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Tom
Tom, HVAC Technician
Category: HVAC
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Experience:  16+yrs HVAC/R service. Residential Commerical Industrial
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I want to replace a Mears M21DP thermostat with a Honeywell

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I want to replace a Mears M21DP thermostat with a Honeywell RLV430A programmable thermostat. The Mears unit is connected as follows to the power source wires: black to black, red to black, red to white, black to white. How do I wire the Honeywell unit (2 black wires) to the power source?

Tom :

Hi,,,I"m pulling up a manual on the Honeywell thermostat...I'll be back shorty to see if I can assist. Tom

Customer:

OK

Tom :

Having trouble finding what I need....I'll opt out so others can assist.....If I find what I need...I will reply back shortly.

Tom :

Tom

Customer:

Tom, the two red wires are connected to line wires (one black, one white).

Tom :

I can't find the install manual..just the operating book.

Tom :

On the Honeywell thermostat......this is obviously a line voltage stat.....it only has 2 wires?

Customer:

yes, two black wires.

Tom :

Ok.....then is there a terminal or something to connect outgoing wiring to your baseboard?

Customer:

I assume that the 2 black wires from the Mears unit are going to the baseboard, since they appear to be connected to the load wires.

Tom :

Ok....is there a place to put the load wires on the Honeywell thermostat.........or is it just the 2 black wires and no where else to connect any other wires?

Customer:

just the 2 black wires, nothing else to connect any other wires

Tom :

Here is what it sounds like to me.

Tom :

You have 220 volt baseboard.

Tom :

220 into the Mears stat......breaks both lead......220 out to the baseboard.

Tom :

It sounds like the Honeywell unit is only breaking one side of the 220 line..

Tom :

Which would mean you would connect one of your line (supply wires) directly to one of the load wires on the baseboard.

Tom :

Does that make sense?

Tom :

Do the black wires on the Honeywell have any labels? Line, Load, L or T or anything?

Customer:

so you suggest to connect one line wire directly to one load wire and to connect the two Honeywell wires to the other line and load wires; the manual says that either wire can be connected to the load or to the power supply; no labeling on the wires

Tom :

Yes..and since the manual states it doesn't care which one...should be no problem. What you have is an 'electronic' switch......the thermostat is going to break the black wires in the stat with an electronic connection (no mechanical relay)... pretty cool....

Tom :

In the old days..they would break both lines of power. Not 100% necessary,,,but a nice way to do it. Probably for cost considerations..Honeywell is only breaking one side...it will work fine that way.

Customer:

so I connect black line wire to black load wire and connect the Honeywell wires to a white line wire and a white load wire.

Tom :

That should work fine. You're dealing with ac current...so it doesn't really care about the colors.

Tom :

Just get a line to load.....and a stat line to line........and a stat line to load.....and it should work

Customer:

OK thanks; will hit ACCEPT from here.

Tom :

No problem....reply back to this conversation if you have more questions as you go and I'll do my best to help. Tom

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