Heat Recovery from Wastewater Using a Gravity-Film

Heat Exchanger

Technology to capture heat from wastewater

DOE/EE-0247 Revised

Introduction

There are a number of uses of hot water in buildings,

including showers, tubs, sinks, dishwashers, and

clothes washers. In virtually all of these cleaning

applications, the wastewater retains a significant

portion of its initial energy—energy that could be

recovered and used. Estimates based on the U.S.

Department of Energy (DOE) test procedure for

water heaters indicate that the equivalent of

350 billion kWh worth of hot water is discarded

annually through drains, and a large portion of this

energy is, in fact, recoverable. To capture heat from

wastewater produced by all sources in a dwelling

and put it to use would require a regenerator-type,

double-walled heat exchanger—one that can capture

heat from wastewater generated by one fixture or

appliance (e.g., a clothes washer) and apply this heat

to assist another hot water demand that may occur at

a later time. If wastewater generation is concurrent

with the need for hot water (e.g., a shower), a

nonregenerative, straightforward heat exchanger

can be used. Heat exchanger systems of each type

are available for use in buildings.

The Gravity-Film Heat Exchanger

(GFX)

 The GFX is a simple heat exchanger design for heat

recovery that was developed under a grant from the

DOE Inventions Program. This straightforward

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Hot Drain Water From

Showers and Sinks

To

Fixtures

To Water

Heater

Preheated

Water

Incoming

Cold Water

Drain Water

Dishwasher

13%

Shower

43%

Clothes washer

20%

Bath

6%

Small sink 

usage

15%

Unidentified

3%

Domestic

Hot Water

Dishwasher

13%

Unidentified

3%

Small sink

usage

15%

Shower

43%

Bath

6%

Clothes washer

20%

design is a vertical, counterflow heat exchanger

that extracts heat out of drainwater (usually warm)

and applies it to preheat the cold water entering the

building. The GFX is installed into a section of

available, vertical drain line in a dwelling. The

design consists of a 3- or 4-inch central copper pipe

(that carries the warm wastewater) with 1/2-in. copper coils wound around the central pipe. Heat is

transferred from the wastewater passing through the

large, central pipe to cold water simultaneously moving upward through the coils on the outside of the

pipe. The coils are flattened a little where they touch

the pipe to increase the contact area and improve heat

transfer. The key to this patented device was the

inventor’s observation that wastewater clings in a

film-like fashion to the inside wall of the pipe as it

undergoes gravity flow in the open drain, and this

warm, falling film transfers heat through the pipe
wall to the incoming cold water that

passes through the copper coil wound

around the pipe.

The GFX has a number of advantages for

wastewater heat recovery:

• Rugged, no moving parts

• All copper construction

• Compact; replaces about five feet of

vertical drain line; can be installed

where drains are piped, including inside stud walls

• Sweat connections at each end of the

coil where line pressure exists; rubber

connectors to attach each end of the

copper pipe to the drain

• Available with multiple parallel coils

outside the central pipe to reduce pressure drop.

Because the GFX has relatively little thermal mass, it is unable to store much heat

energy for later use. Consequently, the

GFX is designed to work best when the

production of warm wastewater and the

need for hot or warm water coincide. For

example, a GFX would not be particularly beneficial for preheating water for a

bath, but it is ideal for use with showers

where the use of hot or warm water for

the shower and the production of wastewater from the shower occur at the same

time. Small sinks could also benefit from

a GFX if water flows to and from the sink

at the same time.

GFX Performance

To evaluate the performance of the

GFX in a typical residential application,

we installed a 60-inch GFX system in

the basement of a single family home in

Knoxville, Tennessee. We instrumented

the system and performed experiments to

measure its performance. The basement

layout of the water heater, entering water

piping, and distribution piping to the

home were such that the hot and cold water to the shower (as well as all other fixtures) first passed through the GFX, as

shown in Figure 1. For the evaluation, we

ran the shower at several settings, ranging

from the warmest shower temperature

available (no cold water to the shower) to

the coolest reasonable shower

temperature (90oF) by adjusting

the hot and cold water valves.

During this time, we measured

water flowrates (M) and water

temperatures (T) including the

shower temperature, T6, and the

ambient air temperature, T7. We

maintained each setting for several

minutes to allow temperatures to

reach constant values before measurements were recorded.

