons before and after

TRVs were installed building-wide in the summer of 2013. Each billing period corresponded 

roughly to a calendar month. Because the interruptible oil billing periods are based on delivery 

dates and do not necessarily correspond to the usage period, oil records were not collected. To 

discount unknown fuel oil heating consumption from the analysis, gas billing periods with more 

than 10 hours of <20°F outdoor air temperature (the fuel switchover temperature for this 

property) were removed from the analysis discussed below.

Daily base load usage was estimated as the average daily use of the main gas account during 

non-heating season billing periods (June 1 to September 31). This approach neglects the 

influence of seasonal variations on cold water inlet temperature and occupant behavior, which 

may be significant. Average heating therms/day were calculated for each heating season 

(October 1 to May 31) billing period by subtracting estimated daily baseload usage from each 

bill. Monthly billing period average temperatures were calculated from LaGuardia airport 

weather station records. The graph in Figure 8 shows average heating therms/day plotted against 

average monthly temperature for the heating seasons before (in orange; includes 2 months from 

the 2011–2012 heating season as well) and after (in blue) the building-wide TRV installation.

Linear regression lines for the pre- and post-building-wide TRV installation data points are also 

shown in the graph.

2.4 Building Energy Optimization Program Conceptual Payback Model

Energy savings from reduced average indoor space temperature were estimated using Building 

Energy Optimization Program (BEoptTM) software. The indoor space heating set point for a

typical two-bedroom apartment with three radiators and a single exterior, uninsulated masonry 

wall was varied from 80°F down to 71°F (the baseline condition consistent with the Building 

America benchmark) to determine the reduction in annual natural gas heating therms per degree 

of reduced space temperature. The present dollar value of the therms saved over the useful life of 

the TRVs was compared against their installed cost to determine the minimum level of 

overheating that would allow TRVs to have a positive payback. Modeling assumptions are given 

in Table 10 in the Appendix, and the financial calculation assumptions are given below in Table 

4.
16

Table 4. Cost-Benefit Financial Assumptions

3 Results

3.1 Effect of Thermostatic Radiator Valves on Indoor Space Temperature

The pre- and post-TRV installation space temperature averages given in Table 5 show a 

reduction of 2°F for apartment 1A (in which the new TRVs were installed) and an increase of 

1°F for apartment 1F (in which the old TRVs were installed). Three rooms in the study 

experienced a reduction in space temperature, three had no change, and in two rooms the average 

recorded space temperature increased.

Table 5. Average Pre- and Post-TRV Space Temperatures

Apartment 1A—New TRVs Apartment 1F—Old TRVs

Room Pre Post Room Pre Post

Kitchen 79 77 Kitchen 75 77

Small Bedroom 73 70 Small Bedroom 71 71

Living Room 76 76 Living Room 73* 75

Master Bedroom 77 74 Master Bedroom 71 71

Average 76 74 Average 73 74

*data where near-window temp < 55°F was excluded to account for window opening

3.2 Thermostatic Radiator Valve Performance in Mild and Severe Heating 

Weather

This analysis was intended to measure TRV performance during varying weather conditions, but 

the space temperature averages and standard deviations given below (Table 6) appear to illustrate 

other aspects of the building’s heating system. From mild to severe cold weather, both apartment 

averages show a 3°F reduction in space temperature, likely indicating that the slope of the boiler 

outdoor reset control does not match the rate of change in heat loss experienced by the building 

from mild to severe cold. The average standard deviation for apartment 1A is 2.6°F in both mild 

and severe weather, while it ranges from 3.5°F to 3.6°F for apartment 1F. Because the space 

temperature swings are nearly constant from mild to severe cold outdoor conditions but vary

between the two apartments, this may be caused by a tendency for the residents in apartment 1F 

to open their windows more often.

