Help installing Nest on millivolt system

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  #1  
Old 05-11-17, 07:01 AM
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Help installing Nest on millivolt system

Hello all,
This relates to this discussion and my issue:
http://www.doityourself.com/forum/th...ansformer.html

I'm trying to run a nest setup where I have a Gas - Forced air system.
Just reading the above thread, I have the same system and I assume I could use the same method.



Just a few questions.
  1. Why do you common wire to the W wire through the coil?
  2. What SPST relay do I need?

Like this? https://www.jaycar.com.au/24v-spdt-10a-relay/p/SY4067
OR THIS:
https://www.jaycar.com.au/24vdc-spdt...relay/p/SY4051

If not please find one on this list:
https://www.jaycar.com.au/search?tex...3-0a2c2a74e111


:Old Wiring
:Gas Valve

The brown wiring on the left front and back 'terminal' is going to the thermostat not sure what the other wires are, I assume the green wire is the fan?
 
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  #2  
Old 05-11-17, 02:59 PM
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The reason it has to be wired that way is because the Nest needs power from the transformer in order to work. It gets this from C and R. It can not operate off the power running through the Millivolt system, and the Millivolt valve can not be connected to the 24V output of the thermostat. When the thermostat calls for heat, it connects R to W, which in a conventional system would complete a circuit with C inside the furnace.

The relay coil has C on one side and when the Nest calls for heat, it connects R to W, completing the circuit and energizing the relay coil. This closes the Millivolt circuit and turns on the furnace.

That said, the relays you listed are PCB mount - meaning they're designed to be mounted to a circuit board. You can solder wires to those legs, but they are very fragile and will not tolerate much bending. I can't find a proper one on Jayco because their site search sucks. 24VAC coil relays are not real common outside of HVAC, so you really don't have a lot of cheap options other than like fan relays. Call around to local HVAC contractors and see if they carry a White Rodgers 90-293Q or equivalent. They cost about $10USD ($14AUD).
 
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Old 05-11-17, 04:10 PM
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Welcome to the forums.

Yes.... that diagram is exactly what you need.

Just to answer some of your questions.

On that gas valve are three distinctive pairs of wires.
The short black ones are the gas valve.
The green wires go to a pilot generator that sits in the pilot flame.
This is where the millivolt power comes from to open the valve.
The last pair of wires goes to your stat.
These three sets of wires are a complete loop.

The fan is controlled by a heat sensing thermostat in the furnace air plenum.

You need C power to the nest so that it charges correctly. A single relay may not keep it charged. Although we are dealing with AC voltages here.... to make it easier to understand.... C is like the negative and R is the positive. When the stat calls for heat.... it puts out 24vac on the W wire. That W wire and the C wire is the 24vac to operate the relay.

You can use any single pole N.O. 24vac relay.
Remember.... since this is AC.... there is no polarity to the relay.
Packard-Fan-Relay-SPDT-VAC
PACKARD-PR290Q-Relay-Coil-Contacts


A little late to the party. Got called to the phone and forgot to send
 
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Old 05-11-17, 07:02 PM
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Hi, thank you very much for your help!
Went down to my local electrical store and showed them the relay you suggested and showed me this:
hing, although this is Dual pole. -DPDT



Please advise me if this will work.

If it does I have a few questions.
From the millivolt valve it has two black wires and one turns red, does it matter which wire goes into load side in terms of W/RH which was attached to my old thermostat. (Would this connect to 13 AND 14)?

And the coil side would be 1 AND 4 OR 1 AND 9?

And leave : 5,7,9,12 unconnected?

Would I be correct?

Thanks again for your help!
 
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Last edited by X1XNobleX1X; 05-11-17 at 07:22 PM.
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Old 05-11-17, 07:15 PM
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You need a relay with a 24VAC coil..... not DC. That link is to a DC relay.

In your original post you said the thermostat wires were brown.

Name:  millivolt.jpg
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Last edited by PJmax; 05-11-17 at 07:42 PM.
  #6  
Old 05-11-17, 07:17 PM
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Sorry, on top of mine it actually says AC24V



I linked the wrong one.
http://au.element14.com/idec/rh2b-ul...10a/dp/1384681
 
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Old 05-11-17, 07:43 PM
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Ok....I updated my diagram to match your relay.
 