We found that the GFX could raise

the inlet temperature from 60oF

to 85oF by maintaining shower

conditions at 2.0 GPM and 120oF

(water heater setpoint; see

Figure 2). We noticed that, under

these conditions, the temperature

of the wastewater inlet to the GFX

unit (T3) was 12 Fo lower than the

shower temperature (T6) due to heat

losses between the shower drain and

the GFX. From measured temperatures

and water flows through the GFX during

this experiment, we calculated the total

amount of heat transferred by the GFX

and the heat provided by the water heater.

The GFX preheats the cold water to the

shower as well as the inlet water to the

water heater, and the distribution of preheating depends on the end-use (shower)

temperature, as shown in Figure 2.

In Figure 2, the total preheating by the

GFX is shown by the two lower segments

of each bar, and the upper segments indicate heat provided by the water heater.

With cooler shower temperatures, more

of the GFX contribution goes to preheating the cold water, whereas for warmer

showers (e.g., 120oF), most of the energy

from the GFX is used for preheating hot

water. Over the range of shower temperatures studied, the GFX saved about 40%

of the total energy needed for the shower

in this experiment.

Installation Hints

We used data from the experiment to develop a system model of the GFX. Based

on the system model, we determined how

several installed piping configurations to

and from the GFX as well as other factors

affect the performance of the GFX.

Balanced vs. Unbalanced Flow. There

are three basic ways that piping to a GFX

system can be installed: balanced flow,

unbalanced flow for preheating cold water, and unbalanced flow for preheating

water to the water heater. In balanced

flow, all of the water that passes into the

building also passes through the GFX

before the split to the water heater occurs.

This arrangement ensures that the

incoming water is balanced by an equal

flow of drainwater. The GFX in the

experimental study was installed for

balanced flow; accordingly, the GFX

preheats all of the shower water, including flow to the cold water side of the

shower faucet as well as flow through the

water heater to the hot water side of the

faucet. Where the flows on both sides of

the GFX heat exchanger are equal (balanced), the heat exchanger performance

is higher than for any other flow arrangement. In balanced flow, the temperature

drop of the drain water always matches

the temperature increase of the incoming

water.

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T6

T3 T5, M5

T1, M1

T7

T4

T2

GFX

Water

Heater

Figure 1. Layout of GFX system in

residential application.
and an incoming water temperature of

58oF. The results of this analysis, shown

in Figure 3, indicate that the GFX saves

more energy under balanced flow conditions for all shower temperatures—

approximately 50% savings in water

heating energy required for a shower.

In the unbalanced flow cases, we found

that the energy saved by the GFX

depended on the end-use (shower)

temperature, as shown. Savings ranged

from 30% to about 45% over all

reasonable shower temperatures for

the unbalanced flow case. Where it is

impractical to install the GFX for

balanced flow, using the GFX to preheat

water to the water heater (HW preheating

in the figure) is a reasonable option. Further, there would be no tempering of the

cold water delivered to the fixtures in the

dwelling.

Wastewater Temperature. Based on

the experiment, we determined that the

temperature of the wastewater stream

into the GFX has a large effect on GFX

performance. The temperature of wastewater entering the GFX from a 120oF

shower was appreciably cooler than the

shower temperature due to heat losses

from the drain line. Had it been

possible to insulate the drain line between

the shower and the GFX, its performance

would have been higher. In the systems

modeled, we assumed that the shower

temperature was 12oF higher than the

wastewater stream entering the GFX unit.

The impact of unbalanced flow on

GFX performance is less obvious. In

unbalanced flow configurations, the GFX

preheats the water to the cold side of the

faucet or the water through the water

heater but not both. In this scenario, the

wastewater flowrate through center drain

pipe of the GFX is higher than the water

flow through the tubing on the outside of

the GFX because the wastewater includes

cold water that bypasses the GFX. Thus

for equal shower drain temperatures and

flowrates, the rise in temperature of the

freshwater through the GFX is greater

than the change in temperature of the

wastewater as it flows through the GFX.

This apparent advantage is lost, however,

because the fresh water flow through the

GFX is smaller for unbalanced flow than

for balanced flow at a given wastewater

flowrate.

To understand how balanced or unbalanced flow affects the performance of the

GFX, we modeled three cases for a single

shower: (1) balanced flow piping to the

GFX, (2) unbalanced flow where the

GFX preheats only cold water to the

shower, and (3) unbalanced flow where

the GFX preheats water to the water

heater (see Figure 3). In accord with the

DOE Water Heater Test Procedure, we assumed a water heater setpoint of 135oC

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Figure 2. Heat exchange from GFX.

Figure 3. Energy saved by using GFX according to flow.

