Common on Nest not starting boiler

kylegansel

Hi all, I’ve been reading numerous posts on this website and can’t really find my answer. I have a new boiler system with a two wire transformer. I had a nest that was on the old boiler for about 2 1/2 years and it just recently died because after all my research, I’ve determined that it definitely needs a common. I have a four thermostat cable where only the red and white wire is being used. I grabbed one of the other spare wires in the thermostat cable that is not being used and connected that to the common on the transformer and then connected that the common on the nest. I’m not showing any errors on the nest, but the boiler will not fire up.

After numerous steps of troubleshooting, I found that the valve for that zone that has a nest on it does open and close when the thermostat calls for heat, but the boiler will not fire up. After speaking with a few HVAC friends they told me that it should be that simple but I’m curious if anybody knows why the boiler is not firing up when the common is now wired to the nest.
If I remove the common from the nest and the transformer, the boiler then fires up normally but now the nest is not getting powered. Looking for help!!!

I’ve attached a few photos that my for me as this is at our about 45 minutes away from my home.

Jamie Hall

Are these three terminal zone valves? If so, try swapping red and white at the zone valve. No guarantees.

Big Ed_4

Right now with this cold weather put back the old thermostat or jump out the thermostat wires for heat .. You need to run a new thermostat wire to run a internet thermostat .. It needs clean voltage …

HVACNUT

Whats the Green wire connected to?

The 3 Yellow wires from the zone valves spliced together with the Yellow wire nut SHOULD be Common. The additional wire in that splice SHOULD land on C on the transformer in the bottom pic. I cant make out much more.

If it's wired that way and not working, try switching R and W at the thermostat.

Nest has had issues when only using Rh. For some reason they're biased towards Rc to handle switching.

kylegansel

This is a new Nest so the battery will be good for a while. The old one lasted 2.5 years. @HVACNUT I know it's a bit messy as I had the neighbor take photos for me. The green was wired to the common on the transformer and then wired back to the Common port on the Nest plate. It's not a the moment cause I couldn't get the boiler to fire up. So honestly switching the R and the W wouldn't hurt anything? Or should I switch from RH to RC?

Something else to note. When I first tried wiring the Green back to the Nest as a common from the transformer I kept getting no power errors. Either E74 or E79 on the Nest. The yahoo's that ran the cable used whatever they had. I found two splices just taped together. In order to get by I wire nutted them together. It is my plan to run new thermostat cable but could these splices be causing problems? I did check the voltage at the transformer and the Nest and both locations provided me with 24v. The problem is I couldn't figure out the load side. When using the multimeter I would only get 24 v when I'd touch both the R and C. When I tried testing separatly I got nothing. Just trying to share everything I know so I can get this figured out.

@Big Ed_4 FYI on the thermostat cable.

@Jamie Hall I'm hesitant to swap those wires as all three zones fire up fine when there is not common wired to the Nest zone. I have a Livingroom, Bedroom, and Bathroom zone. The Nest is on the Livingroom Zone. As soon as I wire a common to that Nest the valve for that zone opens and closes but the Boiler will not fire up.

SuperTech

Possibly the zone valve is in bad shape and using the transformer to power the thermostat inhibits it's ability to open all the way and close the end switch to bring the boiler on. I know I see enough problem with those zone valves to be suspicious of them.

Another good idea would be considering a switch to a thermostat that's less problematic, which would be any other thermostat.

Jamie Hall

I'm suggesting swapping R and W at the zone valve head for that specific zone — not at the thermostat, not at the transformer, not for any other zones, just for that specific zone valve head.

On voltages — they are always relative to something. At the thermostat, you should have 24 volts AC from R to C at all times. You should have no or floating voltage from W to C in the thermostat is not calling. If the thermostat is calling you should have 24 volts AC from either R or W to C.

kylegansel

@SuperTech What other Wifi Thermostat would you recommend?

SuperTech

I like all the Honeywell thermostats, T6 T9 or T10 all have Wifi capabilities. The Ecobee is a good option and I haven't had any problems with them unlike the Nest thermostats.

kylegansel

@SuperTech Do those require a common?

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1824821#Comment_1824821

So far as I know all "smart" thermostats require constant power — so yes, they require a common or an independent power supply which is alwyas on.

kylegansel

@Jamie Hall I've got to figure out the power issue first then…

SuperTech

https://forum.heatinghelp.com/discussion/comment/1824882#Comment_1824882

Use a Fast Stat FSCM00 if you don't have enough conductors on your thermostat wire. I use this kit all the time on boilers when there's only two conductors on the thermostat wire to the boiler.

HVACNUT

109A_5

Hello kylegansel,

" The green was wired to the common on the transformer and then wired back to the Common port on the Nest plate. "

How many transformers does the system have ? Is the NEST common connected to the transformer that actually powers the zone valves ?

So this green wire goes to this transformer ??? Not seeing it.