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Old 05-11-17, 08:39 PM
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Thanks for your help!
Although, I have plugged in the way of your diagram and it doesn't seem to be working/switching?
When the 24V transformer is turned on the fan is contently running and blowing hot air out.

When I turn off the transformer, the blower stops.
 
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Old 05-11-17, 08:59 PM
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With a millivolt system..... the stat doesn't control the blower. The burner should light and the blower will start in a minute or two when the furnace heats up.

I see your picture but I can't the socket numbers.

Does your nest show a heating (W) connection ?
Those two wires marked gas go directly to the gas valve.... correct ?
 
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Old 05-11-17, 09:08 PM
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Hello!
The nest does show the W connection.
And the two wires do go to the gas valve.

The picture of the base is below:
But the numbers on the socket don't correlate to the image on the top of the relay. I have put the wires into the numbers that are shown in the socket.



 
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Old 05-11-17, 09:25 PM
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The socket does match the relay.... although it does look to be inverted.

So with the relay not plugged in... the burner is off.
Try this.... disconnect your gas wire connections.
See if the nest is controlling the relay. Easier to have someone set stat to heat and see if relay clicks in. It should click out if when the stat is not calling for heat.

If you are by yourself.... take relay out. Put stat to no call for heat. Now plug relay in... there should be no click when the relay is plugged in.

Put stat to heat.... now plug in relay. It should click.
 
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Old 05-11-17, 09:35 PM
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Okay!
So, basically it immediately clicks every time the relay is plugged in it, even if the thermostat is off or on.
Wiring is wrong maybe?
 
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Old 05-11-17, 09:43 PM
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You do have a C wire to the nest....correct ?

The nest naturally steals power thru it's W connection. I'm wondering if that relay is too sensitive and the nest just being connected to it trips it.

I've only ever used HVAC type relays.
 
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Old 05-11-17, 09:49 PM
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Yes, the C wire is connect to '13' on the socket.


Is there anything else I can do to test this? Or do you think it is my relay I specifically purchased.



Hoped I could get that a relay you suggested in Australia, because it costs $50 AUD to ship to here.
 
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Old 05-11-17, 09:55 PM
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You have the red C connected to the relay socket and it also needs to go to the transformer.
 
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Old 05-11-17, 09:57 PM
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Yes, one side of the transformer is also connected to the C wire.
 
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Old 05-11-17, 09:59 PM
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Do you have any resistors handy. Maybe something around 150 ohms.
 
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Old 05-11-17, 10:02 PM
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Not on me, although I could purchase some tomorrow.
What would I need to connect that to?
 
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Old 05-11-17, 10:25 PM
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Connect it as a test across red and white at the relay. Instead of all the current flowing thru the relay.... some will pass thru the resistor. If the relay works correctly then.... we'll know that it's too sensitive for the nest.

Get something a little higher...... get a 330 ohm at 3 or 5 watts. If that works you can leave it there.

I don't have any nests here or I'd try it for you.
 
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Old 05-12-17, 12:14 AM
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Is this okay?
https://www.jaycar.com.au/330-ohm-5-...istor/p/RR3286
[ATTACH=CONFIG]80656[/ATTACH]

Sorry about all the probably stupid questions, but I'm just wanting to make sure.

You have been really helpful!
 
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Old 05-12-17, 11:24 AM
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The part in the link is correct.
 
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Old 05-12-17, 04:37 PM
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What do you mean by Nest taking power from w?
Thought it took power from RH AND C?
 
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Old 05-12-17, 04:46 PM
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A normal HVAC relay has a certain amount of resistance so that an electronic type stat can draw power thru it to charge. Your relay is not like the HVAC relays.... it is more sensitive.

I don't work for nest so I don't know 100% how their stat works but it appear that even with a C connection it draws a slight bit of power thru the function terminals... like W.

If that resistor doesn't work for you then my only idea is to get an HVAC type relay like I liked to earlier in the thread.

The HVAC type relays work fine with the nest. That I do all the time.
 
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Old 05-12-17, 08:21 PM
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Thank you for your assistance!
Although tried the resistor and it is still turning on immediately as soon I it in.


Is that correct? the resistor seems to be getting warm/hot when turned on.

If you have no other ideas, which of the relays should I purchase? The 293Q OR THE 290Q? As the cost difference between the two is minimal.
 
