Nest Thermostat with Taco Zone Valves and Aquastat L8148E


  #1  
Old 07-21-22, 09:42 AM
A
Member
Thread Starter
Join Date: Jul 2022
Posts: 5
Received 0 Votes on 0 Posts
Question Nest Thermostat with Taco Zone Valves and Aquastat L8148E

I am in the process of replacing an older digital display thermostat with a 3rd gen Nest thermostat. My current heating system consists of two Taco zone valves (model 572-2) and an aquastat L8148E control unit (see image "Old Setup"). My plan is to move the older digital display thermostat and have it control zone #2, and install the new Nest thermostat to control zone #1 (see image "New Setup"). I would appreciate a second pair of eyes to look through these schematics/drawings before I begin re-wiring the system.

It seems that in the older setup, coming from the transformer, they had the hot "R" and common "C" wired opposite of what is recommended in the Taco manuals (but I guess polarity doesn't really matter with the two-wires thermostat setup). You will see that in the new setup I tried to correct this. Please let me know if this makes sense for the Nest.

While browsing this forum and others, I also noticed that some folks have suggested using a relay between the Nest 'R' -> 'W' and 'R' -> '1' in Taco valve for further protection (see third image attached). Is this recommended or even necessary?

Thank you for your assistance!

-AB



 
  #2  
Old 07-21-22, 09:52 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 70,064
Received 2,954 Votes on 2,665 Posts
Welcome to the forums.

Polarity is not important when using two wires. It's very important with three wires.
Your diagram looks good.

I haven't used a relay to isolate the nest from the valve but it couldn't hurt.
 
  #3  
Old 07-24-22, 01:07 PM
A
Member
Thread Starter
Join Date: Jul 2022
Posts: 5
Received 0 Votes on 0 Posts
Thanks for the feedback PJmax, I think I will go without the relay.
-AB
 
  #4  
Old 07-25-22, 07:35 PM
A
Member
Thread Starter
Join Date: Jul 2022
Posts: 5
Received 0 Votes on 0 Posts
Future reference for anyone that might come across this post and have a similar setup, here is the reason why the isolation relay is not necessary with a taco zone valve: the circuits are already isolated. Each circuit flows only in that circuit (see diagram below from Taco Heat Motor Zone Valves manual).


 
  #5  
Old 07-25-22, 09:56 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 70,064
Received 2,954 Votes on 2,665 Posts
You previously showed a relay isolating the nest from the zone valve.
That is different from what you are discussing now.

If you read the fine print from your boiler.... you are supposed to use an isolation relay between the zone valves and the boiler TT terminals because there is a second transformer.
 
  #6  
Old 07-28-22, 06:46 AM
A
Member
Thread Starter
Join Date: Jul 2022
Posts: 5
Received 0 Votes on 0 Posts
@PJmax yes, you are correct in that the two are different. My only concern was ending up wiring the transformers in series and therefore suppling 48 VAC to the nest.The diagram that I included in my last post shows that the two circuits only make contact at terminal 2 of the taco zone valve, such that neither of the transformers can cause a current or voltage driving force on the other circuit.

Yes, I understand that the relay on my original post was to isolate the nest from the taco 555 power head, which some folks have pointed out that the 555 switching mechanism can cause the nest to throw an error. Please see the following post. I was wondering what your experience has been connecting a nest tstat to the 555 power head?
https://forum.heatinghelp.com/discus...co-zone-valves

Also, doesn't the Aquastat L8148E already include the relay that you drew in your last post?

Thanks for all the help!

-AB
 
  #7  
Old 07-28-22, 11:32 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 70,064
Received 2,954 Votes on 2,665 Posts
The L8148E has it's own internal transformer.
Tv is the R supply from the transformer to the thermostat.
T is the W return back from the thermostat.

Your circuit would not put the transformers in series but they will share a common.

You should connect terminal 2 of the zone valves which is that transformers common to Tv.
You should measure between terminal 2 and Tv before connecting the wire to insure close to 0v.
If you see close to 24v..... then swap the leads on the zone valve transformer.
The idea is to keep the transformers in phase.

Rarely is this done and usually there isn't a problem.
 
  #8  
Old 07-30-22, 06:57 AM
A
Member
Thread Starter
Join Date: Jul 2022
Posts: 5
Received 0 Votes on 0 Posts
@PJmax so I should be fine with my original schematic, right?

Thanks,
-AB


 
  #9  
Old 07-30-22, 12:29 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Jersey
Posts: 70,064
Received 2,954 Votes on 2,665 Posts
You haven't had any problems so you should be fine.
 
 

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