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I'm seeking assistance regarding connecting a C-wire (for Ecobee) to a new boiler using the Fast-Stat common maker. The new boiler currently has two thermostat wires available for connection. However, my thermostat wire has only two wires, and replacing it is difficult due to its location on the second floor. I'm wondering if it's feasible to utilize the Fast-Stat common maker in this situation. It is one zone system with one circulator. Any guidance or advice would be highly appreciated. The control board is 109632-01 and the boiler is RRG 84% AFUE Natural Gas Water Boiler with 100,000 BTU Input and 73,000 Output. Thank you so much for your help!

My thoughts are to: 1. splice PI (pink) and connect it to the red thermostat wire to make it R. Then 2. splice BR (brown) and connect it to black (C) common maker. I attached the picture from what I think is the closest from the diagrams.

In red, two wires corrently not connected

enter image description here

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  • Is there not a ladder wiring diagram available for your system? I'm concerned that incorrect wiring could inadvertently bypass safeties in the system or leave the thermostat in a confused state due to lack of power should something trip.
    – ThreePhaseEel
    Jul 30 at 3:56
  • Put an answer below. Also: have you done the math on 84%? Usually going to something in the 90s has a quick enough payback to be worth the extra cost, and a lot of those units will have more complicated controls than a two-wire system. Since it's a new boiler it's at least worth doing that math before you install.
    – KMJ
    Jul 30 at 7:19
  • @ThreePhaseEel - The first picture is the diagram of the system.
    – Kamsu
    Jul 30 at 14:04
  • @KMJ - Besides math, there are other considerations I have taken into account: not needing to deal with descaling, maintenance, drilling for intake/exhaust in the foundation, etc. I think it's worth peace of mind not to deal with call backs from renters. Besides, I would still have the same question/issue with the common maker even with the condensing boiler.
    – Kamsu
    Jul 30 at 14:04

2 Answers 2

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This is fairly simple

According to the ladder diagram for your system, the thermostat simply connects transformer R to the call-for-heat signal on your control board, so this is straightforward. You can tap C for the common maker using P9 terminal 3 as per the ladder diagram, or splice into the brown C wire from the 24VAC transformer with a wirenut or tap splice if you'd rather not use up a terminal on the board for it.

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  • If I tap C for the common maker using P9 terminal 3 this will handle Common Maker's black. Where should the R (from the thermostat) be connected?
    – Kamsu
    Jul 30 at 16:09
  • You should be able to tap R from the yellow wire connected to pin 7 on the P5 plug (the one with the black wire next to it, not the one with the blue wire next to it). You don't need to treat this system as "dry contact", btw
    – ThreePhaseEel
    Jul 30 at 19:16
  • Is my understanding correct? Red wire from the thermostat => pin 7 on the P5 , Red wire from the Common Maker => pin 7 on P5, Black wire from the Common Maker => pin 3 on the P9, White wire from the Common Maker => pin 9 on the P5
    – Kamsu
    Jul 31 at 0:11
  • Yes, your understanding is correct @Kamsu
    – ThreePhaseEel
    Jul 31 at 0:36
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Since this is a two wire heat only setup, I wouldn't use the Common Maker. Instead, add a 24VAC transformer near your ecobee. There's instructions on how to do it on the ecobee site, and I've included the diagram below. The Rh and W1 wires will be the two wires from your boiler. This is a supported setup for a heat-only ecobee with no separate fan control.

picture of wiring of 24VAC transformer with ecobee

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  • There are no outlets near the thermostat so adding an adapter is not an ideal solution. I would rather figure out the common maker as per my question.
    – Kamsu
    Jul 30 at 13:59
  • In that case, I think your diagram in the question is spot on. The red wire from the thermostat would go to P9 terminal 2 alongside the CM black wire on terminal 3. That gives your Common Maker both sides of the power and a control line, and gives your thermostat both sides of power and a control line. If it doesn't work, I'd swap P9 terminal 2 and 3. There's nothing I see on the schematic that suggests interacting with the thermostat and P9 can cause any sort of issues with lockouts and such. Worst case appears to be that it just won't work, which you will know right away.
    – KMJ
    Jul 30 at 17:10
  • Based on your answer, this is my understanding: Red wire from the thermostat => P9 terminal 2, Black wire from Common Maker => P9 terminal 3, Red wire from Common Maker => P5 terminal 7, White wire from Common maker => P5 terminal 9. Am I following your suggested answer correctly? I really appreciate your time.
    – Kamsu
    Jul 31 at 0:22
  • That's what I'm seeing.
    – KMJ
    Jul 31 at 3:31

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