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PART 2- Help Installing Nest on Millivolt System Using 24v Transformer

PART 2- Help Installing Nest on Millivolt System Using 24v Transformer


  #1  
Old 02-07-14, 02:30 PM
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PART 2- Help Installing Nest on Millivolt System Using 24v Transformer

continuing from http://www.doityourself.com/forum/th...ml#post1934092

I received the latest nest in the mail and was told rh and rc are shorted in the inside(hidden) of the t-stat.

can i just do rc and c for the 25vac trans then rh and w for the boiler?

gonna be cleaner than piggybacking/splicing 2 wires to rh.

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  #2  
Old 02-08-14, 08:24 AM
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Yes, wire it as you have posted.
The stat should see that you have 2 different R terminals in use and show this on the stat screen when you are running the setup.
 
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Old 02-09-14, 09:24 AM
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Thanks.

after testing without the w attached and just attaching w to volt meter- im getting voltage towards the boiler. Gonna need an spst relay after all.

Also found that i was lacking just 10mA when using power from boiler hence the additional transformer.
 
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Last edited by xl883nest; 02-09-14 at 11:21 AM.
  #4  
Old 02-09-14, 09:29 AM
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Also found that i was lacking just 10mA when using power from boiler hence the additional transformer.
If you actually have a millivolt system then there is no where near enough power to run the nest.
 
  #5  
Old 02-09-14, 05:52 PM
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that is why i'm using the 25.2vac transformer with 450mA

my question now is attaching the relay, can i do this:

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instead of this - http://www.doityourself.com/forum/th...ml#post2079680

im using rh to the relay instead of c
 
  #6  
Old 02-10-14, 11:51 AM
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couldn't edit my previous post but i was referring to INSTEAD OF USING:

[ATTACH=CONFIG]26478[/ATTACH]
 
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  #7  
Old 02-10-14, 05:55 PM
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The second diagram looks like it should work if your nest thermostat will accept it.
 
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Old 02-20-14, 10:54 AM
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Thumbs up

Ended up with the original diagram from hvactechfw. HUGE THANKS!
post- 01-22-12, 06:24 PM

Been running great for almost 1 week since 2/15 Saturday.

my new setup:

[ATTACH=CONFIG]27064[/ATTACH]
RadioShack® 25.2V CT 450mA Std Chassis-Mount Xformer
Model: 273-1366 | Catalog #: 273-1366
Price:
$7.99 - you can get it for $3 with $5 discount when you let RS check if your eligible for phone upgrade (non-tmo), you can ofcourse decline the upgrade and still get the discount


Fan Relay - 24V - 4 Terminal - SPST-NO 120/240/277 - Heavy Duty - New - USA (ebay arnold896)
$6 shipped

[ATTACH=CONFIG]27063[/ATTACH]
Dorman - Autograde 22-18 Gauge Female Slide Disconnect Terminal, .250 Inch Red
$3

Volt meter testing done across C and W from nest before connecting coil side wires:
  • set nest temp high to activate heating which read 12vac+
  • set nest temp low to deactivate heating which read 25vac+(possibly even more, don't remember)
Used nest android app for controls.


Testing done before connecting load side wires from furnace to relay was:
  • set nest temp high to activate heating which closed relay, click sound
  • set nest temp low to deactivate heating which opened relay, back to it's Normally Opened state


I was afraid that the nest was sending constant 20v+ voltage to the relay and would cause heating to be on all the time - as I was told that some relay would trigger few volts earlier than it's rated for. Surely enough based on the wiring diagram, I'm only getting 12v+ when temp is low (for off, no heat) and 25v+ (maybe even higher for heat on) - this is across wire C and W from the nest, before I put the relay in place.

*Next project is to move all heating equipment power sources to UPS backup.
 
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Old 02-21-14, 02:00 PM
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HUGE THANKS to hvactechfw

Used the original wiring diagram by hvactechfw, thank you!!!
Posted 01-22-12, 06:24 PM
[ATTACH=CONFIG]27115[/ATTACH]

Been running solid since 2/15/14.

