Added transformer for smart thermostat?

Dscheck

I’m planning on installing a smart thermostat to my 2 zone navien combi boiler. There are two 007-zf5-9 taco circulators which each have their own built in switching relay. There is a common connection on the relay and the thermostat wire has a common wire. The question is whether the 24 volt transformer has adequate capacity to power a smart thermostat? Suggestions for what model thermostat are welcome. The transformer specs say 1VA but I can’t tell if that @120 volts or 24 volts. If I do need added 24 volt capacity do I also need an isolation relay? Is there a transformer with an isolation relay in one unit? Thanks for any insight.

mattmia2

The 1 va would be on the output but that would only be 40ma so i suspect that is the current of one of the control loops, not the transformer. Where is this transformer, in the boiler? part of the circulator? part of a zone control? standalone?

EdTheHeaterMan

It is difficult to know because it is difficult to find the needed information on the thermostat before you purchase it. You need to know the power requirement of the thermostat. and compare it to the power output of the control that is providing the power.

if you have a Taco Priority Zone Circulator the electrical info is easy to find


The thermostat manufacturers (like NEST) don't have that info easily available. The best you can get is if there is someone that is somewhat intelligent at tech support for your thermostat to give you that info.

The old-school way to test the power usage of a thermostat is to use a clamp-on amp meter. Wrap a coil of thermostat wire around the jaw 10 times to multiply the amperage signal to the meter. Once you have that reading device it by 10 to get the actual reading. Then you Ohms Law to do the math. If it is within the transformer rating, then you're good to go.

You want to measure the common wire (between C on the control and C on the thermostat.)

.VA rating is Volts x Amps. So if the actual volts between R and C is 26.2 volts and the actual Amps is .03 then you are good to go for 1 VA

If the thermostat draws .15 amps at 26.2 volts then you need at least 4 VA and the 1 VA won't work

In either case, the Taco zone relay that comes on the circulator is no good unless the power draw is below .5 VA. That is unlikely... so get an isolation relay for the WiFi zone

Respectfully submitted
Mr.Ed

EBEBRATT-Ed

@Dscheck

Best way out to be sure is just put your own 120/24 volt 40 va transformer and a RIB #RIBU1C relay you can get both of them from supply house.com. Some do it with a fan center transformer relay but that is more $$

The relay and transformer will cost you 15 cups of coffee since we can't discuss price here

I doubt the TACO panel will support the thermostat with it's internal transformer

Dscheck

Okay, thanks. Ecobee thermostats for example draw 3.5 VA so it appears clear the internal transformer doesn’t have the capacity. Does anyone know where I can find a wiring diagram for adding the transformer and isolation relay for this instance? All the diagrams I find seem to be for adding a c-wire which is not the issue I have.

EBEBRATT-Ed

@Dscheck

I think I have a diagram. Check back later this evening

Dscheck

Picture of one of the existing switching relays that are part of the circulator assembly.

EBEBRATT-Ed

Here you go

Dscheck

Thanks so much for the diagram. Just what I needed.

Dscheck

An associated question. Is there any reason not to use one transformer of appropriate size for both zones? I assume I would need an isolation relay for each zone.

EdTheHeaterMan

One transformer to power ALL isolation relays is acceptable Must all be wired identically so set up a rule like; All thermostat R terminals go to the transformer R. All thermostat C terminals go to the C on the transformer. All thermostat W terminals go to the relay coil wire, and the other relay coil wire goes to the C on the transformer.

Dscheck

Is there a reason to use a hard wired transformer vs a wall wart style for this application?

EdTheHeaterMan

Not really. You can get an inexpensive 15 or 20 VA transformer that will operate the relay. All you need is to get the contacts to close. You can even use different relays. A simple electronics hobby store (Like the old Radio Shack) relay will do the job

Dscheck

Thanks for all the reply’s and help.