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Existing nest hooked up to new furnace. No power to rh terminal. 2 wires: w1 and rh. 0 Recommended Answers 7 Replies 1 Upvote
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Installed new boiler furnace. Existing nest thermostat displays error e74: no power to rh terminal. 2 wire system: w1 and rh. 24 to 28 volts at thermostat. Furnace constantly runs. Does not shut down when target temp is reached.
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Doug,
 
What voltage do you get when you measure W1 and Rh wires at the thermostat?
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Hi Patrick,
Thanks for responding. I get 24 volts when I test between the the connected w1 and rh terminals. The furnace installer thought that the base might have a short in it. I don't want to buy a new base and find the same issue.
Doug
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Doug,
 
Move the W1 wire to Y1 and the Rh wire to Rc. Does the furnace still turn on? Call for cooling and the furnace should turn on.
 
Which Nest thermostat do you have?
What make and model is your new boiler?
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I switched the wires as instructed. Now the furnace keeps running and the nest says e73: no power to the nc terminal.
Boiler is made by NTI. Model # Tft85. Any other suggestions?
I have a 3rd generation learning Nest thermostat.
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Doug,
 
Thanks for the information on the boiler.
 
The NTI TFT85 boiler isn't compatible with any of the US/Canada Nest thermostats connected to it directly.. It appears that this boiler uses a proprietary interface that none of the Nest thermostats support.
 
Where are you located? In US/Canada or the UK/Europe area?
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I live in Waukesha, WI - USA.
The boiler is manufactured in Canada. I read on-line that European heating systems require an extra module that is located between the furnace and the thermostat. Most of Europe uses 240v. Does the module accommodate for the higher voltage?
Would connecting a common wire to the furnace and the thermostat solve the "no power to rh terminal" issue?
Note that the existing nest thermostat worked fine when connected to the previous Munchkin boiler.
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Doug,
 
Ah, wait a minute!!! Looks like I miss something.
 
It seems your boiler has a 24VAC I/O terminal block that is separate from the Communication and Sensor I/O terminal block. 
 
So here is my suggestion on how to wire your boiler. You need to add an external relay as your boiler is looking for dry-contact switch closure to turn it on and off
 
 
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