Nest Thermostat and Taco Zone valve z075c2-1?

Endlessknight

Greetings,

Back in 2012 I installed a first gen Nest in my condo, two wires R and W. It worked reasonably well for years then in 2016 we changed the boiler to a Buderus Logano G215 with Taco zone valves (model z075c2-1). The wiring to the thermostats were not changes, same two wires. After that I started experiencing an issue where the heat would come on in my unit without my Nest calling for it. Since the Nest didn't turn it on it would stay on until I manually closed the valve or tricked the Nest into turning it off by setting it at temp higher than what it was in the unit then turning it back down again.

At first first I thought my Nest was bad so I replaced it with another, I think it was a 2nd gen. But it had the same issue. So I went back to a Honeywell rth5100b1009 and that's what I've been using for the last year.

Anyone know of a solution to this issues? Would one of the new Nest, 3rd gen or E, work? If not is there another wifi thermostat that would work?

Thanks in advance for any help/info offered.

ratio

The Nest (and other power stealing stats) trickle a small amount of current through the heating wire to charge their internal batteries. It sounds like the new electronic controls are sensitive enough to occasionally see this as a call for heat. The best fix is to add a third wire, called the common wire, to allow the stat to charge it batteries without interfering with the heat calls. If that's not an option, there are several other things you could do, bearing in mind that the WiFi radio nearly always requires the common wire.

A Honeywell thermostat with RedLINK wireless communication can be run with AA batteries and coupled with a RedLINK gateway plugged in to your home router will give you WiFi access through the Honeywell Total Connect Comfort app. There are add-a-wire devices that may allow the wires to be used for power and heat at the same time. You might be able to find a WiFi thermostat that runs on batteries, but I am unaware of any that do.

HVACNUT

Like @ratio said, with only 2 wires, your Nest is a power burglar.
If you have an extra wire, connect to C on the stat and C in the Taco zone board.

If not, you can put a resistor between the thermostat wires and the board terminals.
I had that trouble on an EK once, and it
drove me nuts (constant SH circ) until I placed a call to my trusty EK Territory Manager. (Thanks John!). The resistors were at my shop the next day. How's that for service?!

I don't remember the part number, but I'll be going through my stock tomorrow and if I come across one, I'll post back.

Endlessknight

Thanks the input. Unfortunately running a third wire is not an option. That would require a lot of fighting with the HOA.

I used the Nest for 4 years on the old two wire system and the WiFi worked fine. Any chance the new Nest E, which I believe has a lower power requirement, would work in my situation? What should I search for to find one of those "add-a-wire devices"?

Thanks

Endlessknight

Think I found what I need: link

HVACNUT

What is the operating control? Typical aquastat or Logamatic 2107?
What is the 24v power source for the zone valves? Direct from a transformer, or ZV zone board?

ratio

The Venstar power supply looks like it would solve your problem. Good find.

HVACNUT

I don't know if that's what @ratio was referring to.
That kit looks like it only offers additional switching, not a common, but not sure. It is hard wired via transformer to a 115v outlet, so maybe it picks up the neutral?

Also, that's exposed wiring. Is the cord 5 feet long to mount the stat 60" from the floor?
Is there a convenient 115v outlet near the stat? If you have to change its location, no exterior walls, and leaving it on the coffee table is no good either.

Just saw your post @ratio. (Had to feed the dog).
Does it use the neutral?
And it is exposed wiring, correct?

ratio

The wall watt powers the thermostat. The relay provides isolation between the wall wart power & the boiler power. It's likely that the thermostat can be wired in a way that the relay is not required, but having the relay in there won't hurt in this case, and there are certain applications where it would be required.

The power cord will be exposed. Maybe some of that l'il plastic cord hide that the big box stores carry will pretty it up.

HVACNUT

Sorry @Endlessknight. I don't have any resistors in my truck stock.
I'm sure you can call Buderus, Nest, or Taco and they'll let you know what you need. It's not uncommon.
Better than exposed wiring IMO.

Endlessknight

Thanks guys for all your help. Just bought a Nest E, it using less power that a standard Nest. Going to try that with running at extra wire. If that doesn't work going to go to a 3rd gen Nest with the Venstar power supply. Found good instructions here: Link

Will update just in case others are following along and dealing with the same issue.