Furnace Issues when it is very cold

[Croghanguy]

My daughter recently started having problems with her gas furnace. It seems that when it is very cold outside (temps have been in the teens on and off the last few days), the furnace doesn’t always cycle on, even though the air temperature in the room is 15-20 degrees what the thermostat is set to.

She has been in the house for 7 years, and the Rheem furnace is at least 12 years old. She has been using a Nest thermostat for the last couple years, and as far as we can tell is working okay; it seems to function properly manually, and, you can communicate with it via smartphone.

The furnace normally seems to run/cycle fine . But it appears that if it is really cold outside, and the furnace cycles off normally, it will not cycle back on without manual intervention. That intervention is nothing more than putting slight pressure on the ductwork right above the furnace itself. See area indicated in attached picture. We happened on to that by chance, but seems to work every time.


We did a little testing. It is currently 18 degrees outside. By causing the furnace to cycle off, and then a minute or two later, changing the thermostat to cause it to come back on, it does! But, if there is a long delay time before asking it to start (10-15 minutes or so (and maybe things cool down?)), it will not cycle on by itself. The manual intervention (touching the ductwork) seems to work. This test has been pretty repeatable.

Thought about taking the side panel of the ductwork off to see what is behind the 'sweet spot' that we are touching, but there are about 2 dozen hex-screws holding it in place. And, if we got it off and didn't see anything, we're afraid maybe our manual workaround would stop working when we put it back together.



Any thoughts?

 

[John Gayewski]

When the room temp drops do you hear the thermostat click? There is a series of events that start a furnace. You need to know where, in this series, the problem exists.

There could be a pressure switch that is not working or the tube is partially blocked. Touching the duct causes a pressure change and trips the pressure switch. Or a loose wire is being wiggled. But those are guesses.

 

[Sponsor]

 

[Croghanguy]

When the room temp drops do you hear the thermostat click? There is a series of events that start a furnace. You need to know where, in this series, the problem exists.

There could be a pressure switch that is not working or the tube is partially blocked. Touching the duct causes a pressure change and trips the pressure switch. Or a loose wire is being wiggled. But those are guesses.

I'm not sure if there is a sound/clicking at the thermostat. Standing by the furnace, there is no sound until the ductwork is touched, then after maybe a second, it sounds like a motor kicks on. A couple seconds later, it sounds like a burner lights, and a few seconds after that the main blower fan starts up.

 

[Fitter30]

Do u have a volt meter? Thermostat connections bottom furnace panel need to check voltage. If thermostat is calling
R tp C 24vac
R to W 0 vac if there is 24vac thermostat is either not calling or a bad connection or wire
W to C 24 vac
also check power connections and ground wires for a loose connection

 

[Croghanguy]

Ended up calling in a Pro for this problem. Turned out to be a loose connection between the pressure switch and the control board. He took care of that, and things seem to be working fine now. BTW, if it had something to do with outside air temp, our temps locally were in the single digits overnight, and no issues.
Thank you all for looking at this!