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Replacing a 2 wire thermostat with a 4 wire

Posted by grasswhacker (My Page) on
Thu, Dec 13, 07 at 19:43

I want to upgrade my old mechanical thermostats with new digital programable types. The old ones have only two wires which are black and connected to two black wires in the outlet box. The two white wires in the box are connected to each other. The new units that I want to install have four wires, two white and two black. The new units are of correct volts (240) and amp capacity. Can I simply connect the black wires from the thermostat to the black in the outlet box and white to white? So the question is - Can a four wire thermostat replace a two wire unit with this wiring configuration? Thanks for any help!


Follow-Up Postings:

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RE: Replacing a 2 wire thermostat with a 4 wire

Been awhile since I wired a 'stat, but if memory serves me correctly I believe the 4 wire stats are for heating and cooling. I think you need a simple digital stat for heating only. Check the stat you bought and see if it's heating only or heating/cooling.


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RE: Replacing a 2 wire thermostat with a 4 wire

Judging by the fact that these are 240V thermostats, I'm guessing that you are talking about in-line 'stats for electric heat. True? If not, skip the following.

[I think Bigbird's comment about the four wires being for heating and cooling might have been assuming a low-voltage thermostat on transformer power. If I'm wrong, Bigbird, feel free to come back and give me a whack upside.]

It sounds to me like your old 'stats are single-pole switches and the blacks and the whites are both hot (i.e., two poles of a 240VAC circuit). But it's possible that the whites are neutrals in a 120VAC circuit. THIS SHOULD BE VERIFIED!

Since you say the thermostat has four wires, it is probably a double-pole switch in which case you'd connect all four wires to the thermostat. (A double-pole switch means that there will be no power in the radiators from either pole when they aren't producing heat.)

But there's another possibility and that is that it is a single-pole switch and the digital programming electronics need a neutral. I think that's unlikely, but could you get back with more facts (e.g., the type and model of thermostat)?


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RE: Replacing a 2 wire thermostat with a 4 wire

No Tom, I think I need the whack upside. Of course they're electric baseboard. I was sleeping when thinking it was a low voltage whole house stat. 240V...duh!!!


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RE: Replacing a 2 wire thermostat with a 4 wire

The thermostats control a zone baseboard heater in each room. I called Honeywell and got two different answers - yes and no for compatibility. The wiring diagram on the website shows simple black to black, and white to white connections It seems to be okay, but I want to be very sure before ordering and installing the new thermostats.


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Bigbird - Thermostat

Bigbird - The model is TL 8230A1003 (Line Volt Pro 8000)


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RE: Replacing a 2 wire thermostat with a 4 wire

I had a look at the instructions for your new stats. Do as Tom suggested: take a voltmeter and test the black and white wires entering the box. They should each have 120V when tested to ground. If so, then you can use the new stat wired exactly like the instructions. If only one wire has 120V, then you've got a 120V heating system and you need a different stat.


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RE: Replacing a 2 wire thermostat with a 4 wire

Bigbird - I took the cover off one of the heating units and the label says 240 volts and the number of watts for that unit. Does that confirm compatibility for the thermostat?


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RE: Replacing a 2 wire thermostat with a 4 wire

I'd still test it or at least confirm that a double pole breaker supplies both the black and white wires for that cc't with 240V. You never know how somebody wired something without checking it yourself. I have seen stuff so ridiculously wired that I have occasionally walked away and not gotten involved.


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RE: Replacing a 2 wire thermostat with a 4 wire

As I mentioned in my original post, there are two 12 guage cables coming into the box. The two blacks from the cables go to the present two wire thermostat. The two whites are connected to each other. Thanks again for your help.


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RE: Replacing a 2 wire thermostat with a 4 wire

grasswhacker...what bigbird is saying is to actually confirm you have 120V on each leg of your cable. Confirm it with a volt meter as he suggested.

Bigbird...shouldn't he also get 240V on his meter by checking across both hot legs? That confirms he is connected to opposite busses in the main panel? Please correct me if I'm wrong.


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RE: Replacing a 2 wire thermostat with a 4 wire

I do not have a voltmeter and would rather not be fooling around with live wires if I can help it. In the breaker box each zone has two breakers connected with a label "20 amps". Does this help?


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RE: Replacing a 2 wire thermostat with a 4 wire

Just an observation, but if you'd "rather not be fooling around with live wires" and a voltmeter, maybe you shouldn't be fooling around with thermostats. A voltmeter is pretty much a "must have" for any DIY'er.


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RE: Replacing a 2 wire thermostat with a 4 wire

"I do not have a voltmeter and would rather not be fooling around with live wires if I can help it. In the breaker box each zone has two breakers connected with a label "20 amps". Does this help?"

And just how do you plan on ensuring the circuit is off before working on it?
This is past what you know how to do and are comfortable with.
Pay someone to perform the work.
Do you have a way of measuring the torque of the screws used for the wiring?


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