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How to ID a millivolt system

Jshine Member Posts: 19
How can I tell if I have a millivolt system?  We have a gas furnace that produces steam for heat - no fans - no cooling.  Thermostat is a 2 wire Honeywell T87F. One wire is connected to the pressure control unit - the other wire connects to the low water cut off.



  • JStar
    JStar Member Posts: 2,752
    edited November 2010
    First sign.

    Millivolt systems will not have a shut-off switch on the boiler, or a breaker in the panel. The boiler will most likely not have any high voltage connections on it.
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    Give me the numbers

    from the gas valve. If it is a Honeywell powerpile (750 Millivolt) system it will have an s in the numbers say like a VS-820 valve?

    Are you the same person with the question about T8775 thermostat?
  • Jshine
    Jshine Member Posts: 19
    millivolt question

    Thanks -

    Yes - I do have a shut off switch on the boiler

    Gas valve is  VR 8304M 3152  24v  60hz

    so - looks like "not" a millivolt system ??

    and yes - I have the post up about the T8775A  -  after that stat failed to work I got out the boiler manual and read a section "electronic thermostats" - "certain types of electronic thermostats may lose their memory or shut down." the manual goes on to say - "with PS-802 probe type low water cut offs, this may occur each time a low water condition is detected".  ...goes on to talk about installing a R8222A relay etc.. so even though it doesn't sound like my situation - now I'm getting spooked about trying another brand new T8775 or using any digital stat.  So looking at other stats and reading some posts here - I keep coming across posts that say you can't use a certain stat with a millivolt system - power robbing and so on and so on... pretty confusing for me..that's why I posted the millivolt question. 
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    If you still have your old T87F put it

    back it was one of the most reliable thermostats Honeywell ever made.
  • Jshine
    Jshine Member Posts: 19

    That exact thought has crossed my mind - The T87F has worked perfectly - maybe I'll just place a hook next to it to hold a good magnifying glass.

    Thanks again for all the good info.
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