Installed Cost of Three TRVs $420

Useful Life 15 years

Discount Rate 3%

Cost per Natural Gas Therm $1.10
17

Table 6. Mild and Severe Heating Weather Space Temperatures (°F)

Apartment 1A

Room

Mild Weather 

Space 

Temperature

Mild 

Standard 

Deviation

Severe Weather 

Space 

Temperature

Severe 

Standard 

Deviation

Kitchen 76 2.9 76 2.9

Small Bedroom 71 2.9 67 2.7

Living Room 76 2.2 73 2.3

Master Bedroom 75 2.3 73 2.4

Average 75 2.6 72 2.6

Apartment 1F

Room

Mild Weather

Space

Temperature

Mild 

Standard 

Deviation

Severe Weather 

Space 

Temperature

Severe 

Standard 

Deviation

Kitchen 76 4.6 73 4.7

Small Bedroom 71 3.2 68 3.5

Living Room 75 2.5 72 2.6

Master Bedroom 72 3.8 70 3.7

Average 74 3.5 71 3.6

3.3 Effect of the Building-Wide Thermostatic Radiator Valve Installation on 

Space Heating Energy Use

Analysis of natural gas consumption for the winter seasons before and after the building-wide 

TRV installation shows that the number of heating therms used per day, normalized by average 

outdoor air temperature, stayed nearly constant (Figure 8). Possible reasons for the lack of 

heating energy savings are given in Section 4.
18

Figure 8. Heating fuel usage versus average outdoor temperature before and after building-wide 

TRV installation

3.4 Cost-Benefit Conceptual Model Results

A median cost of $140 per installed one-pipe TRV was obtained from consultant interviews and 

contractor quotes for materials and labor, making the total cost of three TRVs in the modeled 

apartment unit $420. Using 71°F as the post-retrofit target space temperature, annual heating 

therms saved, and the present value of the dollar savings over the useful lifetime of the TRVs for 

pre-retrofit overheating conditions are shown in Table 7. Following these modeled savings 

projections, the installation of TRVs would have a positive payback in cases where the existing 

space temperature exceeds 76°F and the post-retrofit target temperature of 71°F is achieved. 

Table 7. Present Value of Fuel Savings over Life of TRV

Pre-TRV 

Temperature

Annual 

Therms 

Saved

Present Value 

of Fuel 

Savings

71°F – –

72°F 4 $53

73°F 8 $105

74°F 15 $197

75°F 23 $302

76°F 31 $407

77°F 38 $499

78°F 44 $578

79°F 50 $657

80°F 55 $722
19

3.5 Resident and Superintendent Interviews

The residents of apartments 1A and 1F were given an initial brief survey to gain some insight 

into their prior behavior and attitudes with regard to the building’s heating system. Both parties 

had lived in the building for 5 or more years and despite experiencing some discomfort related to 

both over- and underheating, neither had brought the problem to the attention of the building 

superintendent. The elderly couple in 1A commented that their overall thermal comfort was 

good; the family living in apartment 1F reported that while the apartment was comfortable in 

mildly cold weather, during severe cold the bedrooms became too cold. Both groups stated that 

they opened windows to mitigate overheating.

An informal follow-up with these same residents revealed that they perceived the TRVs 

positively. However, the residents in apartment 1F still reported underheated bedrooms, and the 

data collected show that the windows were still being opened regularly in both apartments during 

the heating season even after the TRVs were installed.

The building superintendent also conveyed his opinion that the TRVs had positively affected 

comfort in the winter following the building-wide installation.
20

4 Discussion

The results of the analysis at the apartment and building levels do not suggest that TRVs 

significantly reduced average space temperature and heating fuel consumption. However, it 

would not be reasonable to conclude from the data collected that TRVs do not effectively control 

space temperatures. Several critical cofactors that appear to drive space heating levels and fuel 

use at the test site were identified: the building’s heating system equipment, condition and 

operating parameters, and resident behavior. General considerations regarding TRVs also are 

discussed in Section 4.1.

