The graph above shows how often our heating systems are drastically oversized here in the Seattle area.
The numbers at the bottom indicate the outdoor temperature and the numbers on the left show the heat output of a furnace in Btu’s.
The diagonal line between the grey and green sections is the heat curve. We know that we don’t have an actual heat loss until the outdoor temperature is 65 degrees or colder. As the temperature drops, we need more heat to offset the heat lost by our homes.
We are using an 80,000 Btu input furnace as our example. The output of the furnace varies based on the efficiency of the furnace (AFUE or Annual Fuel Utilization Expense). The single stage 90% efficient furnace (in RED) has one heat output and that is 72,000Btu. If we follow that line across to see where it intersects the heat curve we find that particular furnace is perfectly sized when the outdoor temperature is 22 degrees. The problem is that we very rarely hit 22 degrees here. So what happens is that when the indoor thermostat tells the furnace to come on, it does and overheats the home very quickly and shuts off after a minute or two of running. A few minutes later it comes on and does the same thing again, this is called short cycling. This is very hard on your furnace and very uncomfortable to live with. If your furnace is an 80% efficient furnace (these furnaces have a metal exhaust pipe), this can actually cause condensation to collect in the exhaust system and the heat exchanger and destroy the furnace prematurely. If this furnace is in a cold area like a garage the problem is even more damaging.
The furnace manufacturers figured this out fairly quickly and came out with a two-stage furnace. This furnace (in orange) comes on at about 60% capacity in the 1ststage and this is perfect when the outdoor temperature is 42 degrees. This is so much better for the furnace and more comfortable for the homeowner. The furnace burns less fuel, blows less air, makes less noise and doesn’t overshoot the thermostat nearly as much. It also runs for long enough to heat up and condensation isn’t as much a problem. The only real trouble with two stage furnaces is that they are oversized when they go to second stage (41 degree outdoor temperature) and they aren’t correctly sized again until the outdoor temperature is 22 degrees outside.
York solved this problem by engineering a 98% efficient modulating furnace (in green). This furnace fires at 35% capacity and the modulating gas valve increases the heat output based on the actual run time. This smart technology actually teaches itself how long it takes to heat the house and increases the heat output and blower output based on the actual outdoor temperature. This furnace is sized properly at 52-degree outdoor temperatures and is correctly sized at every outdoor temperature between 52 and 22 degrees! Talk about comfort! No more hot and cold swings, no more excessive blower noise and no more wasted energy! This furnace runs so quietly and circulates the air inside the home so evenly that it eliminates those pesky hot and cold spots in the house. Instead of hot and cold rooms, you will have even temperatures throughout the home because the blower is running for a longer period of time with a lot less airflow and noise.
Adding a heat pump or reverse cycle chiller can lower the energy costs even more while providing the most comfortable indoor environment possible.
This system is also ideal for adding an air filtration system because the blower is on for a longer period of time which means that you recycle the air inside your home more frequently and you remove more contaminants from the home!
