Sooner or later, you will replace a furnace—and when you do, the issue of size must be a consideration. It may seem odd, but a furnace with excess capacity is not the one you want. Here is some background:

About 35 million homes in this country heat with forced-air natural gas, and many older furnaces are so inefficient that 30 percent of their fuel energy goes up the flue instead of heating the house. Fuel produces fire and fire produces heat, but not all that heat gets to where you want it.

Furnaces are rated by the amount of fuel energy consumed when they're running, called "input BTU." Furnaces of the same input BTU can produce vastly different output, though, depending on their efficiency. Furnaces manufactured after 1992 are required to turn at least 78 percent of their fuel into heat, but efficiency was a price-driven variable before then.
Say it's time to replace your furnace and you live in an older home. The old furnace is probably oversized, to as much as four times its ideal capacity. That's because contractors used bigger furnaces to offset heat loss in the days before tight windows, weather-stripping, and heavy insulation.

As you consider the replacement furnace, beware of bidders who will check your existing nameplate (the unit's label, stating the BTU-per-hour input) and then propose to sell you one just like it or, even worse, one that's larger.

For illustrative purposes, say the old furnace has a 40,000 BTU nameplate and is 78 percent efficient. An Internet search shows that major brand furnaces now are typically 95 percent efficient, so for openers you could downsize to 33,000 BTU on efficiency alone. And if you have upgraded your home's windows, siding, doors, or insulation, the home's heat losses are now well below what the old furnace was sized for. If you buy in kind, you will have a greatly supersized furnace, and that creates a problem because the furnace will get your home to its designated temperature right away, then turn off before the heat is uniformly distributed. This is called "short cycling"; it is inefficient and it will reduce the furnace life. Moisture accumulates in the heat exchanger, and running in short bursts doesn't get the insides hot enough to vaporize the water.

Another problem with short cycling is humidification. Many of us use a furnace-mount humidifier to keep the house from drying out, but in a short cycle it doesn't run long enough to do the job. And finally, a furnace circulates air to overcome cold spots around the edges of the house, under windows, and near doors. In a short cycle the furnace blower isn't on long enough to fully displace the air there.

To get a furnace with ideal capacity, require all bidders to calculate the optimum size, then compare their numbers. We are told that some utility companies provide this service at little or no charge, but regardless of who does it, calculations should be based on "Manual J, Residential Load Calculation," published by the Air Conditioning Contractors of America.

One final thought: Always install a carbon monoxide detector near your furnace and near a vent. Even a properly installed furnace can introduce carbon monoxide into your home.

Select information was obtained from the U.S. Department of Energy.