Convert nest to support millivolt

andrewco

I'm sure this has been answered - multiple times - but I wasn't able to find my specific answer. I want to convert a millivolt system (gas fireplace) to be able to run on a nest. Currently supplies Rh and W wires. Would adding just a C power wire make this work or is it more involved?

Jamie Hall

Good luck. You are going to need an independent 24 volt supply for the Nest, and the Nest will then control a relay which, in turn, will control the millivolt system. Don't even try to hook them together.

andrewco

So the nest will use it's 24v control to open and close the relay and the relay will control the millivolt system? Anyone out there done this....I'm sure it's been done.

Jamie Hall

Exactly. The Nest, bless it's pointy ears, will think it's controlling the heat -- but all it will be doing is turning the millivolt system on and off through a relay, instead of the old thermostat turning it on and off through its own switches.

Make sure when you activate that Nest that you have a really good secure password for access to it -- and that you change it regularly. Anything connected to the internet is a target, and Nests are very enticing to the burglary fraternity...

SWEI

A Honeywell R8845U should do the trick...

andrewco

The R8845U looks like it will fit the bill. Nice all-in-one unit. The only thing outstanding is that I need to supply some power to the thermostat as well. I assume I can probably get power off of the R8845U for this as well?

SWEI

Yes, it supplies fused 24 VAC. It also has a dedicated set of low resistance NO contacts for Powerpile and millivolt systems.

andrewco

Would I wire that 24 VAC directly to the C terminal on the thermostat to provide power to it?

andrewco

I just received the R8845U and it looks like the wiring is pretty straight forward - but I figure it can't hurt to confirm:

L1/L2 = hot/neutral AC power in (GND is in there too :-)
W, R & C get wired to the Nest (C providing 24v input power)
X1 & X2 are the millivolt side going to the gas stove/fireplace.

SWEI

Yes, that is correct. Let us know how it works out.

andrewco

Worked perfectly. Excellent solution for running a millivolt system with a NEST.

Onto the next project. Probably should start a new thread but here's the challenge. I've got a garage heater (Dayton 3UG73) that has a line voltage thermostat. It's a 240v/21A electric heater. I'd like to control the device with a standard 24v thermostat.


Ideas?

BobC

The easiest way would be to use a 24v transformer and a relay, this relay draws reasonably low current off the 24v and is rated at 30a / 277v so it should handle the heater.

http://www.newark.com/panasonic-ew/he1an-ac24v/power-relay-24vac-30a-spst-no/dp/90B7726

The complete specs are the data sheet they list.

Bob

SWEI

http://www.functionaldevices.com/building-automation/display.php?model=RIBM02ZNDC has it all in one package (and would interrupt both ungrounded current carrying conductors, which is good form.)

andrewco

Some great options. It would be even better if the RIB had 24v output for a similar thermostat that would need 24v to power it.

I didn't want to get into in my initial question but I'll be putting another contact closure inline (24v side) that will be triggered from an X10 signal based on the garage door being open. We have some...ummm....little ones that don't quite get the idea that the heater will run unfettered if the garage door is open.

I figure the logic would go....if the thermostat calls for heat but the garage door is open the heater won't run. I think I'll put a 5 min delay on closing the garage door sensor contact in the off chance that the door opens and closes in short time.

andrewco

SWEI said:

http://www.functionaldevices.com/building-automation/display.php?model=RIBM02ZNDC has it all in one package (and would interrupt both ungrounded current carrying conductors, which is good form.)

That RIB unit - so are all the terminations to the screw terminals or are some to the spade connectors on the relay itself?

SWEI

Line voltage and contact closure inputs are on the barrier strips. The 30A DPDT power contacts are made directly to the relay (30A at 277 VAC is a lot of juice for a PCB trace.)

andrewco

Would there happen to be a board similar to the RIBM02ZNDC that also supplies the 24v as well?

SWEI

Nothing that I am aware of. I'd probably use a separate power relay -- maybe even a solid state relay.

andrewco

Wait....a separate relay for supplying 24v back to the thermostat's C connection? And I assume I can drive the dry contact input directly off the thermostats control?

SWEI

Sorry -- a separate relay for switching the 30A 240V load. You could use the same Honeywell R8845 you used for your Nest. I would install the power relay as close to the load as practical and run 24VAC to there (less 10ga wire to mess with.)

andrewco

SWEI (I don't what to call you otherwise :-) can you give me some guidance on the wiring for this? Since I'm familiar with the R8845, I think I'll use that and just add on the separate relay. I will put the relay probably inside the heater and just connect it in between the two leads for the line-voltage thermostat.

So I know the wiring from the R8845 to the thermostat and the wiring for powering the R8845. But can you help me with the wiring from the relay for the line-voltage thermostat to the R8845?

