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two-stage gas valve query
Hello. I've asked a few people this question, and haven't been able to get an answer.
I thought I'd try here.
On a furnace that has two-stage gas valve, what causes the furnace to run at the lower btu stage versus the higher btu stage?
Answers I have received include, "it does it when its colder outside," or, "when there's a higher demand for heat it runs the higher stage."
These don't quite answer my question.
They make me think: how does the furnace know its colder outside? and, colder than what? colder than yesterday? colder than the inside? and, how does the furnace know there's a higher demand for heat?
As far as I know, a thermostat only tells a furnace "start" or "stop."
Do modern thermostats also tell the furnace a demand level in addition to "start"?
One guy said, "oh, well there's a computer in the furnace that does it all." OK. I accept that. But computers are not all-knowing. A computer is simply a decision making entity. It takes inputs and makes decisions. For example, a furnace computer could have an outside air temperature gauge that it reads to know the difference between the outside air and the inside air (assuming it also has an inside air temperature gauge). Do furnaces these days require wires to the outside for temperature sensors?
My question is, what tells the furnace when to activate the lower stage and when to activate the higher stage?