I had a frustrating afternoon. My garage furnace doesn't want to run. And its really not fun trying to work in an ice cold garage. This afternoon I swapped out both a thermocouple (I was sure that was the issue) and a wall thermostat. Both got it to work for a couple cycles and that was it. I hate calling a furnace dealer but I'm out of ideas. I can't understand why the wall thermostat wants to quickly read low battery when I put in brand new ones (truth be told I've tried three different sets, including going so far as to buy a brand new package)..... and it did this on both the original thermostat and the brand new one. Last I checked the pilot light is working fine since I changed the thermocouple but the main burner does not want to follow the 'program', including in the hold mode. Any suggestions are welcomed.

PS: Wife just asked how old is it (50k btu natural gas Reznor)? I said, not that old! Then I looked it up - 1994. Hum, 23 years.
PSS: When all else fails, check the manual. Seems I replaced the Reznor in 2000, under warranty, so its only 17 years old.

 

Check the transformer. If it is not working the valve will not open and the thermostat has to run off batteries (which could show low due the the cold temp).

 

Scoots, thanks. You may be on to something. I'll look into that tomorrow.

 

Sorry to hear especially with the cold weather. Hope it's the transformer, Good Luck.

 

 

Thermostat can be bypassed easily to determine if that's the problem. Battery powered models I've owned get their batteries replaced maybe every 5 years so I doubt the thermostat is the problem.

-- Chuck

 

From what I can gather online, I doubt its the transformer. I've got an auto tester; however, since I have both 120v and 240v service in my garage I'm not sure what the input is to the transformer. The output I believe should be 24v. I'm not comfortable checking the resistance unless I know better what I'm doing and how to do it when it comes to checking the step-down transformer while installed.

It may simply be a dirty flame sensor but I cannot locate where it is. I cannot locate it from any inspection views; perhaps the burners themselves have to be removed for access. Its a ceiling heating unit and not the easiest thing to inspect and work on.

Chuck, I know its not the thermostat. I have had the furnace running with both the old thermostat and the new thermostat. But sooner or later it goes off again. The batteries read low with these digital wall thermostats because they rest near a 2" opening in the wallboard which is ice cold unless the furnace is running. If I bring the wall thermostat into the warm house the low read goes away and the batteries are fine.

At this stage I guess I'll be calling a heating dealer on Tuesday.

 

Dave, take a look at YouTube first. I have found tons of helpful info, you might find your exact problem or maybe the answer to your question about 24v, whatever that is.

 

If you take the batteries out of the thermostat while still hooked to the wiring does the display go blank?

 

It does not work that way. There is a base on the wall with pins to connect the 'juice' to the furnace. The batteries are in the face plate that you remove, so the display is always on if on the wall or off the wall.

I'm trying a final tactic . . . I filled the hole in the back of the base plate with a little 2x2" piece of cotton towel. It should prevent the really cold draft from coming through the hole to reach the thermostat batteries. At the moment the furnace is running great and it's 57° in the garage. I told it to run the program which is to maintain a constant 45° all the time unless I want to work in the garage. If the furnace is still running in the morning, I will know I solved the issue. TBD.