Old (T/T) Taco SR506 with 5 zones, looking to convert to 5 NESTs--transformer questions

30dayfreetrial

I'm new to dealing with home wiring, and have done several hours of research, but can't quite find the information I'm looking for. I have an old--ie T/T, no individual common terminals for each thermostat--Taco SR506 hooked up to my boiler, with 5 zones configured. Heat only, no AC.



I don't want to try to run 5 NESTs with no common wire, so rather than setting up individual (ie plug-in) transformers at the thermostat end for all 5, I wanted to set one up on the boiler side, and run a common wire to each thermostat.

The SR506 has a 24V 15VA transformer with a common terminal that I could theoretically use.



However, from my research, that is enough to power 1 NEST, but not 5 (in a couple posts here, it seems like a 24v 40VA transformer can run 2). It seems that even if it had individual common terminals for each thermostat, the problem would remain the same--the built-in transformer couldn't power that many NESTs anyway. As such, I was looking at some 24V ~100VA transformers to wire up instead--for example:



This is where I'm stuck--I'm not sure where to go from here. Getting the common wires seems easy--use the yellow transformer output to a 6 conductor push wire connector, and use the other 5 conductors to run the common wires to each thermostat. But...the on-board transformer is still responsible for the main power to each thermostat--which it couldn't handle--right? Could I just do the same type of thing for the red wires for each thermostat? IE use the yellow/white transformer output to a 6 conductor push wire connector, then steal the red wire from each thermostat terminal on the SR506, and run them to the push wire connector? Or would the fact that the SR506 no longer had any power outputs to the thermostats break everything? I suppose I could try this with a zone I didn't care about or use very often...but getting some help up-front would be great.

Please forgive me if my terminology is off, or if this question is too basic. While I have a mechanical engineering degree and have no problem messing with machines and trial-and-erroring my way through things I have no experience with, I've managed to avoid wiring my entire life until now, and just don't have enough knowledge to feel comfortable (enough) to try this yet. That said, I'd like to avoid paying an electrician if I can

Ironman

You don't wanna use Nest, or any setback with hydronics or a modulating boiler.

30dayfreetrial

Ironman said:

You don't wanna use Nest, or any setback with hydronics or a modulating boiler.

Hi Bob, is there any particular reason for that? Google says NEST is compatible with that kind of system, and I know some folks personally (and I came across many more during my research) who are running them in such systems without issue. It never even occurred to me to doubt whether I *should* do this.

STEVEusaPA

Google says a lot of things. Search this post for the bazillion problems people are having with Nest.
And not to be a wiseguy, but did you try calling their tech support to see if they can help you? Or the people you know personally with the same set up as you have?

JUGHNE

Not to be a wiseguy also, but can't just Google for answers?

It seems like they would have their own "Geek Nesters" on line.

30dayfreetrial

STEVEusaPA said:

Google says a lot of things. Search this post for the bazillion problems people are having with Nest.
And not to be a wiseguy, but did you try calling their tech support to see if they can help you? Or the people you know personally with the same set up as you have?

I've read tons of things, here, and on other forums--some with problems, some with none--but everybody seems to have different hardware on the boiler side, so nothing I've found is completely apples-to-apples. Those I know personally don't have the same or even similar switching relay hardware, and Google didn't have any wiring recommendations for specific switching relay setups. Prior to posting, I found several other posts here which imply that nothing more than pulling a common from an external transformer would be enough--but then I see others saying that the common must come from the same source as the primary power source (though I very well could be misunderstanding the context).

I can try Taco support, and see what they say--none of the items in their FAQ relate to powering any more than one thermostat (though at least my original suspicion of using that COM terminal I circled in my picture to power a single thermostat is confirmed there).

Ironman

30dayfreetrial said:

Ironman said:

You don't wanna use Nest, or any setback with hydronics or a modulating boiler.

Hi Bob, is there any particular reason for that? Google says NEST is compatible with that kind of system, and I know some folks personally (and I came across many more during my research) who are running them in such systems without issue. It never even occurred to me to doubt whether I *should* do this.

