I just bought a home with a three-zone radiant heat system controlled by Nest thermostats. There's only two wires (red and white) going to each thermostat. This works, but is problematic — in the past two days, I've had to charge two of the three thermostats via USB in order to get them back on the wifi. It's going to be a long winter if I have to keep doing that!

It looks like the wires straight drops through the wall to the basement, so pulling a new one shouldn't be hard. I'm worried about where to hook it up, though. I found this article from Ecobee support: Taco SR503-4 Three Zone Switching Relay which.. is not encouraging. It suggests that I need a separate transformer and then should control the actual switch with an isolation relay.

But with the power requirements of one or more ecobee3 thermostats this transformer will be insufficient in capacity. As a result another external transformer will be needed to power the thermostats and an isolation relay per zone to connect to the R & W terminals of each zone.

However, I found this newer diagram directly from Taco, which has a common terminal clearly marked (where the old one just says "24 VAC Power"). And it doesn't have any warnings about capacity. So........

  1. Maybe this was a problem with older SR503s but not newer ones? If so, how do I tell?

  2. Is this a problem particular to Ecobee? Will I be okay with Nest?

  3. Currently these are gen 1 and gen 2 Nest thermostats. If the current ones okay, might future ones not be?

Anything else I need to be aware of?

  • Are all three zones on individual transformers, or do they share a transformer? Nov 23, 2018 at 21:22
  • As I understand it, they all share a common transformer in the SR503-4.
    – mattdm
    Nov 23, 2018 at 21:23
  • 1
    Updated link tacocomfort.com/documents/FileLibrary/102-379.pdf; newer model Taco with the C-wire "EFFECTIVE: January 18, 2017". There doesn't seem to be any way to tell from the outside of the box which version it is, but mine was installed in 2018, so I'm hopeful.
    – Liam
    Nov 18, 2023 at 14:06

2 Answers 2


Use a separate transformer, across Rc and C on the Nest

What I'd do here is have the Rh and W terminals on the Nests connect to the zone R and W terminals on the Taco zone board, while the Rc and C terminals on the Nests are connected to a 24VAC transformer (a standard 40VA one should be ample to power 3 Nests) that is independent of the Taco board. This way, the Nest will turn off its internal Rh/Rc bridge and should power itself from Rc and C at that point, while it switches Rh to W to turn on the heat zones. It also avoids overloading the internal transformer on the Taco, and should eliminate the need for isolation relays from the system.

  • Will Nest properly understand this? Will it think there is an AC attached? But I guess also... Is this necessary? Is there a real concern about the transformer power?
    – mattdm
    Nov 24, 2018 at 3:03
  • @mattdm -- it should figure out that there's no AC present by dint of nothing being on Y1 or Y2. As to whether this is necessary, I'd need to ask Taco tech support how much current the internal transformer can support (Nests are said to pull somewhere around 200 to maybe 400mA a piece) Nov 24, 2018 at 3:12
  • FWIW, this is what I did. Seemed easy enough and better safe than sorry.
    – mattdm
    Dec 28, 2018 at 21:14

Regarding question 1: Older SR503 relays do not have a connection to common, while newer ones do make common available. For SR503s that do make common available, elsewhere on the web https://www.youtube.com/watch?v=IcvKK0yYtws there are suggestions to place a 500 ohm to 1k ohm 0.5 watt resistor between the SR503 common connection and white to the Nest thermostat. The concept is to provide an adequate 'trickle' current to maintain the thermostat battery. However, the Nest can be rather current-hungry compared to a relay, so if more than one thermostat is powered that way it could cause issues with SR503 function.

Concerning question 3 regarding Nest versions: I have three Nest Gen 3 'learning' thermostats (2021) powered by one separate 800ma 24v (20VA) transformer between the thermostat Rc and C connections. Each of the three successfully control a heat-only system into a 2006 version of the SR503, which does not make available a connection to common.


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