Ctgriffi Posted January 5 Share Posted January 5 (edited) I've been going round and round with Airxcel tech support on this subject and am getting a bit frustrated with them, so I thought I'd drop the question here: I have a Coleman Mach roof A/C unit that's ~10 years old. It works well generally and is controlled with a mode selector switch as well as a manual thermostat dial. My unit has a simple, add-on heat strip which comes in handy for cool weather camping, and the heat function is activated by turning the mode selector switch to "Low Heat." The problem is that, when using the heat strip, the thermostat dial doesn't seem to have any bearing on its function—the unit just blows warm air constantly and always pulls about 12A, regardless of the thermostat setting and regardless of the ambient temp in the RV. That can get real toasty, real fast, in a tiny RV! And that is not how it was designed to work; the thermostat is intended to turn the heat strip off and on, in order to maintain a set temperature of course. The question is, "Why is it not working correctly?" If you look at the highlighted portion of the attached wiring diagram you can see that, in Low Heat mode, the hot/black wire is always connected to the Heater Plug. Then, when the thermostat is calling for heat, the white/neutral supply wire makes a connection to the blue wire, which is the other side of the circuit at the Heater Plug (in other words, it's designed to switch the neutral off/on, rather than switching the hot). I have tried completely replacing the thermostat, which didn't help at all. I then tried pulling the blue wire completely off the thermostat, left it dangling... and the heat strip still continued to burn merrily away! Bottom-line, it appears that the heat strip circuit is being completed via another path, bypassing the thermostat somehow and making it irrelevant. Which seems dangerous. How does this happen and what's the fix? Edited January 5 by Ctgriffi Quote Link to comment Share on other sites More sharing options...
extech Posted January 5 Share Posted January 5 obviously the diagram isn't showing everything. since it is an add on i would be looking at the wiring where the heat strip is. someone grounded the return Quote Link to comment Share on other sites More sharing options...
Ctgriffi Posted January 5 Author Share Posted January 5 (edited) The heat strip wiring is just a 2-wire plug that connects to the ready-made socket on the unit (all factory). The heat strip itself has a steel bracket with a single bolt that secures it to the roof unit's chassis, in the path of return air. I also checked the main power connection, which is 12GA Romex coming into the ceiling control panel: black-to-black, white-to-white, and ground-to-ground are all wire-nutted together correctly. Edited January 6 by Ctgriffi Added photo of heat strip Quote Link to comment Share on other sites More sharing options...
extech Posted January 6 Share Posted January 6 the factory ready made socket has a lead going to the return Quote Link to comment Share on other sites More sharing options...
extech Posted January 6 Share Posted January 6 can't read the diagram, does the yellow wire go to the thermostat? Quote Link to comment Share on other sites More sharing options...
extech Posted January 6 Share Posted January 6 if it is the thermostat on the left, it shows it as a simple switch and it is in the always on position. i think the t stat part only works the ac Quote Link to comment Share on other sites More sharing options...
Ctgriffi Posted January 6 Author Share Posted January 6 (edited) Sorry the diagram looks fuzzy; this system degrades images quite a bit. So, the thermostat switches the common wire (neutral) to the yellow wire when calling for cold or to the blue wire when calling for heat; it’s a brand new unit and was bench-tested before installing. (That blue wire is connected directly to the heat strip through the 2-wire plug.) The other side of the heating circuit is the black/hot wire which is always connected to the heat strip via the selector switch, when in Low Heat mode. Edited January 6 by Ctgriffi Quote Link to comment Share on other sites More sharing options...
extech Posted January 6 Share Posted January 6 so at your thermostat there is a yellow and a blue wire Quote Link to comment Share on other sites More sharing options...
Ctgriffi Posted January 8 Author Share Posted January 8 (edited) Figured this one out finally... the heat strip (photo up above) has a little metal box containing a high-limit switch. I opened that box up and found some sketchy wiring that was creating a path to ground through the box and mounting bracket! Anyway, once I cleaned up the wiring in that box and made sure nothing was gonna ground out again, I put it all back together and tested: Thermostat dial is doing its job now, switching the neutral supply wire off/on as desired. Good deal. 😅 (I tested the little high limit switch in the box too, using a heat gun, and it's doing the thing. If that switch goes bad, you can end up with a completely open connection, with zero heat output.) Edited January 8 by Ctgriffi Quote Link to comment Share on other sites More sharing options...
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