Changing thermostat

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Old 10-26-05, 05:56 AM
cwbell1
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Changing thermostat

I have an old Lux S.P.S.T AT10-1141 thermostat for my gas furnace. I want to install a digital programmable thermostat to conserve fuel. Without looking inside, I purchased a Hunter 44360. When I opened the cover of the old one I found that it was controlled by a mercury switch powered by a standard electrical house wire leading from a step-down transformer in the basement. I had expected to find the normal 4-wire telephone like wiring. I checked the voltage and found it to read 14.8 volts.
It looks like I am going to have a major problem installing a programmable thermostat without rewiring the whole system. I am open to any input or suggestion as to what I can/must do in this situation.
 
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Old 10-26-05, 12:36 PM
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Line voltage??

How old is your furnace? Make/model of it.
 
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Old 10-26-05, 01:40 PM
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I checked the voltage and found it to read 14.8 volts.
Line voltage??
I Hope not.

There are a few battery operated programable thermostats made that would work in this situation, but any chance you might be able to run a new 4-wire thermostat cable? It might make things easier.

Doug M.
 
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Old 10-26-05, 07:38 PM
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Doug,

He has line wire at his current t-stat, then goes down to the transformer...


I'd say the best thing to do is to pull new wire... But if his transfromer suppse to power the furance... but give us more info on the furnace itself.
 
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Old 10-27-05, 05:47 AM
cwbell1
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My furnace is a Burnham hot water boiler type. Model # P-206A-WNI, Ser.# 17333447. Inside I have a two-wire cable coming in from my step-down xfmr on the basement wall to a larger xfmr inside the unit. The wires are Red and White.
Running another wire would be difficult but I've done the "impossible" before. I would rather not do this if possible. If I did run a new 4-wire I wouldn't know which ones go where. Unfortunately I'n not an "electricukel engineer".
 
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Old 10-27-05, 06:29 AM
cwbell1
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Sorry, I forgot to mention that it is a 1991 model.
 
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Old 10-27-05, 07:07 AM
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He has line wire at his current t-stat, then goes down to the transformer...
Jay11J, We're both reading this a little differently and I'm not sure which of us is correct. I'm reading that the line wire is carrying 14V from transformer through thermostat back to furnace (wrong type of wire was used). I think you're reading that the wire is carrying 120V from power source back to transformer. Since, in either case, there's no neutral or negative to check against, I'm not really sure how we can accurately determine which of us is correct and it's quite critical that we know before giving any further advice. I'm guessing cwbell1 measured between the 2 thermostat wires initially (in series with control or transformer) and could potentially have gotten 14V on either of our scenarios. The only thing I can think to suggest, cwbell1, is that you measure between each of the thermostat wires, one at a time, and a good ground source like the ground prong (round hole) of a near-by receptacle. Hook one wire from the test meter to the ground connection and the other wire to one thermostat wire at a time. If you get a reading of 120V from either wire, it must be line voltage and you'll need a line voltage thermostat. If you don't get 120V from either wire, it's probably low voltage and we'll have to figure out what the true voltage is (by measuring between the terminals at the transformer itself) before going any further OR install a battery operated low voltage thermostat.

Jay, do you have any better ideas? Anyone else?

Doug M.
 
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Old 10-27-05, 09:39 AM
cwbell1
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OK gentlemen, let's try this. I don't have a ground close enough to the wall thermosat to do a measurement. I went to the basement to my xfmr there.
I have a 120v source going to the xfmr. Out of there I have my wiring going upstairs to the wall thermostat from that xfmr reads 14.8 volts, which is also the same voltage I am reading across the mercury switch.
Out of the same xfmr in the basement, I have two wires going to the gas heater. That measurement reads 25.9 volts. Hope this is clear to you. I'm doing my best to give you the info you need.
 
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Old 10-27-05, 02:42 PM
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I'm not sure that clarifies everything, but it tells me enough to be 95% confident that the only solution is a battery powered non power stealing low voltage programmable thermostat. The one you bought wouldn't even work if you rewired with 4 wire thermostat cable and provided a common wire. The voltage is too low (24V is the standard). You will probably need to pig-tail some smaller wires to the thermostat cable coming from the wall to get them to fit the terminals of any of the available programmable thermostats. A couple makes and model numbers that should work are: Honeywell CT3200 and White-Rodgers 1F80-361. The keywords to look for are non-power stealing or battery powered.

Doug M.
 
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Old 10-27-05, 03:50 PM
cwbell1
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Thanks Doug. I 'll return this one and try to locate the ones you referenced. That's what I like about old houses. They are never plumbed or wired in a way that makes things easy.
Many thanks to you and Jay.
Charlie
 
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Old 10-27-05, 07:39 PM
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I guess you were right, Doug! ;-)

Anyway, i agree with Doug's suggestion about t-stat.
 
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