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I foolishly got myself into this mess. Any suggestions?

1923 house with original steam heat (gas). The addition (~1998), has hot water (baseboards). I tried to replace the addition's thermostat, and got myself into a situation...

The addition had one of those old dial Honeywell mercury thermostats. After removing it, I was left with 4 wires (here) - black, red, yellow, green.

My new thermostat is a Honeywell RTH2300B, and calls for this wiring, which is white, red, yellow, green. After much discussion with Honeywell's support and some online research, I decided to take a chance and see if the black would substitute for the white. Here's what I did: photo. Yes, I'm an idiot and don't know what I'm doing.

Anyway, I clicked the front of the thermostat on (with the power switch off), and my heat turns on. The only way for me to turn the heat off is to take off the front of the thermostat, so it's like this.

So, my question is: other than trying to get an electrician here on a Sunday of a long weekend, should I try another wiring configuration?

Any suggestions? Thanks. It's going to get down to 18 deg F tonight.

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  • Also, not sure if it helps, but I believe (not positive) that the wiring of the old thermostat had the red and black together on one screw, and the yellow and green together on another. I think. Maybe.
    – user980
    Commented Jan 15, 2017 at 19:57
  • Red Green Yellow Black is telephone cable. Phone line 1 is red/green (Christmas). Phone line 2 is yellow/black. (bumblebee). But he didn't follow that. It's quite likely he doubled up the wires because they're thinner than standard thermostat cable. If you ONLY have heat, you should have two (effective) wires.
    – Harper - Reinstate Monica
    Commented Jan 15, 2017 at 22:32
  • Got a voltmeter? Check the voltage from every wire to every other wire. 6 measurements in total. RG, RY, RB, GY, GB, YB. I would expect some 0V, and some either 24 volts AC, or a fraction of a volt DC.
    – Harper - Reinstate Monica
    Commented Jan 15, 2017 at 22:53
  • What's the make and model of the boiler? Is it a multi zone system (one boiler, multiple thermostats)? Can you locate the thermostat cable at the boiler?
    – Tester101
    Commented Jan 15, 2017 at 23:55

2 Answers 2

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Since you have baseboards, you don't have a fan. That means it's probably a 2-wire thermostat. Check the voltages with a voltmeter to make sure you're not playing with 120/230V line voltage. (it's likely either 24V or millivolt).

The thermostat you describe is for a 24-volt system. Those simply short wires together. To call for heat, they short R, G and W. To call for air conditioning, Y, G and W. If it shorted all 4, it would run heat and A/C against each other, which would dry out the air in a hurry. (your car's "defrost" mode does exactly this). Here's the upshot: If you short random wires together, nothing terrible will happen. That should remove some fear.

If your thermostat is millivolt, it either shorts the wires, or does not. Again, shorting will do no harm and probably cause the heater to fire.

Your system doesn't have a fan, so G is irrelevant. You don't have A/C, so Y is irrelevant. So, if you short the right two wires together, the heat should fire. That's it.

Here's the only wiggy bit: They doubled up the wires. That's because they're using telephone cable which is maybe too thin for thermostat service. You should probably double them too. How do you find the right pair?

First, find the right singles. Short wires 1 and 2 and see if the furnace fires. Don't do it for any longer than it takes to find out. Next, try wires 1 and 3. Does it fire? OK, then, logic it out. It must mean 2 and 3 are equivalent. Try every combination. If you get it wrong, the heat will run continuously.

Once you have them paired, attach one pair to "24VAC" (R) and the other to "Heat call" (W) on the thermostat. Doesn't matter which one, since the thermostat just shorts them either way.

(it will matter on a thermostat with 3 or more functioning wires, be it Fan (G) or common (C) for a smart thermostat.)

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  • Thank you! I need to buy a voltmeter. If I'm not mistaken, the two pairs in the old thermostat were red/black & yellow/green. But I'm not 100% sure.
    – user980
    Commented Jan 16, 2017 at 3:08
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You might have to look at the wire diagram at the hearing source to determine how the wires were hooked up there. From there you can change or match up your thermostat wiring.

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  • (Certified idiot here) Couldn't find any wires near my furnace.
    – user980
    Commented Jan 15, 2017 at 20:31
  • 2
    Is there a water circulating pump somewhere? It's entirely possible your furnace proper is millivolt, and monitoring demand from a pump. Does your heat work during AC power failures?
    – Harper - Reinstate Monica
    Commented Jan 15, 2017 at 23:00
  • Do you still have steam and hot water heating in your house or has it been updated to all hot water? If you have separate systems in the house ( house and separate addition ) then this addition may have its own zone valve.
    – d.george
    Commented Jan 16, 2017 at 13:28

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