Honeywell Smart Thermostat Wiring Question

Steam_Guy56

Hello All,

Background: Over the winter I bought one of the honeywell smart thermostats in an effort to fix the short cycling of the boiler (combined with an uncalibrated PA404). I got the system to work, but when the low water cutoff comes on or the vaporstat cuts out the burner, the thermostat shuts off because the R wire (power) is connected to the hot t1 terminal.

My question is: how should I wire it so that the thermostat remains powered when the low water cut off comes on or the vaporstat cuts out, without bypassing those safety features?

I have attached pictures of the cyclegard wiring diagram, boiler diagram, the thermostat connections, as well as the boiler model information for reference.

ratio

You didn't say which Honeywell stat you have, but the easiest solution would be to install batteries int the stat if it will take them. What's going on is that the safeties are wired into the circuit before the power goes to the thermostat, so when one of them opens it kills the power to the stat as well. The batteries would allow the stat to remember what's going on until the power came back.

If batteries aren't an option, you will have to do one of three things:

1. rewire the controls so that the safeties are after the stat (not recommended)
2. power the stat from a different, constant, source and add an interposing relay to control the boiler
3. if the stat has separate Rc and Rh terminals, and there is an extra wire from the boiler to the stat (seems like there is), you can do the equivalent of 2. above without an additional relay. But it depends on the stat.

Steam_Guy56

thanks for the ideas,

It is one of the T5 thermostats, which doesn’t seem to take any batteries.

I was thinking that a relay might be the best option, but I wasn’t sure which one to get. Would this relay work: https://www.grainger.com/product/6CWY7?gucid=N:N:PS:Paid:GGL:CSM-2295:4P7A1P:20501231&gad_source=1&gclsrc=ds

I have previously done the 3rd option, but after connecting the air conditioner to the same thermostat, I ran out of wires.

ratio

Looks like it would, all you need are a 24 vac coil & contacts rated for pilot duty at 24 vac.

I'd probably use a https://www.functionaldevices.com/product/ribu1c/ instead, for ease of mounting.

Steam_Guy56

Gotcha, I ordered the ribu1c.

From the wiring diagram on the side of the relay, I was planning to connect the white/blue wire directly to the 24v transformer and then the thermostat white wire to the relay’s white/yellow common. And then connect the yellow and orange relay wires to the t1 and t2 terminals on the boiler.

The red Tstat power wire I was also planning to connect to the transformer. Does this look correct?

ratio

You're correct with the yaller & orange, they'll go to T1 & T2; however, the thermostat side of the relay will be a little more involved. First, take the white and red off the T1 & T2 terminals and the blue wire off of the transformer on the boiler. That transformer isn't going to feed the stat any more and interconnects between transformers can cause problems.

It sounds like you have a separate transformer for the cooling side of things, does that system have an extra wire running to the stat that could be repurposed as a common/C from that system? If so, put the Rc-Rh jumper back in & connect the cooling-side common, along with the blue heating common, to the C terminal of the stat. Then connect the blue at the boiler to the white/yaller of the RIB, and the white/blue of the RIB to the white at the boiler. Done.

If you can't use the cooling transformer to feed the stat, then you'll need to add a new transformer (somewhere near the boiler to make the wiring easy), fed from a constant-hot circuit. Then just take the blue that was on the boiler transformer to the C terminal of the new transformer, along with the white/yaller from the RIB, the red wire to the R terminal of the new transformer, and the white to the white/blue of the RIB.

The transformer should be 40VA, larger would work (cost more), but I wouldn't go less. 24 VAC secondary of course, and primary whatever voltage you find (probably 120, but 240 if you want!)

With RIB relays, I sometimes take the back off them, pull the circuit board out, & cut off the unused wires (blue NC & white/black 120V coil) to keep from accidentally using them. Probably voids the UL listing though. Be careful stuffing them back in, the LED has a tendency to get misaligned.

Steam_Guy56

Thanks @ratio, lots of good information I didn’t know. I am not sure how familiar you are with the T5 stat, but it doesn’t exactly have a Rc Rh jumper. It has this plastic tab that flips up and down for “2 R Wires” or 1 R wire. I assume there is some kind of jumper inside of it, but I haven’t taken it apart. Currently I have the thermostat wire for 1 R wire, and a shared common between the air conditioner and the boiler. Everything is working at the moment, just that the thermostat shuts off when the power to the T1 terminal is cut off by the low water cutoff. I had it set up this way so that the thermostat could still call for heat from the boiler. But the relay will solve that issue.
I think switching the wires to come from the air handler like you said will simplify everything. Thanks again for the advice and clarification.

ratio

Ok, I haven't had a T series in my have in a while. I'm not 100% sure with that stat, but other Honeywells are powered from the cooling R when they are set up for 2 transformers. If that's the case, when the thermostat is set to "2 R wires", the heating R and W are just a set of contacts & won't care if the power to them goes away. That would be the simplest solution.

