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