Nest Thermostat with Taco Zone Sentry Valves - 2 wire

robg1987

Hello!

I am hoping someone might be able to help me. I just bought a new (old) house that uses an oil boiler for baseboard heating with 3 zones. The zone valves are Taco zone sentry Z075C2-1 valves. The thermostats are all honeywell mercury thermostats that I am guessing are original to the house (1977). All three thermostats have only two wires behind them - red and white.

I am looking to replace these thermostats with nests (the newest cheaper model) and have been researching today for hours on whether or not it will work with the wiring I have in place. I have read that it is necessary to add a C-wire. I have seen other answers say to put a resistor in somewhere and I have seen an option to add a transformer and plug it into the wall for power. The transformer route is a no go, as there are not outlets close enough to the t-stats to make it viable and frankly I don't want wires hanging out of them.

I see other posts where people have some kind of a relay board for the zone valves - I do not see anythign similar to that in my setup but I am also not very knowledgeable in the HVAC world, especially in baseboard type heat. From what I can tell, the three valves run to a transformer and the thermostats and that's it.

Any help is appreciated!

Thanks,
Rob

Jamie Hall

First off, what is wrong with the mercury thermostats? If they really are mercury T87s, they are as close to a bulletproof reliable thermostat as you can get.

OK, I get it. They aren't glitzy.

Don't try to run a Nest without a C wire. While there are workarounds which sometimes work, in general, no, sorry, they don't. What you will need to do, if you don't want to run a wall wart for each Nest, is to determine which terminal on that transformer on the heating system is the common hot power terminal, and run a wire from that to each of the Nests, in addition to the R and W wires which are already there.

You don't need an extra relay board. That wouldn't help matters anyway.

Then when you get the Nests powered up, be rather cautious about programming them. The Nest is a very good thermostat -- for forced air heat. It's at best so-so for hot water baseboard type heat, and you will find the best results will come if you set it properly (I believe the setting is radiant, but I'm not sure) for hot water baseboards, and then defeat its "learning" and "occupancy" sensing so it doesn't try to ramp the heat up and down all the time, though you can allow it to do an evening setback if you like.

HVACNUT

You state "the cheap model Nest". I don't know which model that is but I recently installed 3 Nests for a client that came with 3 AA or AAA batteries so they didn't need 24v from the equipment. 
I don't know the model but I know I could put one "top shelf" from the blue line.

geno907

the Wc connect will need to be connected in the nest thermostat.

pecmsg

I'll take those old T87's off your hand!

robg1987

Thank you all for the comments thus far. I went back into the basement last night and pulled the thermostat wiring further out from the wall to find that I DO have a third wire in the bundle going to each thermostat - both in the basement near the boiler and at each thermostat.

There is a mess of wiring between the zone valves, thermostats, and 24V transformer that I need to decode, but where would I need to put the third wire downstairs to use it as the C wire? The "end switch" side of the zone valves each have a red and white wire, which are tied together with wire nuts and then the red and white go to a hydrostat box on the boiler. The C and W/Y side of the zone valves each lead out to the mess of wiring that seems to connect to the transformer and thermostats, but that's the piece I need to clean up a little. Don't know if that helps enough to give me an answer but that's what I've got for now.

Thanks in advance!

As a note, the old thermostats are T822D1008

Jamie Hall

Those old thermostats are very high quality thermostats and, if properly adjusted, do an excellent job of providing even stable heat. They will have been doing a much better job than the Nests ever will in that regard, but so be it. That's your choice.

Now to wiring... the first thing you need to do is to tidy up the wiring downstairs, at least so that there are no tangles or loops or neatly wrapped indecipherable bundles. The next step is to make a little diagram of how the wires are connected to what.

The end switches on the zone valves are usually plain vanilla switches -- if there are four terminals on the zone valves. The other two terminals provide power to operate the zone valves, and it is those two terminals that you are interested in.

robg1987

So bear with my on the drawing...I'm an accountant by trade.  In short, the C terminal on the zone valves connects to the white wires leading to the thermostats. The W/Y terminal on the zone valves connects to the C terminal of the 24V transformer. The Load terminal of the transformer is connected to the red wire for each thermostat.

Jamie Hall

Do you have a way of measuring the voltage on the red wire at the thermostat and the C terminal of the transformer with the thermostat off? I'm not the real expert on how to wire these things, but I suspect that if you go from C on the thermostat to C on the transformer you may be good to go.

HVACNUT

The drawing looks right.
The third wire connects to C on the thermostats and C on the transformer. 
Just make sure as the drawing shows, R from the transformer to R on the thermostat.