Adding Nest Thermostats to 3 Zone Boiler

spilltim

First post here. Hoping to save some money and help others that share my question.

I just bought a home and had a little bit of work done on one of the zones. We were having issues with the radiant floor zone while the two other baseboard zones worked without issue. (we have a separate pump for each zone)(No DHW, so priority is off) The issue was fixed but while the tech was working I asked if my system would be compatible with my Nest thermostats that I already own.

The technician told me that my switching relay was an older argo and didn't have a spot for a common wire which is needed for my Nest thermostats. He mentioned that a resistor could potentially be used, but a better solution would be to just upgrade the board. I am fine with this. I would prefer to upgrade the switching relay anyway.

I asked the tech for an estimate to replace the current switching relay with a newer one that would be compatible with Nest. I got the quote to install the "6 Zone Switching Relay For Pumps (Direct Replacement)". I didn't know much about it so I looked into the Switching Relay and found that the part (Taco SR506-4) was relatively cheap. The remainder was labor and it was high.

Next thing I realized is I do not need 6 zones. I need maybe 4 zones max in the event I ever decide to install indirect DHW. So now I am looking at the Taco SR504-4. After taking a look at my existing switching relay and the documentation for the SR504-4, it seems like this should be rather plug and play.

What I am trying to determine is if I still need to use a resistor when connecting the C wire. I am under the impression that the resistor is only needed if there are currently only 2 wires (R,W) running to from the switching relay to the thermostat. If I have enough wires (3) running from the switching relay to the thermostat, which I do, I should just be able to connect R and W to the switching relay and thermostat, and connect the additional wire to the C on the switching relay and thermostat without needing a resistor. Does this sound correct? The diagrams on the documentation (linked at the bottom) are a bit misleading.

As a follow up, I am looking at both the Taco SR504-4 and the TacoSR504-EXP-4 (only a few dollars more.) I have noticed that the EXP model has more bells and whistles that I am not sure I need, however the EXP model has a C port on each zone as opposed to the non EXP model that has only a single C connection. Does this make any difference? If I went with the non-EXP model would I just connect all 3 C wires from the thermostat to the single C port on the switching relay? Or is there a specific reason that there is a separate C port per zone on the EXP model?

http://docapi.tacocomfort.com/tmp/tacopdftmp-210213112813.pdf

Appreciate any and all help. My wallet does too.

Alan (California Radiant) Forbes

It all sounds correct. I'd go with the EXP if it has the additional "C" terminal, especially at the price. You won't need the resistor if you use the "C" terminal. BTW, we're not supposed to talk about money on this website, so if you could edit out the amounts...........
A good technician will always specify a control with additional zones in case one relay goes bad or you want to add and extra zone or DHW.
BTW, his pricing doesn't seem that far off, especially since he knows what he's talking about.

spilltim

Thanks for the heads up Alan, I made the edits. I also didn't think about the case of one relay going bad so you would have one or two backups. Thanks for pointing that out.

I am kind of surprised to hear that you felt his pricing was fair, however I don't really have anything to base off my argument that I felt the price was high. I am a new home owner so I assume I will become more familiar with what to expect as I encounter more things that need to be repaired.

Jamie Hall

May I humbly ;mention that if that one zone which you describe as "radiant" is, in fact, radiant floors, the Nest will be worse than useless for that zone, unless you disable all the learning, cycling, setback and occupancy features.

pecmsg

Any t-stat besides N E S T

Radiant heat operates off an ODR control. as far as a t-stat set it and forget it!

spilltim

I guess I should have mentioned that. I am familiar with the issues while using the Nest learning features with the radiant floor heating. I am not using the learning features on any of my Nest thermostats. I am primarily using nest for the convenience of remote access as well as having it integrated into the Nest ecosystem I have at home (smoke alarms as well as one or two cameras.)

I will have to do some research into ODR control. That is something I am not familiar with. 

Alan (California Radiant) Forbes

Here is a good read:

https://www.heat-timer.com/outdoor-reset-control-savings/

spilltim

Thanks Alan that was an interesting read. I had the technician out today as a checkup to ensure the radiant flooring was now working as expected. I asked him about the ODR and apparently my boiler doesn't have it or is not using it. I think I have the answer that I came for but I'll add some pictures of the existing system for anyone that stumbles across this post. 

SteveSan

How many Nest are you planning on installing on the new control? Any more than two Nest will require an external transformer to be wired in. Also make sure the Nest have been pre-charged and in some cases the resistors are still needed. Our won't work according to Nest, they suggest using a 220ohm and up to 5watt resister with their t-stats. Any questions please feel free to contact Taco 401-942-8000 Mon-Fri 8am-5pm EST.

spilltim

Yikes really? I anticipated having 3 Nest connected to the new control: one for each zone. I am going with the Taco R506-EXP-4 which has a separate common terminal for each thermostat zone. I would hope that since the control board is built for 6 zones that the transformer would have been been built powerful enough to support numerous thermostats without issue.

My understanding was that since I have enough legs on the thermostat cables (R,W,C) I would not have to use a resistor. I was under the impression that a resistor was only needed when there were not enough legs and the C from the thermostat would have been tied into W.

