I have searched this site for info on 2 stage thermostats but haven't found the answer I have been looking for so I thought I'd ask. How does a 2 stage thermostat work? How does it control the staging? Time? Outdoor ambient? Reason for asking is everyone says 2 stage stats are much better than having the 2 stage furnace board control staging (12-15 minutes in low stage?) but no reasoning to it. How are they superior?
A 2 stage thermostat will kick the furnace into 2nd stage immediately if the set point is a few degrees higher than the room temp instead of having to run in 1st for 15 min first. It will also kick down from second stage when the room temp gets closer to the set temp which reduces the chance of overshooting the stat. This is reason enough to insist on a 2 stage thermostat
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The control schemes vary betwen different brands ans models. Some are more advanced and contorl tighter than others. Equipment size and the characteristics of the home as wel as thermostat location will all have an impact.
So I e mailed my thermostat manufacture, Radio Thermostat (3M-50 wifi) about staging operation and got a great explanation on how it works. For example if the thermostat is set to 70* and the swing set to 1* (which mine is) at 69* the furnace will start in low stage. In order for high stage to start it has to drop to 68* and then it will stay in high stage until satisfied at 70* and turns off. Okay, now what happens when its cold enough for single stage to stay at 69 but not cold enough to drop to 68, kinda the "limbo" state? Is it more efficient to run constant than to cycle as if it were a single stage thermostat?
Not sure about the 3M but most thermostats have sufficient logic to keep the proper setpoint and switch stages as needed. If you have a furnace it's doubtful that high stage will EVER be needed except for recovery from setback. Furnaces tend to be oversized (2-3X the size needed isn't uncommon) in general.
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