c-wire

rickb54

Which terminal would I connect a c-wire to on a Honeywell L7224 aquastat?

Alan (California Radiant) Forbes

Looks as though they don't give you an available "C" terminal.

HVACNUT

rickb54 said:

Which terminal would I connect a c-wire to on a Honeywell L7224 aquastat?

You don't. 
What are you trying to do, WiFi thermostat?

Teemok

If I really needed a C terminal from that control board I could get one. There is a way to tap off the transformer directly. Most people are not be that adventurous but it can be done. You could use a relay board in between and it would have a C terminal for you on it.

Teemok

You would need a 120v outlet you can plug into and a light gauge appliance pig tail, some 2 wire thermostat wire and a romex connector or two. See "typical wiring" in the instructions and ignore the circulator wiring part. The 24v terminal that is next to R and W is the C terminal.

https://www.supplyhouse.com/Taco-SR501-4-1-Zone-Switching-Relay?utm_source=google_ad&utm_medium=pmax&utm_campaign=pmax_best_sellers&gclid=EAIaIQobChMIq8P1-rzigQMVXwOtBh2wDAMtEAQYASABEgLmufD_BwE

rickb54

Yes, I need a c-wire for a wifi thermostat

Jamie Hall

You can't get power from an aquastat. It's a switch, not a power source. Depending on the rest of your wiring, there may be a way to connect from the system transformer and get a C (common return) wire. If you have a relay board, as @Teemok suggested, you may be able to get power from that.

Teemok

The straight forward route is spend money and assemble the kit above. Or for almost free you can hack: Obviously electricity is dangerous. If you have no knowledge or experience get some help from someone who understands the following. One of the 4 connections of the transformer is the C wire. 2 are obviously not it being 120v hot and neutral. Figuring out which of the two low voltage points is + is the trick. There might already be a fuse in on the R side of the TT circuit. If not you can put an inline fuse on the C wire. If we rename the T T terminals R and W, R being the + You just need a wire soldered to the C terminal of the low voltage side of the transformer Or easier, the C is technically ground. "IF" you know which terminal is R (+) macke sure it is connected to the R of your thermostat the other T will be W. You can power the thermostat by grounding your thermostats C wire to the chassis ground of the control board. R to ground will measure as 24v ac. W to ground has no voltage difference. C is the wifi t-stats way of charging by (completing the circuit R to ground) without sending voltage through the W terminal. When your thermostat "calls" for heat it closes a switch that sends R power to the W and that activates the boiler control. Charge current without control activation is your goal.

EdTheHeaterMan

I can't be sure because it has been a long time since i did this but I once pulled one of those apart and looked on the back of the circuit board. I believe that the T terminal that is also marked 3 is R and terminal 3 is Common while the other T is W. Just a guess at this point because I wanted to know the answer to that same question years ago. File this under the forgot part of: "He forgot more about Oil Heat that you'll ever know"

Not a very powerful transformer, so I'm not sure if there is enough power for more than one extra piece of equipment like a thermostat. Depends on how much power your thermostat is drawing

EBEBRATT-Ed

That is an electronic aquastat. You not getting a c terminal off of it.

If you want to add an additional transformer and relay you can do it. and get a common terminal

See the attached drawing.

Teemok

EBEBRATT-Ed said:

That is an electronic aquastat. You not getting a c terminal off of it.

If you want to add an additional transformer and relay you can do it. and get a common terminal

See the attached drawing.

Don't believe the C side of the 24v transformer on that board is grounded? I suppose Honeywell might not do that so the only C is the solder point of the secondary coil of the transformer, it is there. I didn't believe it either until I proved it. Every blue wire in a Trane furnace control is ground to the cabinet. Similar to how all 120v neutral / white wires are grounded at the breaker panel.

Teemok

The power draw to charge a nest, ecoee or the likes capacitor is tiny.

Jamie Hall

@JohnNY posted a very simplle diagram as to how to go about wiring up a wi-fi thermostat. You can find it here: https://forum.heatinghelp.com/discussion/193709/the-last-word-about-c-wire-thermostats-and-wiring-diagrams#latest VERY simple. Just do it and quit messing around with trying to make fancy boards do what they aren't meant to do.

Alan (California Radiant) Forbes

Teemok reminds me of icesailor, except on quaaludes. Super smart, but polite. I'm a fan!