4.1 Influence of Steam System Configuration on Thermostatic Radiator Valve

Performance

4.1.1 Air Venting Imbalance

The radiator temperature data exhibited signs of uneven steam distribution between radiators, 

likely caused by uneven or inadequate venting on the steam mains and risers, and air vent failure 

at the radiator. Consistent steam was not entering at least one of the eight radiators monitored 

before the TRVs were installed (Figure 5), and even after the installation, the data show that 

steam was not arriving as quickly to some radiators as it was to others (Figure 6). This is a 

common problem with one-pipe steam systems (Choi, Ludwig, & Brand, 2012). If this is indeed 

a building-wide issue at the test site, TRVs installed in underheated rooms would not be expected 

to limit or lower the average space temperature. 

4.1.2 One-Pipe Convector Radiator Considerations

There are several key differences between the application of TRVs in a one-pipe convector 

radiator system versus those with two-pipe systems and/or cast-iron radiators. With two-pipe 

steam, the TRV can be placed at the radiator inlet where it can block steam entirely from 

entering the radiator when the TRV is closed. In a one-pipe steam application, TRVs are 

installed at the vent side of the radiator, principally because of debris and condensate clogging 

issues at the inlet. A one-pipe TRV in this position can work in either of two ways: 

• If the space temperature has already satisfied the sensor and the TRV is closed before a 

steam cycle begins, it can prevent the entire mass of air in the radiator from exiting the 

radiator body. The cycle runs and no heat is added to the space (with one caveat that will 

be discussed in Section 4.1.3).

• If the TRV is open at the beginning of a steam cycle and closes when the air has been 

only partly displaced from the radiator body, it will effectively limit the volume of 

radiator body that can receive steam during the remaining steam cycle.

The second scenario requires that once a steam cycle begins, the TRV sensor is able to actuate 

the valve in a shorter time span than that in which the air is displaced from the radiator. A large 

cast-iron radiator can take 8–10 minutes to fill completely with steam, whereas convectors will 

fill in 2 minutes or less, as shown by the data collected for the test apartments. The reaction of 

the TRV has to happen, therefore, in less than 10 minutes from the beginning of the steam cycle 

for cast iron and in less than 2 minutes for convectors. The sensor material begins to expand 

immediately upon a change in sensor temperature (Boss, 2014); therefore, the limiting factor is 

more likely to be the time required to establish a convection current within the room that bathes 
21

the TRV sensor in air that exceeds its set point temperature. The location in which the TRV 

sensor is placed within the room is then a critical factor in responding to the higher temperatures 

in the convective current. Also, the TRV temperature set point may, relative to the desired space 

temperature, impact this response time. 

Precise data showing that room convection current establishment times allow TRVs to function 

in this regard with relatively fast-filling convector radiators were not collected as part of this 

study, but could be useful in providing guidance on TRV installations, particularly sensor 

placement.

4.1.3 Boiler Control Set Points

Boiler operating pressure and outdoor reset curve settings could affect both the ability of TRVs 

to control individual radiator heat output and the building-wide effect of TRVs on heating fuel 

usage. Although the TRV manufacturer’s recommended limit for operating steam pressure was 2 

psi above atmospheric pressure, the steam boiler in this study was set to operate at a range of 2–5

psi. Neglecting for pressure drops along the steam mains and risers, 5 psi at a radiator would 

compress the trapped air volume by approximately 25% and, therefore, allow that radiator to 

emit 25% of its maximum heat output, even when the TRV has successfully closed. If the 

operating pressure could be reduced to a maximum of 2 psi, this would allow only 12% of the 

radiator to fill with steam in the same case, increasing the ability of the TRV to limit radiator 

heat output.

