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Old 03-04-2013, 04:50 PM   #1
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Help with thermostat replacement on hydronic system


Hi - I'm new here and not very familiar with hydronic systems so I'm hoping someone can help.

I recently purchased a home with hydronic heating. The main floor is under-floor and the bedrooms each have fans/blowers built into the wall. The main floor and each bedroom have individual thermostats (analog line voltage double pole) and then there appears to be a "master" thermostat in the upstairs hallway that is digital/programmable.

I am looking to replace the analog line voltage thermostats with digital ones so for increased accuracy and so that I know the current room temperature.

The old thermostats are King M502 (120V/240V Line Voltage Double Pole)

I am replacing them with the Honeywell TL7235A1003 Line Volt Pro Non-Programmable Digital Thermostat with Electronic Temperature Control

When I removed the old thermostat, I found the following connected to it:
three black wires (two paired together)
two red wires (paired together)
one ground wire (connected to thermostat grounding screw)

There were also three white wires capped together in the box.

I'm unclear how the four wires from the new thermostat connect to the three pairings I found from the old one. The old thermostat had two load wires (black) both connected to the paired black wires (all four connected together) and two line wires (red) - one connected to the remaining black wire and the other to the paired red wires.

How can both load wires be connected to each other?

Any help is greatly appreciated! Thanks.
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Old 03-04-2013, 04:55 PM   #2
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Help with thermostat replacement on hydronic system


One black to each black of the stat, and one red to each red of the stat.

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Old 03-04-2013, 05:11 PM   #3
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Help with thermostat replacement on hydronic system


What about the extra black wire?
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Old 03-04-2013, 05:34 PM   #4
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Help with thermostat replacement on hydronic system


I said that wrong. I was looking at the old stat, and forgot about the wires you said were originally connected to it. Your old stat probably lost its primary contacts(they stuck open) so they re wired it to use the safety contacts. You can either wire it the same as the old one. Or separate the 2 reds. The red that is part of the romex that the single black is in is your hot wire. It and the black would go to the red wires of the stat. The remaining red to one of the blacks, and the pair of blacks to the other black of the stat.
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Old 03-04-2013, 06:42 PM   #5
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Help with thermostat replacement on hydronic system


Ok - I'm still confused by two things:
1) How can both load wires go to a single wire?
2) The old thermostat had two black wires for load and two red wires for line. The new one has a red/black pair for load and a red/black pair for line. I assume I need to account for that when re-wiring (not just pair red-red/black-black)

Thanks!
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Old 03-04-2013, 06:49 PM   #6
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Help with thermostat replacement on hydronic system


Are there 1 or 2 electric baseboards in the room. not uncommon to run 2 baseboards off the same stat.

The old stat should have been the same pairing.

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