5Kw w/line voltage therm heater controlled from Nest

andrewco

I'm trying to convert over a 15Kw garage heater to be controlled by a nest thermostat that I have. I picked up a R8845U and figured out the connections for the thermostat and of course the power to the R8845U, but I'm kind of stuck with how to have it (R8845U) control a 24vac contactor that will open and close the line voltage thermostat on the heater.

Here is the info on the switching relay.

Abracadabra

Unless I'm misunderstanding you, the r8845 will only handle 2KW. You'll need one that's rated higher.

Or are you trying to use the r8845 to control another 24vac coil contactor? In that case, why not use the nest to control the contactor directly?

andrewco

Yea - you are misunderstanding me. I'm using a 24v contactor to open and close the line voltage thermostat. I just can't figure out how to control the 24vac contactor FROM the r8845 to do that.

I've got the 110v input wired and the thermostat that controls the r8845 wired but I can't figure out how to control the 24vac contactor from the 8845.

andrewco

and Abracadabra - sorry if my last post came across terse - I didn't intend it to be :-)

Paul48

You need a 24 v relay with contacts rated for the load. The nest powers the relay. You don't need a line voltage t-stat.

andrewco

Sorry - I'm not explaining this well I guess. I have a 15kw heater (garage heater). It has an integrated line voltage thermostat. I've put a 24vac properly rated contactor between the two leads to the line voltage thermostat. I now need to control the 24vac contactor. On the other side, I have a R8845U which is wired to the Nest. I have power going to the R8845U (110vac). So - all that is left is trying to get the R8845U to open and close the 24vac contactor when the nest tells it to. I'm need to know how to wire the 24vac contactor (relay in a way) to the R8845U.

Robert O'Brien

The 8845 has two sets of cold contacts. Com & NO and A&B. Power A from the 24V source, B to one side of contactor and the other side of contactor to 24V source. When 8845 pulls in , A&B will make sending 24v to contactor

andrewco

So - the 8845 will not supply 24vac to the contactor? I need an external 24v source?

Robert O'Brien

The contacts are cold, they need to be supplied from somewhere.

Robert O'Brien

If the integral transformer can handle load, you can pick it up from R&C

andrewco

Okay - sorry for the the dumb question. What leads should I wire the contactor off of the 8845? And just to confirm - I'm already running some of those connections to the nest thermostat.

andrewco

Okay - I see that R&C supply 24vac and can power the contactor - the only issue is that I need to switch the 24vac based on call for heat. What's the trick for that.

unclejohn

What is powering the nest now. And why can't you just run common from your power source for the nest to one side of the properly rated contactor you have and then run the other side of the contactor to the w output of the nest?

andrewco

I'll give it a try. The R8845U is powering the nest. It needs C in order to operate because of the radio (wifi). So you are suggesting wiring the Nest AND the Contactor to the same terminals on the R8845U. So C & W -> contactor R,C & W -> nest?

hot_rod

maybe a drawing of what you have and are trying to do.

You want the Nest to control the 115V unit heater? In that case why not eliminate the 115 V thermostat currently on the heater.

The Nest pulls in the Relay, relay powers the unit heater. You need a 24V transformer to power the Nest.

If you don't have a 24V source, a switching rely with 24V built in would simplify all this.

andrewco

nope. The heater is a 15kw 208v heater and the line voltage thermostat is right off of that. That's why I need the 24v contactor rated for the heater's line voltage. I'm going to draw it out right now and post.

andrewco

Here's a diagram to explain.

hot_rod

Same circuit, different voltage. Mainly you need a contractor that can handle that KW, with a 24V coil.

The Nest operates on 24V, so a 208- 24V transformer is also needed.

andrewco

Okay - really confused now. I have a contactor that is 24v coil and can handle the 208v heater line voltage (at the line voltage thermostat). All I need to do is switch said contactor. If I apply 24v to the contactor - then the heat goes on, if I remove 24v then the heat goes off. I'm not following why I need ANOTHER relay for the 208v circuit.

andrewco

For clarification, I would be connecting the contact AT the line voltage thermostat connection but would remove the line voltage thermostat. I don't want to switch the 208v main power, because the heater has a pre-heat circuit which I don't want to by pass.

JUGHNE

If I understand correctly. This is a 15 KW 208/240 VAC hanging heater that previously used a line voltage T-stat. He wants to control it with a 24 VAC T-stat, (Nest).

The contractor (probably 240 VAC coil) for the 15 KW is already in the heater. Just needs a 24 volt source with single pole pilot duty relay. The old standard fan relay with transformer would work but is usually 120 volt transformer. It sounds like he has relay r4845u? is that coil voltage 120 vac or 24 vac?

Any info on the 15KW heater itself?

andrewco

Yep - here's all the info on what I have:

The heater: 3UG73 (attached wiring diagram)
The transformer & switching relay is an R8845U1003
The contactor is a Packard C130A 1 Pole 30 Amps 24 volt Coil

JUGHNE

That wiring diagram does not show an internal contactor which any 15KW heater would have. 15KW is 15,000 watts and draws over 60 amps per leg. (15 KW heats an entire house in the Midwest)

The diagram also indicated the max heating rating to be 5000 watts at 240 volts.

Your existing thermostat (is it unit mounted ?) handles the full current thru it. The 5KW load amp flow thru the existing T-stat could be as high as 21 amps, it may be rated for 25 amp load.

