Ecobee thermostat with Taco 570 zone valves

sre92

I'm looking at a boiler with 2 zones that currently has old round Honeywell thermostats. They are connected to a 40VA 24VAC transformer, and power the wax motor in 2 Taco 570 series zone valves. The end switches of these valves are connected together, and go to a Taco SR 501 for some reason. I don't know why they installed that rather than a zone valve control, but that is what is there right now. The SR 501 sends the T-T call to the boiler, and also switches the single circulator.

I'd like to put at least one smart thermostat in the system. I see from this thread:

https://forum.heatinghelp.com/discussion/comment/1785090#Comment_1785090

that Taco 570 series valves need to be wired carefully because of the common terminal (not a "C wire") on the end switch. I am confident that I can follow those instructions.

However, it looks like the 570 motors use a lot of power (0.9A*24V=VA) which is actually more than the 40VA transformer currently in the system is rated for. And if I add another 3.5VA to that transformer for the Ecobee, that only makes things worse.

1. Can the Ecobee power these zone valves directly, or does it need to be through a relay (or an off-the-shelf zone valve relay)? I haven't been able to find a current or VA rating on the W1 terminal on the thermostat.
2. Is 40VA enough for two zone valves plus the thermostat? By the specs it doesn't look like it, but Taco puts 40VA transformers in their 3 and 4 zone ZVC403-5/404-5.
3. If 40VA isn't enough, then the cleanest solution is probably the 5-zone Taco ZVC405-5 which has 2*40VA transformers. Caleffi makes the ZVR103 which can have an additional transformer added in, though that ends up being more expensive than the Taco ZVC405-5.

Jamie Hall

@EdTheHeaterMan will come up with a wiring for that. Better him than me.

However… unless you really want the programming, the Ecobee — which is about the best of the programmables — won't do any better at control than the "Old round Honeywell thermostats" do… but with a lot more hassle.

sre92

It's for a ski condo, so remote monitoring (and control) is why a smart thermostat is desired. Condo maintenance can check on the condo while everyone is away, but at $25/check-in, a smart thermostat will pay for itself pretty quickly.

EdTheHeaterMan

WOW. Somebody reads my old posts… Thanks

According to Taco you can operate 3 Taco 570 heat motor valve actuators on one 40 VA transformer.  If you add more valves, you need to use a larger VA transformer or a second transformer.  But I know that is not your question.  You want to add the VA draw of the ecobee thermostat to the equation.  Since the total current draw of all three zone valves can happen all at once after a power failure (or turning off the switch for a service procedure) when the power is restored, then there is the possibility that you will be drawing over 55 VA from just the zone valves alone.  

With that information in hand, I would recommend a separate transformer and isolation relay for the ecobee thermostat’s 3.5VA current draw, or change the transformer to a 75 VA model. 

Are you using a zone valve relay or are these wired independently of a relay so the wires look like a confusing bundle of spaghetti?

JayWisth

Use an isolation transformer. In my own house, I combined 4 first floor zones that were similar enough in heating load to control off one thermostat, my EcoBee. These were Erie/TCA/Schneider zone valves with a stated draw from 5-6.5 watts. However, the thermostat would get notably warm and short cycle the heat like some kind of a misadjusted heat anticipator! I used a good quality four pole "ice cube" relay that draws a very small current compared to four zone valves.

Jay Wisth

3 years retired HVAC Tech - Been lurking here, keeping the brain active; first time poster….

HomerJSmith

@EdTheHeaterMan , I read every one of your posts, religiously. Even tho I not that religious. Some of the best posts on this site, without exception, are your contributions. Thanks

EdTheHeaterMan

Depending on the model of the smart thermostat, you may get an error message (No Power, or Power Failure) as a result of the Taco normal operation.  When that heat motor is activated, wax expands inside a piston that opens the valve.  It also operates two switches by way of a cone shaped cam.  As the valve opens the end switch closes to operate the heating system or circulator pump. 

As the valve opens more the power from the #1 terminal to the heat motor is opened to stop the heater from overheating the wax in the piston.  As the wax cools the piston starts to close and the cam switch closes to heat the wax again.  This on and off action of the heating element is normal operation for the 570 valve actuator but a smart thermostat may find that loss of power thru the valve as an error.  Another reason I recommend the isolation relay.

I find that this relay Fan Control Center is the best value for an isolation relay. Less than $35.00 you get both the relay and a 40 VA transformer.

HomerJSmith

@sre92 , This may help. Forget the Nest designation. This is a 3 wire setup to the Ecobee.

HomerJSmith

@sre92 -If you are going to use more than one ecobee, with @EdTheHeaterMan recommendation for a isolation relay, using a SP/DT fan center will not work. You will need two isolation relays for two ecobees & one transformer. If you only have two thermostat conductors to you current thermostat, you will need a special wiring diagram unless you can pull a new three conductor thermostat wire.

https://www.supplyhouse.com/Jard-92290-SPST-24V-General-Purpose-Switching-Relay

HVACNUT

Replace the 40 va with a 75 va that has a built in circuit breaker.

sre92

https://forum.heatinghelp.com/discussion/comment/1885871#Comment_1885871

Here's a better diagram of what is going on as it exists. Definitely some spaghetti… I didn't open the lid on the SR501 so I'm doing some assumption, but the end switches on the two 570s are tied together so that either can turn on the single zone switching relay.

I can picture wiring with a transformer/relay as you and HomerJSmith describe. It would add to the spaghetti, but only cost ~$35 vs ~$180 for the 5-zone 80VA Taco ZVC405-5.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1885881#Comment_1885881

This will work with 2 Taco 570 Zone valves. You might be pushing it with three Taco 570 valves on a 40 VA transformer.