PDHW dual temperature using EPH CP4

[old duffer]

There does not appear to be any mention of OpenTherm or PDHW on their website, so I don't see how your comment is relevant to my original post - please could you explain where OpenTherm and PDHW come into your recommendation?

its a water heater LOL...you enter the temp you want and it outputs that temperature....it self modulates...its a water heater for DHW...The combi is different and comes with its own AP and IoT centre. https://www.factoryheaters.co.uk/store/ZEN-35-Combination-Boiler-35kW-p524509697

 

[old duffer]

The point was that @Notch7 above suggested using the built in weather compensation function, as an alternative to OpenTherm, in order to achieve PDHW. The OP (@Cleph) said that the boiler isn't capable of doing that because the SL2 on this boiler doesn't allow it. I was pointing out that it must possible. It is even mentioned in the user guide (see below). Presumably the wiring would have to change.

View attachment 333398

My Rinnai combi allows me to select thermostat heating or flow heating. It also allows me to select temperature set point or it uses air temp monitoring and flow return temp for automatic regulation. I use the auto function on flow temp. Rooms are 17-18 degrees @ 53% humidity....very cosy...switch it on in late Oct and switch it off end of Feb...gas consumption is now 67% lower than the old baxi

 

[old duffer]

I am encouraged to hear that my room stats would still work if weather compensation is fitted, but I guess the question is whether adding a weather compensation kit would disable the OpenTherm control from the EPH stats. Here's the page from the EPH user guide for setting OpenTherm parameters:
View attachment 333559
And the climatic curves from the next page are:
View attachment 333560
Which go in a similar direction to the Ideal WC curves that @MNW67 showed above (if you allow for the reversed axis).

If the Ideal WC Kit worked sensibly, i.e. took account of the OpenTherm information from the EPH stats, then it would be great. But if it's not sensible then it will override the OpenTherm values and I will have spent money unnecessarily. I guess I need to think of a question, but if anyone has experience of OpenTherm with the Ideal WC kit I'd welcome any input.

using 21st century kit means you don;t have to think.,...just switch on and forget....

 

[Notch7]

A lot of plumbers install boilers and have no interest or knowledge on how to set them up. They sling the boiler on the wall, join the pipes and leave. Most heating systems in the uk seem to be either s plan or y plan, neither system is good for a condensing boiler or heating system efficiency in general.

It doesn’t make a lot of sense that non condensing gas boilers were phased out in 2005 but most of them fitted since don’t actually condense or are overpowered.

 

[Notch7]

My Rinnai combi allows me to select thermostat heating or flow heating. It also allows me to select temperature set point or it uses air temp monitoring and flow return temp for automatic regulation. I use the auto function on flow temp. Rooms are 17-18 degrees @ 53% humidity....very cosy...switch it on in late Oct and switch it off end of Feb...gas consumption is now 67% lower than the old baxi

That’s interesting.

The marketing on heating controls seems to be towards zoning, timing controls and smart technology.

But in actual fact the most efficient systems have no zone control just a pretty low flow temperature linked to external temperature and leave the system on most of the day and let the building absorb the heat then maintain the house temperature by keeping the rads just slightly warm….and the boiler is running at maximum efficiency, low heat. Output and minimal stress on the boiler components.

I’ve been keeping all the TRVs fully open apart from my bedroom at night and allow the system to flow unchoked, it’s working well with return temps hovering around 40 deg mostly.

 

[chrisk801]

I've recently had a new Ideal boiler installed (Ideal Logic Max Heat 2 H18) and plumbed for PDHW (priority hot water). I use it with EPH CP4 series: CP4i, additional Combipack 4, CP4-HW-OT, wired and configured as per EPH instructions, covering downstairs, upstairs and hot water tank, with the boiler controlled using OpenTherm.
The boiler target flow temperature is set on the master CP4 using HHbO (t-5) in the OpenTherm parameters and the boiler display shows that this is working.
However, for PDHW the aim is to use a lower boiler target flow temperature when running central heating (e.g 55°C) to keep the boiler in condensing mode and higher when heating the hot water tank (e.g. 69°C) to get the tank above the Legionnaires temperature. Unfortunately the EPH set does not support dual temperatures, so I set HHbO to 55°C and use a relay across the OpenTherm wires when HW is on, which causes the boiler to run at its set flow temperature (69°C).
This produces the desired result, but is a bit naff (and the boiler display is wrong when heating HW). Has anyone found a better way?

