Well color me impressed! (LONG post)

I recently stumbled across a reference for use of a zone relay board to have multiple zone valves control the call to heat on a boiler, providing isolation of the two low-voltage circuits. FYI I have a single circulator with three zone valves (each with its own thermostat).

HIstory: in 2019, nine years after installing the boiler, one morning the boiler was not running. The logic board was dark, long story short I found that the boiler's transformer was failing on a call to heat. I determined that the best approach was to isolate the zone valve low voltage circuit from that of the boiler, which I did with a simple relay. This failure was confusing to me, since I had followed the WR instructions, and the boiler had worked fine for years.

I could not remember where I read about the zone relay board, so I decided to ask Gemini (Google's AI interface). I was amazed at the response. It was disconcerting to have Gemini initially recommend the wiring setup that led to the failure in 2019, then have it recover nicely when I fed it more information.

With more info Gemini was able to more accurately assess the situation by identifying a problem with direct wiring and the 2-wire zone valves.
Note: I later asked about the source of the direct warning issue. Although the warning was NOT in any of the documentation I received with the valves, I DID find it with a Google search, WR must have updated their technical documentation.

I feel better with an understanding now of what was actually going on.

—Eric (long read below)

Q: My boiler system has three zone valves each with its own thermostat. The boiler has a contact closure for the call to heat with two terminals labelled TT. Should I use a relay board between the zone valves and the boiler?

A: This is a classic wiring question for multi-zone hydronic heating systems.

No, you typically do not need a relay board between the standard zone valves and the boiler's TT (thermostat) terminals. The zone valves themselves are designed to handle the control logic.

Here is a breakdown of how the system is typically wired and why it works:

1. 🌡️ Zone Valve Operation

Standard two-wire or three-wire zone valves (like Honeywell V8043 or similar) contain two key components:

• The Motor/Actuator: This opens the physical valve when the corresponding thermostat calls for heat.
• An End-Switch (or Auxiliary Switch): This is a simple, dry contact (a switch) that closes only after the valve has physically opened enough to allow water flow. This switch is the critical link to the boiler.

2. ⚡ Wiring the Call for Heat (TT Terminals)

The two TT terminals on your boiler are the low-voltage (usually 24V AC) input that tells the boiler: "Ignite the burner and/or run the circulator pump." The boiler will fire whenever these two terminals are electrically connected (closed contact).

• The Solution: You simply wire the end-switches of all three zone valves in parallel across the boiler's TT terminals.

3. 🗺️ How the Logic Works

1. Thermostat A calls for heat: Thermostat A powers the motor on Zone Valve A.
2. Valve A opens: The motor opens the valve to let hot water into that zone's radiators/baseboard.
3. End-Switch A closes: Once fully open, the end-switch inside Valve A closes, completing the circuit across the boiler's TT terminals.
4. Boiler fires: The boiler receives the signal (closed contact) and starts the heating cycle.
5. Simultaneous Calls: If Thermostat A and Thermostat B call for heat at the same time, both of their end-switches will close. Since they are wired in parallel, the circuit to the TT terminals is still closed, and the boiler still fires.

4. ⚠️ When Would a Relay Board Be Used?

A separate relay board (often called a Zone Control Panel or Zone Controller) is typically used in two main situations, but often not strictly necessary for just three zone valves:

1. To Manage Multiple Pumps: If your system had three circulator pumps (one for each zone) instead of three zone valves and a single main pump, the relay panel would manage which pump to turn on.
2. To Simplify Wiring and Troubleshooting: A dedicated Zone Control Panel offers a clean, central point for all the wiring and often includes indicator lights for which zone is active, making installation and maintenance easier. If you plan to expand the system later, this is a good investment.

Conclusion:

For three standard zone valves, the most straightforward and traditional method is to wire the end-switches directly in parallel to the boiler's TT terminals. You can buy a simple zone control panel for convenience, but it is not technically required to make the system function correctly.

Do you know the make and model of your three zone valves, or would you like a detailed wiring diagram for connecting the three end-switches to the boiler's TT terminals?

Q2: The zone valves are White Rogers 2-wire model 1361. I originally wired them according to their specification but then during one heating system I found that the boiler transformer was failing. So I isolated the zone valves using a single relay.

A2: That's a very helpful piece of information. The White-Rodgers 1361 is a specialized two-wire valve, and your experience with the failing transformer points to a common issue with that style of valve when wired directly.

The fact that you already installed a single relay to isolate the zone valves from the boiler's TT terminals means you have already implemented the correct and safer long-term solution for your particular setup.

💡 Why Your Relay Solution is Best

The White-Rodgers 1361 zone valve is a 2-wire valve, but it acts like a 3-wire by using a rotating switch and a warp heater to regulate its position.

The key terminals on the 1361 are:

• Terminals 1 & 2: 24 VAC Power from the transformer.
• Terminals 2 & 4: Thermostat connection (closing this circuit calls for heat).
• Terminals 2 & 3: The Auxiliary/End Switch that closes when the valve is open.

Crucially, on the White-Rodgers 1361, the auxiliary switch (Terminals 2 & 3) is NOT a dry contact. Terminal 3 becomes energized (carries 24 VAC power) when the valve is fully open, and Terminal 2 is the common/neutral wire from the transformer.

The Risk of Direct Wiring (What likely failed your transformer):

If your boiler's TT terminals are also fed by a 24 VAC transformer (which is very common), and you wire the 1361's energized Auxiliary Switch (Terminals 1 & 3, or 2 & 3 depending on which wire is the common/hot) to the boiler's TT terminals, you are effectively connecting two different transformer circuits together.

• This can create a dangerous "backfeed" situation.
• The phase relationship between the zone valve transformer and the boiler control transformer may clash, leading to high circulating current that can overload and eventually destroy one or both transformers.

✅ Conclusion: Keep the Relay

You should absolutely continue to use a relay (or a zone control panel) to isolate the zone valves from the boiler's TT terminals.

Your current single relay acts as a buffer:

1. Zone Valves Call for Heat: When any of the three zone valves opens, its energized auxiliary switch (Terminals 2 & 3) sends 24 VAC to the coil of your isolation relay.
2. Isolation: The isolation relay energizes.
3. Boiler Call: The dry contacts on the isolation relay close, completing the circuit across the boiler's TT terminals.

This setup protects your boiler's internal transformer/control circuit from the potential power conflicts of the energized White-Rodgers auxiliary contacts.

If you have all three zone valve end-switches wired in parallel to the coil of that single relay, your system is correctly protected and optimized for those specific White-Rodgers 1361 valves.