Wiring a Nest Thermostat to a Boiler with Zone Relays: Where the C Wire Comes From

By HVAC Cheat Sheet Team · 5 min read

Older hot-water zones were wired with two conductors: the thermostat is just a switch across two terminals. A Nest needs constant power, so the question on every one of these jobs is where the common comes from. On a boiler with an aquastat and switching relays, the obvious-looking answer is the wrong one.

Safety: Aquastat and switching relay enclosures have 120 V terminals next to the low-voltage ones. Shut off power to the boiler before opening them. This is work for a trained technician.

The aquastat's "C" is not a common

On the Honeywell L8124A, the terminals are:

Terminal What it is Voltage
T, T Low-voltage thermostat 24 V
C1, C2 Circulator Line voltage
B1, B2 Oil burner relay or gas valve Line voltage on the L8124A
ZC, ZR Zone control (to switching relays) Line voltage
L1, L2 Power supply Line voltage

Honeywell's installation instructions (95-6571) label C1 and C2 "line voltage circulator." A thermostat wire landed there puts 120 V on the thermostat cable. There is no 24 V common terminal on this control: the thermostat circuit is the two T terminals and nothing else.

On a multi-zone system the room thermostats don't go to the aquastat at all. Each one goes to T-T on its own switching relay, and the relay tells the aquastat to fire through ZC, ZR or the aquastat's T-T, depending on the layout.

What the zone relay can supply

It depends on the relay. Open the cover and read the low-voltage terminal strip.

  • Argo AR861-2II / AR861-3II: the Class 2 terminals are thermostat pairs only (T1-T1, T2-T2, and T3-T3 on the three-zone board). Argo's manual shows a 15 VA transformer on the two-zone board and no common terminal. Older AR861 relays are similar. There's nowhere on this board to get a C.
  • Taco SR501 and other SR relays: terminals are T, T and COM. Taco describes COM as the common side of the transformer, to power some setback thermostats. With a third conductor, this gives you R (T), W (T) and C (COM).
  • Other relays (Argo ARM series, Honeywell R8888 and similar): many newer boards have 24 VAC or common terminals. Check that relay's own instructions for which terminal is which and how much thermostat load the transformer can carry.

Count the load. Several Nests charging from one small relay transformer can overload it; check the VA rating against the thermostat maker's requirement.

What Google says the Nest needs

Google's guidance for the Nest Thermostat and Nest Learning Thermostat (4th gen): heating-only, cooling-only and zone-controlled systems require a C wire or a compatible power accessory such as the Nest Power Connector. For a zoned system with more than one thermostat, Google says to install the Power Connector at the zone controller, not at the boiler. Google also warns that some third-party C-wire adapters can cause problems or damage the thermostat.

Options that work

1. The relay has a common terminal and you can get a third wire

Pull a new cable (or use a spare conductor) and wire R and W to the relay's T-T as before, and C to the relay's common terminal, following the relay's diagram. This is the cleanest fix.

2. Only two wires in the wall

Use the Nest Power Connector at the zone relay, following Google's installation guide for a zoned system. Read its guide for what it needs at the controller before you buy; if the relay has no common terminal, go to option 3.

3. The relay has no common terminal

Two usual approaches:

  • Separate transformer and isolation relay. A 24 V transformer powers the Nest (R and C). The Nest's W energizes the coil of an isolation relay, and that relay's dry contacts close across T-T on the zone relay. The zone relay's own transformer never meets the new one. Honeywell's R8845U switching relay instructions (69-1292) describe this kind of intermediate switching from a low-voltage controller.
  • Replace the zone relay with one that has a common terminal and a transformer sized for the thermostats, then wire as in option 1. On a system that's getting four smart thermostats, this is often the better long-term answer.

Don't tie two transformers together to "share" a common. Unless they're phased and the manufacturers allow it, that risks a blown transformer or a damaged thermostat.

Common mistakes

  • Landing C on C1 or C2 at the aquastat. Those are 120 V circulator terminals.
  • Using ZC or ZR as a common. Also line voltage on the L8124A.
  • Powering four thermostats from a 15 VA relay transformer without checking the load.
  • Jumping a new transformer straight onto the zone relay's T-T. Use an isolation relay.
  • Assuming every "AR861" is the same. Read the terminal strip on the relay in front of you.

FAQ

Can I wire the Nest Power Connector to the C terminal on the aquastat?

No. On an L8124A, C1 and C2 are the circulator's line-voltage terminals. The aquastat has no 24 V common for a thermostat.

Do I need a separate 24 V transformer?

If the zone relay has no common terminal, yes, with an isolation relay between the thermostat and the zone relay's T-T. If the relay has a common terminal with enough capacity, you don't.

Will a Nest run on just R and W on a boiler?

Google says heat-only and zone-controlled systems need a C wire or the Nest Power Connector for the Nest Thermostat and the 4th-gen Learning Thermostat. Use Google's compatibility checker for the exact model.

Where does the Power Connector go on a zoned system?

At the zone controller (the switching relay panel), per Google, not at the boiler control.

Sources

Parts:ThermostatTransformer

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