Trying to troubleshoot some Nest thermostats and 6 Zone Taco Box

DustyGmt

Member
Location
Vermont
New member as well as a new electrician, I finished my 4th year of electrical school a few years ago and am working on my own here and there and troubleshooting these kinds of things are not a strength. I guess ill just outline the problem as best I can.

Lady called and said she has 4 nest thermostats that arent working and the hydronic baseboard heaters arent coming on. First thing i did when I got here was pulled one of the dead nest T stats and plugged it into a micro USB to recharge it and then took a voltage reading on the Red and White wires, I was only getting 4V between the red and white at each of the 4 locations. I then went to the Taco box and tested the Line voltage and had 120v, then I tested each of the two transformers and was getting 27V, then I tested Zones 1-4 and was getting the 4V (no surprise) but Zone 6 which does the Hot Water Heater was reading out at 27V between the red and white and that is working properly. There are two transformers in this Taco Box and 2-5A Fuses and I tested those as well and they are working.

I then pulled all of the thermostats off the walls and went back to the taco box and started getting 9v on the red and white zones 1-4. I took the wires off zones 1-3 and was getting a reading of 13V on Zone 4. I apologize if this is scattershot, I'm certain my lack of experience troubleshooting Low Voltage controls is really showing here but just trying to walk you through my probably somewhat misguided process. Also turned the circuit off and put my meter on the Continuity Tone function and rang out each R/W Zones 1-6 and the only Zone having Continuity was Zone 6, the only Zone getting proper voltage and working properly.

Sorry for the giant wall of text, I'm out of my depth here and am ready to punt it to someone more qualified but its kind of eating at me that I cant figure this out, I want to become more skilled at troubleshooting these things but I'm inclined to believe that this may be a bad control box or maybe somewhere in this ugly rats nest of LowV wires there is a problem but it was working fine last winter so I'm not sure. Any tips or help from an old pro would be appreciated. Thanks. -Dusty
 
Not sure what's going on here. The terminal blocks on the bottom connect to the zone valves. 1&2 is the motor and 3&4 connects to the end switch. When the thermostat calls for heat, there should be 24VAC on 1&2. When the valve is all the way open the end switch closes "shorting" 3&4 which, through the Taco closes a relay and turns on the circulator through line voltage contacts marked "pump end switch" bottom left and also closes the low voltage contacts marked XX at the top left. XX would go to TT on the boiler which would control the circulator. Looks like this is the way it's done here.

The terminal blocks at the top connect to the thermostats. I see R&W which when "shorted" should activate that zone's zone valve as above. The "C" can be used to supply power to the thermostat instead of batteries.

Simple.

So, Check for 24VAC at the red and yellow from each of the two transformers.
Then check for 24VAC between R&C on each thermostat connector block.
If this checks out jumper R&W together on zone 1 and look for 24VAC between 1&2 on the zone 1 block at the bottom.

That should tell you if the controller is working properly. After that, expand your troubleshooting to the thermostat, thermostat wiring and zone valve wiring.

Here is a link to the Taco controller installation instructions.

https://www.tacocomfort.com/documents/FileLibrary/102-399.pdf

-Hal
 
Did it ever work with the nest thermostats ? How many VA is each nest thermostat 4?
The instructions @hbiss posted show the transformer A is 40VA and powers zone valves 1-3 so thats 12VA of load for the thermostats.

Also turned the circuit off and put my meter on the Continuity Tone function and rang out each R/W Zones 1-6 and the only Zone having Continuity was Zone 6, the only Zone getting proper voltage and working properly.
Smart thermostats should be wired with at least a 18/3 to deliver a 'common' for a 24 VAC power at the stat. Like pulling a neutral to a switch box. A old dumb thermostat can be on a two wire like a wall switch. Nest may send trickle thru the zone valve to work off a 2-wire not sure how reliable that would be or if Taco supports that.
You photo shows what looks to be 18/4 to the thermostats with only red and white connected I would connect the blue wire to the common terminal so between red and blue at the nest is 24 VAC.
I am guessing it worked fine with the old thermostats and the nest are unable to use whatever trickery to power its electronics by leaking current thru the zone valve.
Connect the blue wire to common and I bet they will work.
 
Did it ever work with the nest thermostats ? How many VA is each nest thermostat 4?
The instructions @hbiss posted show the transformer A is 40VA and powers zone valves 1-3 so thats 12VA of load for the thermostats.
Each thermostat only powers one zone valve, and zone valves are designed to be controlled directly by a thermostat. Not sure how the Taco controller works, are there relays that go between the thermostats and zone valves? So, you may be right about how the Nests are powered.

But you do make a valid point, did those Nest thermostats ever work? They are finicky and AFAIC not worth the trouble. Also, I question why the Taco controller was used. Normally a conventional thermostat just controls the zone valve directly. Thermostat turns on the motor/actuator and the end switch contacts close and turn on the boiler and circulator through the TT terminals on the boiler controller. Only use for that Taco controller is if the boiler didn't control the circulator, and I'm not seeing any evidence that the line voltage contacts are used.

I would probably get rid of the whole thing as well as the Nests.

-Hal
 
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