120vac to 24v for Smart Thermostat (with pictures)


  #1  
Old 09-13-15, 12:46 PM
F
Member
Thread Starter
Join Date: Sep 2015
Location: USA
Posts: 5
Received 0 Votes on 0 Posts
120vac to 24v for Smart Thermostat (with pictures)

I've been doing a lot of research trying to figure this problem out, and you guys seem to be the ones to ask. I haven't seen anyone with this specific setup, but came close to it with a few other posts.

I have a 120vac gas boiler/water heater hooked up to a steam radiator (forced steam/hot water?) system in our 1947 Garrison style house in Portland, ME.

We received a Lyric thermostat as a wedding gift last winter (wanted the nest, but free is free ) and have been waiting to figure out how to hook it up. I finally made some headway when I found the AUB RC840T-120 (pictured below) with the 3 wire hookup.
Name:  rc840t-240-1.jpg
Views: 632
Size:  10.3 KB

Our setup was previously comprised of a 2 wire, white/red physical switch, so it's the same basic story of needing the physical switch and transformer to run.

Long story short, can someone please guide me in the right direction to hooking this up? I am capable, but the diagraming and instructions have me a bit worried that I will fry something more expensive than I am able to afford. Here are the diagrams and photos that have been helpful in other posts.

RC840T Diagram
Name:  RC840T wiring.jpg
Views: 1352
Size:  17.5 KB

Laars Mascot LX Diagram
Name:  laars.34 (1)-page-001.jpg
Views: 3994
Size:  47.5 KB

Actual Wiring Photo
Name:  image1.jpg
Views: 1978
Size:  38.3 KB

Please let me know if I need to provide further information, or if you need bigger pictures or sources.

Thank you all for any help you can provide, this has been a bit frustrating.
 
  #2  
Old 09-13-15, 01:09 PM
Geochurchi's Avatar
Member
Join Date: Nov 2012
Location: United States
Posts: 5,030
Received 91 Votes on 86 Posts
Maybe it's me but I don't understand what you are trying to accomplish.
Geo
 
  #3  
Old 09-13-15, 01:18 PM
F
Member
Thread Starter
Join Date: Sep 2015
Location: USA
Posts: 5
Received 0 Votes on 0 Posts
Trying to hookup a smart thermostat to a line voltage system. The smart thermostats require constant 'on' power where line voltage systems (at least to my understanding) use physical switches, mercury based I think. The RC840t converter replaces the physical switch and adds a transformer to supply 24v current to a thermostat and take orders to set the on/off of the physical switch. It basically lets you use a newer thermostat with line voltage system. The wiring, however, is not so clear.

I can easily figure out how to put the w/r lines in, but I am concerned with hooking up a the third wire of the RC840T model as I don't see where i would tie that into the system. I think it's the 'C' or common wire, so i'd have to hook it up somewhere on the boiler itself, and that is where i am left scratching my head.

Again, this is all based on random research, i'm no electrician, which is what concerns me.
 
  #4  
Old 09-13-15, 01:37 PM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
Post a picture of your thermostat.
If you can also post a picture of the boiler end of these wires it would help.
Does the thermostat run directly to the boiler T terminals or does it hit a relay?

Name:  boiler control connections.JPG
Views: 1949
Size:  39.6 KB

Do you have a volt meter to measure the voltage at your thermostat?
 
  #5  
Old 09-13-15, 02:02 PM
B
Member
Join Date: Mar 2015
Location: Canada
Posts: 186
Received 0 Votes on 0 Posts
That strip is indeed all low voltage.

The manual states the outer connection strip is low voltage (24 volts) and the connection strip on the inside is (on the other side of the lower panel) is line voltage.

Your schematic indicates a 24 volt transformer powering the control board. You can tap into that for your constant voltage.
 
  #6  
Old 09-13-15, 02:43 PM
F
Member
Thread Starter
Join Date: Sep 2015
Location: USA
Posts: 5
Received 0 Votes on 0 Posts
As far as I can tell, the thermostat runs directly to the T terminals on the boiler. The thermostat was originally, as installed, an old fashioned line voltage. My best estimate is that the RC840 unit would connect in between the line coming from the T terminals and the 24v thermostat, but I am confounded as to where to connect the black wire from the RC840 to the boiler.

http://pixel.brit.co/wp-content/uplo...Thermostat.jpg
this is the thermostat I am trying to install. The previous one was only running the w/r two wire setup, but we have a whole bundle wired through the house so upgrading to a 3 wire is actually an option if necessary.
 
  #7  
Old 09-13-15, 03:30 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,917
Received 1,485 Votes on 1,373 Posts
You may need a relay. It won't be the high voltage one you've shown. Hang tight. Looking at the schematic now. You will need three wires between the boiler and the stat.
 