Let me ask you something: how many boilers/hydronic systems has Google installed or worked on? How long have they been in the hydronic business?

Now I realize they are just the search engine to sites where people offer their opinions, but therein lies the problem: they are just opinions. Most of them offered by folks with no real experience or training. And, you know what they say: opinions are like armpits; everybody has a couple and they usually stink!

Now to your inquiry as to why the Nest is not a good choice:
1. Hydronic systems respond slowly compared to forced air. They primarily heat objects, not the air. Forced error furnaces heat the air and not the objects or mass of a structure. It takes far less time and energy to heat the air for a short period of time than to heat the mass of the structure. For this reason setback doesn't work well with hydronic systems and can actually be less efficient and less comfortable.
2. The second reason is that the Nest doesn't play well with modulating equipment. It's designed for "bang bang" equipment (non modulating). Because Americans are used to this type of equipment, they think keeping its run time to a minimum will maximize efficiency. However, this is not true with modulating equipment which seeks to run as long as it can on as low of a modulation as possible. It also from its ODR curve seeks to keep the SWT as low as needed to provide better efficiency and comfort. Turning the thermostat setting up and down throws the logic of a mod/con on its head.

hot_rod

check into the Ecobee stats, they seem to be more hydronic heat savvy.

pecmsg

NEST has

Better marketing then MIKE HOLMES and the 3M filtrate furnace killer!

Alan Welch

I have the Honeywell wifi thermostat and love it, not for setback because we keep a constant temperature. The ability to monitor the house temp while away is important to me. Also turning on the A/C when coming home after being away in the summer is great.

SuperTech

> @Alan Welch said:
> I have the Honeywell wifi thermostat and love it, not for setback because we keep a constant temperature. The ability to monitor the house temp while away is important to me. Also turning on the A/C when coming home after being away in the summer is great.

Yes. A Honeywell wifi thermostat is about as high tech as I'd like to see with hydronic heating systems.

I'm a service tech and I despise Nest thermostats because I get more annoying service calls about them then all other thermostats combined. The owner should be the smart part of the heating system, not the thermostat.

pecmsg

Alan Welch said:

I have the Honeywell wifi thermostat and love it, not for setback because we keep a constant temperature. The ability to monitor the house temp while away is important to me. Also turning on the A/C when coming home after being away in the summer is great.

its cheaper in the long run letting that A/C run during the day at say 74 and control the humidity during the day then try and drop the temperature quickly.

Alan Welch

It doesn't use much electricity when off for a few days, and doesn't run long to bring it up to temperature.

30dayfreetrial

Ironman said:

Now to your inquiry as to why the Nest is not a good choice:
1. Hydronic systems respond slowly compared to forced air. They primarily heat objects, not the air. Forced error furnaces heat the air and not the objects or mass of a structure. It takes far less time and energy to heat the air for a short period of time than to heat the mass of the structure. For this reason setback doesn't work well with hydronic systems and can actually be less efficient and less comfortable.
2. The second reason is that the Nest doesn't play well with modulating equipment. It's designed for "bang bang" equipment (non modulating). Because Americans are used to this type of equipment, they think keeping its run time to a minimum will maximize efficiency. However, this is not true with modulating equipment which seeks to run as long as it can on as low of a modulation as possible. It also from its ODR curve seeks to keep the SWT as low as needed to provide better efficiency and comfort. Turning the thermostat setting up and down throws the logic of a mod/con on its head.

Thanks for this information Bob. So, I definitely left out some important information in my original post. I don't have a mod/con boiler--it's a Smith Cast Iron Boilers 8 series, oil-fired, with a Carlin EZ-1HP burner, also with a Hydrostat 3150 (to which the Taco SR506 is connected) set at 140/200. So, there doesn't appear to be any provision for an ODR curve, and the SWT will (should?) never go below 140, regardless of whether the thermostats have triggered a circulator in any given zone to send hot water there. Unless I'm missing something, there doesn't appear to be any difference between me setting the temperature at a thermostat, and a "smart" thermostat doing it for me (along with the additional functionality they provide).

