DHW Priority

mattbee24

I just installed an HTP UFT-80W boiler with an indirect tank. I have 2 heating zones (1 for 1st floor and 1 for second) along with the DHW zone. It is not installed as a P/S setup as the boiler doesn't require it. There is a Taco SR504-4 controlling the zone pumps and the DHW pump is controlled by the boiler. In this setup, is there really a way to set the DHW as a priority? My thinking is that there really isn't a way since the boiler really isn't controlling the zone pumps. Am I missing something?

NY_Rob

Just looking at the schematic for the SR504-4, if you placed a DPST 120vac relay on the hot wire out to your pumps and connected it's pull-in coil to the 120vac CH terminals on the boiler that would enable the CH pumps for spaceheatng and kill power to the CH pumps when the DHW priority kicks in and kills power to the CH terminals on the boiler.

Maybe Taco has a pre-made module for that application or it's simple enough to find a decent 120vac DPST relay online.

Another thought... I don't know what the jumpered "ZC" and "ZR" terminals on the SR504-4 do, but if making or breaking the connection between them interrupts power to the CH pumps... you could use a SPST relay there rather than the DPST relay in the hot line of the CH pumps.
You'll have to either check those terminals with a VOM or call Taco to see what they do.
Maybe one of the pros here know what the "ZC" and "ZR" terminals do?

G Averill_2

Look at the boiler display item 24:DP (DHW Priority Timer) for this. This function has a default setting of 30 minutes, with a range of
0-60 minutes. Setting the range at zero (0) allows the DHW to always have priority.
If I can also add, we always recommend to our customers that this boiler be piped with a primary circulator. The primary circulator is not large but helps to insure a longer life to the heat exchanger.
FYI.

NY_Rob

^ the OP's problem is that the boiler is not controlling the CH pumps- the zone controller is and they will not turn off during a DHW call because the zone controller has no idea there is a DHW call.

mattbee24

NY_Rob said:

Just looking at the schematic for the SR504-4, if you placed a DPST 120vac relay on the hot wire out to your pumps and connected it's pull-in coil to the 120vac CH terminals on the boiler that would enable the CH pumps for spaceheatng and kill power to the CH pumps when the DHW priority kicks in and kills power to the CH terminals on the boiler.

Ahhhh...don't know why I couldn't think of that!! Thanks!!

Canucker

Is there an advantage to letting the boiler run the DHW pump? Why not let the Taco control it all?

mattbee24

Canucker said:

Is there an advantage to letting the boiler run the DHW pump? Why not let the Taco control it all?

I thought about that, but I'm pretty sure there is just a TT out on the control panel. The boiler wouldn't know that it was the water heater calling and not the heating zones.

Canucker

mattbee24 said:

Canucker said:

Is there an advantage to letting the boiler run the DHW pump? Why not let the Taco control it all?

I thought about that, but I'm pretty sure there is just a TT out on the control panel. The boiler wouldn't know that it was the water heater calling and not the heating zones.

I could see that being an issue if you're using the outdoor reset. Makes for real slow DHW recovery. Is that the concern? I'm not that familiar with the UFT-80 but I'm surprised it can't be wired to do this fairly easily. I'm sure there are people on the board that have done it. I'm off to look at wiring diagrams, see what I can find.

SeymourCates

mattbee24 said:

There is a Taco SR504-4 controlling the zone pumps and the DHW pump is controlled by the boiler. In this setup, is there really a way to set the DHW as a priority? My thinking is that there really isn't a way since the boiler really isn't controlling the zone pumps. Am I missing something?

Edited:

The "X-X" terminals in the SR504 connect to the "T-T" terminals on the boiler. This is the signal that allows the boiler to fire. The boiler will automatically provide DHW priority each time the tank calls.

To prevent any of the pumps on the SR504 from operating, remove the jumper between ZC and ZR on the SR 504 and connect one wire between ZC on the SR504 and "L" on the CH system pump for the UFT.

Nothing else required.

HVACNUT

An easy way would be to install a RIB relay 2401D DPDT in an unused knockout in the SR504.
Use the 115v coil wires on the relay and wire it to the primary circ terminals on the boiler.
Break the SH circulators hot wires in the SR504 through the N.O. contacts on the RIB. (1 N.O. contact per zone)

On a DHW priority call, if there's a SH call, XX will close in the Taco, but because the boiler is programmed for DHW priority, internally, CH will not close, therefore keeping the primary pump circuit open.

When the DHW zone is satisfied, and there's a SH call, CH will close, allowing 115v to the primary pump terminals, energizing the 115v coil on the RIB and closing the contacts to the SH pumps.

Now rip it all out and do P/S piping.

mattbee24

@HVACNUT

I think that is the same thing NY_Rob is suggesting, and probably what I will end up doing.

HVACNUT

> @mattbee24 said:
> @HVACNUT
>
> I think that is the same thing NY_Rob is suggesting, and probably what I will end up doing.

Oh yeah. Sorry @NY_Rob.

NY_Rob

I would still look in to the "ZC" and "ZR" terminals on the SR504-4. If pulling that jumper makes/breaks the power to the CH pumps... it would be better to put the relay there vs. breaking the two hot leads to the CH pumps directly.

Rich_49

What indirect are you using ? Does it use an aquastat or sensor ?

