HVAC intermittently losing power

Hi! I have a problem with a furnace (located in a ceiling attic) that loses power when it gets very hot outside, say over 100 degrees. This happened last year a few times, this year it is much worse – is happening daily. I’ve checked the breakers, the float valve, and everything seems fine. I’ve also replaced the fuse on the furnace board, and it doesn’t make a difference. The thermostat will go completely dark in the late afternoon and then light up again some time later in the evening when it cools off. We think it is heat related because that is the only pattern visible. Flipping breakers doesn’t bring it back to life.

AC man came out today and talked to my wife, he claims we lose power to the furnace because the outside unit overheats. I’m skeptical because it seems even if the outside unit could not run, why would that prevent the furnace & thermostat from receiving any power? I was really hoping it was the float valve given there is some water in the pan but that valve seemed OK. Interested in any ideas!

Does the power for that system come from the outside unit.

I don't think so because if I turn the breaker for the outside unit off, power still reaches the furnace & thermostat.

Have you actually checked to see if you have power up there when it shuts off. Or if the is power at the load side of the breaker.

Yes, power is at least getting to the light switch. I confirmed today during a time period when the furnace was working that it is NOT the float valve. When the float valve trips, power does still get to the furnace as expected. I suspect the problem may be the door switch on the furnace going bad because the plastic housing gets really warm to the touch. I got a replacement switch today and will see if that does the trick. If so, that is a $10 solution that the AC man wanted a $10,000 solution for!!

Hmm...if it is of any help, I dealt with a similar issue a couple of years ago. It was winter time, in the 30's outside and the furnace and blower were intermittent in turning on. Eventually they stopped coming on at all. Sometime during the night, with the thermostat still set to Heat, I heard a single "attempt" at trying to turn on. I heard the furnace blower power up for a few seconds like the furnace was starting and then it went off and stayed off.

I figured it was something major that shorted out. The culprit? A lightswitch type of deal that the HVAC people installed where they could flip the switch to cut power to the unit if checking certain things. It had shorted out and the unit wasn't getting any power. Hopefully yours is likewise something very easy. When the HVAC guy came out, he said, "It isn't even on! It isn't getting any power beyond the switch!" When he changed that switch, it worked just fine. Apparently those switches are prone to doing that. He even made a deal with me for the service call: "Renew your yearly service contract and I won't charge you anything, plus you'll have another year of service checks and discounts on service calls, parts and labor."

Sounds like a tstat problem to me . A digital tstat is typically powered by battery's , and all it really is , is a switch . When it sences a call for heat , it switches on . Even if you shut off the breaker to your furnace . The tstat itself will still function. Make sure your battery's are good and in securely .

If you open it up and touch the red and white wires together , the furnace will fire up (heat )

When we have a unit in the crawl space, we have a switch in the hall in the living space, like jmr I have found that switch faulty.

Thanks everyone for the ideas! A second AC man came out and this one told my wife the problem was the float valve cutoff switch. He wanted to come back and blow out the pipes at a cost of $200. This weekend, I proved him wrong in 5 minutes by causing the float switch to trigger and as expected...tstat still worked. Just no cold air. Power was not lost to anything.

So, I went and replaced the furnace door safety switch to see if that would help. A $10 part!! Lo and behold, no problems this weekend at all, the hottest weekend of the year. I guess something was overheating in the switch I had.

The part that really ticks me off is that 2 guys came out, one who wanted to sell us a $10k replacement system, one who wanted to charge for a fix that would not have worked. Neither had any friggin' clue what the problem was because they spent zero time really diagnosing anything. They just looked around & came to conclusions that would cost $ and not address the root cause of the problem. And, even if the float valve WAS bad or the outside AC unit was bad, it took literally minutes to prove that would not have caused power loss to tstat & upstairs unit. Pays to be a DIYer I guess.

Unfournately there are a lot of service companies in all of the trades that are sales companies and not service companies. Finding qualified service technicians is the hardest part of being in business. There are many homeowners that have no clue to the mechanical workings of a structure and fall for the don't fix, replace mentality. I'm glad you got your problem sorted.

It's easy in a society that is willing to throw money at anything for convenience., I have seen people buy a new lawn mower because their current one is hard to start.