Need some extra eyes and advice on how to add a common wire to our current boiler setup. We have a Triple Aquastat L8148E that has 2 wire that connect to our thermostat. Like many before me, I'm trying to figure out how to connect a Nest to our boiler; I've already attempted with the 2 wires and the Nest is not holding a charge.

From what I've read, it sounds like I might need to add a 24v transformer and then a separate relay between the Aquastat and Nest. If I do this, do I just splice in the transformer on the power coming to the boiler (pictured).enter image description here

Before I do all this, I'm wondering if I can add a c-wire on the Aquastat to the "TP" terminal since currently there is nothing connected to it? Does this terminal need to have 24v for a C-wire? enter image description here

The "W" and "T" are the wires that run to our thermostat currently. The previous owner or HVAC person used 18/5, but cut the extra wires.

Here is the schematic that is under the lid for the Aquastat...not much of this makes sense to me so not sure if it's helpful for this question or not. Would appreciate any insight or explanations on what to do, thank you! enter image description here

  • "The previous owner or HVAC person used 18/5, but cut the extra wires." In the one photo, it appears that there is plenty of extra 18/5 if you just cut back the cable jacket. I can't tell if that's a splice at the bottom or not; does the 18/5 exit the box? Commented Jan 21, 2019 at 13:12
  • @MikeWaters so the 18/5 is spliced into old two-wire between thermostat and aquastat Commented Jan 21, 2019 at 15:01
  • @toekneeoeeo -- was the thermostat always wired to T and W, or was that wiring changed recently? Can you get us an exact model number for your aquastat for that matter? Commented Jan 22, 2019 at 2:51
  • @ThreePhaseEel so I double checked model number and I actually have the L8148J 1009...one brief google search seems like a C-Wire connection is possible. does the theermostat need to calling for heat for me to chefk voltage? what terminals do i need the probes to touch to find 24v? Commented Jan 22, 2019 at 13:31
  • Is the existing thermostat a mechanical (analog) thermostat, or an electronic (digital) one? Commented Jan 23, 2019 at 0:06

1 Answer 1


You should check out this video: https://www.youtube.com/watch?v=BZJ2tch3_oc. The B2 connector is the common connection for the 24V circuit of this relay. It's based on the diagram for L8148J, so it may be not apply to the L8148E.

  • 2
    Welcome to Home Improvement. Please edit your post to include some detail from the video since it could disappear at any moment. As it is, that could be a link to some ad for something totally unrelated, too, so some might consider it potential spam, and it effectively meets the "link only" criteria for deleting answers.
    – FreeMan
    Commented Dec 1, 2020 at 15:24

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