Priority domestic hot water PDHW

[fixitflav]

here is the wiring diagram for a logic
View attachment 43843

Could be wrong, but I don't think either of them gives PDHW. The Y-plan doesn't, and if you want
PDHW you would go for W-plan with either/or diverter valve. Cheaper and simpler.
With S-plan as shown there, if CH and HW are called, both valves open.
Interesting that there's now talk about PDHW, as that was the system way back, as in my first house in 1973, but then Y-plan and S-plan came out, presumably thought to be an improvement as no need to wait for HW to be satisfied before CH available.

 

[SJB060685]

As above, neither of them look like they give PDHW

 

[EvilDrPorkChop]

Have quite a bit of experience of PDHW on systems;

Firstly you can make any traditional system PDHW without any changing of valves with a NC DP contactor quite easily. Basically on a HW demand the DP contactor opens the circuit and kills the power to the Heating zone valve, closing that and reheating the HW. When the HW stat is statisfied the DP resets to NC and the heating fires back up - simple. Dependant on how many zones you've got will determine which DP contactor you require. This is obviously a solution for a traditional system with one set flow temperatures.

The advantage to this is that you can have CH without HW on the clock, where as most PDHW systems require you to have HW & CH on at the same time.

I have this on my own system and it works well - I have it on a switch so you can knock it on and off as required.

Secondly; Vaillants systems will use HW priority when using their controls & wiring centres. You don't just need the GreenIQ 4 pipe boiler to do it. Any other their heat & system boilers will do it when using the Wiring centre and Vaillants control. You can either use a diverter or 2 two port valves, and I like to use the Probe on the tank too (VR10 I think). You can set DHW temp then, along with DHW flow temp (For reheat) and maximum time spent reheating DHW.

I don't have as much experience with Viessman but from my limited experience their system is very similar to Vaillants.

Thirdly; external controls will offer the same solution. Honeywell Evohome are just in the process of adding it to the next firmware update which does HW priority software based. I'm sure there will be a couple of other controllers that will do the same.

HW priority IMO isn't right for every system, it some situations it can be work really well. In other's I don't think there's much advantage.

 

[John.g]

Is there any simple way that you can also increase the DHW flow temp on a weather compensated boiler which may be only running at 40C when the hot water needs a boost, say maybe by having a additional resistor (controlled by another relay) wired into the outside air temperature sensor to fool it to increase the boiler SP temp.

 

[SJB060685]

My understanding John is its preset in the PCB, so when you have a HW demand it ramps up boiler output and flow temp for fast recovery times. When the cylinder sensor is satisfied it will close HW zone valve and re open CH, which then reverts back to temperature curve set for weather compensation

 

[John.g]

OK thanks, so (does) that implies that all boilers with weather compensation also have PDHW.

 

[SJB060685]

As you know I'm an oil man, I'm not the one to ask about gas. What I know is what I've recently studied and PDHW is still fairly new. Whether its done with two 2 ports and the CH a normally open with cylinder thermistor, or with the now 4 pipe option from some gas manufacturers which has a heating flow and return and a flow and return for HW, each with their own pump i believe, or a way to separate the flow direction? Its not something I'm aware of in the oil industry as we are still technically a long way behind gas.

 

[EvilDrPorkChop]

OK thanks, so (does) that implies that all boilers with weather compensation also have PDHW.

In theory they should do, but some of the control systems don't implement this and always work. I know there's been issues with Opentherm and determining a CH and HW demand with some manufactures.

This was why when the new ERP energy laws with new controls only came into force on Combi systems - traditional systems where there is stored HW there was a concern where if the Flow temperature was limited due to a weather compensation system or Open Therm then there was a slim possibility cylinders may not be heated to the required temperatures to kill off legionella.

Is there any simple way that you can also increase the DHW flow temp on a weather compensated boiler which may be only running at 40C when the hot water needs a boost, say maybe by having a additional resistor (controlled by another relay) wired into the outside air temperature sensor to fool it to increase the boiler SP temp.

In short no. Some boilers have a terminal where you can wire a live in on a HW demand where the boiler goes into full load. Others are software based when used with their own controls.
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As you know I'm an oil man, I'm not the one to ask about gas. What I know is what I've recently studied and PDHW is still fairly new. Whether its done with two 2 ports and the CH a normally open with cylinder thermistor, or with the now 4 pipe option from some gas manufacturers which has a heating flow and return and a flow and return for HW, each with their own pump i believe, or a way to separate the flow direction? Its not something I'm aware of in the oil industry as we are still technically a long way behind gas.

The issue with the 4 pipe systems is that they aren't always practical to do, especially retro fitting in existing properties. On a standard system you're always only going to have a flow and return back to the airing cupboard/cylinder usually then CH pipes from there to rads.

 

[John.g]

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.?.

 

[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.

 

[EvilDrPorkChop]

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.

I think the main reasons being is that the boiler isn't large enough to reheat the HW and CH at the same time. For example - I have a customer with a 30kw Logic Plus system. The coil is rated around 15kw, the output of the rads must be in the region of 30kw nearly. So when the HW AND CH was calling the HW wouldn't reheat and the CH wouldn't get warm (in fact if the cylinder was remotely warm it would take the heat out the cylinder). So it would just try and reheat everything and basically not get anywhere. PDHW allows the cylinder to reheat efficiently then input the full 30kw into the heating system when completed.

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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Yes the NC DP contactor is in addition. As i've said quite simple to add in, if OP would like a wiring diagram I will try and sketch something up.