After our talk on short cycling, we came across people with the opposite complaint: their furnaces were running constantly. As opposed to constant short cycling, these people were experiencing some form of long cycle. Their furnace would fire up, warm the office, and then keep running. There would be constant, mechanical sound from the machine. This is exactly what it’s supposed to do.
In a single stage furnace, the thermostat calls for heat, the furnace runs, and once the target temperature is reached, it all shuts down. This is horrendously wasteful. Your furnace has just burned fuel or used electricity to get a heat exchanger red hot. When the furnace shuts down, that heat exchanger is still roasting, glowing red, but its not using that heat. The heat exchanger will just sit there, cool down, and an extra ten, twenty, or even thirty minutes of warmth has just been completely wasted.
The excess heat in the exchanger will go into the ambient air in the furnace. It might heat some of the basement or perhaps the furnace will have a tiny bit less work to do next time. In any event, it’s resources spent that aren’t making you more comfortable. This situation is analogous to approaching a red light, flooring the car, and then slamming on your brakes. All that gas to speed up is probably being burned for nothing.
Using Idle Heat
There’s not really much reason to let that heat just sit there and disburse. We need a way to take advantage of that glowing, hot mass of metal and put all its heat where it counts. We need to let the blower keep running after the burner has shut down. In the most simplistic heating system, that’s not going to work. The furnace was designed to run at either full power, or no power. It doesn’t have anything in between.
To run the blower after the system shutdown, we add a “stage”. This gives us a two stage heating system. During the main heating cycle, the burner and blower will both run as they would normally. Once the target temperature has been hit, the burner can shutdown and we change stages. We’re not burning any fuel now. In this stage, the blower will continue to run, but it may step down to a lower RPM so as not to overheat the building.
Depending on the configuration, the blower may remain on for hours after a burner shutdown. This is good, it means you can make the most of your fuel supply and save a good penny in the long run.
So Is the Furnace Running Too Much?
In a two stage configuration such as this, if you hear the blower running often, that’s not a bad sign. The thing to worry about is when a furnace runs the burner too often. We’ll be checking in on that soon to cover the warning signs and possible fixes to an over-active furnace.