Pressure and volume of a gas in a vessel share the relationship: 

Vnnn = Vooo × Pooo ÷ Pnnn

In the case of 5 psi of steam pressure at the radiator, this would compress the trapped radiator air 

to a percentage of its original volume:

1 × 14.7 ÷ (14.7 + 5) = 75%

In the case of 2 psi of steam pressure at the radiator, this would compress the trapped radiator air 

to:

1 × 14.7 ÷ (14.7 + 2) = 88%

The TRVs used in this study were built to withstand up to 15 psi without risk of mechanical 

failure. (This refers to the close-off pressure of the valve. Of course, 15 psi as an operating range 

for the steam system would be grossly inappropriate for the reasons discussed above.)

The outdoor temperature boiler reset curve set point of “I” was maintained throughout the preand post-TRV installation analysis periods. This may be desirable and necessary—even with 

TRVs installed on a well-balanced and properly maintained steam heating system—to allow

longer radiator output times for spaces that experience higher rates of heat loss while the 

building as a whole is satisfied sooner, allowing the boiler to shut down earlier in most cycles

and save energy. Conversely, if the collective action of the TRVs causes the boiler to short cycle 

at the original setting, the reset curve set point may need to be reduced and the resulting 

reduction in “cycle-on” time would be expected to reduce fuel consumption. Unfortunately, 
22

boiler runtime and potential short-cycling cannot be inferred from the radiator temperature data 

that were collected in this study, because steam distribution systems are designed to maintain 

steam pressure for some time after the burner turns off.

Lastly, the manufacturer’s operation instructions for the outdoor temperature reset boiler control 

indicate that the control should be set for 30-minute cycles with convector radiators, as opposed 

to 60- or 90-minute cycles for cast-iron radiators; however, the control at the test site was set for 

60-minute cycles for the duration of the study. Because convector radiators have significantly 

less mass than cast-iron radiators, they do not store thermal energy, and continue to release that 

energy into the conditioned space once a boiler cycle is complete, in the same manner as castiron radiators. Shorter, more frequent boiler cycles are recommended for use with convectors for 

this reason. The longer steam cycles in use at the test site may increase the daily temperature 

swing within the apartments and decrease the frequency with which the TRVs can close off a 

partially filled radiator at the beginning of a steam cycle.

4.2 Existing Level and Character of Overheating

The space temperature data collected from the two test apartments do not show extreme 

overheating before the TRVs were installed in those two apartments. The average pre-TRV 

installation space temperatures were 76°F and 73°F, respectively, in apartments 1A and 1F. If 

these temperatures are representative of the whole-building average, this would reduce (but not 

eliminate) the potential for the TRV retrofit to reduce building-wide space heating demand 

compared to buildings where the space heating average temperature is 80°F or higher. 

Further, the pre-TRV installation difference of 3°F average space temperature between the two 

apartments is slight. If the variation in average temperature throughout the building is similarly 

small, this would also present less opportunity for TRVs to reduce overheating compared to a 

building in which heating demand varies greatly from apartment to apartment. For example, in a 

building where some apartments have large south-facing windows (high solar loads), a building 

with a wing exposed to high wind-wash, or one in which the residents’ thermal comfort levels 

vary greatly, the superintendent is typically forced to maintain a high average space temperature 

to satisfy the coldest apartments. TRVs could reduce this need by limiting heat emitter output in 

rooms with less heat demand while the boiler output continues to satisfy rooms with greater heat 

demand. With this consideration in mind, it is worth noting that the two test apartments both had 

predominantly northern exposure; this may have legitimately increased the demand in those 

apartments relative to other parts of the building where the TRVs may have more effect.

4.3 Resident Behavior and Perception

Although difficult to quantify, frequent window opening by the residents in apartments 1F and 

1A was apparent from the collected temperature data both before and after the TRVs were 

installed. Depending on the frequency and degree to which window opening occurs, a given 

TRV sensor might never exceed its set point temperature and therefore, never close to limit 

radiator output.

