Help using ZC for DHW Priority

pjsnyder

Hi all,

I have two Dunkirk 75000 BTU boilers wired to cascade (150k bTU total) for my radiant floor heat and DHW (indirect tank). I have the factory outdoor reset sensor installed on the boiler and typically run 90-130 degree water for the radiant. The heating thermostats and radiant circulators are wired to a Taco SR506 relay, and the DHW aquastat and pump are wired directly to the boiler. The problem is that the boilers don't know when there is a DHW call and therefore don't shut off the circulators.

I have been reading posts about wiring the CH output on the boiler to the ZC on the Taco and removing the ZC/ZR jumper. Would this work to let the SR506 know to shut off during a DHW call? I don't want to send overly hot water to the radiant zones, when this does happen, the boiler fires for a long time and struggles to make up the DHW demand. Should I wire all 3 wires (H→ZC, N→N and G→G) from the CH terminals to the SR506?

The CH pump terminals on the boiler have 120V when there is a call for heat. With DHW priority set to "ON" on the boiler and a call for DHW, the CH pump terminals drop to 8V, and the DHW pump terminals have 120V. Photos of the boiler terminals and the SR506 are attached.

Please let me know if I misunderstood the previous posts about the use of ZC, or if you see any unintended consequences or other flaws in my proposed approach. Thanks!

EdTheHeaterMan

Since you are full on the SR506 with CH zones, you can't use the DHW priority on the Taco SR

Let me look at the control wiring for the Dunkirk. What model number do you have and what ODR control are you using and what boiler cascading control are you using?

Heading out for some time, Will look for your reply after I return in a few hours.

pjsnyder

Thanks Ed. I have two DKVLT-075 boilers. I am just using the outdoor reset and the cascading control logic in the boilers themselves (i.e. I do not have a Tekmar or other standalone ODR or cascading controllers).

pjsnyder

All - here is the previous thread that got me considering this path forward. I couldn't find it earlier today when I made my initial post. Thanks to all who responded to this earlier post, the discussion really got me thinking.

https://forum.heatinghelp.com/discussion/162746/dhw-priority

EdTheHeaterMan

OK it appears that Dunkirk has a "Multiple Boiler Guide" that we need to reference to get this sorted out. I ma looking for the document now

I have your DKVLT-075 boiler I/O manual and need more information. If you have that manual there are several multi boiler piping designs. I altered this one to see if you have something close to this. it is a primary/secondary design where the boiler is the primary loop with a CH system pump and all the CH zones each have a zone pump. (that would mean the internal boiler pump must operate, then the CH primary loop pump would operate to heat the primary loop, then the zone pump would direct the heated primary loop water to the individual zone.

If you have that set up then I can work with that for your solution. I just need to know if the DHW tank is piped as a secondary like all the CH zones or is it connected like i show in the diagram. A pump from the supply and the return back to the return making that loop a ladder type zone (not a secondary loop zone)

Or draw a sketch of your design as best as you can… or take a photo and post it here.

depending on the piping setup for your system, there will be different control logic I would employ to get you 180° water to the DHW zone without sending any high temperature to the CH zones.

Waiting for your response.

HVACNUT

Yes. A 120 volt wire from L on the DHW PUMP terminal to ZC with the jumper removed will work.

Do the boilers know DHW is priority, or does it have to be programmed ?

pjsnyder

@EdTheHeaterMan - Annotated photo below. Is my understanding correct that the boiler has a primary loop and then the DHW is a secondary loop, just like a CH zone? I don't believe that there is a dedicated CH system pump, but my understanding is that the boiler pump is on whenever there is a CH or DHW call, and then the zone pumps push the water to the respective zone.

PS - You'll notice 2 Tacos…I have a lot of zones, and assuming the ZC plan is viable, I plan to run a single 14/2 line from the boiler to a junction box, then split it to run a separate line to ZC on each SR506 board.

@HVACNUT - Should I wire L, N, and G from the boiler terminals to the SR506 board?

The DHW priority is currently set on the boiler. I believe that it's set for 30 minutes @ 160F, but I know how to change it thru the menu if it needs to be modified.

Thanks for the replies.

EdTheHeaterMan

From studying your pipes, this is whai I believe you have.

This helps to determine what to do next.

All of your CH zones are part of the primary loop and each has a mixing valve to set the low temperature to that zone. This means that you can operate a higher temperature boiler water without any of the high temperature water ever getting to the low temperature zones. That is a good thing. The DHW circulator is connected as a primary loop zone also. and it is the closest to the boiler so in the summer, there is no need for the hot boiler water to go anywhere the other CH zones. That is a good thing.

