Nest and Two Stage Furnace

[braberley]

Hi,

I bought a house with a 2nd Gen Learning Nest thermostat and a two stage furnace that were installed three years ago.  Like a lot of new furnace installs using Nests, they didn't have a wire for the high/second stage (W2), so wired it the same as the old single stage furnace using just W1 for the low/first stage.  They then configured the furnace to turn on the second stage after 15 minutes if the set point temp is not reached.  The fan schedule is disabled on the Nest, so the furnaces turns on the fan when it turns on the burners.

So I have three conditions for the furnace:

1.Off where there is no air flow, the fan is off.

2.Coolish air when low stage is on, and the fan is on.  I assume this is the low stage.

3.Warm air when the high stage is on, and the fan is on.  I assume this is the high stage.

When the temp on the Nest drops about 0.5 degrees below the set temp, the fan comes on and coolish air blows.  I assume this is the low stage.  The Nest display stays black and does not say "heating to x".  It still says "set to x".

After another 15 minutes, when the temp on the Nest reaches 1.0 degrees below the set temp, the air becomes warm and the Nest display color changes to orange and says "heating to x".

When the Nest turns on the low stage, is it normal that the display does not change?  I would have thought it would turn orange and say "heating to x".   Why does the display only change once the high stage come on?

Is it possible what I think is the high stage is actually the low stage?

[johnCNA]

Personally, I think your installer did a poor job.  A roll of 6 or 7 conductor thermostat wire is under $30 and it's quite easy to pull a new replacement wire to fully support the features of your new furnace.  If it were me, I would demand they return and set that up correctly.  Since they jerry-rigged the furnace itself to operate in a non-standard fashion, you're not getting the full benefit of 2 stages or the operation of the Nest.

I have a 2-stage gas forced air furnace and integrated central A/C.  So here's how a typical 2-stage furnace works.  The house gets 1 degree below the thermostat set point, and the furnace quietly and gently starts blowing warm air. This is really nice because you don't actually feel the temperature swings between heat on and heat off.  My system rarely kicks into Stage 2 heating.  For reference, I'm in Chicagoland. Temps in the teens now but we have already had a few below 0 days.  Even then, our furnace cruised along 98% of the time on Stage 1.

If we open the front door and let a blast of cold air in, or if I move the set temperature to 3+ degrees higher, the furnace shifts to Stage 2 and we can hear the blower running at a higher speed.  As the room temp gets within 2 degrees or so of the set temp, the furnace throttles back and quietly continues to generate heat.

On the Nest display or in the app, it shows 'Heating' for Stage 1 and 'Stage 2 Heating' for stage 2.  Your installers have wired it to behave as a single stage furnace.

[braberley]

So you find your stage 1 air warm?  Do you find it that much less warm then stage 2 air?

What I am calling stage 1 is definitely cool, not warm.  Right now stage 1 is a waste of gas.  You can see the temp falling as what I call stage 1 is running.  Right now, that is more concerning to me then whether the Nest or furnace turns on stage 2.  

I'm starting to think that what I think is stage 1 is not.  That the fan is coming on for some reason and just blowing cool air.  What I thought is stage 2 is stage 1.  Its coming on when tstat temp reaches 1 degree below set point, which lines up with what you are seeing.

[johnCNA]

Yes, your "stage 1" is definitely not working right.  Like I said, my stage 1 heat is all that's used 98% of the time, gently blowing warm air.  And only once in a while kicking into Stage 2.  So to me it sounds like however they rigged the furnace controller, it's not igniting the burner but starting the blower.

[braberley]

Definitely something not right.  Have HVAC tech coming in a couple of days.  Thanks for your help.

[braberley]

Think I found my problem.  Looks like not using C wire with Nest (even though they say you MAY not need it) can cause all sorts of weird problems.  Like turning on fan.  See link below.

http://greyghost.mooo.com/nest-thermo/

[Ryan_G]

Hey folks,

Thanks for visiting the Google Nest Community. 

Since this thread hasn't had activity in a while, we're going to close it to keep content fresh.

If you have additional questions, feel free to submit another post and provide as many details as possible so that others can lend a hand. 

Hope this helps!

Kind regards,

Ryan

[Ryan_G]

Hey everyone,

Thanks for being here.

It's great to see that the issue has been sorted by a common wire. 

If you have any other questions, please don't hesitate to write it up here at the Google Nest Community.

Best regards,

Ryan