why the different options for connecting a smart thermostat?

lost11

why don't they just work the same?

Argo requires an additional transformer and isolating relay to properly connect smart thermostat.

Taco will require a similar setup to Argo for Eecobee (https://support.ecobee.com/s/articles/Taco-SR503-4-Three-Zone-Switching-Relay) but will work with a resistor for NEST (https://heatinghelp.com/assets/documents/Nest-Wiring-Applications-TACO.pdf)

the original transformer wasn't designed to supply enough power for these smart thermostats?

super_snop

Different thermostats draw different power. To further complicate things if there is no common wire the thermostat will essentially cycle the heat to steal power fir its battery. Thats where the resistor come in. I would never setup a smart thermostat without a C wire. 

JK_Brown

Check out "How Many Zone Valves Can Be Put on a Transformer?" in the help center thermostat section. And take note that Smart thermostats use a lot more power for their wifi transceiver so it is a greater load than a standard electronic thermostat. And the old mechanical thermostats drew no power.

And you can't just increase the transformer size without running into the limits for the "intrinsically safe" (don't have enough power to give serious shock or create enough heat to start a fire in normal install) low voltage wiring in the NEC.

Zman

I don't think any of the models draw tons of power. All models work better with a common wire. In most cases, the marketing department has a 10-mile lead on the tech department.

Jamie Hall

You claim that a Nest will work with a resistor. Sometimes. On some systems. And not reliably.

super_snop

Jamie Hall said:

You claim that a Nest will work with a resistor. Sometimes. On some systems. And not reliably.

You are correct. I have installed a few nests on 2 wires with a resistor. Haven’t had the best results. 

HVACNUT

I believe Argo has a 15 va transformer while the Taco panel uses a 10 va transformer. 
Maybe enough for one "smart thermostat", but not multiple. 

EBEBRATT-Ed

@lost11

Some of the smart thermostats have an internal batter that needs charging and it takes a little power to do this. Nest's original approach was to hook it up 2 wire (with no common C wire) with the idea that the thermostat would charge by "power stealing" when the equipment is off between calls for heat.

This has proved unreliable coupled with the fact that the small transformers in some controls are not large enough to do both jobs (operate the controls and charge the Nest) at the same time. Some of the other gimmicks to provide a fix have not proven to work

The sure fix if someone insists on using these stats is to install a seperate isolating relay and an additional transformer. It's only about $$ worth of parts and sme labor

hot_rod

And if you are zoning with valves, some of the new capacitor ZVs draw a lot of power when they recharge. Add that to the stat draw and you need some serious current If all happen to pull high current at once.

Jamie Hall

One reason -- not excuse -- for why the Nests sometimes work is that they were designed to operate systems with frequent short cycles -- forced air. The internal batteries are simply too small -- no matter how they are charged -- to reliably run the device during a longer call for heat, when charging isn't possible over a two wire setup.

mygardenshed

EBEBRATT-Ed said:

@lost11

Some of the smart thermostats have an internal batter that needs charging and it takes a little power to do this. Nest's original approach was to hook it up 2 wire (with no common C wire) with the idea that the thermostat would charge by "power stealing" when the equipment is off between calls for heat.

This has proved unreliable coupled with the fact that the small transformers in some controls are not large enough to do both jobs (operate the controls and charge the Nest) at the same time. Some of the other gimmicks to provide a fix have not proven to work

The sure fix if someone insists on using these stats is to install a seperate isolating relay and an additional transformer. It's only about $$ worth of parts and sme labor

I just installed a Honeywell T5 which was very cheap after rebate from PSEG. Basically wanted to see what all the fuss is about and I’m a habitual tinkerer.
I installed it using a 24v/40va plug in transformer connected to ‘C’ and ‘RC’. I didn’t install a relay. This is on a Force steam boiler 103,000 btu. There is no control board per say. Should I still place an isolating relay on the ‘W’ and ‘RH’ circuit going back to the heater? Seems to work fine without it but I don’t want my tinkering to make any trouble for myself.
Thanks

jad3675

I have three ecobee 3 lites, 4 Caleffi z-one valves and a ranco aquastat hanging off my Caleffi ZVR104 panel with a single transformer. I've never had an issue.

Jamie Hall

The key thing to take away from all this is that sometimes a given smart thermostat will work with a given arrangement-- and sometimes it won't. And one brand -- or even model -- of smart thermostat will work in a given installation, and another won't.