To get the boilers to do Priority for DHW with a higher temperature is not a piping design problem. it is a control logic problem and that is what I need the second manual "Multiple Boiler Guide" to clarify the control logic. i believe that your DHW zone needs only one boiler to operate unless the water tank is over 50 gallon and the pipes feeding that tank are larger than 1". It is tough to tell with the insulated pipes

EdTheHeaterMan

One more request.

The top Taco SR506 appears to have only 5 of the 6 zones connected. Can you take a closeup photo of the wiring inside that box?

Finally, do the pipes at the lower right of the photo stop there? (yellow arrows) Or do they turn and gos somewhere I can't see at this angle?

pjsnyder

@EdTheHeaterMan - Thank you. I believe your schematic and description is correct. The indirect tank is a 65 gallon Bradford White Dual Wall, and the DHW supply and return pipes are 1", reducing to 3/4 inch at the tank. I had the boiler running the way you described, with a set temp of about 170 degrees. The problem with that was twofold. 1) I had to crank the mixing valves down substantially, and for some reason, when I did this they seemed to be introducing a lot of air into the system, and 2) I wanted to take advantage of the outdoor reset, eliminate the need for the mixing values, and run the boiler at a lower temp.

pjsnyder

@EdTheHeaterMan - You are correct on both accounts. The upper Taco has Zone 6 available and the pipes end on the lower right side of the photo at the second boiler. The DHW tank is just off the photo to the right. My concern with running Priority on Zone 6 of the upper board (and not from the boiler) is 1) How does the boiler know to fire at a higher temperature and 2) how does the lower board know that the upper board is on priority?

One other thing to note, I do have two zones for hydronic coils on my Unico airhandler (upstairs and downstairs zones) for some makeup heat if the radiant can't keep up. We are in a pretty windy area and have a lot of windows, so my HVAC contractor added that as a backup. That is typically when it is very cold and windy out, so the boiler is running on the upper end of the outdoor resent. I realize the hydronic coils want water that is about 160F, and I was thinking of using the priority zones to run those pumps if needed.

EdTheHeaterMan

Understand.

But you should have the mixing valves adjusted for the desired temperature for each loop anyway. Once you set the mixing valves, they will never let the water temperature going to the zones go higher than the set point. For example: if the set point for a zone is to let only 120° water into that loop, then when the boiler sends 100° to that valve hot in. the valve will open completely and will not acce3pt any water form the cold side of the valve, so the MIX side will send out 100° to the loop. When the boiler temperature increases to 130° then that mixing valve will open slightly to take some 130° boiler water and some return water from the loop on the cold inlet to mix it down to 120°. Now when you get the DHW priority fixed and the boiler still has some 180° water in the pipes the mixing valve will open more to let less boiler water into the zone and more colder return water to the cold inlet automatically to render 120° out the MIX port to that zone loop.

And each mixing valve can be adjusted differently for each zone. Carpeted floors need higher temp than hardwood floors and different from ceramic tile floor, etc., etc., etc. (stole that last part from Yul Bynner's character in The King and I.)

Adjusting the mixing valve should have on reason to cause Air to form in your system. That seems odd unless there was a pocket of air in the space between the return pipe tee fitting to the cold inlet on the mixing valve. Once that was purged, you should no longer have an air problem.

Our goal is to get the boilers to operate on ODR for CH pumps and have a max temperature of about 130°F then when there is a call for DHW, the boiler will increase to 180° for the duration of the call for DHW. (or a time limit for calls longed than 60 minutes) So you will want the mixing valves set properly.

I am still reviewing the information form Dunkirk

EdTheHeaterMan

@pjsnyder Said: "One other thing to note, I do have two zones for hydronic coils on my Unico airhandler (upstairs and downstairs zones) for some makeup heat if the radiant can't keep up. We are in a pretty windy area and have a lot of windows, so my HVAC contractor added that as a backup. That is typically when it is very cold and windy out, so the boiler is running on the upper end of the outdoor resent. I realize the hydronic coils want water that is about 160F, and I was thinking of using the priority zones to run those pumps if needed."