0

10

20

30

40

50

60

90 100 110 120

% Water Heating Energy Saved by GFX

Unbalanced Flow (HW preheating)

Unbalanced flow (CW preheating)

Balanced Flow

0

10

20

30

40

50

60

70

90 100 110 120

Supplemental energy delivered to water heater

Preheat to water heater by GFX

CW preheating by GFX

Note: 2 gpm shower; T1 = 60F; T7 = 65F

Btu/h (in thousands)

Shower Temperature (°F)

Shower Temperature (°F)
Disclaimer

This report was sponsored by the United States Department of Energy, Office of Federal Energy Management

Programs. Neither the United States Government nor any agency or contractor thereof, nor any of their employees,

makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade

name, mark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency or contractor thereof. The views and opinions of

authors expressed herein do not necessarily state or reflect those of the United States Government or any agency or

contractor thereof.

Had we assumed that there was no heat lost by the

shower drainwater before it reached the inlet to the

GFX, the water heating energy savings would be ten

times greater than that shown.

Economics

A recent field evaluation of the GFX conducted by

Pennsylvania Power and Light found the simple

payback of a residential GFX system to range from

2 to 5 years. This was based on an installed GFX

cost of $500 and electricity savings ranging from

800 kWh/y to 2300 kWh/y depending on the average

number of daily showers in each home. The

economics of the GFX improved with the number

of daily showers in the residence as expected. In

general, buildings that require large amounts of hot

water for showers (e.g., homes of families with

several children, multifamily apartments, or barracks

with showers on a common drain line) would be

ideal candidates for the GFX and would lead to

shorter paybacks.

In addition to operating cost reductions based on

energy savings alone, the GFX provides additional

benefits. By recovering heat from drainwater and

simultaneously using this heat for preheating water Attn: Dr. Carmine Vasile, President

to the water heater, the GFX effectively shortens the

time needed for the water heater to recover. This is www.gfxtechnology.com

important if the existing water heater is undersized

or if there are more showers than usual taken backto-back. Because heat is extracted from drainwater

by the GFX, the capacity of the water heating system

is increased. This means that it is possible to lower

the thermostat setting on the water heater without

directly affecting the capacity of the water heating

system. These benefits, however, depend on hot

water consumption patterns and the fraction of overall hot water consumption that is amenable to heat

recovery by the GFX.

For More Information

FEMP Help Desk

(800) 363-3732

International callers please use

(703) 287-8391

Web site: www.eren.doe.gov/femp

General Contacts

Ted Collins

Program Manager

 New Technology Demonstration

Federal Energy Management

 Program

U.S. Department of Energy

1000 Independence Ave., SW, EE-92

Washington, D.C. 20585

Phone: (202) 586-8017

Fax: (202) 586-3000

theodore.collins@ee.doe.gov

Steven A. Parker

Pacific Northwest National

 Laboratory

P.O. Box 999, MSIN: K5-08

Richland, WA 99352

Phone: (509) 375-6366

Fax: (509) 375-3614

steven.parker@pnl.gov

Technical Contacts

John Tomlinson

Oak Ridge National Laboratory

P.O. Box 2008, MS 6070

Oak Ridge, TN 37831-6070

Phone: (865) 574-0291

tomlinsonjj@ornl.gov

Randall Linkous

Oak Ridge National Laboratory

P.O. Box 2008, Bldg. 3144

Oak Ridge, TN 37831-6070

Phone: (865) 574-2043

linkousrl@ornl.gov

Produced for the U.S. Department

of Energy (DOE) by the Oak Ridge

National Laboratory

DOE/EE-0247 Revised

July 2005 Revision

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Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% post consumer waste

Summary

The GFX is a simple and effective method for significantly reducing the energy needed to produce hot

water. The savings in water heating energy depends

on the specific installation, hot water consumption

patterns, and whether the GFX is piped as balanced

or unbalanced flow; however, based on our measurements, a 30 to 50% savings in the energy needed to

heat shower water seems reasonable. Balanced flow

and, if possible, an insulated drain line to the GFX

improve its delivered performance. The impact on

overall hot water energy consumption depends on

the fraction of total hot water consumption that

simultaneously produces warm drainwater. Good

candidates for GFX application in the Federal

sector would be dormitories and barracks, health

facilities, and commercial and industrial

facilities that produce waste heat that could

otherwise be used for preheating water.

Where to go for further information

WaterFilm Energy, Inc.

P.O. Box 128, Medford, NY 11763

Tel: 631-758-6271; Fax 631-730-3918

. E-mail: gfx-ch@msn.com