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Nest thermostat says no power

Mike-Mass
Community Member

Ok, so we have 2 separate furnace/ac systems 1st floor and 2nd floor. Installed Nest Learning thermostats in both locations, 2nd floor working perfectly. The 1st floor is problematic. It runs for a while then I get e74 message, no power in the line. Flipping the breaker on and off seems to reset or trigger the system to run so I replaced the breaker in the service panel as a precaution, and also replaced the emergency switch at the furnace. The condensate pump is clean and working fine, and I even snaked a new thermostat wire from the nest to the thermostat. Seemed to help for a day but this morning got the same message. So I’m going nuts, the furnaces are almost identical, same brand, same install and updates. I just switched the red wire over to the other red terminal and it seems to be working, but this has happened too many times to be confident that this is the solution. What else can I try if e74 shows up again? I’m ready to call the plumbing & heating guy who normally services my home. Maybe it’s a transformer or something else? Never had this problem with the old programmable Battery thermostat.

2 REPLIES 2

johnCNA
Bronze
Bronze

Have you swapped the two thermostats around to see if the problem follows the thermostat or stays with the 1st floor furnace?  That would be my first step.

Well, if you ran a new cable, we can assume both ends are connected properly and no bare wires are touching or close to each other.  If you have a condensate pump, is it wired to shut off the furnace if it overflows?  If so, you might have a bad or flaky overflow float switch in the condensate pump.  if it is wired that way, bypass it for a while.  Another place to look is the safety door switch on the door that covers the furnace control board area, if your furnace has one. (most do)  A flaky switch, loose contacts or vibration could cause power drops.  Most furnace control boards also have an automotive style fuse on the board itself.  Might be loose or flaky.

If you eliminate all that stuff, you might have a bad 24v transformer.

Mike-Mass
Community Member

Hey John… Thanks for the reply and suggestions. It’s been tough trying to pinpoint the problem. I have swapped the thermostats and the 2nd floor unit eventually (within 24 hours) reacted the same way.  Switching the red wire over to the other red post helped for 24 hours. Worked all day yesterday, woke up this morning and no heat. Went down and flipped the breaker off/on and the furnace started up within a minute. I finally gave in and called my HVAC guy and they’re scheduling a visit this week. Both furnaces have some kind of electronic board that was installed a few years ago, and I don’t want to mess inside the furnace. I did run a 5-wire rather than a 4-wire so if the guy wants to connect a special common 5th wire, it’s there. Once it’s fixed I’ll post up what the problem was. In the interim I’ll keep flipping the breaker.-Mike