Thermostat

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rnlincourt

Member
Dec 31, 2009
56
Western MA
So I just bought a digital thermostat for my house- (the previous one is an old mercury switch) and while taking off the front plate I noticed that there were only 2 wires connected to contact plates, (I am thinking a 120 source) and a ground. The directions in the new unit says I won't be able to use the new one if it's a 120 source but I am not totally sure it's def a 120 source. Does anyone have any experience with these and do I just need to get a voltage meter to see if it's truly 120?
 
I doubt it is 110. Wha kind of furnace do you have. Do you see the same wires (from the thermostat) attaching to it?
 
Most HVAC thermostats are low voltage supplied by a transformer located next to/on the furnace. Trace the lead (should be similar to 22-24ga bell wire in appearance) to your thermostat back to the furnace to see if it goes to one. If you are lucky, the input and output voltages might be right on the transformer.
 
gzecc said:
I doubt it is 110. Wha kind of furnace do you have. Do you see the same wires (from the thermostat) attaching to it?
it's an ancient weil mclain oil furnace. and it does have a service switch above the unit. I will check when I get home to see if it goes over to the panel. if it doesn't and it goes straight from the service switch to the thermostat will I be able to just plug the remaining wires in?
I think to be on the safe side I will borrow a volt meter to make sure.
 
Post a picture of the furnace wiring set up.
 
How big are the wires at the thermostat? If they are little thin wires (less than lamp cord) it is almost certainly NOT a 110v system - if they are like house electrical wires it probably is. The classic round Honeywell mercury stat is low voltage...

Probably you have a 110v line going to the control board in the furnace to work the burner and / or blower via relays, and a low voltage line going up to the thermostat which produces the "call for heat" signal... This only takes two wires minimum, you will only see more if you have a fancier unit, with things like "fan only" or both heat and AC control functions... Often you will see more strands that aren't connected just because the installer finds it easier to just carry one sort of wire around on his truck and use it for everything, but hooking them up on the thermostat end isn't going to do anything for you if they aren't tied to something on the other end...

The other thing is that you probably should NOT rely on any color codes - look at what does what on the boiler end and make sure that makes sense in terms of the thermostat end...

Gooserider
 
you definitely do have a 24 volt thermostat setup. so to make this easy for you here is what you do. if it is just 2 wires in the cable, what ever color wires you have, take one and attach it to the rh on the thermostat. take the next wire and attach it to the wh or white terminal on the thermostat. that's it. your done. mount the thermostat to the wall. most thermostat wires are colored with, if 2 wires it will be red and white. if 3 wires it will have red whiite and green. if more it will have red white green yellow blue and more. you only use the red and white, unless the wire had problems years before.
 
gzecc said:
Post a picture of the furnace wiring set up.
here it is
 

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DAKSY said:
Kinda dark pic, but it sure looks like a Romex wire for 110....

well the wires are a lot smaller than they look- I did exactly what fbelec suggested and tested things out- seems to be working fine and am still alive! Here is what it looks like wired.
 

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DWBMHD said:
DAKSY said:
Kinda dark pic, but it sure looks like a Romex wire for 110....

well the wires are a lot smaller than they look- I did exactly what fbelec suggested and tested things out- seems to be working fine and am still alive! Here is what it looks like wired.



Hope you have some paint left!!!
 
raiderfan said:
DWBMHD said:
DAKSY said:
Kinda dark pic, but it sure looks like a Romex wire for 110....

well the wires are a lot smaller than they look- I did exactly what fbelec suggested and tested things out- seems to be working fine and am still alive! Here is what it looks like wired.



Hope you have some paint left!!!

how bout it- going to have to patch it up this wknd!
 
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