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? – ThreePhaseEel Nov 23 '18 at 21:22
  • As I understand it, they all share a common transformer in the SR503-4. – mattdm Nov 23 '18 at 21:23

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.

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  • 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 '18 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) – ThreePhaseEel Nov 24 '18 at 3:12
  • FWIW, this is what I did. Seemed easy enough and better safe than sorry. – mattdm Dec 28 '18 at 21:14

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