What Is a Rumford Fireplace?

How the design works

Count Rumford, an eighteenth-century scientist, worked out that an open fireplace heats a room mostly by radiation, the heat you feel on your face from the flames and hot masonry. A deep box with square corners sends much of that heat into its own back and sides, where it soaks into the chimney. Make the firebox shallow and angle the sides wide, and the hot walls throw heat out into the room instead.

The other half of the design is the throat. Instead of a sharp edge where the front wall meets the smoke chamber, a Rumford has a rounded breast that guides smoke smoothly into a narrow throat. Smoke and air flow past that curve without tumbling, so the draft stays steady even though the opening is tall.

How to recognize one

You can usually spot a Rumford at a glance, because the proportions look different from the low, deep boxes most houses have. Many older homes in the Northeast have one, sometimes hidden behind a later, smaller firebox that was built inside the original.

• A tall opening, often nearly as tall as it is wide
• A shallow firebox with a narrow back wall
• Side walls splayed out at a wide angle
• A rounded, streamlined breast leading into the throat
• A narrow throat with the damper above it or at the chimney top

Rumford fireplace dimensions

Builders work from traditional proportions rather than one fixed size. As a rule of thumb, the depth is roughly a third of the opening's width, the back wall is about as wide as the firebox is deep, and the height approaches the width. The throat is kept narrow and the smoke chamber above it rises symmetrically to the flue. Factory-made Rumford components follow their manufacturer's dimensions and listings.

The flue has to be sized to the opening. A tall Rumford opening has a large area, and the flue has to carry that much air and smoke; a flue that is too small for the opening is the classic reason a new or restored Rumford smokes. Because the opening is tall, the hearth extension and the clearance to the mantel and any wood trim need careful attention, and they are set by the local building code and, for listed components, the manufacturer's instructions.

Rumford compared with a conventional firebox

The differences are in geometry, and each one has a practical effect on how the fireplace heats and draws.

The Rumford fireplace damper

The narrow, streamlined throat does not suit every standard throat damper, and a bulky cast iron damper dropped into a Rumford throat can spoil the smooth path that makes the design work. Some Rumfords use a damper made for that throat shape. Many use a top-sealing damper at the chimney top instead, operated by a cable down the flue. That leaves the throat clean and also seals the flue against rain, animals and cold air between fires.

If your Rumford has no working damper, the flue is open all winter and the room loses heat through it. Adding a top-sealing damper is one of the more useful upgrades for an older one.

Why a Rumford smokes, most common causes first

When a Rumford smokes, the causes are usually familiar. An undersized flue for the opening comes first. Next comes a cold chimney, especially on an exterior wall, that needs warming before it draws. Then a throat or breast changed by an old repair, such as a steel lintel set low or a smoke shelf added later. Finally, a fire built too far forward on the hearth, where smoke rolls out under the breast.

House pressure can play a part too. In a tight house, kitchen and bath fans can pull air down the chimney and push smoke into the room. Opening a window near the fireplace while it starts is a quick test.

Restoring an old Rumford

In older houses, a Rumford may have been partly filled in, relined with a smaller firebox, or patched with materials that changed its shape. Restoration starts with an inspection of the firebox, throat, smoke chamber and flue: are the firebrick and joints sound, is the breast still rounded, is the flue lined and big enough for the opening, and is the chimney top in good shape.

Repairs might include rebuilding the firebox to its original angles with firebrick and refractory mortar, reforming the throat, relining the flue with a liner sized to the opening, and fitting a top-sealing damper and cap. Converting a conventional fireplace into a Rumford is sometimes possible by rebuilding the firebox inside the existing masonry, depending on the space and the flue.

Gas logs and inserts in a Rumford

A Rumford can take vented gas logs, and the tall opening shows a flame well. The log set has to suit the firebox depth, and vented logs need the damper open whenever they burn; many installations fix the damper partly open so it cannot be closed by mistake. The flue still needs its yearly inspection, because a gas fire still sends exhaust and moisture up the chimney.

An insert is a bigger change. It closes off the opening, so most of the Rumford's radiant character is lost, but it turns the fireplace into a far more efficient heater. Because Rumfords are shallow, the insert's depth is often the limiting measurement, and the flue needs a liner sized to the insert. Owners who value the look of the open fire usually choose logs over an insert.

Living with one

Rumfords like a fire built upright, with logs standing on end or leaned against the back wall, rather than a long low stack. They burn well with dry, seasoned wood and a warm flue. Keep the fire toward the back, where the angled walls do their work and the smoke has the shortest path to the throat.

The tall opening also means sparks can travel further onto the hearth than they would from a low firebox, so a good screen or glass doors are worth having. Sweep and inspect it like any other wood fireplace: NFPA 211 sets a yearly inspection, with cleaning when deposits build up.