Adding C Wire with Hydrostat 3250 Plus and external transformer
There is a previous thread covering almost the identical setup I have. (See link below)
I am looking to add a smart theermost which requires a C wire.
Fortunately, the wiring for my mercury Honeywell dial is three wire already, with red/white/green, with the green being unused.
The attached photo shows the box where my two zones come in, are mated with the wiring from the transformer and Hydrostat 3250, with red/white ultimately going to their respective T terminals inside the 3250.
The teal line I added to the picture indicates where the C terminal on the transformer (white) is mated to the white wires from my Honeywell, then to Taco zone control units.
My question is, can I simply tie in the unused green wire to the white bundle pictured?
This is a heat only system with two zones. I am only going to install one smart thermostat, because the upstairs only has two wires to begin with. House was originally built in the 50s as a one story home, second floor was added early 70s so not surprising that wiring is different.
There is a separate issue that I will start a new thread if needed if a new thermostat does not solve the problem.
Thank you in advance for guidance
Comments
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As long as your wiring is consistent throughout the control side of the zone valve, and the colors don't mess you up, what you purpose should work. The other thread went into detail, so if you understand what all was going on there you probably have this under control.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
I don't know, @109A_5. I'd be careful there.
You don't want to add your "C" wire to any existing bundle. You want to find the third wire (unused) that goes to your thermostat, no?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
@MarcTheComputerGuy , what model zone valves do you have?
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
OK, OK, I see this as an easy change. Just working with the information given. Zone valve type would help. If given new information I'll try to work with that.
Original IMO;
Changed to this, easy if the extra wire through the building already exists.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System
2 -
If the C wire is the same at the bundle as at the transformer what difference does it make ? I'd simply add (connect) it where convenient.
The circuit above is functionally the same as the the circuits below.
Or this
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Forget the C wire. Why is the aquastat mounted to a removable panel for clean out and chamber access? The sensor wire and a ground wire running through the tridicator opening? Someone wasn't thinking.
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Ummm… @HVACNUT I think that other thread was just an example that @MarcTheComputerGuy found an is using for a reference. That is not the boiler in this thread. You should post that comment on the other thread.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Taco 550-050 24v 0.9a
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@109A_5 I see now what the OP wants to do. I misunderstood. Yes, adding the extra t-stat wire to the white bundle will provide the needed “C” supply.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour
Two btu per sq ft for degree difference for a slab0 -
Whoops. Sorry. Wrong guy.
0 -
Taco 550 24v 9a
As you can surmise, since the line from the up stairs already brings an unused wire into that box of wires, I could just add to the white bunch as pictured with the bit of length left -about 2 inches of green wrapped around the ends both upstairs and in the basement.
Logically it seems doable. What I don't know is if, at a certain point, does the number of wires twisted together create any sort of resistance that would drop voltage, or create any other type of interference. I other words, would it drop the 24v needed by the existing 2 Taco valves in place? I don't *think* there is an issue of amperage as the smart units typically draw under 1 amp.
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If you're using typical thermostat wire and don't have a crazy amount of connections it will be fine. Where wire size, wire length, connections become an issue is with millivolt systems, yours appears to be a 24 VAC system, so much more tolerant.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
Two ways of looking at it.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
CAUTION: The wiring diagrams above can burn out your transformers if you get the wires crossed. Essentially they connect two transformers- the one powering the zone valves and the one in the Hydrostat- together. As long as they are "in phase" this would probably work, but if they're out of phase they will quickly burn out. Why take the chance?
The solution is an "isolation relay" which separates the two circuits. Using the above diagram from @109A_5 , the relay would go where we see "To "T" terminals on boiler control relay". This pair of wires is actually sending 24V out from the ZV transformer.
Any convenient relay with a 24V coil will do. I like the Honeywell (might be called Resideo now) R8222D.
Wire the Coil terminals on the isolation relay to the pair of wires coming from the ZVs. Wire the TT terminals on the boiler control to a "Normally Open" contact on the isolation relay.
When a ZV opens and makes contact #3, the 24V will pull in the isolation relay. This closes the Normally Open contact and tells the boiler control to start the boiler.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
If you just add the wire for the C wire and nothing else has or needs to be changed (the circuit worked before). The existing R wire connects the thermostat's R to one side of the transformer. The added C wire connects the other side of the transformer to the C on the thermostat providing continuous 24 VAC to the thermostat to power it. Nothing else needs to change.
However nothing wrong with adding an isolation relay if desired. However technically it is not needed, and with this configuration the transformer phasing is irrelevant. Adding the isolation relay does require one side of the relay's coil to go to the R terminal of the transformer since the C side of the transformer is switched by the zone valves 'End Switch'.
Per the Taco documentation;
Where issues come in is where unknowingly one side of each transformer is bonded to an equipment ground providing a path that is not realized.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
@109A_5 , unfortunately, out here in real life it's way too easy to get the wires backwards, especially at 11:30 PM on a Friday night. So each of these setups should have an isolation relay. It's cheap insurance.
Another way to do this is by using a "fan center" such as the Honeywell/Resideo R8239A1052:
This mounts on a standard 1900 box, and includes the transformer and relay in one unit. You'd connect all the terminal 3 wires to the G terminal on the R8239, so any one can pull the relay in. The NO contact on the relay goes to TT on the boiler. And the relay itself is replaceable.
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
@Steamhead I'm not against using a relay for the reasons you mentioned in your last post, all I am saying is it can work without it, satisfying the original question.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0
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