So your Packard contactor of 30 amps will be needed for the switching load now done by the old T-stat. It seems you only need a 24 volt power source.

Edit: as I read back above; Abracadaba actually said it simpler.

Zman

OK,
So which heater do you have?
None of those are 15kw.
Are you sure you have 208v power and not 240v.
Essentially all you have to do is disconnect the existing t stat within the unit and replace it with contactor you have.
Are there just 2 hots and a ground to the heater?
That would be a problem with the R8845U1003 as it needs a neutral. The good news is that you don't need it. Just get a universal transformer that will go from 208 or 240 to 24 volt and you are done. I cannot post a drawing at the moment.
Carl

Paul48

I've only ever seen 208 used in lighting circuits in factories. Not that i've seen everything, though.

JUGHNE

He could have 208 or 230. Depends on the utility supplier and transformer connections in the area. The nominal "208" can read as high as 215 volts measured. He needs to put a meter on the L1 & L2 to determine which of the transformer input connections to use . If not sure I would then measure the nominal "24" volts before connecting the nest to it. I wouldn't want to let the smoke get out.

Paul48

I'm lost.....What 24 v are you refering to? There isn't any in that heater circuit.

BobC

Small factories are sometimes wired with 208 3 phase. I've installed step-up transformers to get 240v compressors (on environmental chambers) to give rated outputs on 208v lines.

Bob

andrewco

Okay - let's start over. I provided some misinformation. It is a 5kw heater not 15kw. You guys are probably (more than probably) correct that it's 240v not 208v. The thermostat that is in the diagram for the heater (integrated into it) will be replaced with the 24v contactor that I want to control via the Nest.

andrewco

Oh - and from the diagram above - the line voltage thermostat ON the heater only switches a single leg.

Paul48

What's the VA rating of the coil of your contactor? I think what is confusing matters is the R8845U. I don't think it's the correct component for your application.

andrewco

Paul48 said:

What's the VA rating of the coil of your contactor? I think what is confusing matters is the R8845U. I don't think it's the correct component for your application.

24vdc. As for the R8845U - I need something to provide 24vdc to the Nest and I was using it to act as an INPUT for the Nest. What I'm unclear on is how to switch the 24vdc contactor based on what the Nest thinks.

Paul48

24 VAC........not DC

andrewco

yes - correct. Wow - bad day for me. Yes, the coil is 24 volts Alternating Current. I think I'm going to spell everything out from this point - mostly to keep me honest :-)

JUGHNE

If the Nest requires 24 VDC, I don't think they would be a very popular item. I've never handled one, could you throw up the spec for it??

My guess is that it has to operate on the standard 24 VAC control power that just about everything in residential HVAC uses.

Just get a 40 VA 240/208 to 24 volt transformer. I would leave the switching relay in the box. Make sure the Packard single pole contactor has a 24 VAC coil.

andrewco

JUGHNE said:

If the Nest requires 24 VDC, I don't think they would be a very popular item. I've never handled one, could you throw up the spec for it??

My guess is that it has to operate on the standard 24 VAC control power that just about everything in residential HVAC uses.

Just get a 40 VA 240/208 to 24 volt transformer. I would leave the switching relay in the box. Make sure the Packard single pole contactor has a 24 VAC coil.

Correct - nest is 24VAC NOT DC...again - my bad.

So - power the 40VA 240/208 to 24 v transformer off of the heater itself. Can someone post the wiring for this? Any suggestions on a 40VA 240 to 24v transformer?

Paul48

I can't find any info about the Nest, but as long as it will handle the load of the coil on the contactor, as JUGHNE says, you don't need the switching relay at all.

andrewco

Here are the specs on the Nest.

Zman

208 volt is seen in 3 phase commercial systems. A single family residence would normally be 240 volts.
Carol Fey writes some great electrical teaching books.
One thing she points out is that every simple control circuit needs 3 things.
A power supply, a switch and a load.
On the heater side of things, the t-stat is your switch which will be replaced with the switch side of the contactor.
You need a control circuit. It will consist of a transformer as a power supply, the nest as the switch and the coil side of the contactor as the load. That's it!

Oh, because the nest has a bunch of fancy circuits, it also needs power which it draws from "R" and returns on "C"

http://www.supplyhouse.com/Honeywell-AT140A1018-Foot-Mounted-120-208-240-Vac-Transformer-w-9-in-Lead-Wires-40VA-11014000-p

andrewco

Zman said:

208 volt is seen in 3 phase commercial systems. A single family residence would normally be 240 volts.
Carol Fey writes some great electrical teaching books.
One thing she points out is that every simple control circuit needs 3 things.
A power supply, a switch and a load.
On the heater side of things, the t-stat is your switch which will be replaced with the switch side of the contactor.
You need a control circuit. It will consist of a transformer as a power supply, the nest as the switch and the coil side of the contactor as the load. That's it!

Oh, because the nest has a bunch of fancy circuits, it also needs power which it draws from "R" and returns on "C"

http://www.supplyhouse.com/Honeywell-AT140A1018-Foot-Mounted-120-208-240-Vac-Transformer-w-9-in-Lead-Wires-40VA-11014000-p

Ah, ok. That's what I was looking for. The R8845U stays in the box and I need to pick up a 240v/24vac transformer. I'm close and simpler than I thought. Just need to make sure I wire the 240v transformer correctly :-)

hot_rod

Maybe a volt meter or multimeter would be a good investment also, it's cheaper than replacing mis-wired components.