Replacing a 2 wire thermostat with a 4 wire

grasswhackerDecember 13, 2007

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!

Thank you for reporting this comment. Undo
bigbird_1

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.

    Bookmark   December 13, 2007 at 9:38PM
Thank you for reporting this comment. Undo
terribletom

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)?

    Bookmark   December 13, 2007 at 11:29PM
Thank you for reporting this comment. Undo
bigbird_1

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!!!

    Bookmark   December 13, 2007 at 11:57PM
Thank you for reporting this comment. Undo
grasswhacker

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.

    Bookmark   December 14, 2007 at 6:44PM
Thank you for reporting this comment. Undo
grasswhacker

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

    Bookmark   December 14, 2007 at 6:49PM
Thank you for reporting this comment. Undo
bigbird_1

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.

    Bookmark   December 14, 2007 at 7:05PM
Thank you for reporting this comment. Undo
grasswhacker

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?

    Bookmark   December 15, 2007 at 9:24AM
Thank you for reporting this comment. Undo
bigbird_1

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.

    Bookmark   December 15, 2007 at 10:31AM
Thank you for reporting this comment. Undo
grasswhacker

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.

    Bookmark   December 15, 2007 at 10:55AM
Thank you for reporting this comment. Undo
cobraguy

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.

    Bookmark   December 15, 2007 at 11:04AM
Thank you for reporting this comment. Undo
grasswhacker

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?

    Bookmark   December 15, 2007 at 1:33PM
Thank you for reporting this comment. Undo
bigbird_1

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.

    Bookmark   December 15, 2007 at 3:48PM
Thank you for reporting this comment. Undo
brickeyee

"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?

    Bookmark   December 15, 2007 at 4:25PM
Sign Up to comment
More Discussions
How to get garage freezer to work in cold weather
I have a 7-year-old GE refrigerator/freezer in my unheated,...
amyf5
electrical outlet distance to radiator
how far from a radiator should a wall electrical outlet...
amsayan
Replacing dimmer switch: different wire colors on new switch
My DR lights, which are controlled from two switches,...
msa6
ceiling fan, fan works but lights do not
I have 2 kids and one threw a toy that hit one of the...
katy_bug
Reuse electrical panel
I replaced a 24 circuit Square D panel with a new 40...
zver11
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™