Masonry heaters

What counts as a masonry heater

The Masonry Heater Association of North America (MHA) adopted a working definition in 1998: a site-built or site-assembled, solid-fueled heating device built mainly of masonry, which stores heat from brief fires burned rapidly in its firebox and releases it slowly. Inside are a firebox and refractory channels where the exhaust gives up its heat. The definition adds measurable traits:

• A mass of at least 800 kg (1,760 lb).
• Tight-fitting doors that stay closed during the burn.
• An overall average wall thickness of no more than 10 inches.
• An outside surface that stays at or below 230°F in normal use, except right around the loading door.
• A gas path with at least one 180-degree turn, usually downward, before the chimney.
• A shortest path from the firebox exit to the chimney at least twice the firebox's largest dimension.

Those last two points are what separate a heater from a big fireplace. The exhaust is deliberately made to travel a long, folded route through masonry before it leaves.

How the heat moves

Think of the heater as a battery that is charged quickly and drained slowly. The fire is the charging stage: a full load of dry wood, the door closed and the air open, burning briskly. The hot gases rise, turn, and run down through the side channels, warming the masonry along the way, before the cooled exhaust reaches the chimney.

After the fire is out, the masonry keeps releasing what it absorbed. MHA describes heat radiating into the house for the following 18 to 24 hours, and a surface that is never too hot to touch. For mild weather the advice is a smaller load, still burned fast, instead of a slow, smoldering fire. MHA points out that burning too slowly makes tars and sharply increases air pollution.

How it compares with a stove and an open fireplace

The practical difference shows up in daily routine. A stove rewards steady tending; a heater rewards one or two well-timed fires. Both reward dry wood.

What firing one looks like

MHA publishes a sample owner's manual for heaters, and the builder of a particular heater should supply their own. The sample gives a sense of the routine:

• Door closed. The heater is used only with the door closed.
• The load. Roughly 20 to 26 pounds of wood for a medium to large heater, smaller for small fireboxes, with no piece over 6 to 7 inches across. Wood should be well seasoned, at 15 to 20 percent moisture, and every piece split on at least one side.
• Lighting. A top-down fire with kindling and a little newspaper on top of the larger pieces. No flammable liquids, no grate and no andirons.
• How often. Fires are spaced out. The manual describes waiting 8 to 12 hours before a second fire, or up to three fires a day about 8 hours apart, and cautions against reloading within the cycle simply to get more heat.
• The damper. Many heaters have a damper to hold heat once the fire is finished. MHA's manual ties closing it to the end of the burn, when the wood has fully burned down, and warns that closing it too early can cause a gas explosion. It also calls for smoke alarms and carbon monoxide detectors. Follow your builder's exact instructions here.

A new heater also has to be cured. The same manual describes waiting at least 30 days after construction unless the installer says otherwise, then a series of small fires that build gradually toward a full load.

Planning one: weight, footing, clearances and code

A masonry heater is a building project as much as an appliance purchase. Its weight, its footing and its relationship to the chimney have to be designed in, which is easiest in new construction or a major renovation.

The International Residential Code has a section on masonry heaters. In the 2021 edition, as reproduced in Seattle's residential code, it requires a heater to either follow ASTM E1602, the standard guide for building solid-fuel masonry heaters, or be listed and labeled to UL 1482 or CEN 15250 and installed according to the manufacturer's instructions. It calls for a firebox floor at least 4 inches thick of noncombustible material on a noncombustible footing and foundation. Combustible materials must stay at least 36 inches from the heater's outside surface, with exceptions: a heater with solid masonry walls at least 8 inches thick and channel walls at least 5 inches thick can have combustibles as close as 4 inches, with at least 8 inches between its capping slab and a combustible ceiling, and listed heaters follow their listing.

New Jersey's one- and two-family dwelling subcode is based on the International Residential Code, and the Department of Community Affairs lists the 2024 edition, New Jersey edition, as current. Confirm the requirements that apply to your project with your local construction office before design is final, and expect permits.

Emissions and the EPA

EPA's Burn Wise guidance on choosing appliances says masonry heaters currently do not require EPA certification and refers readers to MHA for more information. That statement describes regulatory status only and says nothing about how a given heater performs. How cleanly a heater burns depends on its design, construction and the way it is fired, which is why MHA stresses fast, hot burns of dry wood and warns against smoldering loads.

Upkeep: the heater, the chimney and the ash

• Annual inspection and cleaning. MHA's sample manual calls for the heater and its flue to be inspected and cleaned at least once a year. For a new heater, it recommends checking the entire chimney installation monthly until a cleaning cycle is established.
• Creosote watch. The manual says that if creosote appears in the flue, stop using that fuel and have the chimney cleaned if deposits are heavy. Our creosote guide explains the stages.
• Ash. Store ashes in a metal container, never on a combustible surface, and remove all ash at the end of the heating season, as the manual directs. See getting rid of fireplace ashes safely.
• A cap. MHA's manual recommends one on the chimney.
• Off-season damper. The manual suggests leaving the damper open when the heater is not in use, to keep it from rusting or freezing shut.

If you have bought a house with an existing heater, try to find the builder's documentation, since layouts and access points vary from one heater to the next. The chimney that serves the heater can be inspected like any other solid-fuel chimney, using a camera where the flue permits. Start with our chimney inspection service, and see buying a home with a fireplace for what to ask during a sale.

Questions homeowners ask

Sources

1. Masonry Heater Association of North America, Definition of a masonry heater (1998)
2. Masonry Heater Association of North America, What's a masonry heater?
3. Masonry Heater Association of North America, sample masonry heater owner's manual
4. U.S. EPA Burn Wise, Choosing the right appliance
5. 2021 Seattle Residential Code, Chapter 10 (2021 IRC text with local amendments), Section R1002 Masonry Heaters
6. New Jersey Department of Community Affairs, Current Construction Codes

This guide is general information. It can't diagnose a specific chimney; an inspection can.