How-to: Honeywell wifi thermostat connect to millivolt gas furnace

Motorapido

Seeking info for connecting a wifi thermostat to a millivolt gas furnace in parallel with a non-powered millivolt thermostat. I'm buying a vacation house deep in the woods of northern Pennsylvania that is heated with an Empire millivolt gas furnace (freestanding, vented). The furnace currently is controlled by a simple millivolt, un-powered thermostat. I want to add a wifi thermostat in parallel with the millivolt thermostat so that I can change the temperature setting while I am hours away from the property by connecting remotely with the wifi thermostat. I'd like to use this Honeywell Home wifi thermostat RCHT8610WF2006/W, T5
https://www.amazon.com/dp/B01LTHM8LG/?coliid=IFK4SMYFZV5D1&colid=QTUA72ZOVO6E&psc=1&ref_=lv_ov_lig_dp_it#productDetails. This wifi thermostat requires a C-wire. Can I simply buy a 24 volt transformer like this one https://www.amazon.com/dp/B00NUI2WGY/ref=cm_sw_r_cp_apa_i_Ey3OCb2W2AFJX to power the wifi thermostat? On another forum, somebody said that you must use the following Honeywell switching relay transformer https://www.amazon.com/dp/B00BP0I7YA/?coliid=I1U29KJAMF9WGH&colid=QTUA72ZOVO6E&psc=1&ref_=lv_ov_lig_dp_it
I can't figure out why I would need the switching relay transformer. Seems like all I would need would be a simple 24v power source.
My thought is that by keeping the un-powered millivolt thermostat wired in parallel, if the power goes out and the wifi thermostat is not operating, the millivolt thermostat will still make a call for heat. I would set the wifi thermostat to maintain the temperature I want and set the millivolt thermostat at 50 degrees as a backup if the wifi thermostat goes offline so the house stays warm enough that the pipes don't freeze. Can somebody school me on how I would go about hooking this up, and if I need the switching relay transformer or just a simple 24v transformer?

ratio

According to the manual I found at https://www.honeywellstore.com/store/products/honeywell-lyric-5-thermostat-rcht8610wf2006w.htm, that thermostat can be set to take 24 VAC power from either the R_c or the R_h wire, so it should be as simple as supplying 24 VAC to R_c & C, setting the power switch appropriately, then switching the millivolt heat call via R_h & W₁. Leaving the existing stat wired in parallel will be fine, too.

There may be long-term reliability issues when switching a millivolt signal, IDK. I would take voltage & current measurements to make sure they're in line with the rating of the contacts. An interposing relay would certainly solve that problem. I'm not sure that the Honeywell R8845 relay mentioned would be adequate, it only offers 9 VA of power, which may not be enough.

Note that the manual specifically says millivolt systems are not supported, which means that your application will be unsupported by the factory.

Motorapido

Thanks for those insights. Looks like an option would be a simple 24 volt transformer powering the thermostat, and I found this relay which appears to be a simple solution -- simpler and less expensive than using the Honeywell switching relay that I originally posted https://www.amazon.com/gp/product/B00BTLN78Q/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=B00BTLN78Q&linkCode=as2&tag=ohmefficient3-20&linkId=a52b33a22abf1cd55379ad1c83270811
Would that simple switching relay work? And I do plan to wire everything in parallel with the millivolt t-stat to handle power-outage heating calls.

pecmsg

Is that relay rated for millivolts?

Jamie Hall

The fundamental problem there -- to which I do not know the answer -- is in the "millivolt" part of the equation. And electronic thermostat -- such as the Honeywells mentioned or, for that matter, any modern thermostat -- may leave a voltage offest across the switched terminals, if they aren't true relays but solid state switches which could defeat the millivolt signal. I'd want to use a relay...

Motorapido

pecmsg said:

Is that relay rated for millivolts?

Hmm. It appears that the power coming into the relay would be the 24v power that comes from the thermostat, which in turn has come from a 24v transformer. When the relay flips, it would simply create a closed switch/continuity of the millivolt wires to the furnace, right? At that point, the 24v would not be traveling down the wires and into the furnace, but instead, the relay would simply be closing the circuit of the millivolt feed from the furnace, if I have this conceptualized correctly.

EBEBRATT-Ed

Don't run the milli volts through the thermostats. Use a relay and put the relay close to the heater. Find a relay that it's contacts are rated for millivolts. The relay coil to be 24volts. Use a trans former to power the stats and pull in the relay coil.

HVACNUT

They've since changed the name to Control Center Relay.
Mount it on a 1900 box on the side of the boiler and your almost done.

Edit: Whoops, screwed up on the millivolt side, but you get the gist.
And I believe you get wire leads on the switching side with the Super Tradeline. 

Motorapido

HVACNUT said:

They've since changed the name to Control Center Relay.
Mount it on a 1900 box on the side of the boiler and your almost done.

Excellent. Thanks. This is to send a call for heat to a gas freestanding/vented heater/furnace with a millivolt gas valve. I suppose I could mount the control center relay in a 1900 box on the side of the furnace/heater, although I'd rather hide it away somewhere. Do I understand correctly that when the 24v power to the wifi thermostat sends a trigger to the relay to close the circuit in the connected millivolt wires from the heater, that's the end of the 24v power, and the relay sends no power of its own but simply closes the circuit between the two millivolt wires from the heater?

EBEBRATT-Ed

I don't think that diagram is going to work. The relay contacts need to be in parallel with the millivolt stat not in series with it. They way it's drawn the relay does nothing

HVACNUT

EBEBRATT-Ed said:

I don't think that diagram is going to work. The relay contacts need to be in parallel with the millivolt stat not in series with it. They way it's drawn the relay does nothing

You didn't see my whoops edit. 

EBEBRATT-Ed

@HVACNUT
Saw it but @Motorapido reposted with a question. Wasn't sure he understood.

HVACNUT

@Motorapido, basically yes. On a call for heat, W on the thermostat will energize the 24v coil G on the control center.
The millivolt connections will be on the Normally Open dry contacts on the switching side of the relay. The 24v and the millivolt circuits are isolated from each other. 
Obviously your millivolt parallel wiring needs to be better than mine.

Motorapido

Would this fan control relay do the trick? https://www.amazon.com/Emerson-90-112-Fan-Control-Center/dp/B002CJHLWC/ref=sr_1_3?dchild=1&keywords=fan+control+center+relay&qid=1612961266&refinements=p_85:2470955011&rnid=2470954011&rps=1&sr=8-3

HVACNUT

Looks good as long as it has a 40va transformer.