Air Conditioning and Furnace Sizing

Just like the cars of old the conventional wisdom of the sixties and seventies was “the bigger the better” when it came to furnace size. But what is the best approach when sizing a furnace? Many customers seem skeptical when given the advice to size their furnace as small as possible. Certainly the furnace size will need to be adequate for that below zero January day but is it in your best interest to have a “454 Suburban” in your basement?

Unless there’s some inherent satisfaction personally gained from watching your thermostat rise five degrees per minute you should probably consider the benefits of sizing your furnace on the small side. Let’s take a look at some of the reasons for sizing “small”. There are three very important ones:

1- Efficiency: Consider the fuel economy your vehicle delivers under these different operating conditions (highway and city). Just like automobiles your furnace is the most inefficient during stop and go operation. The larger the furnace the more often it will cycle and, just as in city driving, the more inefficient it will be. True the monster furnace in the basement will raise the temperature rapidly but it will tend to cycle more frequently as well. Where as the smaller furnace will remain in operation longer (highway driving) trying to achieve the set-point on the thermostat.

During these longer run times a furnace operates at peak efficiency. The reason for this is that every heat cycle begins with an ignition phase and subsequently a heat exchanger warm-up phase. While the furnace is in the heat exchanger warm-up phase the indoor blower is off; and consequently there is little heat added to your home. Before the blower motor turns on, a portion of the heat added to the heat exchanger goes right out the flu and not in your house. Each heat exchanger warm-up phase is accompanied by a loss of heat energy to the atmosphere. Therefore, just as in highway traffic, the less a furnace cycles, (starts and stops) the more efficient it will be. If you could heat your home with a pilot light running continually from October to April your fuel bill would be the lowest possible.

2- Comfort: Not only is your furnace inefficient during stop and go operation, it is also uncomfortable. That furnace we described earlier in your basement, the 454 Suburban, will certainly raise the temperature in the room but consider what that means. Unlike the thrill of going from 0-60 miles per hour in four seconds there is little excitement in going from 70 to 72 degrees instantaneously. In fact when a furnace is sized too large the extra heat mass stored in the heat exchanger will tend to “overshoot” the set point of your thermostat. When the room temperature is high enough, the thermostat will turn the burners of your furnace off, but the extra heat contained within the heat exchanger will continue to heat your room until the indoor blower removes all the heat stored in the heat exchanger. During this time period your room temperature may rise higher than the temperature your thermostat is set at. This will also add to your discomfort since you will experience greater temperature swings- Fun in sports cars but not in your home.

3- Noise: Larger furnaces are usually louder. This is because blower motors for larger furnaces are also larger since they are required to move more air.

The bottom line is: If you want excitement-Buy a sports car. If you want comfort and efficiency-Buy a small furnace.