Like so many things, the thermostat is only part of a system. It has its own characteristics -- but so does the rest of the system. Let us suppose, to give one example, that you design a smart thermostat. Wi-fi, backlit screen, that sort of thing. You know when you design it that it will need a steady power supply to function properly. So... you examine typical heating or cooling systems in your area -- which for most of these widgets, is Sillycon Valley, California. And what do you find? Forced air. With nice short off times and nice short on times. Since you have never been anywhere else, you design your power supply so that it has enough energy storage -- either a battery or sometimes a supercapacitot -- to supply that power for, let's say, a 10 minute on cycle of the thermostat, and then, when the thermostat is off, to use that voltage to recharge -- and you use cutting edge technology to get it to charge in only three or four minutes, using the current which can make it through the relay coil in the forced air system through the thermostat wires -- but not enough to trip the relay. Works like a charm.

Wonderful Now you take that same thermostat and hook it up to (let's be extreme) a middleaged steam system somewhere in New England, where the thermostat on time is going to be perhaps an hour coming out of a setback. The off time will probably 15 minutes to half an hour. Now remember. The internal power supply is designed to supply power for perhaps 10 minutes. After that the thermostat dies. And folks installing them get annoyed.

With the correct wizardry it turns out that you can maintain enough power in the thermostat to keep it running, but that takes more current than you can reliably put through the boiler controls. So you try putting a resistor at the boiler to leak some current back to the transformer. But now you are involved in a very delicate balancing act between pulling enough current to keep the unit charged, keeping the voltage at the control or relay high enough to activate it when wanted -- but low enough to not activate it when not wanted.

If there were some sort of standard on all the other components in the system -- zone valves, furnace or boiler controls, what have you, this might not be hard. As it is, however, there is no standard, either in the equipment or in how -- or how much of it -- is attached.

So sometimes you get something which works all the time. Sometimes it works some of the time. Sometimes it just doesn't work at all.

You can fiddle with the various gimmicks, and very often get it to work at least most of the time.

Or you can make life very easy for yourself and supply that constant power connection which it needs, either with a third wire direct back to the system's power supply, or with an independent power supply and either run it direct or, more reliably, with a dedicated but simple relay which the thermostat controls which, in turn, controls the rest of the system.

You see, it really isn't the fault of the smart thermostat. It's just being asked to do a job it was never really designed for, and so you have to resort to a kludge -- or just relax.

And sit back and relax.

hot_rod

jad3675 said:

I have three ecobee 3 lites, 4 Caleffi z-one valves and a ranco aquastat hanging off my Caleffi ZVR104 panel with a single transformer. I've never had an issue.

The nice thing about Caleffi zone relays is you can plug in an additional 40 va transformer They wire in series so you get 80 va across the zv connections. Molex plugs and a auto reset circuit breaker protection for both transformers

jad3675

hot_rod said:

jad3675 said:

I have three ecobee 3 lites, 4 Caleffi z-one valves and a ranco aquastat hanging off my Caleffi ZVR104 panel with a single transformer. I've never had an issue.

The nice thing about Caleffi zone relays is you can plug in an additional 40 va transformer They wire in series so you get 80 va across the zv connections. Molex plugs and a auto reset circuit breaker protection for both transformers

Yeah, that's why I went with it. Well, that and I'm a sucker for Italian engineering.
I figured the panel was rated for 5 Z-ONE valves. At most I'd have three open at one time (HW priority being the other). The Ecobees are 2A each and the Ranco can't be much more than that. But yeah, for peace of mind I'll be ordering a second transformer.

John

Jamie Hall

hot_rod said:

jad3675 said:

I have three ecobee 3 lites, 4 Caleffi z-one valves and a ranco aquastat hanging off my Caleffi ZVR104 panel with a single transformer. I've never had an issue.

The nice thing about Caleffi zone relays is you can plug in an additional 40 va transformer They wire in series so you get 80 va across the zv connections. Molex plugs and a auto reset circuit breaker protection for both transformers

Um... @hot_rod , are you sure you meant your transformers wire in series?

EBEBRATT-Ed

You can do whatever you want with a smart thermostat. Will it work on your specific system?

Maybe

Maybe not

Hook it up with it;s own transformer and relay and it will work 100%

You can try any # of grab bags of tricks, maybe , maybe not