Hi Cleph, is there any chance you could describe the relay setup you have to get OT working on an ideal system boiler. I'd be looking to use it on an S-plan which I think I theoretically can... Nest calls for heat and modulates properly with OT, but when it calls for water it doesn't adjust the flowtemp of the boiler. I was thinking of a relay setup where the call-for-water, when powered, shorts the OpenTherm wires to cause the boiler to run on manual set flow temperature so i would be interested in knowing how you did this and what relay you used etc.

 

[MNW67]

Hi Cleph, is there any chance you could describe the relay setup you have to get OT working on an ideal system boiler. I'd be looking to use it on an S-plan which I think I theoretically can... Nest calls for heat and modulates properly with OT, but when it calls for water it doesn't adjust the flowtemp of the boiler. I was thinking of a relay setup where the call-for-water, when powered, shorts the OpenTherm wires to cause the boiler to run on manual set flow temperature so i would be interested in knowing how you did this and what relay you used etc.

What would happen using S-Plan in this set up when CH and DHW are both on at the same time? Would the CH zone valve close somehow, or would CH and DHW both receive flow at the manual set flow temperature for the period the DHW is on?

 

[Cleph]

Hi Cleph, is there any chance you could describe the relay setup you have to get OT working on an ideal system boiler. I'd be looking to use it on an S-plan which I think I theoretically can... Nest calls for heat and modulates properly with OT, but when it calls for water it doesn't adjust the flowtemp of the boiler. I was thinking of a relay setup where the call-for-water, when powered, shorts the OpenTherm wires to cause the boiler to run on manual set flow temperature so i would be interested in knowing how you did this and what relay you used etc.

Sure, @chrisk801. I based my setup on a diagram in these forums but can't locate it at present, so I've sketched out what I have, which is much as you describe:

The relay is off when HW is off, thus connecting the boiler OpenTherm contacts to the OpenTherm output of the downstairs CH prog stat, which sets the boiler to run at 55°C. When HW turns on it actuates the relay, which disconnects the OpenTherm from the CH prog stat and applies a short across the boiler's OpenTherm contacts (and because I have set the boiler to 69°C that's what it runs at). Here's another reference: https://www.diynot.com/diy/threads/opentherm-with-s-plan-system.534229/post-5305901
The relay I used was https://www.discountfiresupplies.co...50_60Hz-Coil)-in-White-or-Red-Single-Gang-Box
I've shown a switch in the line from HW Prog Stat On to the relay actuate input, which we keep closed (On). But I could turn off that switch, which would simply mean that the boiler temperature continues to be controlled by OpenTherm from the CH Prog Stat.
@MNW67 - It depends on your wiring. Mine is PDHW by virtue of the wire from the Off of HW Prog Stat to the Com of the CH Prog Stats. But I actually have a switch in that wire so that I can, if needed, run both CH and HW at the same time, for example if we've been away and want everything heated up quickly - I think that's the equivalent of S-plan?

 

[Mister Banks]

What would happen using S-Plan in this set up when CH and DHW are both on at the same time? Would the CH zone valve close somehow, or would CH and DHW both receive flow at the manual set flow temperature for the period the DHW is on?

It’s possible to convert an s plan to PDHW by changing the NC CH zone valve to a NO zone valve and changing the wiring in the wiring centre so that the DHW zone valve controls the CH zone valve. In this setup you will never have both zone valves in the same state but you won’t have the ability to lower the CH water temperature unless the boiler supports it.