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Old 04-24-18, 05:34 PM   #1 (permalink)
Join Date: Feb-2018
Posts: 217
Exclamation Swapping Out Thermostats

I'm trying to swap out my old thermostat (the one with the bulky, unfriendly probe) to a new one with a more streamlined probe. Currently, I'm taping the thermostat probe to the non-sticky, backside of my heat mat, outside the enclosure, using foil HVAC tape.

I set the new thermostat to a max temp of 87 degrees with a 1.5 degree heat differential, plugged the heat pad into the "heating" outlet on the thermostat, plugged the new thermostat into the wall, and taped the probe in the same placed on the pad (again using foil HVAC tape) and suddenly I had high temp alarms going off all over the place and the probe showing a temperature on the pad of 100 degrees. Which was weird because the previous thermostat probe read the pad at 92 degrees just a few minutes earlier.

At first I didn't think much of it, because the prior thermostat had been set to a max of 92 degrees (I have deep substrate and this gave me a surface temp of 82 via IR temp gun) and the old probe didn't have good contact with the pad due to it's less than streamlined shape. I thought the pad would gradually cool down, but the new thermostat probe temp kept going up and up (even though the temp coming from the probe was above the max temp I'd set on the thermostat) until it eventually reached 108 degrees and I pulled it off and unplugged everything.

What is going on? It seemed like the thermostat was heating my heat pad even when it was already well past the max temp. Why would it do that? Should I have waited for the heat pad to cool completely before taping on the new probe?

Last edited by phenyx; 04-24-18 at 05:40 PM..
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Old 04-24-18, 06:08 PM   #2 (permalink)
Join Date: Feb-2018
Posts: 217
Re: Swapping Out Thermostats

Ok, correct me if I'm wrong here but "heating" means that the temperature will go *up* right? Because I've got the thing plugged back in to the thermostat outlet that says "heating", the probe taped back in place, the thermostat display says it's in "heat mode" and the "heating" indicator light is on, but as I've watched, the temp registered by the probe *taped directly to the heat mat* has gone *down* by 2 degrees.

Have the laws of physics changed suddenly? Do I just not understand what "heating" means with regards to a thermostat? Have I moved to an alternate universe where "heating" really means "cooling"? What is heat?

Am I crazy?

UPDATE: Nope, I'm in a universe where the thermostat outlet marked "cooling" has turned on my heat pad.

Last edited by phenyx; 04-24-18 at 06:14 PM..
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Old 04-25-18, 04:11 AM   #3 (permalink)
Join Date: Dec-2017
Posts: 900
Re: Swapping Out Thermostats

I'm not sure what's going on with your Tstat situation, so can't help there.

One thing caught my eye though, and I hope I'm not reading it wrong (again)...

You mentioned the depth of your substrate. Which, really shouldn't matter when checking your hot spot temps with your temp gun. You should be checking the actual surface temps, not the substrate. Also, it is common practice to keep the substrate thin over the hot spot, about 1/4 inch

Anyway, I'm hoping I read that right and was able to help you out with that part at least. But I'm as confused as you are with the Tstat situation
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