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NSPS Regulated Product Applicability/Definition Proposal June 6, 2011 

Prepared for HPBA by David Menotti and Bob Ferguson

Wood Heaters 

A Wood Heater is an enclosed, solid fuel burning appliance capable of and 

intended for space heating (rather than whole house heating), or space heating and 

domestic water heating. Wood heaters may or may not include air ducts to deliver 

some portion of the heat produced to areas other than the space where the wood 

heater is located. Wood heaters include: 

Free-standing Wood Heaters –Wood Heaters that are installed on legs, on a 

pedestal or suspended from the ceiling. These products generally are safety 

listed under UL-1482, UL-737 or ULC-S627. 

Fireplace Insert Wood Heaters – Wood Heaters intended to be installed in 

masonry fireplace cavities or in other enclosures. These appliances generally 

are safety listed under UL-1482, UL-737 or ULC-S628. 

Built-in Wood Heaters – Wood Heaters that are intended to be recessed into the 

wall. These appliances generally are safety listed under UL-1482, UL-737, 

UL-127 or ULC-S610. 

Pellet Space Heaters

A Pellet Space Heater is an enclosed pellet or biomass burning appliance capable of and intended 

for space heating (rather than whole house heating), or space heating and domestic water heating. 

Pellet Space Heaters include a fuel storage hopper or bin and a fuel feed system. Pellet Space 

Heaters include: 

Free-standing Pellet Space Heaters –Pellet Heaters that are installed on legs or 

on a pedestal or other supporting base. These products generally are safety 

listed under ASTM E1509, UL-1482, ULC S627 or ULC-ORD C1482. 

Pellet Space Heater Fireplace Inserts – Pellet Heaters intended to be installed 

in masonry fireplace cavities or in other enclosures. These appliances 

generally are safety listed under ASTM E1509, UL-1482, ULC-S628 or ULCORD C1482. 

Built-in Pellet Space Heaters – Pellet Heaters that are intended to be recessed 

into the wall. These appliances generally are safety listed under ASTM E1509, 

UL-127, ULC-S610 or ULC-ORD C1482. 

Single Burn Rate Heaters [Limited Burn Rate or Utility Heater]

No recommendation at this time. [See background information section] 
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Hydronic Heaters 

Hydronic Heaters are solid fuel-burning appliances intended to heat whole buildings (or 

substantial portions thereof) either as stand-alone systems or as additions to existing heating 

systems. Hydronic heaters may be located outdoors or indoors, may be fueled by wood, pellets, 

or biomass materials, and may include alternate or backup fossil fuel burners or electrical heating 

elements. The heat produced by a hydronic heater is transferred by a heat exchange medium 

through piping from the appliance to another heat exchanger or directly to the spaces being 

heated. Domestic hot water may be produced as well. Safety standards for hydronic heaters 

include UL-2523 and CSA B366.1. Hydronic Heaters include: 

On-Demand Hydronic Heaters– Hydronic Heaters that primarily produce heat in response to 

instantaneous heating needs. These heaters generally cycle as needed to maintain a delivered 

fluid outlet temperature within a specified range. 

Full Thermal Storage Hydronic Heaters – Hydronic heaters that include adequate thermal 

storage capacity (generally in the form of insulated water tanks) to absorb all heat produced 

from at least one full load of fuel. Heat is then released to the heating system as needed from 

the storage. These appliances generally operate at one burn rate and do not cycle. 

Partial Thermal Storage Hydronic Heaters – Hydronic heaters that include thermal storage 

capacity adequate to absorb a portion of the heat produced from a load of fuel. The addition 

of some thermal storage dampens the frequency of cycling in response to heat demand. Heat 

is delivered to the heating system either directly from the heater or from storage. 

Warm Air Furnaces 

Warm air furnaces are solid fuel-burning appliances intended to heat whole buildings (or 

substantial portions thereof) either as stand-alone systems or as additions to existing heating 

systems. Domestic hot water may be produced as well. Warm air furnaces may be located 

indoors or outdoors, may be fueled by wood, pellets, or biomass materials, and may include 

alternate or back-up fossil fuel burners or electrical heating elements. The heat produced by a 

warm air furnace is typically transferred through ducting as heated air. Safety standards for 

warm air furnaces include UL-391 and CSA B366.1. 