Some of the positive perception of the TRVs relayed by the residents and superintendent, aside 

from potential psychological value, may be due to a real improvement in thermal comfort caused 

by higher heat output post-TRV in the case of the room radiator vent that was obstructed and not 

receiving steam before the TRV installation.
23

4.4 Ventilation System

Prewar buildings such as the one in this study are typically not served by a central mechanical 

ventilation system to bring fresh air into the living spaces and exhaust stale air and odors from

the kitchens and bathrooms. Rather, operable windows controlled by building residents are the 

principal means of ventilation. This is a case where the design of the building is intrinsically 

sensitive to tenant behavior; cooking habits and individual preferences are expected to vary the 

ventilation levels. Window ventilation is a separate phenomenon from window-mediated space 

temperature control, but may also contribute to improper TRV function.

4.5 Thermostatic Radiator Valve Considerations

The experiment and analysis are subject to the following potential uncertainties:

• Variety of TRV makes and models. TRVs are manufactured by a number of companies, 

each with multiple models that are subject to design modifications over time. In addition, 

there are two main types of TRV operation: wax and bellows types. Bellows types can be 

either liquid or vapor filled. Under this plan, only one old TRV model and one new 

model were tested, potentially limiting the applicability of the results. Because TRVs 

with vapor-filled bellows are commonly available and reportedly the most accurate

(Boss, 2014), these were used for both the old and new TRVs in this study.

• TRV adjustments. Despite requests not to do so, residents may adjust the knob on the 

adjustable TRVs in an effort to control heat over the course of the study monitoring 

period, potentially affecting the results. Only the TRVs in the two temperature-monitored 

apartments in this study were locked to prevent adjustment after installation. The 

remaining TRVs were not locked, but the thermostatic operators were located within 

convector enclosures, making tampering unlikely. 

• Radiator inlet valve seals and other connections. TRVs, as well as radiator air vents, 

function on the premise that they can control the movement of air within a radiator. To 

the extent that any leaks occur at threaded connections within the radiator assembly or at 

the inlet valve seal, this function may be compromised and/or steam may escape directly 

into the conditioned space.

• TRV sensor placement. The TRVs installed in most of the apartments (except the two 

monitored apartments) were installed with their temperature sensors outside of the 

convector cabinets where they were not in the direct return air stream to the convector. If 

these wall-mounted TRV sensors experienced lower temperatures as a result of their 

location, more heat may have been delivered to the spaces than would have been the case 

if the sensors were installed beneath the convectors; the opposite is also possible. This 

may have affected the utility bill analysis. Infiltration by unconditioned air (drafts) 

through small gaps in window frames and radiator cabinet framing could also cause a 

nearby TRV sensor to read low temperatures that do not reflect the average ambient 

temperature.

4.6 Cost-Benefit Analysis

During interviews conducted as part of this test plan development, a wide range of costs were 

cited by industry members, varying by nearly a factor of three. These represent real costs with 

variation attributable in part to the product used and in part to the installation method employed 
24

(i.e., do-it-yourself or by licensed contractors). As a result, in some circumstances, TRVs can be 

cost effective, while in others they are not affordable for whole-building solutions and possibly 

suitable only for specific problem (i.e., overheated) areas. The threshold of a minimum 77°F 

overheating in order for TRVs to have a positive payback is higher than that found in the 

McNamara (1995) study (75°F). However, this is explained by the much slower rise in the cost 

of natural gas compared to that of TRV installation (in the McNamara study, the total installed 

cost was $50 per TRV, and natural gas was $0.70/therm).

4.7 Impacts and Tradeoffs

Several factors should be considered in the deployment of TRVs (see Table 8).

Table 8. TRV Impacts and Tradeoffs

System Impact and Tradeoff, If Any

Occupant Comfort

TRVs are intended to reduce overheating, which improves occupant 

comfort. They also provide an improved level of occupant control 

over heating. However, slow response times and a lack of 

understanding of how they operate have been reported to lead to 

tenant dissatisfaction (Colgrove, 2012).

Occupant Health and 

Safety

Overheating in winter can result in excessively low humidity levels, 

which can have negative health consequences.

