The new thermostat I purchased doesn't run on batteries, and needs 24 volt power. I have only a red and a white wire connected between the boiler and the thermostat, but there is also a third, unused wire. The instructions from the thermostat say to hook that third wire to the C terminal on thermostat and to the furnace's C, but I have a boiler with no C. Where do I put it on here? Please help I'm desperate.

pic 1

pic 2


3 Answers 3


Unfortunately, the Aquastat you're using does not expose a C terminal. The transformer is likely soldered directly to the board; so unless you're good with a solder gun, you're going to have to buy a different thermostat.

The red wire attached to the T terminal, should be connected to the R terminal of the thermostat. The white wire attached to the T terminal, should be connected to the W terminal of the thermostat. When the thermostat calls for heat, it will connect the R terminal to the W terminal. This will cause the coil to pull a switch closed (relay), which tells the boiler that the thermostat is cold.

If you're not afraid to potentially break things, and you really want to try to get the new thermostat working. You could try to add your own C wire. You'll simply have to connect the new C wire, to the other lead of the secondary coil of the transformer.

enter image description here
Left image shows original wiring. Right image shows where the C terminal should be added.


Modifying the wiring of the Aquastat will likely void the warranty, and could potentially damage the equipment. Proceed at your own risk.

  • If there was a fourth wire, you could use a relay to close T-T using a second transformer wired only to the thermostat. 2 wires would be enough if you found a way to hide a transformer near the Tstat.
    – Mazura
    Commented Oct 30, 2014 at 18:04

If those red and white wires are wired correctly, the +24v 'R' terminal should be the red wire in the upper right, that is under the 'T' screw. Do NOT mess with either of the (12g) wires in the upper left, under the L1 and L2 terminals. Shut the power off to the unit before you go poking around in there.

The C terminal you're after does not seem to be user accessible; it would be one of the solder joints on the back of the circuit board the transformer is mounted to, as old boiler controls weren't designed to supply auxiliary 24v power. I'd have to ask if it is OK to achieve a low voltage ground by attaching it to the control box with a screw. I lack the experience necessary (and the schematic/number for this control) to even attempt to say one of the other terminals on it might work for you, safely.

This doesn't make any sense to me, the thermostat just shorts out the transformer after it goes through a resistor. (top left)

enter image description here

Source of the picture and some good boiler talk.

The manual for a L8124A.

  • 2
    How can you tell the white wire is C and not W? Since there's only two wires in that cable, I would guess those are Rh and W, going to the thermostat. But I'm not really sure.
    – Hank
    Commented Oct 29, 2014 at 22:31
  • The back of the old thermostat has a red wire going to the RH and the white wire going to the W. The photos above are of my Aquastat L8124A. So just connect the unused green wire to the same terminal as the white and then on the back of the new thermostat put it in the C spot?
    – Mike N
    Commented Oct 29, 2014 at 22:43
  • @MikeN: No: if you're saying the thin red and white wires that we see in the upper right go to your thermostat's Rh and W, respectively, then that means this answer is wrong. The C wire should not connect to the W wire, it's a 3rd terminal.
    – Hank
    Commented Oct 29, 2014 at 22:48
  • 1
    @MikeN: I agree with Mazura's latest comments, it looks like the C terminal is not visible in your photos (and possibly isn't accessible at all). Tester101 has a great answer about what the C wire is and where it attaches, which may be helpful in locating yours: diy.stackexchange.com/questions/10482/…
    – Hank
    Commented Oct 30, 2014 at 1:41
  • 1
    @Mazura the little zigzag labeled 1k is a coil, not a resistor.
    – Tester101
    Commented Oct 30, 2014 at 12:16

For T-T connections you need to wire an isolation relay! See https://youtu.be/f0v3FeDZBeY

If you do not have an exposed 24VAC transformer, you'll need to buy one as well as a 24VAC relay (http://a.co/d/aI6iRva). Connect the transformer load to Rh, the transformer common to C, and the W wire to the relay coil control pole. Connect the other control pole back to the common on the transformer. Then connect the T-T terminals to the normally open poles on the relay!

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