smart thermostats

DaveM

I have an oil fired hydronic boiler with three zones running off a taco sr503 relay, the old thermostats all connect with 18/2 wire(red & white). The new thermostats require a c wire Other than running new 18/5 wire through the house, how would I wire these to the switching relay. I know I can use a nest power connector but can the relay handle three of them.

HVACNUT

I had it out with a client yesterday who just installed four 4th gen Nest thermostats. No common, and the main zone isolation heat relay keeps dropping out every 5 seconds. 27 vac going to R on the Nest. 22 vac coming back on W. He's swearing up and down he was told he doesn't need a Common, and the old Nest thermostats worked fine. I rewired the thermostat and air handler and used Yellow as Common, and the problem mysteriously disappeared. Now he's calling his electrician. I love everybody.

As far as YOUR scenario, @DaveM , I believe there's only a 15 va transformer in the SR503, so, no. The Nest 4 draws about 4 va, so three Nests wouldn't work. Or it would be cutting it very close. A seperate 40 va transformer and isolation relay would be needed. I'm not sure if Taco would allow you to replace the 15 va with a 40 va.

DaveM

can you help with a simple wiring diagram Thanks so much

EBEBRATT-Ed

@DaveM PM @EdTheHeaterMan he probably already has the drawing you need stashed somewhere on this site. In the meantime

RIB RELAY & TRANSFORMER FOR NEST.pdf

I will post a simplified drawing I made which will give you an idea

If you add a 40 va transformer you can power multiple Nest thermostats and relays off 1 transformer.

EdTheHeaterMan

If you are asking if the transformer in the Taco SR503 can power everything in the control along with three Nest thermostats I would say NO. You are better off using the power connector or a separate transformer to power the thermostat in each room like this one: Smart Thermostat transformer. You can plug it in a wall socket in the room where the thermostat is located and connect the R and C directly to the Rc and C on the thermostat.  Like this:

You want to use the Rc or R terminal when you have only heat connected to the smart thermostat.  The only time to use the Rh terminal is when there are separate air conditioning controls and heating controls in order to keep the transformers separate.  With heat only this is not the case. 

109A_5

"I know I can use a nest power connector but can the relay handle three of them."

I believe it depends on which version of the SR503 you have.

The latest version:

DaveM

thanks Ed the wall plug-in is not an option taco makes one that connects at the relay itself. I might look into the new version which has larger transformer. How would I wire an external transformer next to the sr503 thanks for all your help

pjg

Good video on this subject, 3 ecobees no problem with 15 va transformer.

I would go with the venstar color touch they draw less than the ecobee.

https://www.youtube.com/watch?v=ZfDIAMBJuVo

DaveM

I understand needing a separate transformer to power my three thermostats but I don’t understand how to wire it to the sr503 relay I only have 18/2 wire coming down from each thermostat to the boiler room thanks for the help

pjg

Running a new 18/5 wire to 1st and 2nd floor is what I did, 1st floor should be easy, 2nd floor will be a little harder but can be done.

Jamie Hall

Can't be done with 2 wire. You need to run at least 3 — and I'd run 5 — wires, as @pjg said.

EBEBRATT-Ed

@DaveM

As others have mentioned you need 3 wires for heating, more if you have cooling. How old is your house?

In the old days thermostat wires were fished in with no staples holding the wire. If that's the case, you can tie a new cable to the old and pull a new cable in.

If the wires are stapled to the studs then its more work. Try pulling gently on the cable and see if it moves easily

DaveM

thank you all for the help. I’ve just been reading about adding a diode on the switching relay between W and COM or a nest power connector which adds a C wire. Was thinking if that would work

EBEBRATT-Ed

They have all kinds of work arounds. I haven't herd of one that works.

DaveM

I’m sure you’re right as usual I’m going to give taco tech a call and see what he says

109A_5

I'm pretty sure it is not a diode. Sometimes a 1000 Ohm 1/2 Watt resistor. The resistance and the wattage may be situational dependent. 3 wires is best practice.

The NEST power connector may work.