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Old 05-12-17, 10:26 PM
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Ok.... if it didn't help.... don't leave it connected.

Something is not making sense. This is so basic a circuit that it almost can't not work.

Attachment 80646

Are you sure your wiring matches this diagram ?
I've double checked the relay numbers and they look to be correct.

When the nest was off the plate.... did the relay engage when you plugged it in ?
It shouldn't.
 
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Old 05-12-17, 10:58 PM
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Yes, the relay turn "click" even without the nest with just the white and red wires connected to the relay and RH,C AND W Connected to nest base
I'll try it without the base and see if that helps

EDIT:
Without the relay base it still clicks immediately


And unplugging the RH or the W wire from the Nest base doesn't power the relay/switch the relay on is that correct?
 

Last edited by X1XNobleX1X; 05-13-17 at 01:01 AM.
  #27  
Old 05-13-17, 04:26 AM
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You have the white wire connected to the transformer. There is no other explanation for that behavior.

White needs to go DIRECTLY from the stat base to the #14 terminal.

The C wire is the ONLY one that should connect to both the relay (#13) and the base.

The RH wire on the stat connects DIRECTLY to the other terminal of the transformer (NOT the relay).
 
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Old 05-13-17, 05:22 AM
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Yep,
- White (W) from Nest base to 14 on relay
- Red cable (C) from to 13 with one cable from the transformer splicing/connecting to the red cable and connected to the C on the Nest base.
-Other cable from Transformer to RH on the Nest base

Followed the diagram provided and still immediately 'clicks'. Without any of the gas valve cables connected.


Probably the best idea would be to purchase a 24V FAN relay as you both suggested.
Does it matter if it is the 293Q OR the 290Q? And which would you choose if you had to pick one.
 
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Old 05-13-17, 12:19 PM
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You say it clicks even with the nest off the base and the wires disconnected from the nest base. That is physically impossible.
 
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Old 05-13-17, 04:32 PM
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Sorry.
What I meant to say was.
If either RH or C is disconnected from the nest base it doesn't click.


EDIT: The relay makes a really loud buzzing sound now and doesn't click. Not sure why?
Seems like I'll just return this relay and buy a 293Q relay.
 

Last edited by X1XNobleX1X; 05-13-17 at 05:18 PM.
  #31  
Old 05-14-17, 02:15 PM
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I've never taken a nest apart before but part of the switching/control circuit may be in that sub base.

Read this carefully..... if you have it wired like in the diagram AND the relay clicks with the nest not installed in the base then you have a bad nest sub base.

When R gets connected to W by the stat.... the relay clicks... energizes. So without the stat in place there should be no R to W connection unless something in the sub base is causing it.

(when I say relay clicks.... that means when you plug it in.)
 
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Old 05-15-17, 03:53 PM
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Yeah the base does contain a fairly complicated circuit board with its own microcontroller, it's not just a pin breakout like most other stats.. So yeah I will second the guess that the base itself is bad.
 
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Old 05-22-17, 04:07 AM
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Just to confirm.
This transformer I have is 24V but I have tested it and it shows 26-28V.

Is this fine?
 
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Old 05-22-17, 05:57 AM
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Yes, it's normal to see 26-28V on a 24V transformer.
 
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Old 05-22-17, 06:26 AM
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Yes, I know.
Although, is it alright to be pushing 26-28v through a 24V relay?
 
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Old 05-22-17, 12:26 PM
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Yes.... that should be ok for the relay.
 
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Old 05-29-17, 06:40 PM
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Got the 293Q relay and should be receiving a new nest soon.

Where do I need to connect the wires on the relay?
[ATTACH=CONFIG]81333[/ATTACH]


I'm assuming C and W go to 3 and 1?
And
The two grey wires from the heater goes to 4 and 2?
 
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  #38  
Old 05-29-17, 07:02 PM
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That's a 90-293Q relay. You are correct and pin 5 is not used.
 
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Old 06-12-17, 10:13 PM
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Well, 4 weeks later.
Finally have a working Nest Thermostat!
Must have been the backplate of the old Nest thermostat.

Thank you guys for all your help!
 
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Old 06-12-17, 11:15 PM
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Awesome news. Thanks for letting us know how you made out.
 
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