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Fan won't turn on while furnace is heating and vice versa

RickG13
Community Member

I recently replaced our old "dumb" thermostat with a Nest unit.  We have a forced air gas furnace for heat.  The wires that are connected are: G,R (RH on the old model),Y,W, and C.

Anyway, under normal conditions, the furnace turns on, heats up for a few minutes, then the blower turns on to push the heated air through the house. 

However, with the new Nest, the furnace will go on, heat up ... and then nothing. Finally, when the furnace itself turns off, then the blower will come on - for a minute or two anyway, doing little more than blowing rapidly cooling air through the vents.

I've reset the wires multiple times, trying to ensure they're in correctly.  And I've even tried turning on the fan manually while the furnace is heating up - to no avail.  One refuses to work while the other is on.  I either have a fan blowing or a furnace heating up, not both.  Starting to get frustrated that this smart thermostat does such a poor job compared to the old one.

Any suggestions?

1 ACCEPTED SOLUTION

RickG13
Community Member

Managed to get it working.  Turned out my old wiring was twisted pair, which Nest technically doesn't support but a lot of people online say doesn't really matter. 
Actually, apparently it did matter.  I decided to run new new 18/5 copper wires down to the basement, removed the old wiring, reconnected everything and now it all seems to work as designed.  Heat and fan as they're supposed to. 

View solution in original post

2 REPLIES 2

sebarger
Community Member

I have a similar system (forced air gas furnace) with the same wiring minus the common C wire. And I have the same issue where my fan won't come on automatically when the heat turns on. I've researched that this is because my HVAC system relies on my thermostat to tell the fan to come on. The Nest default setting is apparently to rely on the furnace to tell the fan or a setting of "Don't Activate".

I found this Nest Pro Installer Guide online and it looks like there is an option to force the fan to Activate when the heat comes on like my current thermostat does (see page 22/23). The only problem is that I can not find the fan option under the Pro Setting menu!?  When I go in through Pro Setup and click on W1, I only see Type (Conventional Heating), Source (Oil in my case), Delivery (forced air), and no Fan setting!

It seems others are experiencing similar issue: https://www.googlenestcommunity.com/t5/Nest-Thermostats/Normal-behavior-Fan-turning-on-after-heat/m-...

RickG13
Community Member

Managed to get it working.  Turned out my old wiring was twisted pair, which Nest technically doesn't support but a lot of people online say doesn't really matter. 
Actually, apparently it did matter.  I decided to run new new 18/5 copper wires down to the basement, removed the old wiring, reconnected everything and now it all seems to work as designed.  Heat and fan as they're supposed to. 

View solution in original post