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Thread: Honeywell RTH-8500 differential problem

  1. #1
    Junior Member

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    Honeywell RTH-8500 differential problem

    I purchased a Honeywell RTH8500 thermostat an it is giving me fits. As is well known Honeywell does not seem to allow changing the differential and who knows exactly what it is. I am using this thermostat on a heat only boiler/radiator system. The problem is that when the room temperature and the called for temperature are the same it randomly cycles the boiler for say 2-4 minutes. Not even enough time for the boiler to heat up. The display always reads 72 for room temperature. Obviously the differential is so small (less than 1 degree) and the way Honeywell averages temperature is so poor that it thinks it needs to run then it decides it does not. I have it set to heat only - menu 0170 = 5 and menu 0240 = 3 which according to the menu is what it should be set to. Honeywell seems to have this crazy idea about cycles rather than differential. Is it possible that setting these menu selection in another way would change the differential?

    I had to reinstall my prior thermostat an old Hunter which had the differential set to 1 degree. That works fine. If the thermostat is set to 72 you see the room temperature kick between 72 and 71. When it averages 71 or stays 71 for several readings if finally kicks in. If the Honeywell worked like this I would be a happy camper.

    I sent email to Honeywell and no answer. I called and they said due to high call volume I would have to call later. It seems they are understaffed and maybe lots of problems.

    If this cannot be resolved I am not adverse to trying another brand. It seems White Rogers Sensi is one that is often used. Whatever it is it can't have tis crazy differential problem I am seeing with Honeywell.

    Any thoughts or help on this would be appreciated.

  2. #2
    HVAC Thermostat Specialist

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    Make sure the wire hole behind the thermostat is plugged up.

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