Priority domestic hot water PDHW

[Scott_D]

here is the wiring diagram for a logic

 

[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.?.