Relay vs transformer and relay


  #1  
Old 08-15-19, 02:52 PM
T
Member
Thread Starter
Join Date: Aug 2019
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Relay vs transformer and relay

I'm trying to install a nest gen 3 with separate heat and AC units. The AC already has 24 v wires connected to the nest with a common to power the thermostat. My heating unit only has 2 120v wires going to the old thermostat. I hastily ordered the aube 840t-120 without doing much research. I'd like to install the aube near the thermostat so I won't need to run new wire thought the house. Since I already have a common to the thermostat, can I just use a relay? If so, can I use the 840t (diagram says I need 3 120v wires) or should I get the aube 840(relay only).
 
  #2  
Old 08-15-19, 04:31 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 64,811
Received 3,922 Upvotes on 3,518 Posts
Welcome to the forums.

The aube 840t-120 requires a continuous 120v so it would not be the best choice to be at the thermostat. Any 24vac SPNO relay will work. Since you are installing the relay near the thermostat it will need to be in some type of enclosure.
 
  #3  
Old 08-15-19, 05:04 PM
T
Member
Thread Starter
Join Date: Aug 2019
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks. Are there any 24v spno relays you'd recommend? Is the rc 840 sufficient?
 
  #4  
Old 08-15-19, 05:11 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 64,811
Received 3,922 Upvotes on 3,518 Posts
Do you have an idea how you're going to contain the relay ?
Just letting hang in the wall wouldn't be the best choice.

The RC840 is fine.
You could install it in a deep cut-in gem box.... plastic or metal..... and put a blank plate on it.

Name:  mgem.jpg
Views: 251
Size:  16.1 KB
 
  #5  
Old 08-15-19, 05:38 PM
T
Member
Thread Starter
Join Date: Aug 2019
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Yeah,that much I do know lol. Thank you.
 
  #6  
Old 08-16-19, 06:56 AM
T
Member
Thread Starter
Join Date: Aug 2019
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Just to confirm my set up would be:

​​​​​​120 wires to relay
24 v wires from relay to nest (rh and w1)?

​​​

 
  #7  
Old 08-16-19, 09:50 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 64,811
Received 3,922 Upvotes on 3,518 Posts
Yes..... 120v wires to the relay.
The low voltage 24v wires go to C and W1.

You'll have to splice the C wires together so that only one connects to the nest.
 
  #8  
Old 08-16-19, 10:03 AM
Geochurchi's Avatar
Member
Join Date: Nov 2012
Posts: 5,073
Received 161 Upvotes on 147 Posts
Hi, it sounds like you had a line voltage Tstat for the heat originally, correct? If so you will need to rework the connections at the boiler so as to have a neutral for the relay.
Geo
 
  #9  
Old 08-16-19, 10:06 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 64,811
Received 3,922 Upvotes on 3,518 Posts
He did have a line voltage stat and now he's using a 24v relay only to handle the switching.
 
  #10  
Old 08-16-19, 10:13 AM
Geochurchi's Avatar
Member
Join Date: Nov 2012
Posts: 5,073
Received 161 Upvotes on 147 Posts
Oh, got it!,,,,,,,,,,,,,,,,,
Geo
 
  #11  
Old 08-20-19, 03:59 PM
T
Member
Thread Starter
Join Date: Aug 2019
Posts: 5
Upvotes: 0
Received 0 Upvotes on 0 Posts
Pjmax,

I was able to set it up at my home with the Nest 3rd gen. Everything works. I'm wondering if you think this set up would also work with a Nest E?
 
 

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