What Do The Number Of Heating 'Stages' Indicate And Why Should I Care?


If you are on this page, chances are your furnace is a single-stage furnace. There are always exceptions, of course, but in dealing with Toronto’s furnace replacement market, it’s been the trend. Now what is the difference with a single or a 2-stage or a 3-stage furnace? What is a modulating furnace?
In a multi-stage furnace, there exists a gas valve which regulates gas flow rate into the furnace burner. In a single-stage furnace, the natural gas is always flowing at its full 100% capacity.
In a 2-stage furnace, the furnace usually – depending on thermostat settings – first comes on the lower heating stage, which is associated with about 60% of its full natural gas flowrate capacity. If the furnace operation was not successful in reaching the thermostat setpoint in, say, 10 to 16 minutes, the gas regulator valve will completely open to allow full flow and therefore increase the supply rate of heat to your home.
The same concept lies under a 3-stage furnace, except that instead of 2 different settings at 60% open and fully open, the gas valve now has 3 settings, another one at around 40%. A 3-stage furnace can often be referred to as a modulating furnace which is different from a true modulating furnace.
Now a true modulating furnace is one that has over 40-50 settings on its gas valve regulator with small incremental change from 30 to 100% opening. Carrier’s INFINITY 98 is an example of a true modulating furnace that will not only modulate up but it also is the only furnace in the world that can modulate down, as well.
So what is the big deal with the number of stages? Why is that beneficial for you?
Well, in answer to this question, let’s start by looking at a scenario. You have a single-stage furnace. You have the temperature reading at 20C and the furnace is now off. You feel a bit cold so you get up, go to the thermostat and set the temperature at 21C. The furnace will kick in and since this is a single-stage furnace you know that the gas valve is fully open and your furnace is operating at its full capacity in raising the temperature by 1C. What you might notice is that it suddenly starts to get too hot. You get up and go to the thermostat again and you see that the thermostat is actually reading 23C and the furnace has just turned off. You will get to be a little cooler as the temperature drops a little under the setpoint of 21C, which is when the furnace wil kick back in. These temperature fluctuations are as a direct result of lack of control over the fuel flow rate to the burners that supply heat. Obviously, when the amount of gas flow rate is not controlled, how can you have a tight control on the rate of supplied heat and therefore, temperature in your home? Surely, you have all experienced these ups and downs if your furnace is in fact a single-stage furnace. Now imagine how the amount of heat which directly affects the temperature in your home can be controlled with a gradual change. The more the number of heating stages, the better control you have on keeping the temperature at the desired setpoint at all times. Now you can imagine the tightest control possible with a fully modulating furnace.
Ask us about the other technology all Carrier multi-stage furnaces possesss to ensure the tightest temp control. It’s a patented technology by Carrier called adaptive logic.
There are generally, then, two main advanatges that multi-stage furnaces bring to your home and the higher the number of heating stages, the more extensive the following:
  • tighter control on temperature resulting in more even temperatures and eliminating fluctuations
  • furnace operation will be quieter because most often the furnace will have to make very small change to temperature – if it’s kept at a constant desired setpoint – and so usually only the first stages of operation will be acting. Lower gas flow rate is associated with lower sound as increase of pressure in flow will be accompanied by more vibration and therefore more noise.

Misconception About Number Of Heating Stages And Saving Energy

Just know that the number of furnace stages do not have a direct effect on the efficiency and rate of gas cunsumption. You always require the same amount of energy in order to heat your house. When the furnace is operating on the higher stages, it will use more gas in a shorter period. Whereas on a lower stage, furnace would end up using the same energy but in the manner of less power in a longer period – which works out to have the same consumption level.
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