How to ID a millivolt system

Jshine

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.

John

JStar

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

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

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

If you still have your old T87F put it

back it was one of the most reliable thermostats Honeywell ever made.

Jshine

T87F

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.