HVAC Thermostat Weirdness

[opus1]

Last night I noticed that the AC kicked on and didn't think anything of it, then about 20 minutes later I realized that the unit had been running constantly the whole time. It was well after sundown and not that hot outside, so it didn't seem right. By the time I'd finished what I was doing and went up to check the thermostat, another 5 minutes went by and everything was still going full-blast. I get upstairs and see the the LCD screen on the thermostat is blank. Unit is a 12-year-old Honeywell TH8000 programmable thermostat. This picture is a newer version, but it looks very similar to this:



Debated whether to pull the thing off the wall while everything was going or go back down to turn off the HVAC, and I was still awake enough to decide to go downstairs and turned everything off with the service switch on the furnace. This is an older house so the thermostat doesn't have a "C" wire for power and runs off of 3 AAA batteries - which I'd changed within the last 6 weeks. I checked them anyway, and they were all good, they had a snug fit between the contacts, and after I cleaned the ends of the batteries and the contacts, and fiddled with it a bit, the thermostat finally woke up. So far, it's been working fine.

Now for the question: I was under the impression that the system doesn't work if the thermostat isn't powered - why didn't everything shut off when the thermostat decided to go into a coma?

Since the touchscreen on the thermostat was acting flaky, and now this event, I've already got a more modernized version on its way from the big-box home store that likes the color orange. Any insight on why things kept running rather than shutting down after the thermostat left the party?

 

[cpayne5]

Sounds like the thermostat failed, but kept the relays actuated. It could have been alive, but the screen dark, for instance. The HVAC should have definitely turned off if green/yellow/orange was not energized.

 

[Oildudeny]

Sounds like the t-stat just crapped out.. always best to turn system off from the safety/service switch than yanking the t-stat off the wall.

 

[mk378]

They are latching relays so they only take a pulse of power from the batteries rather than a continuous draw as long as the A/C is on, which would quickly run down AAA batteries.

So if the thermostat CPU abruptly quits, the relays stay in the state they were.

 

[BlueOvalFitter]

Change the batteries.

 

[Amkeer]

If you have a float switch on the air handler it will cause the thermostat to go blank. The float switch prevent condensation from building up and causing water damage. Its good practice to vacuum out the condensation line a few times a year.

If the air handler keeps running you can also trip the breaker and reset it after a few minutes.

 

[zzyzzx]

Those digital thermostats don't last.

 

[CincyDavid]

I have wi-fi enabled Honeywells in both houses...so far so good. I like them primarily because I can keep the unoccupied house at 85-90 degrees, as long as the humidity doesn't creep up, then when I head to the airport to go there, crank it down to 72, so it's nice and chilly when I arrive.

 

[opus1]

They are latching relays so they only take a pulse of power from the batteries rather than a continuous draw as long as the A/C is on, which would quickly run down AAA batteries.

So if the thermostat CPU abruptly quits, the relays stay in the state they were.

Makes sense - I didn't think about that part.

New thermostat has arrived and is on the wall. I debated going with something different and maybe even wifi but this is in an elder parent's house and I didn't want to cause any duress so I'm glad I was able to find something almost exactly the same. The installer programming was different, but the basic function along with the looks are the same and that's what counts.

 

[Touring5]

Had a Honeywell thermostat that did the same thing. I mentioned to the HVAC tech who was doing a 20-year checkup on my system (lol) that I had a thermostat that had done that. He immediately responded "was it a Honeywell you bought at Menards?" (which it was). IMO, for such an old company that has been involved in things like aerospace, they seem to make a lot of products with marginal quality.

 

[JustinH]

We have a honeywell wifi thermostat. It has worked well however the Geofencing never seems to work right. We have it on my wifes iphone and mine, and sometimes when she leaves the house and I don't, it will think I have left also, and will turn the AC up to 82 degrees.

I need to relaunch the app, and it immediately knows I am at home and adjusts the temperature.

Its a nice feature to have if you have pets, it will let you know if there is a failure and alert you.

 

[97prizm]

Bad batteries can definitely cause this. It's supposed to turn off but sometimes causes a glitch. I have a similar thermostat unit. The honeywell hot water units also occasionally do this. It thinks its on but quits telling the heater to kick in. Woke up to a cold shower