Building and 

Equipment 

Durability

Reducing overheating should decrease energy use, and thus the load 

on the heating system, possibly increasing the equipment’s lifetime.

Building Code 

Compliance Issues

TRVs should maintain building owners’ ability to comply with 

local minimum heat ordinances.

Building and 

Equipment 

Maintainability

TRVs are another component and as such must be maintained;

however, most interview respondents indicated that they require 

little or no maintenance.
25

5 Conclusions

5.1 Research Questions

This research addresses the following questions: 

1. What are common prevailing opinions and perceptions regarding TRVs among the 

building community and how does this affect their deployment?

Several of the market professionals’ assertions recorded in the introduction to this study were 

reflected in the results of this work (see Table 9).

Table 9. TRV Comments and Findings

Comment Finding

Tenants Continue To Open Windows

With TRVs

In the initial winter heating season for which data 

were collected, the two monitored residents 

continued to open windows regularly.

Problematic Remote Temperature Sensor 

Placement

TRV sensors were installed contrary to the 

manufacturer’s suggestion in all but two of the 

apartments at the test building; however, a 

conclusive effect on space temperature has not 

been determined.

Failure To Ensure That Steam System Is 

Properly Commissioned To Work With 

TRVs

This study found some evidence of steam 

distribution imbalance and suboptimal steam 

pressure that would negatively affect TRV 

performance.

Apartment-Level Control Seems To 

Provide Increased User Comfort Mainly 

Due To Psychological Factors

There is qualitative evidence that lends some 

support to this idea.

2. How effectively do new and old TRVs control indoor temperature under various outdoor 

conditions?

Conclusive measures of TRV performance in varying outdoor weather conditions and measures 

of the efficacy of old versus new TRVs could not be obtained from the results of the test plan 

because of the same confounding factors mentioned below.

3. How much space heating energy can be saved through TRV installations and how cost 

effective are TRV retrofits in low-rise multifamily buildings?
26

McNamara (1995) showed an average of 15% reduction heating fuel consumption that was not 

borne out by the analysis of the initial post-TRV installation heating season’s utility bills in this 

study. However, several possible reasons are not attributable to a lack of proper performance by 

the TRVs, including existing steam distribution imbalance, possible TRV sensor location issues, 

a persistent window-opening habit by the monitored apartments’ residents, and a failure to 

optimize the boiler control set points (especially the upper limit of operating steam pressure) as 

an integral part of the TRV retrofit.

The results of the BEopt-modeled TRV savings showed that average space temperature reduction 

needs to be in the area of 6°F in order for TRVs to be cost effective. The assumptions made—

wall and window R-values, wall-to-window ratio, infiltration rate, ventilation rate, heating 

system efficiency, fuel cost per Btu, and number of radiators—should reasonably approximate an 

older multifamily building. However, each of these factors will vary considerably from project to 

project, and the modeled break-even point should be understood as an approximation that cannot 

be applied universally.

The lack of actual heating fuel savings witnessed in this study underscores the need, especially 

given the utility company and state agency financial support for TRV installations, to include 

whole steam system commissioning alongside or as a prerequisite to TRVs in order to maximize 

the effect of public incentives and private investment. 

5.2 Key Lesson: Monitor Space Temperatures and Balance Steam Distribution

Failed air vents and uneven steam main venting are critical to address either in conjunction with 

or before a TRV installation. In some apartments, this is likely to increase heat supply, as 

happened in the test here. An increase in space temperature for underheated apartments would be 

a desirable result of air vent maintenance and steam vent balancing; however, this may lower 

expected heating fuel savings if good space temperature data have not been obtained beforehand 

to provide an accurate picture of existing building overheating. Once underheating has been 

solved, TRVs may still provide a useful upper bound to apartment space temperatures. By 

contrast, radiator and steam main air vents were inspected for function and proper size, and 

replaced as necessary in the buildings that were included in the McNamara (1995) study.