Masonry Heaters 

Masonry heaters are site-built (using local materials or a combination of local 

materials and manufactured components) or site-assembled (using manufactured 

components), solid-fuel burning heating appliances constructed mainly of 

refractory materials (e.g., masonry materials or soapstone) in which the heat from 

intermittent fires burned rapidly in their fireboxes is stored in their refractory mass 

for slow release to building spaces. They typically have an interior construction 

consisting of a firebox and heat exchange channels built from refractory 

components, through which flue gases are routed. ASTM E-1602 “Standard Guide 

for Construction of Solid Fuel Burning Masonry Heaters” provides design and 

construction information for the range of masonry heaters most commonly built in 

the United States. The site-assembled models are generally listed to UL-1482. 
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Coal-Only Heaters 

Coal-Only Heaters are enclosed, coal-burning appliances capable of space heating, 

or space heating and domestic water heating, which have all of the following 

characteristics: 

(a) An opening for emptying ash that is located near the bottom or side of the 

appliance; 

(b) A system that admits air primarily up and through the fuel bed; 

(c) A grate or other similar device for shaking or disturbing the fuel bed or a 

power driven or mechanical stoker; 

(d) Installation instructions that state that the use of wood in the appliance, 

except for coal ignition purposes, is prohibited by law; and 

(e) A safety listing under accepted American or Canadian safety codes, as 

documented by a permanent label from a nationally recognized certification 

body. 

Cookstoves 

Cookstoves are solid-fuel burning appliances that are designed primarily for cooking food and 

that have all of the following characteristics: 

(a) An oven with a volume of 0.028 cubic meters (1 cubic foot) or greater with 

an oven rack; 

(b) A device for measuring oven temperature; 

(c) A flame/exhaust path that is routed around the oven; 

(d) A shaker grate; 

(e) An ash pan; 

(f) A ash clean-out door below the oven; 

(g) The absence of a fan or heat channels to dissipate heat from the appliance; 

(h) A cooking surface area measured in square inches or square feet which is 1.5 times 

greater than the firebox volume which is measured in cubic inches or cubic feet. 

Example: A firebox of 2 cubic feet would require a cooking surface of 3 square feet; and 

(i) A portion of at least four sides of the oven that are exposed to the flame path during the 

heating cycle of the oven while a flue gas bypass will be permitted for temperature 

control. 

Background Information 

Regulated Product Categories 

IN GENERAL: The definitions of regulated heater categories are framed more generally than 

the definition of fireplaces, which are exempt. Our thinking was that the definitions needed to be 

broad enough to assure that nothing in the category was excluded. Beyond that, the issue for 

regulated appliance categories was making sure that the appliance got matched appropriately 
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with the applicable test method and set of emission limits. These decisions are going to be 

made, in the first instance at least, by the test laboratories, and we see little room for 

disagreement in all but the most extreme possible cases. It is unlikely, for example, that there 

will be much difficulty in distinguishing between a hydronic heater and a warm air furnace, or 

between either of these appliance types and room heaters like a wood stove or a pellet stove. 

Wood Heaters

The Wood Heater product category generally continues with the definition that has been in place 

in the current NSPS, with some modest editing for accuracy and clarity, and the addition (again 

for clarity) of definitions of the subcategories that comprise the broader category. These 

products are intended for space heating and include controls to regulate the heat output. Subcategories of wood heaters include free-standing, fireplace inserts and built-ins. The safety 

listings for these products are generally specific to wood-fired space heaters although the safety 

listings can overlap between some sub-categories. For example, in the United States, fireplace 

inserts are tested and listed to the same standard as free-standing heaters (while in Canada, they 

have separate standards). As discussed in the fireplace section, the operating characteristics of 

built-in heaters are very different than those of fireplaces; however, to satisfy local building 

inspectors, some built-in heaters are tested and listed to the factory-built fireplace safety standard

since in outward appearance, they share the same installation characteristics of being recessed 

into the wall with very low clearances to adjacent combustible materials. 

Pellet Space Heaters

Pellet/Biomass Heaters have evolved appreciably since the first NSPS was promulgated. Pellet 

space heaters, intended to heat a room or rooms rather than whole buildings, now include the 

same sub-categories as wood space heaters, including free-standing, fireplace inserts and built-in 

designs. Fuel types have expanded beyond wood pellets to include grains (such as corn), shells, 

hulls and pits and other pelletized biomass such as grasses. The safety listing to ASTM E1509 is 

one way to differentiate pellet/biomass space heaters although heaters in this category are 

sometimes listed to UL solid fuel heater standards instead. The primary delineator, however, is 

the presence of the fuel storage and feed system. Hoppers are included to hold a supply of fuel 

which is automatically fed to the firebox to be burned in a specially designed burn pot. The fuel 

may be fed to the burn pot from above, from behind or from below but in all cases, the same 

result is achieved. That is, the fuel is fed to be burned in the heater on an “as needed” basis in 

response to the heat demand rather than as batch feeding of large loads of fuel typical to cord 

wood heaters. By limiting the fuel feed to only the amount needed to produce the desired heat 

output in conjunction with carefully metered primary and secondary combustion air, 

pellet/biomass combustion provides a more consistent and more predictable combustion 

environment. The unique fuel handing system, the safety listing, and the intended use to heat a 

room or rooms rather than an entire building provide clear differentiation for pellet heaters. 
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Single Burn Rate Heaters [Limited Burn Rate or Utility Heater]

We understand that this product category has been included in the NSPS to help close a loop 

hole that was created in the original NSPS. However, uncertainty on our part regarding EPA’s 

intentions for this category made it extremely difficult for us to provide a definition for the 

appliance category. In the end, we decided that the better course was to hold off on attempting 

to develop a definition until we had a clearer understanding of EPA’s intentions for this 

subcategory. 

