Rumford Fireplaces

Where the design came from

Benjamin Thompson was born in Woburn, Massachusetts, and later became a count of the Holy Roman Empire while working for the Bavarian government, which is where the title Count Rumford comes from. He is known mainly for his work on the nature of heat. He published essays on fireplace design in 1796 and 1798.

According to Buckley Rumford Fireplaces, a builder that has studied and reproduced his designs, the Rumford fireplace became state of the art almost immediately in the 1790s. Thomas Jefferson had Rumfords built at Monticello, and by the middle of the nineteenth century the word "Rumford" was used to mean a fireplace.

The proportions, in Rumford's own rules

Rumford's essay, reproduced by Buckley Rumford, gives the rules in plain terms:

• Back and depth are equal. The width of the back wall equals the depth of the firebox.
• Opening is three times the back. The front opening is three times as wide as the back wall.
• Sides at 135 degrees. The angled side walls (covings) meet the back at 135 degrees, which he called the best position for throwing heat into the room. They should be flat, without curves.
• A narrow throat. He recommended a throat about four inches deep, with three inches sometimes enough in a very small fireplace.
• A rounded breast. Instead of a flat, square edge where the front wall meets the throat, the breast is rounded so smoke turns upward gradually.

A true Rumford also has a straight, vertical back wall. Buckley contrasts this with later variations whose back walls slant forward, which lack the streamlined throat.

The shape in plan and section

Why it puts more heat in the room

An open fireplace heats a room mainly by radiation, the heat you feel on your face when you sit in front of it. Rumford's idea was to expose more of the fire and the hot side walls to the room. The Brick Industry Association's note on Rumford fireplaces describes the same principle: decreasing the depth and flaring the sides directs more radiant heat into the room.

The rounded breast helps the fireplace draw without a large throat. Buckley reports temperature measurements at a Rumford throat of about 75°F right at the curved breast and about 730°F two inches farther back. Its explanation is that room air sweeping in over the fire slides along the curve without mixing much with the hot smoke, so a smaller throat can carry the smoke away.

A Rumford is still an open fireplace. It radiates more than a deep firebox of the same size, but much of the fire's heat still goes up the chimney. If steady heating is the goal, compare it with an insert or a stove.

What the building code allows

For an ordinary masonry fireplace, the 2021 International Residential Code (section R1001.6) requires a firebox at least 20 inches deep and a throat no lower than 8 inches over the opening. A Rumford fails the depth rule by design, so the code includes a specific exception. Rumford fireplaces are permitted when:

• the firebox depth is at least 12 inches and at least one-third of the width of the opening,
• the throat is at least 12 inches above the lintel, and
• the throat area is at least one-twentieth of the cross-sectional area of the fireplace opening.

New Jersey adopts the 2021 IRC for one- and two-family homes through N.J.A.C. 5:23-3.21. The rest of Chapter 10, including firebrick, hearth and clearance rules, applies to a Rumford as it does to any masonry fireplace. One point is worth checking: Rumford openings are large, and the IRC requires a bigger hearth extension when the opening is 6 square feet or more. A 36-inch-wide, 36-inch-tall opening is 9 square feet. See hearths and hearth extensions.

The BIA note itself cautions that some dimensions in its typical Rumford drawings may conflict with local building codes, so plans for a new one should go through the local construction office.

Living with one: draft, dampers and doors

• Smoke when cold. The BIA note says Rumfords have a reputation for letting a small amount of smoke into the room while the fireplace is cold, because the firebox is shallow. Buckley also notes that tall, wide, shallow fireplaces tend to smoke. Persistent smoking deserves a look at the throat and chimney. See why a fireplace won't draw.
• Dampers. The BIA note describes flat steel plate throat dampers or chimney-top dampers for Rumfords. A top-sealing damper keeps the streamlined throat clear. See types of chimney dampers.
• Glass doors. By Buckley's figure, glass in front of the fire stops about four-fifths of its infrared heat, and that heat is the reason to own a Rumford. If you use doors, follow their instructions. See fireplace glass doors and screens.

Is my fireplace a Rumford?

With the fireplace cold and the damper open, look with a flashlight:

1. Measure the opening and depthA Rumford is tall and shallow. If the depth is about a third of the opening width, keep going.
2. Look at the side wallsThey should splay out sharply and be flat, not curved.
3. Look at the back wallA straight, vertical back points to a Rumford. A back that slopes forward toward the top points to a conventional design or a variation.
4. Look up at the front of the throatA smooth, rounded curve on the room side of the throat is the Rumford signature. A flat ledge and a wide throat are not.

Many tall, shallow fireplaces borrow some Rumford features without all of them. Buckley describes one common variation with a slanted back and no streamlined throat as prone to smoke.

Repairs and changes

Most Rumford repairs are the same as any masonry firebox: worn refractory joints, cracked firebrick, damper problems, and water damage from above. The difference is that the proportions are the design. Relining the firebox with thicker brick, filling the throat, adding a sloped back or changing the opening can turn a fireplace that draws well into one that smokes. Repairs should rebuild the original geometry.

Inserts need a firebox deep enough to hold them, so measure carefully and compare against the insert manual before planning one. See firebox repair and firebrick and refractory mortar.

Questions homeowners ask

Sources

1. Buckley Rumford Fireplaces: Rumford Fireplaces (history, design and performance)
2. Count Rumford, Chimney Fireplaces essay (reproduced by Buckley Rumford Fireplaces)
3. Brick Industry Association Technical Note 19C: Rumford Fireplaces (reproduced with commentary by Buckley Rumford Fireplaces)
4. Buckley Rumford Fireplaces: Orton and Rumford fireplaces compared
5. 2021 International Residential Code, Chapter 10: Chimneys and Fireplaces (model code text as published in the 2021 Seattle Residential Code, local amendments marked)
6. N.J.A.C. 5:23-3.21, New Jersey one- and two-family dwelling subcode (adopts the 2021 IRC with New Jersey modifications), via Justia Regulations

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