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  1. #1
    Join Date
    Dec 2018
    Location
    Southern California
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    2
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    Tempstar thermal limit switch question

    Hello,
    New member here, for reference so you know my technical knowledge, I was an airline mechanic for years and now am an engineer. I design aircraft propulsion systems including pressurized flammable gas systems. I am not however a HVAC professional so I have a question before I call a tech to repair my system. I'm not going to attempt to repair a system in my attic that could burn down my house without the proper training and certs but I do want to have some talking points for when the tech arrives.
    Here's my problem:
    I installed Ecobee thermostats on two 1999 vintage Tempstar systems in my attic. They run fine with the AC. Now that I am heating they both constantly reboot. Ecobee says this is due to a thermal limit cutout. I changed filters, looked at evap coils and they are clean. The blower seems to be running fine and I get good airflow out of all my vents. The flame looks good and the exhaust blower seems to run ok. The units run for about 5 minutes then cut out. I measured the thermal limit switch with a meter and it is opening indicating it's getting too hot. (I check right after it trips and it is open, then it resets when it cools down.)
    What I found though is that the limit switch is actually digging into the side of the combustion chamber making direct contact with it (there is a gouge in the switch where it contacts and I can see the chamber is right there through the hole). If I back out the screws a couple turns the limit switch does not trip. What I am wondering is, should the limit switch actually touch the combustion chamber or should it just measure the temp of the heated air inside? I would think the combustion chamber itself will get a lot hotter than 200F. This is the case on both units and it looks like it has been so since the day they were installed. With the old thermostat no one would ever know since it would just keep cycling without any indication on the thermostat itself. The rebooting of the ecobee is what alerted me.

  2. #2
    Join Date
    Dec 2018
    Location
    Southern California
    Posts
    2
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    Thread Starter
    I had a tech look at it this morning. He checked the system and verified the gas pressure was in range. He also said the limit switch should not be touching the combustion chamber. He said it was probably always tripping and resetting but with the older thermostats with batteries you wouldn't see it. He's going to look for a switch that doesn't stick in as far so it won't touch. Hopefully that will fix it.

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    76,253
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    Yeah, they shouldn't touch anything, just the air temp is what they should be sensing.
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