Installation of isolation relay and C wire

zvalve

Any one know of a good online step by step resource more or less on how to install an isolation relay and c wire so a smart Tstat  can be installed. Have a gas fired boiler hydronic system with two zones. From what I understand a new c wire or thermostat wire has to be snaked down to relay on boiler. Need an online hvac course since I have so many questions

Alan (California Radiant) Forbes

It's not step-by-step, but these links may help.

http://www.taco-hvac.com/uploads/FileLibrary/102-397.pdf

https://forum.heatinghelp.com/discussion/145943/thermostat-c-wire-to-com-on-taco-sr502

HomerJSmith

At your thermostat, check the wires and see if you have two or three wires. Why do you think you need an isolation relay? What kind of boiler control do you have? How are you controlling your two zones?

Steamhead

Also need to know the make and model of your boiler, so we can see how it's wired.

zvalve

Steamhead boiler is a gas fired Dunkirk PVWB-4D and photo of it follows with wiring.
Homer there are 3 wires at thermostat green, red and white but I don’t think green is used since it’s a heat only system so green and white were on W terminal and red on R. Techs at Dunkirk on phone said if you want to install a Nest thermostat you need an isolation relay for it. Not sure what you are asking about boiler and zone controls but Boiler control is an Aquastat and zones are controlled by zone valve, see photo

HomerJSmith

If you have 3 wires at the thermostat, you don't need a isolation relay. The green wire would be the C charging wire that connects to one side of the transformer, the other end to the Nest C. The red wire would connect to the Rh at the Nest, the white wire would connect to the W1and the green wire would connect to the C. At the transformer the red wire would connect to the 24 V which goes to the thermostat, the white wire lead from the other 24 V at the transformer to the relay in the aquastat that turns on the boiler, then to the W1 connection at the Nest. That would be a circuit, transformer to the thermostat to the relay back to the transformer. The green wire to the thermostat is the charging wire to the Nest and connects to the C and goes back to the transformer's white wire connection. That's a circuit, transformer 24 V lead to the Nest C back to the other transformer 24 V lead. That's how it works.

I don't know if that transformer on the J box in your photo is running everything. It look pretty hefty at least 40 VA and should be able to run everything. If so, that would be where the 24 V green wire to the thermostat C would connect.

Steamhead

Looks to me like the transformer powers the zone valves. @zvalve , do the thermostats control the zone valves directly, or is there a panel/relay box that controls them?

rsilvers

If you have two wires and can't run a new wire, then this:

https://www.supplyhouse.com/Fast-Stat-FSCM00-Common-Maker-Thermostat-Wire-Extender-Adds-a-Common-C-Connection

HomerJSmith

How do you wire up a isolation relay and what does it isolate? Did Dunkirk tell you?

zvalve

Don’t know and don’t know and Dunkirk didn’t say why or how they just said if you are adding a c line for smart thermostats you need one. My knowledge here is rudimentary and I am learning the basics like what is a relay?

HomerJSmith

Is your boiler a standing pilot or an electronic ignition (intermittent ignition)? I'm thinking standing pilot.

I am wondering about that transformer on the 4X4 Jbox under the red switch box. What does it power? I see 2 zone valves, do you have one thermostat controlling two zone valves or two thermostats, one for each zone valve?

I assume that the aquastat is a Honeywell L8148E 1257 ? What is the model # of your gas valve?

As a side note, you are actually pumping into the expansion tank instead of pumping away from it.

zvalve

Homer 1. Boiler is a standing pilot
2. I believe the transformer is powering the zone valves through the thermostats  and perhaps the aquastat but I don’t know if aquastat has its own power source
3.I have two zone valves and two thermostats , one for each zone valve
4.Yes the aquastat is a L8148E
5. silver base of gas valve has VRS200A 2264 on it but there is also a tan plastic piece above it for the pilot and thermocouple i believe which doesn’t have a model number on it.
6 I believe the sequence is return-circulator-boiler-output with aquastat on it- expansion tank-zvalves- zones

HomerJSmith

I'll try and work out a wiring dia for you in a couple of days. The transformer in the aquastat operates the two pole relay which turns on the circulator and low voltage burner circuit.

The transformer (40 VA) operates the thermostats and zone valves. With 3 wires to the thermostat, that's all you need. The zone valves turn on the boiler thru the aquastat relay.


Per convention, the red wire is connected to Rh, the white wire to the W1, and the green wire to the C (charging connection) at the thermostat.

At the transformer, the red wire is connected to one terminal on the transformer to the thermostat Rh, the white wire is connected from the other terminal on the transformer to one red wire on the zone valve. The other red wire on the zone valve is connected to the white wire to the thermostat W1 connection. The green wire is connected from the thermostat C connection to where the white wire is connected at the transformer.

The green wire doesn't have to be connected at the transformer white wire, per se. It can be connected at the ZV red wire with the white wire that comes from the transformer.


The yellow wires at the zone valve are connected to the TV & T connections on the aquastat.

And...that's how it's done in the big city.

zvalve

Thanks Homer, learning while doing and making mistakes. On one thermostat coming out of wall  I have a green,  white and red wire. There is 27.8 Vac across green and red,  2.8 Vac across white and red and 1.4 Vac across gr and white. Don’t see any green wires down by boiler? Zone works with a good thermostat and red on R and green on W or green and wht on W. I think Green and wht were originally on W terminal in an old programmable thermostat’s wall plate so I kept it that way not really knowing what I was doing.

HomerJSmith

Here is a diagram of the wiring.

HomerJSmith

Between Red and Green, the voltage should be 26+Vac at the thermostat location.

Between Red and White, the voltage should vary based upon the resistance of the windings in the zone valve motor, but much less than 26 Vac at the thermostat location.

Between White and Green, the voltage should be 0 Vac at the thermostat location.

These voltages are base upon my drawing.

Some times a tech will cut off the green wire if it isn't being used. Look for it at the thermostat wires at the boiler area. You might have to skin back the thermostat cable sheathing to find it.

The third green wire is essential for a charging thermostat to function correctly. Otherwise, a different strategy must be used.

Jamie Hall

As a slight addition -- only the red to green voltage is really reliable. There are too many other things -- including your meter -- which can affect the other two. That said, when the thermostat is calling for heat -- but only then -- red to white should be 0, and red to green should stay at about 24.

zvalve

Thanks Homer and Jamie I am starting to understand. Applaud your generosity of knowledge. Homer you’re the best.  How would you clean up the chaos of wiring at the zone valve? A terminal strip?