Unusual single-loop zone system using 3-way valves to bypass rooms - one valve staying on

Chi

My 1955 house originally had steel pipe radiant floors that failed a long time before I got it. Someone retrofitted baseboard radiators but the whole house was on one single loop so there was no way to zone any of the rooms. I came up with what seems like an unusual approach to retrofitting the single loop system to make zones using 3-way zone valves to bypass particular rooms.

This system has been working for years with 2 bedrooms on one thermostat but with the addition of the second thermostat and valve head, I am having an intermittent problem with the guest room valve staying on even when the thermostat is disconnected.

I attach the plumbing diagram as well as the wiring diagram and would appreciate thoughts on what might be causing the problem. The last time it happened, I measured 27.5v between the white and blue wires from the thermostat even though I had removed the thermostat!

EBEBRATT-Ed

Blue is your "common "AKA neutral if it was 120 volt wiring.

Your two way valves are open all the time weather the nest calls or not. Don't know why you even need those valves.

You will have 24 volts between red & white with the zone stat removed. I assume these zone valves are power open and spring closed.

Try jumping red and white with the stat removed the valve should open.

The valve may be stuck open

Chi

EBEBRATT-Ed, I have been wondering why I even have the "Back of House" 2-way valve except that without it, hot water will circulate through both circuits (front and back of house) when the Nest is calling for heat in the dining room/living room. Currently, the "back of house" 2-way valve is actuated by the 3 bedroom zone valve end switches. This, I think, is an unusual setup and may be the cause of the current trouble but it has worked fine for 7-8 years without the guest room thermostat and valve connected (I wasn't heating that room so just manually left the valve closed).

I recently added a new Nest to the guest bedroom and wired up the valve. It generally works; the valve opens when the thermostat calls and closes when the set temperature is reached. Just sometimes the valve stays open, the boiler and circulator keep running, and there is 27.5v between the white and blue wires with the thermostat removed. Maybe there is a problem with the valve actuator and it doesn't close even though the thermostat isn't calling anymore. But where does the voltage between white and blue come from?

The valves are Taco 4-wire motorized valves (power open, capacitor close).

EBEBRATT-Ed

Here's the Taco wiring to those valves that is why you're probably getting a back feed. It will explain it better than I can.

The way it looks on your diagram the front and back valves are both powered off the transformer so they are open all the time, unless I am mistaken

Mad Dog_2

Such smart answers you get here. The best in the business reside here.  Mad Dog 🐕 

hot_rod

Power stealing stats can “bleed” back some current. That is why those “pull down” resistors are used.
Different opinions on what size resistor is required. If you have the 3rd “common” wire, no need for the resistor

Without the resistor you may get a false heat call. But evert statement is different, it depends on the electrical devices in the circuit.

Chi

Happy New Year.

@EBEBRATT-Ed, I am using the 4-wire motorized taco "Zone-Sentry" valves not the 3-wire heat motor version. I am suspecting some sort of "back feed" but just can't spot it.

@hot_rod, I did have trouble with the first Nest thermostat in power steeling mode but was able to connect a common to it and all the subsequent Nest thermostats.

The "back of house" 2-way valve works as diagramed; it opens after the individual room zone valve opens and closes after the room valve closes. My diagram may be confusing because I show the relay and transformer as one unit. It is a White Rodgers 90-113 fan control center where the relay load is the boiler/circulator rather than a fan.

Thank you both for considering my problem.

Chi

The Taco Zone-Sentry brochure.