With a multi-stage system, which of the two stages spins the fan faster? Which is the default and which is triggered when the temperature is further away from the specified value?

  • Hello, and welcome to Stack Exchange. You'd get better responses if you included the make/model of your system, as well as some other details (e.g. is this heat? A/C? both?). Aug 19, 2017 at 16:00

1 Answer 1


Not sure you are giving enough information here to know where to begin to help answer. But I'm going to make some assumptions as I think I know where you are going.

Are we talking about a forced air furnace? I'm assuming so.

Are you meaning to ask "with a variable speed" or "2 speed" or "multi speed" blower motor what is the trigger for the speeds to kick in? Assuming so:

This is going to depend on furnace manufacturer, thermostat model, and the system installer. For example, furnace Control boards and smart/programmable thermostats are programmed to react *(in some cases as a smart-learning mode) to what speeds affect the temperature stability in the home and will self regulate. Large swings in temperature can trigger high speed regardless of program.

Some systems also allow you to set it static, either high, or low. Also, some installations have external temp switches and will only use high speed when the delta temp outside is below or above a set point.

So there are many things that can control fan speed. When it comes to multistage, this can be referring to dual heating units, or different set points for BTU generation.

  • Actually, I mis-interpreted what a multi-state system is. For air conditioners, it might change the compressor used for instance. It does not alter the fan speed.
    – Sawtaytoes
    Aug 18, 2017 at 5:59
  • Are you asking about of an actual problem you have or a hypothetical problem for people to try to solve. If it is an actual problem you need to be specific about what it is you are talking about. I can not understand the questions you are asking or decipher just what you are saying.
    – d.george
    Aug 19, 2017 at 13:52
  • If you are actually asking "in a forced air system, what causes a fan with more than one speed to spin faster" then my answer is correct. If you are asking the same question I just posted above, but asking it about a MULTISTAGE system, then my answer is STILL correct, except that there will be more controls/switches involved in turning on/off the fan, and the speeds it will operate at.
    – noybman
    Aug 19, 2017 at 15:26

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