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I have a problem with a Trane hydro air unit with a Taco zone valve.

[Home owner] First of all, thank you for providing this valuable service. I used your free instruction pages for troubleshooting Taco valves (and fixed one using your instructions). Now I have a more complicated problem I need your help with, so I am using the 24/7 Help Line.

Hereís the problem. We have a hydro air system with multiple zones, Trane air handlers, and Ecobee 4 thermostats. Everything used to work fine until we decided to replaced two of the the noisy Honeywell zone valves with Taco valves. 

On one of the zones, the plumber who replaced the valve wasnít familiar with the wiring of Taco valves. The thermostat connections were wired to terminals 2 and 3, and the boiler control connections were wired to terminals 1 and 2 by mistake -- AT FIRST.

This resulted in a burned Ecobee 4 thermostat (expensive mistake).

I corrected the wiring and got a new Ecobee.

Now, the cooling side and the fan work OK. However, when the thermostat calls for heat, I hear the blower come on for a second and then the entire system shuts down and resets (including the thermostat which is powered by a C wire from the air handler).

The Trane air handler is a 4TEC3F18B1000AA, if thatís of any use.

Iíll appreciate any guidance you can provide. We live in the Boston suburbs, and itís practically impossible to get an HVAC professional here unless itís an emergency (they are all booked months out). And obviously heating isnít an emergency in May!

Sincerely, Home owner.

A follow-on question. When the Taco valve was miswired and the thermostat applied 24V across terminals 2 and 3 of the Taco valve, what damage might it have caused? I did replace the Taco head to make sure the problem wasnít with the valve, and it wasnít.

[Me] Hi I am thinking on this, does the thermostat wire go to the zone valve or the air handler first?

[Me] I think the wiring across the 2 and 3 wire would have burned out the transformer if the zone valve was open, but I don't think it would have damaged the zone valve head.

[Home owner] The W1 wire from the thermostat goes to the zone valve first.

Which transformer? The transformer inside the air handler isnít connected to the zone valve as far as I can tell.

[Me] A transformer opens the zone valve, what ever transformer, is connected to the thermostat that opens the zone valve, would have shorted out if the zone valve was open, but the zone valve could not have been open, because it was wired wrong.

If you find that using email is confusing, I would be glad to call you.

[Home owner] Sure ó letís talk tomorrow if thatís OK. Feel free to call any time after 11 am Eastern time.

[Me] Okay I will give you a call tomorrow. it will go faster this way.

[Home owner] Yup - thanks. We could also do FaceTime/Skype video etc if youíd like to see the wiring.

[Me] Good morning Home owner I ran your problem through my head last night, and I think I have the answer, we can talk about this when I call you later. It may be, the transformer in the unit that is failing, has been damaged by the cross wiring, so it will show that it has 24 volts, (this is why the thermostat works) but when a load is put on it, it cannot handle the load, and it drops to 0 volts, and this causes the thermostat to reset, as well as the unit. This is what I have come up with at this point. I will speak to you later.

[Home owner] Thanks, Tom. But in that case, wouldnít you expect it to reset under an AC load as well? The unit works fine when the thermostat calls for AC or for fan only. It only resets when it calls for heat.

[Me] Normally yes, but the Taco zone valve adds .4 amps to the load, that extra load may be too much, assuming the transformer was partly damaged. I assume the other 2 units work fine with the Taco zone valves.

You can do a test, you can remove the wire from the number 1 terminal on the Taco zone valve and open the zone valve manually to see if it will work. you may need to turn one of the other units up to make the boiler fire.

[Home owner] Good idea - I will try that and report the result.

Bingo! You were right. I did what you said, and heat came up OK without resetting. So, itís the additional 0.4 amps of the Taco valve thatís pushing it over the edge. Iíll order a transformer from Amazon, I guess.

[Me] That is great, please remember this is my best guess, I am almost sure the transformer is the problem, but I could be wrong, it's possible, the zone valve head is pulling more than the others and it could be the zone valve head, it may be worth your time to swap zone valve heads with one that works, just to eliminate the zone valve as a possible cause. also you may want to check to see if the plumber switched wires in the air handling unit, make sure it is wired just like the other 2 units. Remember to allow the zone valves heads to cool, before you swap them.

[Home owner] I already tried swapping zone valve heads, and the behavior is the same. So I am pretty sure itís not the zone valve head. I double-checked the wiring and itís just as the same as the other units. So, the transformer theory is a good one. I already ordered a replacement transformer. It might take a week to get here. I will let you know what happens once I replace it. Thank you very much for your help!

[Me] Very good. Thank you for using my service, if you have any other needs please let me know, and I look forward to hearing how it works out with the new transformer.
Thomas Petito Reardon

[Home owner] Hi Tom:
You are a genius! I replaced the transformer and the heating circuit is back in business. Thank you again, and happy (belated) Memorial Day. Please let me know if you'd like me to put in a good word for you on the internet and where.

[Me] I am so glad we got it just right. If you would like to send me a testimonial about your experience using my service, that would be great., I will post it on my website, along with our conversation. (less your personal information). Thank you again, for using my service.

[Home owner] Sure thing ó here is a blurb.
We had a strange low-voltage glitch in our heating system following a zone valve installation. After desperately looking for a local HVAC company that understands wiring issues (and failing to find one), I turned to Tomís Open Dialog service for advice. After a number of email exchanges and a couple of troubleshooting steps, Tom speculated that the odd behavior might be caused by a failing transformer in the air handler. I replaced the transformer and sure enough, the system is good as new once again! Tomís experience and insight spared us a major headache. Thank you for this valuable service!

[Me] This is great Thank you, let me know if you have any other questions