  #8  
Old 09-13-15, 04:35 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,917
Received 1,485 Votes on 1,373 Posts
You should be able to connect the thermostat directly to the boiler but it will require a little detective work and a voltmeter.

If you want to connect it without any ID'ing or the need for a meter then you will need a fan center. The fan center supplies 24vac to the thermostat and a set of contacts for the boiler.

White Rodgers 90-113 - Fan Control Center, 120 VAC Primary 24 VAC Secondary, SPDT Relay
 
  #9  
Old 09-13-15, 07:36 PM
B
Member
Join Date: Mar 2015
Location: Canada
Posts: 186
Received 0 Votes on 0 Posts
As far as I can tell, the thermostat runs directly to the T terminals on the boiler. The thermostat was originally, as installed, an old fashioned line voltage.
I'm pretty sure a line voltage stat was used simply because it contains a switch operated by thermostat. Your manual clearly states that connection strip is all low voltage. The line voltage one is inside the cabinet. Anyway... the voltage is irrelevant because it's self supplied all that's needed between T and T is a switch. If one side of the original stat is connected to T and the other side connected to the other T then the voltage (what ever it may be) is being supplied by the boiler itself. If it's 120 volts (which I doubt) then you should be able to see that with a mutlimeter connected from one of the T terminals (only one... the other will have 0volts) to ground.

 
  #10  
Old 09-13-15, 09:19 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,917
Received 1,485 Votes on 1,373 Posts
The point here is the OP wants to connect a WiFi thermostat. The TT terminals are 24vac but he needs a Common connection for the stat.
 
  #11  
Old 09-14-15, 02:37 AM
B
Member
Join Date: Mar 2015
Location: Canada
Posts: 186
Received 0 Votes on 0 Posts
Well, the schematic does show a 24vac transformer powering the board. He simply needs to locate it.
 
  #12  
Old 09-14-15, 08:42 AM
F
Member
Thread Starter
Join Date: Sep 2015
Location: USA
Posts: 5
Received 0 Votes on 0 Posts
The point here is the OP wants to connect a WiFi thermostat. The TT terminals are 24vac but he needs a Common connection for the stat.
This. I had picket up the AUB RC840T unit after looking at this post and seeing similarities between the two for converting and adding a C wire. From the sound of the discussion, i'm not sure I needed it now, or maybe just got the wrong thing?

Wow, you guys are wicked smart. I'm at work right now but I will wrap my head around this tonight and try out the voltmeter to see what I can figure out.
 
  #13  
Old 09-14-15, 09:12 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,917
Received 1,485 Votes on 1,373 Posts
That AUB switches 120v or 240v. You don't have that. You have 24vac.

Yes...there is a transformer shown on the connection diagram. If the transformer is not mounted on the board it should be connected to the board with a red and yellow wire.

I wasn't going to type up the ID work you'd need to do until I knew if you had a meter.
 
  #14  
Old 09-14-15, 11:59 AM
Houston204's Avatar
Forum Topic Moderator
Join Date: Oct 2009
Location: Texas
Posts: 7,425
Received 17 Votes on 14 Posts
A 40 va 24volt elk Transformer plugged into a standard wall socket can supply 24vac to RC and C.

Rh and W can connect to the 2 T terminals.

Name:  Honeywell Lyric with external transformer n boiler.jpg
Views: 7717
Size:  28.8 KB
 

Last edited by Houston204; 09-14-15 at 07:46 PM.
  #15  
Old 09-14-15, 12:01 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,917
Received 1,485 Votes on 1,373 Posts
Thanks for the info. I had completely forgotten that connection method. maybe old age settin in
 
  #16  
Old 09-16-15, 12:55 PM
F
Member
Thread Starter
Join Date: Sep 2015
Location: USA
Posts: 5
Received 0 Votes on 0 Posts
OOOOOHHHH, OK! It finally clicked!

I think I get it now! The TT connections that the stat was originally hooked up to were downgraded inside so really I was trying to use the AUB unit to convert 120 to a 24 that was already at 24 (120 didn't actually 'exist' where I thought it was). That makes so much more sense. I figured a line stat = line voltage by default I'll reconfirm what I now see to be the case, that the 24v was already running and I really just needed the C wire addition.

In that case, I'll use this method and post some working pics once I get it going.

A 40 va 24volt elk Transformer plugged into a standard wall socket can supply 24vac to RC and C.

Rh and W can connect to the 2 T terminals.
Man, talk about overkill, now I'll just see if I can find anyone that needs that AUB unit Thanks so much guys, real pros
 
  #17  
Old 09-16-15, 01:10 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,917
Received 1,485 Votes on 1,373 Posts
You're welcome. Keep us posted on your progress.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: