We discovered this week that programmable thermostats for your furnace have batteries in them, and if the batteries die, your furnace will run without shutting off. We figured out what was happening before we went to work, so we don’t know how long our furnace would have run, but we speculate that it would have run all day, or until something broke/melted/caught on fire/etc. :eek:
Also, programmable thermostats apparently start flashing when your batteries are about to die. I saw the blinking but I didn’t know what it meant.
I just wanted to share this information so you’ll know what it means when your thermostat starts blinking at you!
Maybe your particular brand/model of programmable thermostat behaves that way. Mine (made by Honeywell) doesn’t. I haven’t put batteries in it since we moved in about seven years ago, and I’m not sure when/if the previous owner did, either. Pretty sure it draws power from the wires that connect it to the furnace.
Must be a brand thing. My old one-just replaced with a Nest (awesome)- would do the opposite. When the batteries were dead it’d go into a sort of energy saver mode and drop the temp to 35.
The batteries in thermostats run only the control circuitry and the display UNLESS they are on a two-wire connection to the heating system. Then the batteries are also used to close a relay (or electronic equivalent) whenever the heater is supposed to run.
(I had never seen anything but three-wire connections, where you have a power, a neutral and a control wire, until this house… and the three zone heating thermos use only two wires and a mechanical thermostat. I have been trying to find a digital replacement for these that doesn’t rely on batteries and have come up dry.)
If the OP’s thermostat uses two wires, battery failure should mean no heat, not continuous heat. If it’s three wire, it’s possible a failing battery could make the circuitry latch in the on position, but very unlikely. I’ve never seen a fail-to-on design for such things.
I’ll be damned, I checked the manual for my thermostat (Honeywell Model CT3400/CT3455), and yes, it does use batteries. But they last a damn long time: I really haven’t replaced them in about seven years. The manual says that when the batteries are near the end of their life the display will flash “REPL BAT,” which is pretty unambiguous; I haven’t seen that happen yet.
A couple of brand-name AAs will power the tiny clock and temp-monitoring circuit for a long, long time. They’re likely to fail long before they just run out of juice.
We had this exact situation one night, with runaway heat on. It was over 90d F when I woke up and killed the system. Replaced the batteries the next day and all was well.
My programmable does not have batteries. We had an HVAC failure two years ago, and the thermostat was the first suspect, we went to check the batteries and there were no batteries to replace.
When it happened, we discussed whether it was worse to have the furnace keep on running or stop running - it’s kind of a toss-up. No furnace, we could have come home to frozen, burst pipes and a huge clean-up bill. Furnace running all day, it could have burnt the house down. So, in closing, I think I might go shopping for a better programmable thermostat that doesn’t have quite as much destructive potential.
Okay, I looked at the thermostat - it’s a Noma brand device. It was in the house when we moved in here three years ago - I don’t know how old it is or what features it does or doesn’t have.
ETA: For any doubters, we replaced the batteries in our thermostat, and that fixed the problem. That tells me that it was the batteries that were the problem.
We have a Honeywell - and it requires batteries. It was installed about 3 years ago.
Annoyingly, it’s VERY VERY VERY hard to remove the faceplate to get to the batteries. We wound up mostly ripping it from the wall (pulled out the drywall anchors) in the attempt. Fucking stupid design.
Now, now, just because you don’t have four fingernails to push in all the locking tabs simultaneously while holding down the CANCEL button and twisting in a counter-clockwise direction, don’t blame the design…
I did some testing on my thermostat a few years ago after the last programmable thermostat /battery thread, and found the following - if I pull the batteries from the thermostat, it will maintain whatever condition it was in at that moment. If the heat was off, it would stay off. If it was on, it would stay on. So it left the switch in whatever state it was in at the time of battery failure.
