Priority domestic hot water PDHW

[EvilDrPorkChop]

You & others above mention where the boiler goes to full load, if the boiler is directed to full load and if its output is higher than the coil output, then what controls the boiler before it reaches its hi limit safety stat limit, does it cycle on/off until the cylinder temp is achieved.?.

Most boiler controls systems with PDHW allow you to set the coil rating of the cylinder - well Vaillant’s do. If you’re using an external control the boiler will modulate accordingly as it would usually.

 

[John.g]

That's interesting as any coil output is not constant while the cylinder is heating up, there will be, IMO, a big difference between a cold and hot cylinder, no so much if the HW setpoint hysteresis is say 5 to 10C so should work reasonably well under these (re heat) conditions but one would think generally that the coil rating would need to be understated to enable the HW SP to be reached before the boiler reached its hi limit setting, but obviously it works.

 

[fixitflav]

You & others above mention where the boiler goes to full load, if the boiler is directed to full load and if its output is higher than the coil output, then what controls the boiler before it reaches its hi limit safety stat limit, does it cycle on/off until the cylinder temp is achieved.?.

Depending on the boiler, it either stops/starts on the control-stat, or modulates the gas flow down. And then stops if the minimum modulated flow gives more heat than the cylinder takes.

 

[John.g]

So full loading is basically just increasing the boiler temperature SP to its max?? and in the Vaillant's case it limits the max boiler output to the inputted coil rating?.

 

[SJB060685]

As Dr said above John, certain boilers have set options to set the boiler output to coil output, or a % of coil output as well as setting flow temperature. As you know most modern cylinders coil rating is based on a flow temperature of 80-85°c with a minimum flow rate and as you also know as the delta T changes so does the coil output.
The sensors used at the cylinder would be a thermistor whos resistance would change depending on the temperature, the resistance signal received at PCB would shut the burner down, or revert to CH if that was called for at the lower temperatures. At no point should the flow temperature exceed the preset value and come close to high limit stat setting, obviously system boilers high limit stat can be set to over 100°c.
As Dr also said if thermistors aren't used and a traditional cylinder stat is then the burner will modulate down when the delta T decreases.
Thats my understanding anyway and having said all that a diverter valve or mid position used as a diverter valve will give PDHW for older systems/boilers without this modern fancy stuff

 

[fixitflav]

Firstly you can make any traditional system PDHW without any changing of valves with a NC DP contactor quite easily.

Is the NC DP contactor in addition to the cylinder and room stats? I think the OP would need a wiring diagram.
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So full loading is basically just increasing the boiler temperature SP to its max?? and in the Vaillant's case it limits the max boiler output to the inputted coil rating?.

If yours is a reply to my #33, yes. The control-stat setpoint adjustable by the user. The boiler must have controls to stop or reduce gas flow when system demand is below boiler max output, whether in HW or CH mode.

 

[John.g]

The way I see PDHW operating is on HW demand (1) the HW zone valve opens and the CH zone valve closes and (2) that the boiler temperature SP is automatically increased to ~ 83/85C and the boiler modulates to maintain this 83/85C until the cylinder HW temperature is reached. (The Vaillant seems to introduce a refinement on this?) So what set point are you referring to here "The control-stat setpoint adjustable by the user."

 

[fixitflav]

a diverter valve or mid position used as a diverter valve will give PDHW

It might be possible to wire up a mid-position valve so it works either/or, but as the way it's intended to be used it allows flow to both CH and HW simultaneously. That's the whole point.

will give PDHW for older systems/boilers without this modern fancy stuff

If it's old enough it will be PDHW (eg Honeywell W-plan) because that was before Y and S-plans came in.

The OP has gone a bit quiet, but the way I read him he just wants to replace a boiler in a system with existing 2-port valves. Not sure why he wants to go PDHW, that and both-on have pros and cons. Unless weather compensation is used, when PDHW (or at least separate HW and CH cycles) is unavoidable. My sketch in #18 is a suggested way to do it.

 

[SJB060685]

It might be possible to wire up a mid-position valve so it works either/or, but as the way it's intended to be used it allows flow to both CH and HW simultaneously. That's the whole point.

I know a mid position is designed for flow through both ports, i was just pointing out that one could be wired in such a way to act as W plan and give priority to HW.

 

[fixitflav]

The way I see PDHW operating is on HW demand (1) the HW zone valve opens and the CH zone valve closes

That's right, but the circuit to achieve it has to be designed.

So what set point are you referring to here "The control-stat setpoint adjustable by the user."

I was assuming boiler has an adjustable control-stat, either old-fashioned type with a knob, or temperature set on a screen. When on weather compensation there's an additional control to stop or modulate the gas at some lower temperature, depending on outside temperature. Bypassed when HW called, to allow temperature to rise to
control-stat setpoint, via an input to the boiler to tell it it's HW mode. Obviously the control-stat would be set high enough to give realistic HW recovery time.
But no doubt there are variations from different manufacturers.