My setup:


radioshack 25.2vac 450mA, $7 or $3 if you let them check if your eligible for phone upgrade (no tmo), you can declined the upgrade but still get discount
[ATTACH=CONFIG]27117[/ATTACH]
spst normally open 24vac relay from ebay for $6 shipped, made in US
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female quick disconnect $3 for wires to relay
[ATTACH=CONFIG]27116[/ATTACH]



Testing done with volt meter for wires c and w from nest before attaching to relay:
  • read 12+/- vac when nest temp is set low
  • read 25+ vac when nest temp is set high


*Used nest android app for controls

Testing done with same wires c and w connected to coil side of relay:
  1. when nest temp is set high, would hear click for circuit closure confirmation
  2. when nest temp is set low from high, would hear click for relay back to normally open position


Was afraid that nest was sending constant 25vac to the relay and would turn on heat all the time but sure enough wiring did what i needed it to and stay 12vac unless nest temp goes up, also was concerned that some relays are sensitive and would trigger few volts before it's rated but no issue there.

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Next project is to move all heating equipment(vent motor, water feeder, nest transformer, boiler transformer) power connections to UPS device.
 
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Last edited by xl883nest; 02-21-14 at 02:21 PM.
  #10  
Old 03-01-14, 05:18 PM
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question on using hvactechfw's diagram...

My only concern is that my millivolt fireplace (by Regency) is a DC system (not an AC system) and this transformer takes a 120V AC current to a 24V AC current. So if my fireplace wants a DC power source, isn't a problem to send it an AC power?
 
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Old 03-01-14, 08:24 PM
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Welcome to the forums.

The reason a relay is being used is to isolate the AC and DC components. The fireplace leads go to the relay contacts and the thermostat switch lines go to the relay coil. The two never meet.

The relay just closes the contacts shorting out your two wires to your fireplace. The relay doesn't supply any power to the fireplace.
 
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Old 03-01-14, 11:32 PM
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thanks!

Ah.... makes much more sense now. Thanks for the input! By the way, my relay has 2 posts on the coil side and 4 posts on the load side (it's the same relay that the earlier posts discuss). Does it matter which two I use on the load side? Thanks again for your help.

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  #13  
Old 03-02-14, 04:39 AM
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Yes, it is important to use normally open contacts for the load side.
This would be the top 2 terminals. Looks like it is 2 and 4 in your picture.
 
  #14  
Old 03-02-14, 07:02 AM
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xl883nest - From the pictures there is exposed 120v wiring. There are some options but best is to use something like this combination transformer/relay that mounts directly to the 4x4 electrical box that you already have.
90-118E - White Rodgers 90-118E - Fan Control Center, 120 VAC Primary 24 VAC Secondary, SPNO/SPNC Relay
 
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Old 03-13-14, 07:08 PM
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no luck with installation -- any suggestions???

Well, I finally was able to do the installation today, but the NEST received no power whatsoever. So... I have three questions that I'm hoping someone much smarter can answer...

1) in case the relay is defective, is there a way to test it? It's the one in the photo I posted earlier...

2) i used the original wire diagram from "hvactechfw" -- but there is one thing i'm not sure about... his transformer shows "line" on one side as C -- but I thought C usually meant common, which would be the neutral wire. I tested the transformer and determined which wire was hot, and i assume that goes to the R and not the C. (I actually tried it both ways and got no power to the NEST either way...)

3) does it matter which way the wires go on the coil or load side of the relay? the diagram does not specify anything here either...

I'm open to any further ideas. Thanks so much!!!!
 
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Old 03-13-14, 07:20 PM
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You are using the nest to control a millivolt fireplace..... correct ?
You are using a separate 24vac transformer to run the nest.... correct ?

The transformer needs to be connected to the R and C terminals of the nest for it to power up.
With the AC transformer there shouldn't be a hot or neutral...... both wires together deliver the 24vac and there would be no polarity.
The relay coil connects from C to W on the nest.
 
