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wiring a thermostat

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Fred f
Fred f Member Posts: 1
in THE MAIN WALL
A T8400B1018 Honeywell electronic power robbing thermostat requires a 1000ohm 3watt resistor be placed between the lower TT terminal and the lower FF terminal on a system with a R8184G primary control. I accept the instructions , I just want to know why and what the relationship is between those two terminals. Thanks in advance.

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  • BillW@honeywell
    BillW@honeywell Member Posts: 1,099
    resistor

    It is used to prevent the thermostat's display from blanking out whenever the primary operates.
  • Big Ed
    Big Ed Member Posts: 1,117
    Hmmm...

    Good question. Bill gave us the reason but what is the relationship ? Both are a power source .The TT is 24 V while the FF is 3.5V . I know if the Lower T makes contact with the upper F it will lock out..Good to know when looking for a safety reason.. I will assume(and I hate assuming) the resister will allow a trickel of juice for maintaining voltage for the thermostat's program???
  • Glenn Harrison
    Glenn Harrison Member Posts: 405
    The reason is...

    according to the internal wiring diagram I just looked at, the bottom "F" is common going back to the internal transformer, and the bottom "T" is the same as "W" coming from the t-stat to energize the primary relay. So it is essentially the same as coonecting the resistor between "W" and "C" on a scorched air system.

    And if your curious, you can go to:

    http://hbctechlit.honeywell.com/techlit/PDF/69-0000s/69-A0617.pdf

    to see the wiring diagram I looked at.

    Glenn Harrison, Residential Service Techician

    Althoff Industries, Crystal Lake, Illinois

    Althoff Industies Link
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