I'm guessing I'll be using something like this as mentioned above?

http://www.newark.com/panasonic-ew/he1an-ac24v/power-relay-24vac-30a-spst-no/dp/90B7726

SWEI

SWEI is short for Southwest Energy Integrators. I'm Kurt.

That relay will work, but should really be installed inside the appliance. Outside the appliance, a double pole relay (one on each of the hot legs) would be a better idea. As far as the R8845U, just wire the constant 24VAC (R & C terminals) in series with the A & B terminals and the coil of the power relay.

andrewco

"but should really be installed inside the appliance" was that a typo since I mentioned that is what I actually planned to do?

SWEI

Ya, sorry -- missed that.

andrewco

Sorry if this is a dumb question but the line voltage thermostat that is on this heater has two leads. Would those both be hot and hence the need for the DP relay?

SWEI

What thermostat make and model? Many of them (definitely most of the Canadian-made ones IME) interrupt only a single wire. To interrupt both, there would need to be four wires going to the stat.

andrewco

The thermostat is actually part of the heater. I'm attaching the diagram from the heater. I've confirmed that there are only two leads to the thermostat. The thermostat is a SPST thermostat.

I was thinking more about this and I assume I can just put the relay in place of this thermostat. Again - as you suggested, I would mount the relay outside the heater - but there's plenty of room for it in it as well.

Two images attached: 1) heater wiring 2) thermostat image

SWEI

If by 'thermostat' you mean the high limit switch, do not remove it. Instead, wire your power relay contacts in series with it.

If there's a separate adjustable thermostat, you could either wire in series with it and just turn up the stat to its highest setting or (if it's likely to be messed with by clueless occupants) wire your relay in place of it.

andrewco

It's an adjustable thermostat - so wiring the relay in place of it would be the way to go. I was honestly thinking of putting the relay inside right next to the thermostat. Is that just bad practice?

SWEI

That's probably how I'd do it. Stick a note (or better yet, an updated wiring diagram) in there, or at a minimum clearly label the wires -- for the next guy.

bitpushr

Like many others, I'm trying to figure out how to use a Nest with my millivolt system. But perhaps unlike others, I already have a relay in place. I have a heat-only house.

I have a newer Honeywell thermostat with two wires, and it's connected to the R & W terminals of a Taco SR503 relay. I checked with a multimeter and those terminals are always energized with 24VAC. The Taco relay controls my circulator pump via a separate, older Honeywell aquastat.

Because my thermostat has a constant 24VAC, does this mean I can connect a Nest with just two wires -- RH & W? There is also a separate 24VAC common terminal pair which I could tap into if needed.

Thanks!

Jamie Hall

Does that thermostat always have 24 volts to it? If it's calling for heat it should have zero volts (closed switch) and then the Taco will see the voltage and close...

andrewco

bitpushr - I believe you will need to supply 24v back to nest on C as well in order to actually power it.

jjkid

Andrewco - excellent post and exactly what I am doing with my nest and millivolt gas gravity furnace. Maybe a stupid question but have to ask it. X1 and x2 are for the millivolt system. Which "x" takes the red wire and which takes the white wire?
- thanks

andrewco

I'll let the experts confirm this but x1 and x2 are just contact closures so I believe it's arbitrary. See attached image that just shows a contact closure between x1 and x2.

SWEI

Doesn't matter which wire goes to which terminal -- either will work.

jjkid

That's what I thought and that's why I thought it was a stupid question.
J

darrendavid

Hi all-

Resurrecting an old thread here as it seems like the best place for this. Based on advice in this very thread I have purchased a Honeywell R8845U to connect a Nest to our ancient millivolt gravity heater. I've followed the instructions (see attached photos) but I've got no action from the Honeywell. I've tried two different units and checked both fuses, to no avail. I'd expect that if I push the "TEST" button the green LED should light up and the heater should switch on. No dice. I've confirmed there's power to the unit and the transformer is getting warm, but the Nest remains completely dark and there are no lit LEDs in the R8845U. Any idea what I could be doing wrong?

Thank you all in advance for any and all help!

Darren

Upstairs in the house...


Downstairs in the garage... (wires leading to the Nest in the image above are on the left below, naturally)

andrewco

If the nest is dark that means it's not getting power. I seem to recall that it has wiring diagnostics as long as it has some remaining power (battery) to get that far. That being said if you hit the test button and don't get anything then there's something else wrong. Have you checked if you get closure (meter) between X1 and X2 when pushing the test button? If you don't get that, then it's the R8845U. You can also confirm you are getting 24v on the C terminal of the unit and then back to the terminal block of the Nest.

darrendavid

Solved my own problem - short in the wiring to the Nest! All good now.

andrewco

I was going to mention buzzing out continuity on each of the wires to the nest but I figured you did that already :-)