Furnaces should have no place in energy efficient homes. Here is why – when you add enough insulation to the building and tighten up the envelope, your heating load falls faster than the cooling load. This does make sense because the temperature difference between the outside and the inside is much higher. The infiltration rates too, are usually higher.
However, this creates problems when sizing the furnace. The furnace should be sized to deal with the cooling CFM. This may mean choosing a model with a much higher heating capacity.
In regions where the climate is hot, it is not uncommon to see oversized furnaces. In Phoenix or Tucson, you might even see furnaces that are oversized by 300 percent or 400 percent.
Manufacturers of unitary equipment have shown hardly any interest in building small capacity furnaces. Status quo furnaces, on the other hand, haven’t changed much over the last few decades. They are also meant for cold climates and inefficient homes. The available choices are usually large, larger or even larger. This has been a problem in mild climates. And now that the homes in the colder regions are becoming tighter and more energy efficient, this is becoming a big problem in the cold regions as well.
In other words, a gas powered furnace is a bad choice for energy efficient homes. To put it simply, oversizing causes several comfort issues. When people invest lots of money in a home, they also expect it to be comfortable.
Consider this situation. What will happen if on a really cold day, the furnace runs only 15 minutes in an hour? Whenever the heat cycles are off, the house will begin to cool down from the outside. The thermostat is usually located away from the exterior walls and the windows. By the time, your thermostat notices a temperature drop and activates the furnace, the perimeter zones will have fallen by several degrees.
That is not all. Almost all modern furnaces come with electronic controls which dictate a minimum run time. When you have a grossly oversized furnace in the house, it will experience massive overshoot during fall and spring.
Many dealers suggest costly variable-speed furnaces for energy efficient homes. They claim that these multi-stage furnaces will resolve these issues. It is true that multi-stage furnaces do not normally have capacity issues. However, there are cheaper models that are better at capacity matching.
Why heat pumps are better
Heat pumps are better systems to heat high performance homes. Because when you use a heat pump, it is easy to closely align the system capacity with the load.
In warm climates, heat pumps are the best solution. They have relatively less operating costs and in a tight, well insulated home, they are much more comfortable. What’s more, the low supply temperature, which is considered the biggest problem with heat pumps, is preferable in a well-insulated home.
The heat pump produces less heat when the outdoor temperature drops. So except in the warmest regions, you will need supplemental heat. However, in energy efficient homes, the need for extra heating is much lower. This also makes heat pumps a better alternative to furnaces.