The main effects of an over-sized furnace are: reduced amount of time that the furnace operates, increased repetitive

loads on the furnace, and, in most cases, increased electrical consumption of the furnace.

When a furnace only runs for a short period of time, long runs of ducts in a heating system may not heat up fully

before the furnace cuts out. Due to this, any parts of the home supplied with heat by these ducts may be cooler than

other parts of the home.

Another problem that can occur due to the short time the furnace spends operating is condensation in the chimney.

Condensation can occur in the chimney if the furnace does not run long enough to warm the entire chimney. When

the combustion gases from the furnace come in contact with the cooler parts of the chimney, condensation can

develop. This can damage the chimney and the drips of condensation that run back down into the furnace can damage

the heat exchanger. With some older-style chimneys, the condensation can turn into ice and cause the walls of

the chimney to collapse, creating a serious health hazard. Condensation can be a problem with conventional and

mid-efficiency furnaces. High-efficiency furnaces are designed to intentionally cause condensation and do not have

this problem.

Conventional and mid-efficiency furnaces operate more efficiently the longer they run if properly serviced and maintained. When they are over-sized, and

have shorter run times, the combustion efficiency is reduced.

In addition to shorter cycles, over-sized furnaces tend to turn on and off more frequently, which can cause the

mechanical components in the furnace to wear out sooner, requiring additional maintenance calls.

The blower fan motors in an over-sized furnace is typically larger that what is installed in a properly sized furnace.

Although the effects of this are minimized by the reduced run time of the oversized furnace, there can be additional

electricity costs if the fan is used to provide air-conditioning or to continuously circulate the air in the house. Furnace styles types.