Thermostat loses power after boiler fires

psbalogh

Hi,

We bought a new Honeywell T9 Thermostat for our Dunkirk gas boiler. Before I installed the new thermostat, I re-ran the thermostat wire to add a C wire and tested it with the old thermostat. It still functioned and the boiler would fire when it called for heat, but the thermostat would occasionally lose power for a minute or two. I installed the new thermostat with the same wiring setup, and the T9 is having the same issue. After paying more attention, what seems to be happening is: the boiler fires and runs for a normal amount of time (10-15+ minutes). Sometime near the end of the cycle (either just before or after the boiler shuts off), the thermostat loses power. After a couple minutes it restarts and works fine.

Since it was happening with the old thermostat, it leads me to believe it is an issue with the wiring. When I added the C wire, I left all the other wiring as it was when we moved in. I connected the C wire to terminal 2 on the CycleGard LWCO based on my understanding from reading other forum posts.

Below is my best attempt at a wiring diagram, plus photos:

neilc

nothing to do with your thermostat issue, but,
damn, that water looks bad

109A_5

Hello @psbalogh,
You have two semi-incompatible devices, both want constant power, but to wire them up so they are both happy takes some finesse. Basically your LWCO is shutting off your thermostat to do its goofy test.

Also IMO it should have been designed to interrupt the thermostat's W wire, not the R wire.


https://hydrolevel.com/wp-content/uploads/2018/12/CG400-Instructions-web.pdf

109A_5

Hello @psbalogh,
You could add a relay something like this. The thermostat's R wire goes to the transformer's R wire, so the thermostat never losses power. The relay opens the thermostat's W wire to shut down the burner for the LWCO test. There may be other ways.

KarlW

Another option is to return the thermostat and get a battery powered thermostat. If you are looking for the remote sensor idea, then something like an Emerson Sensi would work. I had a good experience with this on my boiler prior to going with my self-imported only European Netatmo thermostat that can sync with the thermostatic valves. Before my Sensi I had a 1st-Gen Nest which had an internal battery that would could bridge the water cutoffs without shutting down.

An Ecobee would not work as it has no battery backup.

It seems like an AA or AAA powered thermostat is easier than a new relay, but your mileage may vary.

psbalogh

neilc said:

nothing to do with your thermostat issue, but,
damn, that water looks bad

I know...I had the boiler cleaned and drained at the start of the season, and it looked ok then...pretty bad now.

psbalogh

109A_5 said:

Hello @psbalogh,
You could add a relay something like this. The thermostat's R wire goes to the transformer's R wire, so the thermostat never losses power. The relay opens the thermostat's W wire to shut down the burner for the LWCO test. There may be other ways.

Thank you! That is good to know about the LWCO test. I'll try the setup you recommended.

psbalogh

I decided to just buy a 24V plug-in transformer and wire the thermostat to that for power. I ordered this product from Home Depot. The one they sent me is different and doesn't have a UL logo.




Anyone know what the F logo in the middle is? This product has the same model number but is still different. Think the one Home Depot sent is safe to use?

109A_5

Hello @psbalogh,
Safety isolating transformer


Fail-safe safety isolating transformer