Jamie Hall

With a traditional boiler, you don't have the problem of the Nest cheerfully defeating most of the advantages of a mod/con -- so that's not really a problem. However, you do still have the problem -- with any hydronic system -- that it responds slowly to being turned on. Or off. Which limits -- though not eliminates -- the energy saving advantages of setbacks. It is possible, with some savvy, to configure a Nest so that it doesn't do short term setbacks, and limits the longer ones to a reasonable amount -- say 3 degrees or so.

But then, why hassle with the Nest in the first place?

Now, however, that leaves out the glitz factor, and some folks just want a Nest because it is new and cool. No problem with that. If that is what you want, the easiest and simplest way to use a set of 5 Nests with your system is to use relays. What you will wind up with is that larger transformer you mentioned. C to all the Nests, and R to all the Nests. Then the W -- output from the thermostat -- goes to a relay coil, one for each Nest -- and the relay coill also returns to the transformer common. Each relay will now behave exactly as any other thermostat would, and you can connect the normally open contacts from the relay to the appropriate connection on the Taco and off you go.

As soon as you get it up and running, be sure to change the passwords and make sure that your WiFi network is very secure. Otherwise you are telling the world when you are and are not home (you're telling Nest and Google anyway -- can't help that -- but why tell your friendly local burglars?).

30dayfreetrial

Jamie Hall said:

If that is what you want, the easiest and simplest way to use a set of 5 Nests with your system is to use relays. What you will wind up with is that larger transformer you mentioned. C to all the Nests, and R to all the Nests. Then the W -- output from the thermostat -- goes to a relay coil, one for each Nest -- and the relay coill also returns to the transformer common. Each relay will now behave exactly as any other thermostat would, and you can connect the normally open contacts from the relay to the appropriate connection on the Taco and off you go.

Hi Jamie, I appreciate the response!

Please see my hacky drawing below, representing an example circuit for zone 1--did I parse your suggestion correctly?



And my understanding of this configuration is that the transformer is powering the NEST 100% of the time, and the relay is being used to bridge R and W to energize the heating circuit when the thermostat calls for heat--is that correct?

HomerJSmith

I have install Nest thermostats on radiant sys and baseboard without any problems. Most of the concerns I have seen is with the wiring of the Nest C to the control.

I appears, freetrial, that the picture of your box shows that the thermostat wires are 2 conductor which makes it more difficult as that 3rd wire make the connection to the Nest C connection.

Below is a diagram that I did for a radiant sys with a transformer and relays to operate Thermo actuators on a manifold. I hope that it will shed some lite on your problem. Ignore the RBI relays or substitute anything in its place.

Dave H_2

@30dayfreetrial
Here is how we suggest with an external transformer.




Dave H

Jamie Hall

Use @Dave H solution. That works.

30dayfreetrial

Dave H_2 said:

@30dayfreetrial
Here is how we suggest with an external transformer.
Dave H

Thanks Dave! That's pretty simple, aside from the one sanity question I sent you directly about how/where the wire connections can be made (ie on the board or separately via wire nuts/push-in connectors).

One additional question--given that I want to install one of these thermostats for each of the 5 zones, is it acceptable to branch the R and C coming out of the external transformer to 5 separate conductors each (provided the transformer can satisfy the power requirements)? IE, branch the one common output from the transformer to 5 common wires, one running to each thermostat, and branch the one 24v output wire from the transformer to 5 R wires, one running to the R terminal of each zone?

Dave H_2

Here is the diagram for the older SR controllers with the metal covers. The previous diagram is for the newer SR panels with plastic covers.


Dave H

HomerJSmith

Be aware the the auxiliary transformer must be on the same buss in the electrical sub-panel as the SR-504. You might want to make sure that the polarity of the auxiliary transformer and the the transformer in the SR-504 match.