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Old 03-13-14, 08:07 PM
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clarification...

Thanks PJ MAX, I appreciate your help! Here are the answers to your questions...

Maybe I'm missing a beat here!

You are using the nest to control a millivolt fireplace?
Yes -- a Regency minivolt fireplace

You are using a separate 24vac transformer to run the nest?
Yes. Here is the item:
Amazon.com: Orbit Sprinkler System Power Source Transformer 57040: Patio, Lawn & Garden

And I am using this relay:
PACKARD PR380 Fan Relay 24 VAC Coil Voltage SPST NO NC Contacts: Amazon.com: Industrial & Scientific


If I understand you correctly, it doesn't matter which of the transformer wires goes to the R and C terminals on the NEST?

I am connecting one of the transformer wires directly to the R terminal of the nest.

The other transformer wire is pigtailed to two wires: one that goes to the C on the Nest and one to the coil side of the relay.

The other coil terminal on the relay goes to the W on the nest.

The white and black wires coming from the fireplace are going to the load side of the relay (top terminals 2 & 4)
 
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Old 03-13-14, 08:36 PM
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Are you using Rh and C for the transformer ?

I think 2 and 4 on the relay are correct. I don't have a diagram of that relay. You could always check those terminals with an ohmmeter to see if they have continuity when the nest is calling for heat.

The thermostat should power up with just the Rh and C connection.
 
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Old 03-13-14, 08:45 PM
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yes -- using Rh & C for NEST -- but C is also tied in to coil side of relay...

but no power to NEST...

i am using doubled up CAT 5 wire from the nest to the transformer... about 20 feet run... maybe that's not enough to carry the current?

if i hook up the transformer directly to the NEst (without the relay) -- i suppose that would test the transformer? without damage to the nest?

also, if i try the ohm meter (which i have) to test the relay... perhaps you can tell me the settings to use? i'm a bit behind the curve on that...

sorry for my ignorance here...
 
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Old 03-13-14, 09:05 PM
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No problem.........

I'm not 100% up on the workings of the nest. I would think that just power on Rh and C would power it but maybe it needs to see a switching load on the W1 wire.
Just connecting 24vac to it won't hurt it. Try that first. Also.... when you have the nest connected to the 24vac... see what it measures.

You can also use that same 24vac right to the relay to measure for the contacts you need.
You set your meter to Rx1. Apply power to the coil. You should hear a click from the relay and your meter should show continuity on 2 and 4. When you remove power that continuity will be lost.
 
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Old 03-13-14, 10:22 PM
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finally!

okay! it works...

i traced the wires for the 10th time and found the W wire was not connected properly. once i fixed that, it worked like a charm. so i guess the W wire is necessary after all!

thanks everyone for your help!
 
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Old 03-13-14, 11:09 PM
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Good job .........................
 
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Old 04-28-14, 10:39 AM
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@Astuff

2 months, so far so good.

the only 120v wires is literally under the 25vac transformer. everything is in there tucked in. 2 yellow wires coming out and a tiny black wire (not used) for secondary voltage option on the transformer.

thanks for the info tho.

it's so nice to be able to set nest away mode remotely when we're gone for few hours. let the savings begin!
 
  #24  
Old 11-30-14, 07:23 PM
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Need the C wire?

Hi,

Have installed 4 (!) nests in my houses, using just the R and W wires without difficulty, so clearly the nest can obtain the current it needs from those connections.

Now would like to control a millivolt fireplace.

Can I just go from 24V transformer to R, from W to coil side of relay, and from other side of the coil back to transformer?

And of course the millivolt lines from the fireplace to the switched side of the relay.

Thanks.
 
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Old 11-30-14, 07:33 PM
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Welcome to the forums.

so clearly the nest can obtain the current it needs from those connections.
No...that is not true. It's supposed to be able to steal power to keep charged but that doesn't happen in many applications. Count yourself lucky.

You can certainly try your method without hurting anything. I'm not sure the relay has enough resistance to run the thermostat. If not...the other circuit is clearly posted previously in this thread.
 
 

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