Ensuring that the operating steam pressure and boiler control set points are appropriate for TRVs 

and the distribution equipment (convectors or cast-iron radiators) involved is also a must for onepipe steam retrofits.

Monitoring the existing space temperature spread and establishing the causes of variation before 

a retrofit strategy is chosen would allow the consultant and building owner to better assess the 

potential benefits of a whole-building TRV retrofit, selective installation of TRVs in some units, 

or simply balancing the steam distribution venting. If feasible, an iterative approach in which the 

venting and boiler controls are tuned first and TRVs are subsequently considered if overheating 

control opportunities remain, would be an ideal means of minimizing unnecessary costs.

5.3 Resident Interaction

Tenant behavior in the monitored apartments proved difficult to predict and influence. If any 

conclusion is to be drawn, it is that space heat imbalance was present at the test site and the 

stated reason for the tenants to open windows. From this, it is plausible that perhaps providing 

balanced steam heat and consistent space temperatures is again a prerequisite to TRV 
27

installation, but for the added reason of reducing the need and habit for tenants to regulate space 

temperatures in this way. The inherent variability in window ventilation still would remain, 

however. The building owners involved in the McNamara (1995) study held resident workshops 

prior to TRV installation to familiarize their tenants with the technology and its proper use; this 

may have had some influence on the more favorable outcomes of that study.

5.4 Future Research

Several compelling questions arose over the course of this test plan that fell outside the study 

scope but could potentially reveal important additional aspects of TRV function.

• The time lapse between the initiation of a steam cycle and TRV closure, and the period of 

time required for radiators to fill with steam, play a large role in the ability of TRVs to 

limit radiator heat output. These time frames are governed by convection current 

establishment within the space, the placement of the sensor to best intercept that current, 

and the volume of the radiator body/flow rate of air through the vent. 

Typical convection current establishment times should be identified and sensor placement 

best practices defined. In conjunction with this, fill times for standard radiator types 

should be better quantified so that practitioners can identify whether given distribution 

equipment will be a “good fit” for TRVs and optimize the installation for maximum 

space temperature control.

• Mutual interactions between TRVs and the steam boiler affect both the ability of the 

TRVs to limit radiator heat output and overall boiler runtime, both of which determine 

fuel savings. As discussed in this test plan, the boiler steam supply pressure range could

affect the volume of steam that TRVs allow to enter radiators in a one-pipe system, and 

this effect needs to be further quantified. The collective effect of TRV action on 

distribution system pressure should, in turn, have some effect on burner runtime within 

each steam cycle; this effect should be understood so that excessive burner short-cycling 

can be avoided and outdoor reset curve set points can be optimized post-TRV installation.
28

References

ARIES Collaborative. (2011). Hydronic Heating Systems Expert Meeting. New York: U.S. 

Department of Energy.

Boss, B. (2014, July 10). (J. Dentz, Interviewer)

Choi, J., Ludwig, P., & Brand, L. (2012). Steam Balancing and Tuning for Multifamily 

Residential Buildings in Chicagoland. Chicago: U.S. Department of Energy.

Colgrove, M. (2012, December 26). (J. Dentz, Interviewer)

Dentz, J., Henderson, H., & Varshney, K. (2013). Overheating in Hot Water and Steam Heated 

Multifamily Buildings. New York: Advanced Residential Integrated Energy Systems 

Collaborative.

Gifford, H. (2003, June). How to make a two pipe steam heating system really work. 

www.ABMA.com, pp. 26-33.

Gifford, H. (2004, January). Choosing Thermostatic Radiator Valves. www.ABMA.com.

Jayne Choi, P. L. (2012). Steam System Balancing and Tuning for Multifamily. Partnership for 

Advanced Residential Retrofit.

Madnick, P. (2014, July 25). (J. Dentz, Interviewer)

McNamara, M. (1995). Thermostatic radiator valve (TRV) demonstration project. NYSERDA 

final report, prepared by EME group.