HVACNUT

> @NY_Rob said:
> I would still look in to the "ZC" and "ZR" terminals on the SR504-4. If pulling that jumper makes/breaks the power to the CH pumps... it would be better to put the relay there vs. breaking the two hot leads to the CH pumps directly.

@NY_Rob. You're the man!

ZR, ZC are typically used in conjunction with a triple acting aquastat, on a boiler that has a tankless coil and needs to maintain a minimum temperature, shutting down the circs on temp. drop.

ZR provides 120v output from the zone board to the aquastat to fire the burner through the high limit.

But ZC, (and here's where your genius applies) is 120v input from ZC in the aquastat to ZC in the zone board, and will keep the pumps relay open until the aquastat is above its low limit setting.

By wiring the CH pump output on the boiler to ZC and neutral on the zone board, (remove any jumpers connected to ZC) the SH pumps will not be energized with a priority DHW demand even if there is a SH call.

No field installed relay needed.

NY_Rob

@HVACNUT ...

Nice!
Your idea is much better than using relays.

Too bad the docs on the zone controller don't include a proper schematic, it would have made things much easier from the beginning.

Just to be sure I get it....
ZC needs to see 120VAC to enable CH pump relays, if we wire ZC to CH hot terminal on the boiler- when a DHW call comes in the CH hot is killed due to "DHW Priority" and 120VAC is no longer present at ZC so the CH pump relays stay open till it sees 120VCA again.

Excellent!

NY_Rob

... never mind

HVACNUT

> @NY_Rob said:
> ... never mind

No, not never mind, or maybe yes, never mind.
But that is correct.
All zone control boards (the ones that I've worked with anyway) have a one relay for the pumps with a 115v coil, energized by ZC.
In a typical cold start application, (not this scenario) ZC would be jumped to H to close the N.O. contacts for the pumps circuit.
The older boards had line and low volt cube relays you could swap out if one failed. Now it's all internal switching.

mattbee24

HVACNUT said:

> @NY_Rob said:


But ZC, (and here's where your genius applies) is 120v input from ZC in the aquastat to ZC in the zone board, and will keep the pumps relay open until the aquastat is above its low limit setting.



By wiring the CH pump output on the boiler to ZC and neutral on the zone board, (remove any jumpers connected to ZC) the SH pumps will not be energized with a priority DHW demand even if there is a SH call.



No field installed relay needed.

Nice!!
So simple! I'm surprised that isn't mentioned anywhere in the manual.
Thank you!!

mattbee24

I think I'm finally satisfied with the installation. I realize some people have had trouble with delta T pumps and modcon boilers, but I figure If I run into problems I can just change them to the speed that works the best.

j a_2

It's a very clean job,that's for sure..love the solder Job and the lack of any Pex in the near boiler piping...

HVACNUT

Nice job!

NY_Rob

Looks good!

Since you have room for it... consider running the UFT off a small UPS. It draws under 100 watts at full output with a circulator running- any small home UPS (make sure it has 3-prong outlets) will be sufficient for the UFT for short term usage.

The boiler control board is basically a small computer motherboard running off a boot flash IC... it's pretty sensitive to surges, etc.

Rich_49

Looks like you have gotten past your wiring dilemma , good . May I ask why 2 - VT2218s instead of 1 w/ 2 zone valves ? These are baseboard , series loop zones , correct ? What is the design heat loss of each zone ?

mattbee24

NY_Rob said:

Looks good!

Since you have room for it... consider running the UFT off a small UPS. It draws under 100 watts at full output with a circulator running- any small home UPS (make sure it has 3-prong outlets) will be sufficient for the UFT for short term usage.

Good idea. I'm thinking one of these would work well.

mattbee24

Rich said:

Looks like you have gotten past your wiring dilemma , good . May I ask why 2 - VT2218s instead of 1 w/ 2 zone valves ? These are baseboard , series loop zones , correct ?

No, I have under-floor radiant. 600 ft. of 1/2" pex on each floor with metal plates.

Rich_49

2 , 600" loops ?
4 , 150" loops ?
6 , 100' loops ?

What delta do you desire for the radiant floor ?

mattbee24

Rich said:

2 , 600" loops ?
4 , 150" loops ?
6 , 100' loops ?

What delta do you desire for the radiant floor ?

One floor has (3) 200' loops and the other has (2) 300' loops.

The upstairs (2 loops) has been completely gutted and insulated. It is approx 625 sq.ft. and has an estimated heat loss of 9,500 btu.

The 1st floor (3 Loops) has no insulation as of right now. It is also approx 625 sq.ft. and has an estimated heat loss of 19,250. We are planning on completely gutting the downstairs, but it may be 2-3 years before we can do it.

I was planning on a 20 degree delta. I laid out the loops so the beginning of the loops are on the outside walls and therefore get the hottest temps.

Rich_49

What SWT will be required at design conditions and at say 40* outdoor ?

Is this your residence ?

mattbee24

Rich said:

What SWT will be required at design conditions and at say 40* outdoor ?

Is this your residence ?

SWT at 40* will be around 110-120. I have it set to SWT of 150 at design temp. (6 Degrees-Northern Ohio). This seemed to work for me the last couple of winters with my old boiler system, except when it was very cold and very windy, then it would only get it up to around 65 on the 1st floor.

Yes, this is my residence.

Rich_49

What are the BTUh requirements at 40* Mattbee ?

mattbee24

At 40 degrees it's around 15,000 BTU total.