Nest Thermostats & older thermostat 6 Zone Boiler

[Kate327]

We recently replaced our some of our older thermostats and installed Nest thermostats on 3 of our 6 zones in the last month or so. (zone 6 is hot water)
Everything was working great. The past week the 2 heating zones which do not have Nests installed aren't heating. The thermostats seem to be working as usual (judging by the screens). They are clicking like they are calling for heat but nothing. I checked the "zone" lights by the bolier and the zones not working aren't lit up either.

Seems odd as for weeks all thermostats were working fine. I do have a hot water zone which is set to priority and have read people have issue w that....but like I said worked fine for weeks.

Thought would be appreciated. Sometimes our boiler service doesn't seem up on the latest technology so I'd like to get an idea of what were dealing with. THANK YOU

 

[WorthFlorida]

What are all these thermostats tied to? Do you have a Taco Switching Relay controller or something similar? Sounds like you do since you mention priority. https://www.supplyhouse.com/Taco-Switching-Relays-195000

The old thermostats, what type and how many wires? Do all thermostats get it 24v from the same source? Older digital thermostats uses a battery for power so it still may click for when it calls for heat.

 

[Sponsor]

 

[Kate327]

this is a picture of the older thermostats. the new Nest thermostats don't use the black wire

 

[WorthFlorida]

The old thermostat is nothing but a two wire basic unit for heat only. At the thermostat not working there should be 24v ac on the RH terminal. thermostat connects the 24v to the wht wire to activate that zone. If there is no voltage at the RH trace it back to the Taco unit where it gets its 24 volts. If 24 volts is on the wht lead at the thermostat trace it back to the Taco unit.

The zones that are not working (thermostat calling for heat) check the voltage at the R and W terminals to a ground or C wire (common) from the transformer, Across the R & W you'll read zero, always. For a test you can with a jumper wire connected any 24v to the W lead of the affect zones. The W terminal (lead) activates the relay for that zone. Since you do not see the LED light on for the bad zones, I first suspect no voltage, second, the relays went bad or the tracers on the circuit board burned. If jumping 24vs to the W lead activities the zone and turns on the circulator, you can hot wire it as is to warm up the space. In an hour to two disconnect the jumper. If this is the case the wires to the thermostat are open somewhere.

It can be assumed that each thermostat has its own wire cable assembly to the Taco unit but with only two wires needed, the bad zone thermostats might be daisy chain to each other sharing only the jacket or the red lead.

This was the best picture I could find.

 

[Kate327]

The old thermostat is nothing but a two wire basic unit for heat only. At the thermostat not working there should be 24v ac on the RH terminal. thermostat connects the 24v to the wht wire to activate that zone. If there is no voltage at the RH trace it back to the Taco unit where it gets its 24 volts. If 24 volts is on the wht lead at the thermostat trace it back to the Taco unit.

The zones that are not working (thermostat calling for heat) check the voltage at the R and W terminals to a ground or C wire (common) from the transformer, Across the R & W you'll read zero, always. For a test you can with a jumper wire connected any 24v to the W lead of the affect zones. The W terminal (lead) activates the relay for that zone. Since you do not see the LED light on for the bad zones, I first suspect no voltage, second, the relays went bad or the tracers on the circuit board burned. If jumping 24vs to the W lead activities the zone and turns on the circulator, you can hot wire it as is to warm up the space. In an hour to two disconnect the jumper. If this is the case the wires to the thermostat are open somewhere.

This was the best picture I could find.

View attachment 67480

Thank you so much for that info. Unfortunately I'm not that well versed in those kinds of repairs. I will have to call someone tomorrow for help.
What bugs me is the two zones (w the older thermostats) that are not working have had no alterations and had been working fine.
The (3) Nest thermostats are currently working fine.
Could the NESTs somehow possibly damage the relays or tracers on the older thermostats???
Again thank you for your time I appreciate it.

 

[Fitter30]

Need a meter for 24 volts old stat RH to W should have 24 volts. If you do jumping RH to W should turn on the panel light. No voltage go to taco panel. Zone not working check R to W voltage? with jumper wire jump R to W zone should light up if it does bad stat wire. Left side of panel 24 v termial strip, com on strip check voltage from com to R of the stat connections should have 24volts if not look at back side of board for burnt marks. That black jumper in old stat is for cooling if you had it.

 

[WorthFlorida]

....The (3) Nest thermostats are currently working fine.
Could the NESTs somehow possibly damage the relays or tracers on the older thermostats???
Again thank you for your time I appreciate it.

No, all thermostats from the wire side work the same. If you had to wire in a separate power supply for the NEST to work, it is only used by the thermostat to work. It can be assumed that each thermostat has its own wire cable assembly to the Taco unit but with only two wires needed, the bad zone thermostats might be daisy chain to each other sharing only the jacket or the red lead. If the red lead are connected, someh=where it might be opened.

 

[Kate327]

Need a meter for 24 volts old stat RH to W should have 24 volts. If you do jumping RH to W should turn on the panel light. No voltage go to taco panel. Zone not working check R to W voltage? with jumper wire jump R to W zone should light up if it does bad stat wire. Left side of panel 24 v termial strip, com on strip check voltage from com to R of the stat connections should have 24volts if not look at back side of board for burnt marks. That black jumper in old stat is for cooling if you had it.

Thank you we’re just perplexed as to how everything worked fine. Then w hook up the 3 Nests all good for a couple weeks. Than today the two zones w the older stats aren’t working. Can’t be a coincidence...

 

[Kate327]

Thank you we’re just perplexed as to how everything worked fine. Then w hook up the 3 Nests all good for a couple weeks. Than today the two zones w the older stats aren’t working. Can’t be a coincidence...

*** I was able to determine that the thermostats are getting power. Used old voltmeter forgot we had. Came up 26. volts...

 

[Fitter30]

Now put stat back together turn to 80* if panel light for that zone is not on go to thermostat strip for that zone R-W should be 0 volts. If not go back to stat put both wires under one terminal check voltage at panel should be 0 volts bad stat.

 

[Kate327]

**Update
I wanted to thank those who replied. The issue was an airlock effecting the two zones w old thermostats was just a coincidence. One of those zones effected had work done over the summer. And was left w an airlock we didn’t discover til cold weather.
So the 3 Nest thermostats had nothing to do w this issue. Just a coincidence we installed them last month and thought they were responsible.