Home & Garden Columns

The Care and Feeding of Floor Furnaces

By Matt Cantor
Friday February 08, 2008

One of the most common features in our early 20th century housing stock is that imperishable ruffian of the heating world, the floor furnace. 

It continues to amaze me how many of these persist in operating as the main source of heat for houses since they do not heat spaces uniformly and pose a greater risk for fire, burns and exhaust leaks than many other types of heater. The answer to this conundrum is, of course, that since they don’t just up-and-die, people keep on using them. 

Natural gas floor furnaces are radiant heaters that heat the spaces that they are in and provide no ducting to heat adjacent spaces. This means that they will tend to bake their immediate environs when they are being used to heat whole houses. Floor furnaces actually heat by convection more than by radiation, which means that they are creating a plume of heat that drives air up to the ceiling above them, down the walls and across the floor back into the unit.  

Since this process pushes air, it also pushes dust, dander and animal hair into the unit where it comes to rest near the bottom. In other words, floor furnaces are central vacuum systems and this is why they are always filled with dust. This material is of course, flammable and adds a lovely allergenic aroma to the air which those of you privy to this experience will recognize immediately.  

Excessive built-up of dust and dander also becomes a fire hazard and should be maintained with the narrow wand of a vacuum cleaner. Many older floor furnaces have a metal cowling or heat shield that can be easily removed for cleaning. Be sure and put it back properly when this job is complete. Some later models have a heat sensor attached to the shield and should only be removed by a professional. You’ll readily see a wire attached to the shield if you try to remove it. 

Floor furnaces get quite hot when being used and small children, especially toddlers are at risk around these floor mounted ovens. Remember that infants do not have well developed heat sensors or reactions and will be burned before they have become aware. 

Flammables should be kept clear of these units and this includes rugs, newspapers, books and drapes. It’s improper to have these devices installed where a door swings across the grill, since even these can catch fire after hours of prolonged heating. 

A feature that’s important to look for with any radiant heater is the presence of a thermostat in the area heated by the device. If the thermostat is located beyond a doorway, the door may be shut when the unit is in use allowing the heater to reach much higher temperatures than those mandated by the thermostat. If this is true for you, have your heating contractor move the thermostat. If you have a unit that turns on and off with a metal floor key, consider having a thermostat installed since without one, the unit can be left on and reach very high temperatures, greatly increasing the likelihood of fire. 

Like all gas heating appliances, floor furnaces generate exhaust gas. This gas is toxic and can contain significant amount of carbon monoxide, a deadly, odorless gas. So it’s really important that these gases be moved outside the dwelling without leakage. I find that lots of these old floor furnaces leak at least a little bit and some leak a lot. One way that leaks occur is through a hole intentionally installed in all of these units right on top. These view holes come with covers and most have a mica or similar clear window in the top. These are often damaged or left open or lost entirely allowing flue glasses to literally pour into the dwelling. Under the best of circumstances this puts lots of steam into the house and under the worst of circumstances this may be deadly. There are also seam leaks on many of these.  

Last in this litany of complaints is the surprising fact that this kind of heating is more likely to create an electrical fire in an older home. This is due to the frequent use of electric heaters in homes that lack an adequate gas heat source. Many of the same homes that have floor furnaces have inadequate or unsafe wiring and the addition of an electric heater (often left on for hours at night) adds the duress that sets off a fire. 

The best response to these many concerns is to spend the money and upgrade. Forced air is the most common replacement but take some time to look at the alternatives before you spend your money. If you do go with forced air, consider using the old floor furnace housing (box) as your “cold-air return” (or intake). It will eliminate the need to repair the floor and generally works quite well. The filter for your “FAU” (forced air unit), can be placed in the gutted box of the floor furnace making it easy to service.  

Although I’ve never seen statistics on this, I wouldn’t be surprised if an FAU used less energy to heat the house than a floor furnace since all rooms are kept at roughly the same temperature with an FAU. The living room or hall is often baking hot when a floor furnace is trying to keep the back bedroom warm enough and this surely used a great deal of excess energy.  

If you are going to keep that floor furnace a while longer consider adding attic insulation to help hold and distribute heat that wants to go right up into space. Also be sure and have it looked at regularly by an expert. 

Despite the charm of our elderly housing stock and my love of restoration, this is one feature that I’d like to see us all part with.