Adding C-Wire to old 2-zone boiler setup? - Honeywell R845A's - This ideal?
I want to add smart thermostats to an old 2-wire boiler setup that has 2 zones currently. Right now I'm running non-smart thermostats with only 2 wires to each thermostat setup. I ran a new 6-wire line up to the thermostats so the extra wire is run. Now, I need to figure out how to patch into this system.
Each of zones has a Honeywell R845A switching relay that does not include a built in Common Wire hookup.
The new wire with plenty of internal wires is run, so that part is done. Now, I need to figure out how to power this. I know there are a few options, buy a separate old school style transformer to feed the current, or, they sell a simple 24V power transformer that plugs into any old outlet and gives the common power line. Then I got looking around and see someone out there went ahead pulled the common wire power from the R845A directly. I saw somewhere else the output of this internal transformer is on the lower milli-amp side, so maybe it is inadequate? Seems like a few people have commented and are doing this successfully.
Instructions on pulling common power directly from R845A
24 Volt Transformer C Wire Power Adapter:
Thermostat May/May Not Require C-Wire
I've read some smart thermostats need common power, others do not. I was considering a dead simple (With Basic Wifi Capabilities) thermostat that says it requires a C-Wire:
Honeywell Wifi Thermostat
Other Ideas?
Pull directly from the R845A? Grab a simple plug in adapter and just use that for ease of install and not pushing the R845A to do what it wasn't designed to do? Something else? All ideas are welcomed.
All tips appreciated.
Comments
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Yes , you can fine the common … But ..You will also need to fuse the circuit now with a 3A. .. I would recommend just up grade to a newer relay , with a common terminal and fused ..
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Hello HandyFS,
As you can see from your research there are many ways to get it to work with various equipment and methods, some better than others for various reasons. Also some thermostats seem to be more problematic than others depending on the method used.
At the end of the day you need to choose the way you do it, so you understand it, build it and it works properly and you can maintain and/or troubleshoot it should it fail.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
You can try any of those plans. If your electrical soldering skills are good, then your lowest cost option is the addition of the C wire to the back of the control you have. If all your wires come from the top or all from the bottom, then you may not need to remove any wires to expose the back of the control to access that C terminal on the transformer. Then you must rename the T on the left to W and the T on the right R, (unless the control is upside down then it the other way around). If you don't rename the T T and you mix up the R and the W from the new thermostat, then it will not work.
If you don't want to do all that soldering and you just want a control that has the C wire already there for you, you can get a Taco SR500 series control. https://www.supplyhouse.com/Taco-SR501-4-1-Zone-Switching-Relay One control will do all your zone circulators. Saving you on purchasing several SR501 to replace each R845A . Like if you have 3 zones you can get a SR503 and connect all the thermostats and all the zone circulator pumps to one box.
I like those multi-zone boxes because it cleans up the wiring and puts diagnostic lights on to help troubleshooting.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Thanks to you all for the feedback so far, all is helpful in helping me narrow this down.
I started going down the path of the Taco's and see they have a 2 zone setup. My system actually has 3 zones now that I think of it, I have a main controller that's also controlling an indirect water heater, and then the 2 separate R845A's for each of the 2 heating zones. Since I eventually may be replacing this entire system, I'm trying to refrain from getting a better controller and the time invested to do all of that since this entire setup may be changed eventually.
If I go the route of soldering in a wire on the R845A's, do you think that internal transformer gives enough power to sustain a thermostat? I've seem some people say it will need more, but there are people on the web doing it successfully as is after adding that common wire so I suppose I may just try it. Like @Big Ed_4 said, would I also need to add in some type of fuse into the mix?
If I decided to just get one of these block transformers that is basically just a plug in transformer with 2 wires, could I just run that directly to the thermostat and power it that way on the R and RC connections, without touching the boiler controls at all? Or must I still blend in some type of relay or fuse? If this is the case, this may be the most simple way so I don't have to go touching anything else, and can just leave the current 2-wire boiler controller as-is, and just separately run up the C Wire (C and RC) directly from a transformer to the thermostat without anything else.
Is this ideal directly from the power block to the thermostat? Is a fuse or relay still required? This image below shows what I'm thinking.
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The block transformer is the better way to go in my opinion. You will not be messing with the internal wiring of the Honeywell control.
As far as the amount of power needed to operate the logic board (or CPU) of the thermostat, that transformer should be fine for operating the relay coil and the thermostat at the same time. The low cost transformer they use in that control may have varied over the 80+ years that control was made by Honeywell, but I think you will be just fine if you go that route.
Remember that some of the folks that so these things are not the best at following instructions sometimes. that can account for the ones that fail, then post about their failure. And it's never their fault, the instructions they didn't follow correctly were the reason it didn't work.
How confident are you with your knowledge of basic electricity, and your ability to follow instructions?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Thanks Ed. All good info and knowledge, I appreciate it. I break out my soldering iron every once in a blue moon, but it does come out and I have a decent understanding of electronics so I could get in there and get that wire soldered on. I have to check, I may even have an old R845A from when a relative switched their machine over from oil to natural gas, so I could always do the modifications on that one and swap it in.
BigEd mentioned I'd need to fuse the circuit with a 3amp fuse, would I just put an in line fuse between the R845A new added C-wire coming off the transformer and the thermostat? I haven't seen anyone mention a fuse and most guys seem to just tap that extra wire and are off to the races. I could easily blend in a fuse if that's ideal.0 -
You would need to fuse the transformer that powers the thermostat .
There was an error rendering this rich post.
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A 3 amp fuse would be used on a 40VA transformer. I think you would be better off with a 1 amp fuse. here is the location forthe fuse in this illustration.
R would be the T on the left if the T T terminals were on the top of the control as shown here.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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see the attached this is my way and its bullet proof and fairly cheap.
You can use 1 40va transformer for both thermostats. Ditch the old honeywell relays the transformer is not large enough. Wire the rib relay to the start stop circuit on each zone. So you need 1 transformer and 2 rib relays.
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On the fuse. Do you NEED a fuse? No, you never "need" a fuse — if the circuit is overloaded, it will just burn something up or out. Do you WANT a fuse? Yup. If something goes amiss it's a lot easier to deal with than a fried board, mtor, house, whatever…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
Looks like three Ed's are not better than one.
@EBEBRATT-Ed says don't solder. Buy new parts
@Big Ed_4 says to solder and use a 3 amp fuse
@EdTheHeaterMan says use a 1 amp fuse
I guess you will just have to switch to Gas and spend $$,$$$.00 on a complete new system.
🤣🤣🤣🤣🤣
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
1
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