new model taco sr504 exp 4 only firing up when storage tank calls for hot water

emgroom

Hello All,

just recently moved into this home and as it got colder in NY, started to use the heat more. the transformer on the old taco unit managed to short and burn itself out so i replaced with a new one (could not find the same model) . was able to wiring everything exactly how it was in the old model but came across 2 wires that was connected to a dedicated 24vac terminal that doesn't exist on the new model. the wires run power to the low cut off valve (mcdonnell miller rb24e 24 v) and complete the circuit. technician came and wired it to zone 4. now i'm not sure if its the wiring or the settings on the dipswitch that is causing the problem i have which is the boiler doesn't fire up unless the storage tank calls for heat. when area zones call for heat nothing happens. red light comes on and circulators are running but no fire. i currently have the dip switches all at the factory settings. i did notice that when i use the reset switch and then put it back to normal operation it fires up the boiler to satisfy the zones call but after that it reverts back to the same issue. if anyone can please help me out i am eternally grateful. there are pictures attached. in there you can see zone 2 calling but nothing was happening.

Jamie Hall

The technician wired the low water cutoff to Zone 4? And he wonders why it only works when Zone 4 is calling?

Good grief.

I'd need a complete wiring schematic to see exactly what should go where -- but the low water cutoff is part of the boiler safety and control circuits (there are -- or should be -- other safeties in there), and should have noting to do with any of the zones individually, although it is possible that their calling circuits are all wired in parallel with the boiler...

Somebody needs to go right back to square one and make sure that everything is wired as it should be. Don't assume that because a certain wire went to a certain terminal on the old controller that it goes to the same one on the new one.

STEVEusaPA

Shouldn't you have had a tech come over and figure out why it shorted in the first place?
I don't even know how @Jamie Hall saw the wiring mistake

emgroom

this is what i was able to pull up from the taco site. the tech came and looked at everything and appeared to think that everything was fine except the power going to the low water cut off.

emgroom

those 2 wires that don't have a home on this board is tapped into the transformer on the boiler itself. this was how it was wired when i came into the home and is the first time i've ever opened it up to take a look. per this schematic it's showing for a 2 boiler setup which i don't have, this was the only one i could find with the pc702 and sr504 exp 4

Jamie Hall

If you think about that wiring diagram further, you will note that the sole purpose of the switching relay board is to control the circulation pumps. Except to send a signal to the boiler to run if the priority zone is energized, it has no other purpose.

The priority zone -- your zone 4 -- will have power on it only when zone 4 is calling; it's not a common 24 VAC output. It's not entirely clear what the terminals labeled n/c for Zone 4 are, but conventionally this would be a pair of terminals which were powered only when Zone 4 was not calling.

The boiler low water cutoff must be separately powered, and be part of the boiler control system -- not the zone control system.

emgroom

Thats what I thought to begin with as well. I tried connecting it to another zone and tried calling for heat and still does the same. I consulted with an electrician at my job and jumped each common terminal in each zone to see if that would correct it since it would energize when calling for heat which didn't change anything. Still only fires up when zone 4 calls but will work properly if I reset and go back to normal setting then back to square one. 

EdTheHeaterMan

All you need to power the McDonald Miller RB-24e LWCO is a 24V transformer. The original installer used the one from the old Taco Zone Relay. That is not the only place on the system to get 24 volts.

The following diagrams from the RB-24e instructions do not use any zone control relays to power the LWCO



Forget about the extra wire in the zone control panel with no place to go. That wire does not really belong there. It may be the cause of the transformer failure. That wire should go to a different 24-volt source as indicated in the wiring diagrams above.

BTW the transformer can be replaced in the old Zone Relay Control.

emgroom

The zone 4 NC is labeled (normally closed) on the board. 

emgroom

Hey Ed,

Thanks!!  I'll have to take a look when I get home from work to see which option was used by the previous owner.  I'll also take some photos to maybe clearly show how the wiring is done on the boiler.   

Joe Mattiello

EMGroom
thanks for reaching out on the Wall. this is a great resource for help with this sort of wiring issue using Taco control. for your convenience, and future reference, I attached the Taco zone control wiring guide.
SR504-4exp allows the use of boiler reset control PC702 which is a two boiler staging control which will also work as a reset control with single boiler operation. the caveat, is SR504 should be set to normal when resetting the boiler. also, zone 4 also AKA DHW zone should be set to priority even is you're now concerned about priority. think of it as a communication medium to let the boiler know there is DHW connected to zone 4 and PC702 will fire boiler to high limit on DHW demand. if it's not set to priority, boiler will reset to temperature necessary to satisfy demand.
as for 24V terminals, they are either 24v in, or out. if you're retrofit includes an external transformer to reenergize the control, please be certain to disconnect failed transformer so the two transformers do not butt heads; that's never pretty!!
in order to support you further, I will have to have a better understanding of your objective, and current wiring schematic.
I recommend reaching out to Taco tech support at 401-942-8000, and ask for tech support. they are very good with diagnosing these types of things. Please reach out to me if you have any issues going forward. 401-261-4890

emgroom

Hi Joe,

I will definitely be able to do that only on weekends though. Thank you for commenting as well!  I was told by the technician that came to not have priority on. I'll try switching it to priority on and see if that fixes the issue.  

The failed transformer was the one on the old taco board. I forgot to mention this arose when the 1amp fuse blew and upon replacing the fuse, everything turned on and then burning smell started to emit from the taco transformer and then it failed.  Extremely hot to touch and no power to anything.  Ultimately I want the boiler to operate and send heat when called for by the zones. As of right now, the boiler will only fire up and send hot water to baseboards (areas to be heated) only when zone 4 is calling for hot water.   

Again, thank you for the wiring guide I will look it over again and thank you for commenting!