Metzger, C. E., & Norton, P. (2014). The Building America Indoor Temperature and Humidity 

Measurement Protocol. Golden, CO: National Renewable Energy Laboratory.

National Renewable Energy Laboratory. (2012). Building America Critical Path Milestones.

Golden, CO: Department of Energy.

New York City Administrative Code §27-2029. (2014). Retrieved December 3, 2014, from Laws 

of New York: http://public.leginfo.state.ny.us

Peterson, G. (1985). Achieving even space heating in single pipe steam buildings, A joint 

research project of Minnegasco and the Minneapolis energy office.

Tahersima, F., Stoustrup, J., & Rasmussen, H. (2011). Eliminating oscillations in TRVcontrolled hydronic radiators. 50th IEEE Conference on Decision and Control and 

European Control Conference. Orlando, FL.

Tahersima, F., Stoustrup, J., Rasmussen, H., & Nielsen, P. (2010). Thermal analysis of an HVAC 

system with TRV controlled hydronic radiator. Automation Science and Engineering.

Trüschel, A. (2002). Hydronic heating systems: The Effect Of Design On System Sensitivity. 

Ph.D. Dissertation. Göteborg, Sweden: Department of Building Services Engineering, 

Chalmers University of Technology.

U.S. Department of Energy. (2013, November 26). Thermostats. Retrieved August 1, 2014, from 

Energy.gov: http://energy.gov/energysaver/articles/thermostats

Weker, P., & Mineur, J. (1980). A performance index for thermostatic radiator valves. Applied 

Energy 6 , (3): 203-215.

Xu, B., Fu, L., & Di, H. (2008). Dynamic simulation of space heating systems with radiators 

controlled by TRVs in buildings,. Energy and Buildings 40, (9): 1755–1764.
29

Appendix

Table 10. BEopt Modeling Assumptions

Input Value Input Value

Total Finished Floor Area 750 ft2 Overhangs None

Bedrooms 2 Air leakage 6 ACH50

Baths 1 Mechanical ventilation None

Left Walls Attached True Central air conditioner None

Right Walls Attached True Furnace None

Back Walls Attached True Boiler Gas, hot water, forced draft, 80% AFUE

Age of Home 90 years Electric baseboard None

Orientation West (8 ft × 30 ft

exterior wall) Air source heat pump None

Neighbors None Ground source heat pump None

Heating Set Point 71°F Ducts None

Cooling Set Point 76°F Ceiling fan None

Natural Ventilation Benchmark Water heater Gas Standard

Interior Shading Benchmark Distribution Uninsulated, TrunkBranch, copper

Wood Stud None Lighting Benchmark

Double Wood Stud None Refrigerator Benchmark

CMU 8-in hollow Cooking range Benchmark

SIP None Dishwasher None

ICF None Clothes washer None

Other None Clothes dryer None

Wall Sheathing None Extra refrigerator None

Exterior Finish Brick, medium/dark Freezer None

Unfinished Attic Adiabatic roof

(R-1500) Pool heater None

Roof Material Asphalt shingles, 

medium Pool pump None

Radiant Barrier None Hot tub/spa heater None
30

Input Value Input Value

Unfinished Basement Adiabatic (R-500) Hot tub/spa pump None

Interzonal Floor R-38 fiberglass batt Well pump None

Carpet 0% carpet Gas fireplace None

Floor Mass Wood surface Gas grill None

Exterior Wall Mass 5⁄8-in. drywall Gas lighting None

Partition Wall Mass 5⁄8-in. drywall Other electric loads 1

Ceiling Mass Simulated concrete 

slab (15 Btu/°F-ft2) Other hot water loads 1

Window Areas 100 ft2 Photovoltaic system None

Windows

Double-pane, clear, 

metal, thermal

break, air fill

EPW location USA_NYLaGuardia.AP.725030_TMY3

Eaves None Terrain City
DOE/GO-102015-4586 ▪ January 2015

buildingamerica.gov