New Taco zone valve constantly opens and closes

TooManyZones

I just recently purchased a home with a seven-zone hydronic heating system and gas boiler, circa 2000 to 2002ish time period. I’m trying to repair the heating system with very little context to go off of. The previous owner passed away so I have had to troubleshoot and investigate everything myself. When I moved in, I found that all zone valves were unplugged and all but one thermostat was disconnected from power; in its place, there was a single new battery-powered thermostat wired directly to the TT terminals on the boiler, and all zone valves manually opened. The sellers installed the new thermostat to get the heat certified as working prior to sale, as the heat could not be turned on during our home inspection and we stipulated that the heat must work as part of the sale agreement.

Unfortunately, that single thermostat meant all other rooms in the house stayed cold, so I reconnected the original seven-zone system with varying degrees of success. The system consists of:

• A gas boiler and single Taco circulator.
• Seven Taco ESP 075C2-1 zone valves manufactured in August 2002, assuming what I think is the date code is actually the date code.
• Six Honeywell T8400C thermostats with two-wire setup.
• One Honeywell Home TH4110U2005 (the new thermostat) with batteries, also on a two wire setup.

The thermostats are wired directly to the zone valve motor control and then into the 40VAC transformer. This hand-drawn wiring diagram was left with the house:

The thermostat/zone valve motor wiring (red & white) and zone valve end switch wiring (blue & yellow) each converge inside a junction box. There is no zone controller in the system. The boiler TT wires are wire-nutted into the blue and yellow end switch wires. See image:

My problem is that some of the zone valves have had an intermittent issue where the valve is either getting stuck open or opening without the thermostat calling for it (I’m not exactly sure which), causing the boiler to run indefinitely until I manually close the valve. After reading a bit about how the ESP zone valves work, my theory was that the old capacitor inside had lost its ability to hold a charge, and so it didn’t have enough power left to close the valve if the thermostat called for heat for too long.

So, I purchased a new Taco actuator head to see if replacing the actuator would resolve my issue. The new actuator fit on the existing valve body and I connected it to the existing wiring. However, now I have a new issue - after getting a call from a thermostat and the call is either satisfied or “cancelled” by turning the thermostat down/off, the actuator continually cycles between open and close, seemingly getting power from somewhere. The actuator doesn't stop cycling until I disconnect the power terminal. I tested this actuator on three different zone circuits and found the same behavior on all of them.

I’m guessing something is wrong with the wiring, but I’m not exactly sure what it could be. Some brief research and reading the actuator’s manual suggests it might be my “power stealing” thermostats, but I’m not sure how to test for that. Is there anything else I should consider? Would installing a zone controller and re-wiring the system for the controller solve my issues?

HVACNUT

The Zone Sentry zone valves are fairly reliable, but its been at least 20 years, so... Has it been that long, already?

My first thought was, how can the other rooms be cold if all the zone valves are manually open? If they are in fact open, and there's no heat in those loops, I would look for air locks, bad air eliminators, low pressure.

As far as the zone valves themselves, they're pretty simple. Does the valve open with 24 volts? Does the end switch close to signal the boiler and circulator? Grab a meter and start checking.

rick in Alaska

According to a quick search I did, the t8400 is a power stealing thermostat, which can mess with the actuator you have. Taco has some resistors you can install that on the zone valve that can help with that. They are 1000 ohm resistors which you can get from SupplyHouse.com

You could try disconnecting all the 8400 thermostats and just use the one th411 and see how it responds. That would give you a better idea if it is the 8400's making the problem. You could also go on the Taco site and see how they wire up to the zone valve. It's been too long for me to rememer.

Rick

Jamie Hall

Aw @rick in Alaska — we're not that old. Though I'll admit it's been a while since I've seen a diagram on a paper bag…

Which, oddly enough, is correct. For two wire thermostats. Each thermostat — a simple switch — powers its own zone valve. The end switches for all the zone valves are paralleled and run to T-T on the boiler. Couldn't be simpler.

However I can easily see how whatever Rube Goldberg wiring may have been added or changed to make the T8400C thermostats work on two wires could very easily result in false signals getting to a zone valve or valves…

HomerJSmith

Those ESP ZVs are series 1 valve and were replaced by series 2. For them to work with a power stealing thermostat, you need a 1/4 watt 910 ohm resister that bleeds a charge to the ESP battery (charging capacitor) that runs the motor in the ZV. I don't remember exactly how I used to do it. You can always replace the thermostats to newer non stealing thermostats. We also used a separate relay in the circuit to isolate the power stealing thermostats. This goes back a lot of years. I would have to look up the process. Honeywell came out with a paper on this. I just dug it out of my Honeywell book. It's titled "Installing a MS1000Q8888 Isolation Relay". Dated 10/17/88 16 pages

I don't think that installing a zone control will solve your issues. The polypropylene capacitors don't go bad or weaken in any way.

When I complained to Taco about the problems with series 1, they sent me 5 replacement Series 2 power heads. I guess they couldn't stand to see a grown man cry.

109A_5

You may have a compatibility issue between the ESP zone valve and the T8400 power stealing thermostat. Is your replacement Head the same model as the original ones were ? Does the zone valve with the replacement head work correctly if you disconnect the thermostat and just touch the R and W wires at the thermostat for zone valve control ?

Here is one example of how to make some of this equipment play nicely with each other. In your case you probably have some compatibility issues, as you suggested may also be age related also, which is why you found the mess you did. IMO they took simple an complicated to the point you are lucky it works at all.