Jamie Hall

spilltim said:

Yikes really? I anticipated having 3 Nest connected to the new control: one for each zone. I am going with the Taco R506-EXP-4 which has a separate common terminal for each thermostat zone. I would hope that since the control board is built for 6 zones that the transformer would have been been built powerful enough to support numerous thermostats without issue.

My understanding was that since I have enough legs on the thermostat cables (R,W,C) I would not have to use a resistor. I was under the impression that a resistor was only needed when there were not enough legs and the C from the thermostat would have been tied into W.

;The transformer on the control board is powerful enough for the control board. Most thermostats are simple switches and take no power at all. The Nest -- and other wi-fi connected devices -- are not just thermostats; they are computers and radio transmitters, and take a lot of power to operate, however, unlike a thermostat, which take no power. Since the folks who make the control board have no control over what additional power hungry devices might be added, they can't anticipate that -- and shouldn't be expected to.

In fact, though, there is enough power on the board to operate ;programmable thermostats. They take very little power (the same thermostat will operate for a year on 3 AAA batteries, for instance), so for convenience they ;do include the C terminal, which is needed.

spilltim

Interesting. This certainly presents an issue with my plans. While Taco may not create boards with a transformer that is powerful enough for multiple wifi thermostats, is there a manufacturer out there that does? I understand what you are saying, but with the developments around smart home devices there must be someone out there that is planning for the future which would include a more powerful transformer to support multiple smart thermostats.

Jamie Hall

spilltim said:

Interesting. This certainly presents an issue with my plans. While Taco may not create boards with a transformer that is powerful enough for multiple wifi thermostats, is there a manufacturer out there that does? I understand what you are saying, but with the developments around smart home devices there must be someone out there that is planning for the future which would include a more powerful transformer to support multiple smart thermostats.

As far as I know, no, and it would foolish. There is no way that a manufacturer or designer could possibly predict how many devices would get attached, nor how much power those devices would draw. Do you provide a big enough supply for 4? 8? 10? But how about a yet fancier one with a camera which takes twice the power? A much more satisfactory solution, overall, would be for the widget manufacturer to include a proper power supply for each unit, which could be used if needed (Rinnai, does, for instance, for the remote thermostat kits they sell for gas wall furnaces).

JakeCK

If I may make a suggestion, ditch the nests and go with a zwave tstat if you want remote operation. Zwave is low power, can connect to any hub you want, and you can go sans subscription/cloud BS.

spilltim

Thanks Jake. I think I may have to as I cannot figure out another optimal solution. I'll report back when I make some decisions. 

spilltim

SteveSan said:

How many Nest are you planning on installing on the new control? Any more than two Nest will require an external transformer to be wired in. Also make sure the Nest have been pre-charged and in some cases the resistors are still needed. Our won't work according to Nest, they suggest using a 220ohm and up to 5watt resister with their t-stats. Any questions please feel free to contact Taco 401-942-8000 Mon-Fri 8am-5pm EST.

I called Taco today and they sent me over the same wire diagram that you provided. I am taking a closer look at the diagram and am trying to figure it out. I would no longer be using the Common terminal directly from the SR506 and would instead plug it into the additional transformer. The R on the transformer though is connected to the R on the tstat as well as the R on the SR506. If there is both power on R coming from the transformer on the SR506 and power on R coming from the external transformer, does this present any issues?

What about when there are multiple thermostats? Based on the provided wire diagram it seems like if I am tying the R from the tstat and SR506 back to the transformer for each zone, all of the zones now appear to be connected via R. Is that a problem? I guess it would look something like this. (in the event I was able to find a suitable transformer) Or perhaps I would need to use a seperate transformer for each Nest.

Jamie Hall

That should work. The thermostats -- get a big enough 24VAC transformer; I'd suggest 80 or more volt amperes since those things just suck power -- are powered by the circuit from the transformer to R on each thermostat then back through C on that thermostat to the transformer. The zone control, on the other hand, is controlled by the circuit from the zone control board's R terminals, through the electronics in the the thermostat which simulate a simple switch (it isn't really, and that can cause problems), back through W to the zone control.

Do NOT ground either 24 VAC terminal of your new outboard transformer. It must float. And don't use it for anything else.

spilltim

Unfortunatley this came to a rather anticlimactic ending but thats OK. I called Google Support in search of some power specs and learned that the Nest Thermostat is not even compatible with my boiler. Weil Mclain GV40+4.

The rep explained that my boiler is not compatible because it has dry contacts and even though it is going through the relay, it would likely not work as expected.

I still plan to update my relay switch to the SR506-EXP since I already ordered it and my existing unit is quite old. Anyone have any low voltage thermostats that they reccommend? Two zones water baseboard and one zone radiant floor.

Jamie Hall

I like Honeywell thermostats -- there is a wide range of models from very plain vanilla up through pretty whizzy. White-Rodgers also make good thermostats. Prices vary all over the place, depending on what features (like programming etc.) you want.

spilltim

Thanks for all your help Jamie. I ended up keeping it simple and went with the Honeywell TH1100DV1000. Hopefully simple means operational and no issues. Fingers crossed.