EdTheHeaterMan

here is a diagram for using a smart thermostat and a RIB relay in this discussion

https://forum.heatinghelp.com/discussion/comment/1784463#Comment_1784463

EdTheHeaterMan

This is what you actually do as long as your posted diagram has not changed over the years.

The problem with steam boiler wiring for smart thermostats is that the thermostat R terminal must be connected to the R terminal of the transformer. Steam boilers rarely have the thermostat R connected directly to the transformer.

When using the isolation relay, you must select one of the transformers as the R wire for the thermostat. In your case that would be the brown wire. That brown wire MUST be connected to the R on the smart thermostat. The common or, White wire in your case, must be connected to the C on the thermostat and must also be connected the the Common of the relay coil, that is the yellow/white wire on the RIB. And the W on the thermostat is then connected to the blue/white wire of the relay. It is important to follow this correctly, for 2 reasons.

1. R on the thermostat and C on the thermostat must be connected in the correct way to the transformer.
2. If you reverse R and C just right, you will let the factory installed smoke out of the transformer.

And just in case you are afraid of that you can install the "Smoke Keeper" shown below.

This is a joke, I really don't sell these. Just buy a fuse holder from the auto store and a package of 3 amp fuses.

EdTheHeaterMan

this is an alternative wiring connection close up

clammy

Usually i just use a rib relay and a separate transformer .It makes it easier for any greenie to show up and trouble shoot it without have to go over all the wiring another easy alternative is to use a taco circ relay ,it helps dummie it all for the rest of us who really dont want to waste our time on a service call over a smart stat not powered correctly and the issues that come w that . The advantage is that there is no altering of the manufactures safety wiring which would be cya should there be issues. I usually look for the easiest way and one that does not over complicate wiring being a lot of installers have issue w wiring controls and some times need others to explain and go over the theory of opposite potential so they can see it in there minds eye , Just because you can figure it out doesn't mean the next guy will be able to in a timely manor without handing the customer a hefty bill. Plus when there is a issue you jump your isolation relay and if see fires up if not it's on the stat end ,trouble shooting 101 ,deductive reasoning ,sequence of operation ,opposite potential, a decent electrical meter and a open mind these are a trouble shooters problem solving tools . Make things easier for others not more difficult or worse over complicated .

i used to work w a group who would do all sorts of crazy silly wiring garbage and when i would question and call them out on it they would state that they do it this for job security but the funny part was they never would return for either future maintenance or repairs if needed so there was no job security and when a issue occured it was awesome to watch them try to figure out the there own birds nest of wiring w zero wiring markings or zero sense .

Be kind always kiss # 1 rule .

peace and good luck clammy

EdTheHeaterMan

@clammy, that is a good rule. however that may cost the customer the retail price of an SR501 relay that is 3 times the cost of a RIB, and most of us that do control wiring understand how the RIB works. although that product became popular at the end of my career, it is a simple device, easy to understand, and does not use much current from the transformer, so it is easily added on to the existing control transformer in most cases.

The diagram I posted above has the same issue if you use a separate control transformer or the existing transformer. R from the thermostat MUST go to the power side of the transformer usually marked R on the transformer. and both the relay coil C and the C from the thermostat MUST go to the C on the transformer. leaving the W on the thermostat to go to the other leg of the relay coil.

No limit or controls wiring is changed in the operation of the steam boiler. The Pressure controls, the vent damper, the LWCO, and spill switch and roll out switch are still in the factory original configuration.

Regardless of the relay used to isolate the thermostat, the change should be made to the wiring diagram attached to the boiler, and to the manual. That way the next guy in can easily find the colors of the wires and the control logic for diagnosing problems.

Steam_Guy56

Here is the update:

I tried the wiring suggested by Ed the heater man first and the relay started flashing; as apposed to remaining constantly on during the heat call. The boiler was firing, but I could hear the relay clicking at 60hz, so I thought that wasn’t a longterm solution.

I then rewired it using the airhandler’s 24v transformer for power and wired the yellow/white common back to the thermostat blue common wire and that solved the flashing issue. Here is a picture of the boiler wiring.

IMG_6099.jpeg

HVACNUT

You could've powered the thermostat with Rc and C, and switching from Rh and W.