Q&A: Oversized Heating Systems

A.Richard Karg responds: Yes, an oversized heating system is wasteful for three reasons: 1) It reduces the annual efficiency of combustion heating systems; 2) it increases the potential for flue condensation in mid-efficiency systems; and 3) it increases the cost of heating systems.

Now for some explanation. First, an oversized combustion heating system will not fire as much as a system that is sized properly. Instead, it will "stand by" more and lose more heat up the flue. The more oversized it is, the greater the stand-by time, and the higher the fuel bill.

Off-cycle losses are greater from a boiler (which heats water) than from a furnace (which heats air) because water stores over 3,000 times more heat energy than air for a given volume. This means that boilers are penalized more than furnaces are for oversizing. Also, note that the higher the efficiency of a combustion heating system, the smaller the penalty for oversizing. (The efficiency of electric heat is not affected by oversizing.)

Second, if a combustion heating system runs less because of oversizing, the flue may not stay warm enough to evaporate flue-gas condensation. This could lead to corrosion of the flue, which is a maintenance problem and could result in flue gases spilling into the house.

Third, the larger the heating system, the more costly it will be to install (for any type of heating system).

Studies indicate that the average combustion heating system in the U.S. is oversized by 2.3 times. Using a conservative estimate of a 5% penalty for oversizing in gas appliances (up to 10% for oil-fired equipment), the savings from accurate sizing is substantial.

Heating systems should be sized for new and existing homes by using design heat load calculations — not rules of thumb or intuitive guesses.

Rick Karg is an energy management consultant in Topsham, Maine, and frequently conducts training seminars on sizing heating systems.