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dual transformer system repeated cycling boiler for 3 seconds

laidlaw-david
Community Member

I have two systems to control with a nest thermostat.  One is A/C with Rc, C, G, Y1, Y2.  The other is heat with Rh and W1.  During heating season (now), the boiler is sometimes engaged by the nest for about 3 seconds every minute or less.  My guess is that it is trying to "sip" power.  But I measure 28VAC across Rc and C, which should provide power, right?  I have been listening to my furnace blower go on and off hundreds of times, and I can't imagine that is good for the blower.

Technical info from web page:

ModelDisplay-3.4
Backplate modelBackplate-5.3
Serial no.09AA01AC31151G32
Software6.2-22
UpdatedNov 2, 20############:04 PM
MAC18b4306bc6de
IP192.168.1.134
Last contactJust now
Battery3.95 volts

and wiring diagram:

laidlawdavid_0-1641156435880.png

Help!

 

PS The ######## are the year, 2021, which was determined by the community site to be illegal HTML and removed...

 

 

4 REPLIES 4

Houptee
Silver Product Expert
Silver Product Expert

Power + Rh connects to W on a call for heat to turn on boiler. You could go to the boiler and put a AC voltmeter on W and other lead to C and watch it see if Nest is sending 24 v AC to W every few seconds.

If it is maybe try doing firmware update then a factory reset and reinstall the Nest go thru the setup questions again and make sure to tell it Rh and Rc are there and not from the same system.


Houptee -- NJ Master HVAC Licensed Contractor

There is no C from the boiler.  The fact that it is coming on means that something is connecting W and Rh.  The only thing that can do that is the Nest, AFAIK.

The firmware reports having been updated in November.  Is there something newer?  From the nest site, that seems to be the latest version.  I hesitate to factory reset and set everything up again unless there is some evidence or experience that that might work.

Houptee
Silver Product Expert
Silver Product Expert

If you look at the boiler transformer there are 2 wires coming from it, one is the supply (Rh +) and the other is the Common.  There has to be 2 wires to make a complete circuit.  So power goes to the Nest on Rh and Nest is just a switch that turns on power to W then the power flows to the boiler on W to activate the boiler relay or zone valve or whatever W connects to at the boiler. Then it returns to Common on the transformer.

If you go into the settings on the Nest it has a screen for the firmware version and says "check for updates".

You might have told the Nest wrong info when you set it up and it is internally shorting Rc and Rh. I assume you have 2 separate transformers one supplies Rc power and boiler supplies Rh power.

Do you have hot water baseboard/radiators? You might need to go thru the setup again and make sure you told Nest correct type of system you have.


Houptee -- NJ Master HVAC Licensed Contractor

Azúl
Community Member

The C wire comes from the A/C, not the furnace. The furnace could be short cycling because nest steals power from the Rh wire while heat is running.