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Question DetailsAsked on 6/23/2016

AC kicks on after lightning strike. Thermostat was set at 78, temp was 76 degrees in house but it still kicked on.

I've had issues with furnace and A/C kicking on after a power surge and/or lightning strike. Last night there was a lightning strike and the A/C kicked on even though the thermostat was set at 78 degrees and it was only 76 degrees in the house. Furnace kicked on this spring after a power surge even though temp in house was above thermostat setpoint. Any ideas why this would happen? My concern is that it could kick on while I'm not home, and run indefinitely until I shut it off manually.

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3 Answers

Voted Best Answer

New one on me - guess you need to call a Heating and A/C contractor (your Search the List category) for help.

Or contact the thermostat manufacturer with your question.

I could see it malfunctioning in that instance, but not it continuing to work correctly afterwards if it got zapped. Unless it is causing the thermostat to reset to factory default setting of say 68 or 70 or so during surges - that could happen if your presets (assuming digital thermostat) had been wiped by a power outage or surge.

You could check your thermostat batteries if the presets were lost and won't hold if you reprogram it - maybe they are dead.

Answered 4 years ago by LCD


Thanks for the input LCD. It all started this spring when there were random power surges (or I've been told it could have been low voltage??). It would be 70 degrees in my house, the TV and a couple of lights would flicker, then the furnace would kick on even though it was set to 66 degrees. There was one instance where it fried the control panel, smart-gas valve and the transformer (the fan would kick on but nothing would happen). I had an HVAC specialist come out to repair it...$1200 down the drain. My neighbors have also indicated they've had lights/TV flickering on and off at the same times that I've noticed it, so it's not something just in my home.

After the repairs the furnace and/or AC will still work after a surge, but my concern is that if it kicks on when I'm not home to turn it off, it will just continue to run :(

I've changed the batteries in the thermostat, and when it does kick on after a surge I always check to see that none of the setpoints have been reset. It's the strangest thing ever!

Answered 4 years ago by Fizmokat


Sounds to me like you need to (maybe with assistance from neighbors - maybe a joint letter or you all call the utility searvice number) you need to contact the utility company about possible problems on their end. Especially if you can talk to neighbors and tie down if they all maybe come off the same transformer (commonly 3-5 houses per transformer for underground service, 1-3 commonly off power-pole mounted ones.) Or maybe you are all on a dead-end service (not necessarily a dead-end street, but one that is feed from only one end by the power company with only one service line, and it has problems.

One other thing - contact the manufacturer of the thermostat about whether your model does a reset and initial turn-on recycle after a power outage - it may be the thermostat doing this after a power loss/browndown. IF ONLY the furnace or the A/C is doing this (depending on which the thermostat is set for use for depending on season), that would be my guess - the thermostat may lose track of where it is and recycle the board starting with an ON cycle until it determines the temperature is past the setpoint for that date/time - then kick it off after the typical 2-5 minute delay period.

I have never heard of A/C or forced air furnace automatically kicking back on to ON (cooling / firing) mode after the power has been off or browned down. Boilers do this if the power has been off long enough for the water to cool below the ON setpoint on their internal boiler thermostat,, but I have not heard of it with A/C's or furnaces, so the thermostat sounds like possible culprit to me.

Of course, next time this happens, look at thermostat and then check where it goes to before the A/C or furnace shuts itself off (assuming it does). If they shut themselves off OK, probably not an issue - other than the fact common surges/browndowns are not good for equipment like that, so if common in your area due to poor power quality, thunderstorms, etc you might consider getting a surge protector or power protector for the furnace (assuming gas fired). A LOT more expensive to do that for electric furnace or A/C because of the high power demand it has to be able to handle - talking many hundreds for 220/240V appliances versus a few hundred for a single 110/120V lower amperage appliance.

If lightning effects are commonly coming through your power lines, a lightning surge arrestor might be in order too - a few hundred $ installed commonly. Not going to protect against a direct hit on your electrical system/house, but generally pretty reliable against garbage surges from hits on powerlines and transformers well outside the house.

Answered 4 years ago by LCD


Hi LCD, thanks for your response. I live in a subdivision and it's no secret that the grid is overloaded in my area. The city has admitted it :( There have been several transformers blown in my area (underground wire), and I've even had them come out to check my meter and the transformer that serves my house. Of course they couldn't find anything because the power was flowing consistently at the time. I have filed a formal complaint with the city as a result of the $$$ required to fix my furnace this spring. I highly doubt any action will ever come from it. You know how that goes!

Sounds like we are thinking along the same lines for a solution. I considered an whole house surge protector, but have opted to try a new thermostat first to see if that works. Going with a smart thermostat that will allow me to monitor temp from my phone while I'm away. After snooping around, it appears that I have the necessary "C" wire for the upgrade so I'm hoping to get a new one installed soon.

If I end up going the route of some sort of surge protector I'll definitely be back with more questions because I know very little about them! Not sure whether a whole house surge protector would be best, versus just protecting the furnace.

Answered 4 years ago by Fizmokat

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