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Electrical Wiring in the Home/Replacing line-voltage thermostat


QUESTION: Hi, I am trying to replace my thermostat which is currently a King Electric D22. I have hydronic heating using in-wall fan radiators. I'm having trouble understanding how my current one is wired and have not been able to wire the new one in the configurations that made sense to me, assuming I can even replace it with the new one. I am trying to replace it with a King Electric HW120, it is a 120V circuit.

Attached in a diagram of the current wiring for the D22. All the white wires are connected together as are the bare wires. Two of wires have a red, black, white, and bare, the third only has black, white, bare.

The HW120 has a black, white, yellow, and red wire.

I have been able to wire it such that is gets power, but my water heater never turns on (it's supplied by a tankless water heater).

Thank you very much for your time!

ANSWER: Derek,

The HW120 is the wrong unit for your situation. The D22 is a 2 pole unit which is for 240 volt heaters which is what I assume you have based on the fact that it is a 2 pole thermostat which is for 240 volt electric heaters and your wiring diagram seems to support the idea that it is a 240 volt circuit. They do however make a version of the HW120 that will work with what you apparently have. They do mention that a 240 volt hydronic installation like you seem to have is "very rare".  They do not say exactly what the model # for the 240 volt version is only that it is available. You will have to do some further digging to find that out.

Check out:

---------- FOLLOW-UP ----------

QUESTION: Thanks Bob. I was confused by the D22 being for 240V as well. But as far as I can tell it is using 120V, that's what my multimeter is telling me and what the fuse box shows. I pulled out the other thermostats in my other rooms and noticed they're not all the same. There's two wall units controlled by one thermostat on my main level so that explains that one. However, in one of my rooms there's only two wires coming out, a red/black/white and a black/white. Still using a D22. I tried connecting the new thermostats I have there but still no luck. There doesn't seem to be any ambiguity to me how it should be connected there so I'm dumbfounded. I guess I'll need to hire someone to come look at this unless you have any insight. Thank you for your time.


Ok given the information I was under the mistaken impression these were line voltage electric element heaters you have. They do make hydronic electric baseboard heaters hence the confusion but this is a whole different animal from what I now realize you must have, which is a hot water boiler system with in wall radiators. Each radiator has its own fan that is controlled by a thermostat that also controls the circulating pump on the hot water system. This is what the wiring diagram for the HW120 shows. I’m sure HW stands for “hot water”.

A 2 pole thermostat can be used as a single pole thermostat and why it was used instead of the HW120 you now have is a mystery to me.

Try this and see how it works. Your existing wiring does correspond to the wiring diagram for the HW120

Connect the black  on the stat to the black in the 2 wire cable. Connect the white on the stat to the white in the 2 wire cable. Connect the red wire on the stat to the red wire in the 3 wire cable. Connect the yellow wire on the stat to the black wire on the 3 wire cable.

The red wire on the stat is supposed to control the pump. The yellow wire on the stat is to control the fan. It should work, give it a go and see what happens.


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Licensed Master Electrician (32 years) 6165B Contractor in Closter New Jersey with 40 years total experience in Residential - Commercial - Industrial work. Emergency Service - General trouble shooting and repair work - ONAN & GENERAC generator installation - Paddle fans - Kitchen & Bath exhaust fans - AC lines - Smoke detectors - Telephone lines - Cable TV lines - Computer network cabling - Exterior lighting - Recessed lighting - Security lighting - (200 - 150 - 100 amp service up grades) - Electric Heat - New circuits - New appliance hook ups - and more ! FREE ESTIMATES 201-358-1552 Fully Insured


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"The Frank Williams School of Electrical Construction". My dear old boss is 100 ! and still going God Bless him. I started working for "Tenafly Electric" right out of high school at 19. He was tough but I learned more working for him in 8 and half years than most guys. We did mostly industrial work while doing some commercial and residential so I was fortunate to be exposed to all 3 areas of electrical construction. The guys I worked with were all good guys and I stay in touch with some of them still. Most of us went into our own buisness's which says something about the caliber of the guys I worked with. We had some interesting clients : Wella Corp. of shampoo fame, Farah Fawcett etc, I didn't buy shampoo for 10 years. It was a great place to work. Pan Am the former airline just before they went under, another great place to work, nice clean environment. C&C Metals, the largest button manufacturer in the US at that time, a not so clean environment but a very interesting place to work, lots of machine's cranking out buttons of all kinds but you had to be on your toes, it was a potentially dangerous place to be . All kinds of action going on around you.

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