Using the priority feature may not be the best option for that. I will look into that option with another approach. The reason this may not be a good idea to use the Priority option is that the priority option may have a time limit of 30 to 60 minutes. (reason is if the DHW thermostat fails in the calling for DHW mode, your CH zones will stay off and the home could freeze if the CH zones are locked off for more that an hour So this feature has a time limit) If you air handlers call for heat for more than an hour, the air handler will lock out the other zones for the entire time it is calling for heat (and that may be when you need the heat the most). Once the air handler is locked out, it will no longer operate until the call for heat from the air handler to the DHW circuit is satisfied. That may take a few days in your case since the floor heat is the only other heat source. (and that one was insufficient for load, as you stated)

EdTheHeaterMan

One more request.

The top Taco SR506 appears to have only 5 of the 6 zones connected. Can you take a closeup photo of the wiring inside that box?

Finally, do the pipes at the lower right of the photo stop there? (yellow arrows) Or do they turn and gos somewhere I can't see at this angle?

pjsnyder

@EdTheHeaterMan - Here are the additional photos you requested. The mixing valves that I have are Taco 5000-3, and I didn't know how to (or if I could) set them to a constant outlet temperature with variable inlet temperature as there are no markings on the valve or adjustment cap.

Regarding DHW priority, the Dunkirk controls limit the priority to a maximum of 60 minutes.

pjsnyder

@EdTheHeaterMan - I'd also be open to adding a separate boiler controller and getting away from the Dunkirk computer. I feel comfortable with the physical wiring side of it, but would need some help developing/understanding the proper wiring diagram. At the end of the day, I just want to be able to run the boilers as efficiently as possible to keep the floors warm, make sure we have sufficient DHW recovery, and reduce my energy demands as much as possible.

EdTheHeaterMan

Here is an idea. You will need a SR502 or SR503 to do this. Not the Expansion model Since you already have the standard model, we will stick with that.

The way that Taco suggests to make both controls talk to each other (all three in your case) is to have all the SR500 series controls set to priority. the Aquastat fro the water heater will operate all the priority zones as indicated in BLUE on the diagram. Then leave the additional SR506 priority zone circulator blank.

Since we need to send a signal to the Dunkirk DHW to operate the DHW priority temperature, you can add a RIBU1c to the unused priority outputs. Connect the Blk/Wht wire and Yel/Wht wire to the N & H of the priority output. Use the NO contacts Yel & Org to send that signal to the Dunkirk DHW input 5 & 6. Since you have 2 unused priority outputs, you can put one on eachSR 500 relay and send the individual signals to the individual boilers as shown in GREEN

Since you have the X X from both of the the SR506's already connected properly to the boiler, I would just parallel wire the new SR relay X X to one of the original SR506 X X terminals as shown. The blue shaded areas indicate that the existing X X to boiler wiring can stay the same.

EdTheHeaterMan

And since your project is so perfectly installed, I might suggest that you mount the new control on an elevated platform above your awesome wiring. In your hands i believe it will be equally awesome. And it beats sending all the needed wires to a remote location

You can pop 2 holes in the rear of the SR502/3 so the wires don't interfere with the control covers of the existing controls. One for low voltage and one for high voltage. You can substitute the RIBU1c with a RIB2401D and eliminate the need to find a place for a second relay on the new control. That is a DPDT switch so it can operate both Dunkirks

EDIT: It does not matter which SR500 box gets the DHW circulator. Connect it to the easiest location to make the wiring stay looking exceptionally awesome.

pjsnyder

Thanks Ed. So, currently the DHW circulator is wired directly to the boiler, and the SR506s are connected directly to one boiler. The second boiler is controlled only with a single wire from the argus link on the first boiler to the argus link on the second boiler.

In your schematic above, I'm assuming I would need to remove the existing CH circulator occupying Zone 6 on the lower SR506 and move that to the SR502? Then make the rest of the connections as you suggest?

EdTheHeaterMan

Yes that is correct. Essentially the SR 506 controls become 5 zone relays with the priority zone controlling the priority circulator for DHW and the SR 502 control becomes a 1 zone control with the priority zone. The call for priority DHW must go to all the SR500 controls in order to cut off the remaining zone circulators.

Knowing that there is a "Argus Link" that connects the boilers lets me know that you only need one RIBU1C relay here is the updated diagram.

The second RIB is gone

Now the only thing you need to decide is the high temperature for the fan coil and then adjust all the mixing valves for the other low temp zones. The correct thing is to set it for the 160° that the air handlers need and mix all the other zones with the mixing valves. That is really the only way to get the air handlers to work properly.

pjsnyder

Thanks Ed. I reviewed over the weekend and will plan to work on this over the next couple weeks (out of town most of this week and weekend). Really appreciate your help on this!