I have a Laars gas boiler that has 3 heating zones. I want to replace the old mercury honeywell tstat on the 1st floor (heat only) with a newer honeywell RTH9580 wifi model. I've taken pictures of my entire system, labeled the wires in the pics to indicate the associated connections, and included the Laars wiring diagram. There is a transformer mounted on the wall near the boiler that is wired in-line to the system (power from the panel –> into the emergency boiler switch –> into the wall transformer –> then to the boiler (low water cut off and boiler circuitry). There is also a transformer within the boiler circuitry itself. Can someone tell me the best method (proper preferred, not necessarily easiest) to connect the C wire?
Connect the C wire from the thermostat when there are two transformers
boilerthermostatthermostat-c-wire
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TR is Common on your unit, Every 24 volt device will have a Common leg as well as the 24 volt hot leg, just as a car battery has pos and neg, common is akin to neg in this case. Common is called this cause every 24 volt circuit terminates upon Common to complete the circuit. The last pic shown is the "time on" "temperature off" fan delay, the 2 lighter colored wires are low voltage, 1 id the heat circuits 24 volt hot leg being W or white, the other being Common or C. The gas valve has the same 2 24 volt leads as well, without completing the circuit nothing will work, every electric circuit requires 2 legs of power be it 1 hot leg and neutral/Common or 2 hot legs as in a 230 volt circuit. Google 40 VA HVAC transformer to see what they look like so you will be able to locate it. Your 2 wire thermostat will have only red and white shown on my diagram
The device labeled "PS-802 ELWCO", is a low water cut-off. The "C" terminal there, is not the C you're looking for.
I believe the white wire from the transformer, is what you'll want to connect your C wire to. I'm not 100% confident though, since I'm not that familiar with this system. You may be able to contact the manufacturer for confirmation.
Best Answer
Okay... After reviewing your new photos, I think I have it figured out.
This is what your system looks like
You'll have to excuse the glitter pens, it's the only thing I could find on my wife's desk.
It's pretty simple actually. When a thermostat calls for heat, the valve for that zone opens, and the boiler is also signaled via the auxiliary switch in the valve.
To get a
Cwire to one or more of the thermostats, you'll simply run a wire from the other side of the transformer on the wall.NOTE: If this diagram is accurate, the white wire to the thermostat is
R, while the red wire isW.The original wire from the transformer should be connected to the
Rterminal on the thermostat, while the newCwire should be connected to theCterminal on the new thermostat. This should supply power to the thermostat, and allow it to operate as normal.The way your thermostat is currently wired, you'll have to swap the white and red wires to get it to work. So white will go to
R, and red will go toW.To make this fit a more common wiring style, I'd probably rewire it a bit. So that the red wires going to the thermostat, connected the transformer and the
Rterminals. And the white wires connected from the zone valves to theWterminals on the thermostats.Rewired to fit a more common style.
Then you'll run the blue wire from the white transformer lead, up to the
Cterminal on the thermostat.