Hydronic Heaters 

Hydronic Heaters as a product category include a number of variations covering where they are 

installed, how they deliver the heat that is produced and the type of fuel burned. Hydronic 

heaters can be pressurized or atmospherically vented. However, hydronic heaters are building 

heaters, not room heaters and are often tied into existing heating systems within the building. 

Outdoor hydronic heaters are located external to the building being heated and typically include 

a weather-tight enclosure. Indoor hydronic heaters are located inside the building envelope. 

There are hydronic heaters that burn cordwood and others that burn particulate biomass fuels like 

wood pellets, wood chips, granular fuels (for example: corn, shells, hulls or pits) or other 

pelletized biomaterials (for example, grasses). All hydronic heaters deliver the heat produced as 

a heated fluid, most often water or in some cases as steam. Hydronic heaters typically include 

safety listings to UL-2523 or to CSA B366.1. 

Hydronic heaters can be further sub-categorized by the way they produce and deliver heat. Ondemand Hydronic Heaters typically include substantial water volume on board the heater 

connected in a loop to the building heating system. They operate in a manner that is similar to an 

oil or gas-fired hydronic heater in as much as the water temperature in the heater is maintained 

between specific control limits by cycling the combustion on and off (or up and down) as needed 

based on the heat demand applied by the building. A second sub-category is for heaters that take 

a very different approach to meeting heating demand. Full Thermal Storage Hydronic Heaters 

include water volume, either on-board the heater or within an external insulated storage tank, of 

sufficient volume to store all of the heat produced from at least a full load of fuel. These heaters 

generally operate at a high burn rate for the full duration of the fuel load combustion. The 

building heat demand is typically satisfied by drawing from the stored heat rather than directly 

from the heater. These systems are typically fired once or twice per day depending storage 

volume and heating demands. Partial Thermal Storage Hydronic Heaters bridge the gap between 

the first two sub-categories. These hydronic heaters typically have relatively high water volume 

as part of the overall system. Some have relatively low on-board water volume but include an 

external thermal storage tank as a required part of the system while others may have all water onboard. Specific installation details can vary from model to model but the concept remains the 

same across this sub-category. These systems supply part of the heat produced directly to the 

building heat load and store any excess heat in an insulated storage tank for later use. If the 

maximum thermal storage capacity of the system is reached before the fuel load is fully 

consumed, these heaters may begin to cycle based on the building heating demand in a manner 

similar to the on-demand hydronic heaters or may simply extinguish combustion in the firebox. 
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These systems can provide the advantage of delaying the on-off cycling until later in the burn 

cycle where fewer particulate emissions are produced. 

Warm Air Furnaces 

Warm Air Furnaces are designed to heat buildings rather than as space heaters. They may be 

stand- alone systems or be tied to existing warm air heating systems. Typical characteristics 

include a plenum with heat exchanger to extract heat from the firebox exhaust, ducting to deliver 

warm air to the building and to return cool air to the furnace, a blower to establish the heat flow 

to the building, thermostatic controls to regulate heat output, a combustion air blower to deliver 

primary and secondary air to the firebox and over-temperature limit controls to insure that safe 

temperature limits for adjacent combustible materials are not exceeded. Warm air furnaces are 

typically located within the building envelope but can be located outside the building. There are 

warm air furnaces that burn cordwood and others that burn particulate biomass fuels like wood 

pellets, wood chips, granular fuels (for example: corn, shells, hulls or pits) or other pelletized 

biomaterials (for example, grasses). Warm air furnaces are typically on-demand type heaters, 

reacting to the heating demands of the building by regulating the burn rate in the firebox or by 

cycling on and off as needed but it is conceivable that warm air furnaces could be used in 

conjunction with heat storage in some form. Warm air furnaces typically include safety listings 

to UL-391 and CSA B366.1. 

Masonry Heaters 

Masonry Heaters are solid-fuel burning appliances that designed to heat by releasing heat stored 

in the refractory mass of the appliance to the spaces to be heated. They can be either site-built or 

site-assembled, involve burning batch-loaded fuel at high burn rates, and typically have designs 

that route flue gases through passages in the refractory mass to facilitate heat exchange and heat 

storage. These characteristics are embodied in the proposed definition. Site-built masonry 

heaters must meet applicable building codes, but are not listed pursuant to US or Canadian safety 

standards; site-assembled models are generally listed to UL-1482. 

Coal-Only Heaters 

The draft incorporates the definition of this appliance type from the current NSPS, 

with some modest editing for greater clarity and accuracy. 

Cookstoves 

The draft incorporates the definition of this appliance category from the current NSPS, with the 

modifications agreed to recently to tighten up the definition and minimize the potential for abuse.