Fan not running on heat

Scd

Thought it was frozen pipe on the attic, it appears not. 1 shared water heater in the basement, 1st floor heating comes from air handler in the basement, 2nd floor heating comes from air handler in the attic. 2nd floor heating has been working until 3 days ago. 1st floor has noissue. 

2nd floor room temp is at 64. 

Set T'Stat to 70 in heating mode - fan doesn't work. Confirmed it calls for hot water - because pipe runs hot at both the furnace and air handler.

Set T'Stat to 70 in heating mode and Fan ON - fan doesn't turn on .  Touch red and green fan runs .

Set T'Stat to 60 in heating mode and Fan ON - fan kicks on.

Set T'Stat to 60 in cooling mode - fan kicks on.

Fan motor speed selector is at Tap 3. Changing to Tap 1 or Tap 2 doesn't make a difference. 

Replaced 2nd floor T'Stat with the one from 1st floor, same problem. 

Any suggestion for next step diagnose? Thanks

pecmsg

What thermostat do you have?

Jamie Hall

I find it a bit odd that the fan runs when you are calling for a cooler temperature, but not when you are calling for heat. Miswiring problem somewhere?

Scd

It has a nest, tried another nest, also the old Honeywell. It was working before for years so wires should be ok

Saw a similar thread somewhere where they had issue with one particular tap, in my case tried all three got same behavior

pedmec

in the nest programming you have to specify they you want the nest controlling the fan on heat.