Some where, someone said they had two HP. with this setup and some changes to the pipes, you could have one HP deal with DHW and CH and the other one just looking after the CH

Allows the best of both 

You can (just like you can have two boilers)  but unless you use vast amounts of dhw to the extent that the dhw is a material proportion of the total daily energy demand, there is not much point.  Better just to tweak your settings to do whatever it is you want to do.

Most of the time your ashp has plenty of excess capacity assuming it's designed to cope with the standard requirements.  If it is getting challenged on the coldest days because it is well matched to the demand, switch on the immersion during a cheap rate period.  That will only be necessary for a handful of days per year.

Sorry I think you lost the point. The main question was about the CH turning off when DHW was being called for. 

So having two HP, that person can have one looking after DHW & CH, while the other one just deals with CH. That way the house will always receive heat and the DHW request will not effect it.

Unless I completely misunderstood you (in which case apologies) I don't think I did lose the point. 

If the dhw only needs to be on for a couple of hours a day it won't materially affect the house when the dhw turns the ch off, unless you live in a tent (or somewhere else with minimal insulation and minimal thermal mass). 

If the dhw needs to be on for substantially more than that to satisfy the demand then it will materially affect the ch, but that then falls into my case where the dhw demand is a material proportion of the daily total demand.

EDIT - OK I look back and see you were referring to someone who already had 2 Heat pumps, which I hadn't realised.  If they only need 1 then yes it could be replumbed this way.  But if they only need 1 then why not replumb it this way and sell the other, unless its marginal and the DHW load is such that, including DHW, 1 isn't sufficient.

Some where, someone said they had two HP. with this setup and some changes to the pipes, you could have one HP deal with DHW and CH and the other one just looking after the CH

Allows the best of both 

You can (just like you can have two boilers)  but unless you use vast amounts of dhw to the extent that the dhw is a material proportion of the total daily energy demand, there is not much point.  Better just to tweak your settings to do whatever it is you want to do.

Most of the time your ashp has plenty of excess capacity assuming it's designed to cope with the standard requirements.  If it is getting challenged on the coldest days because it is well matched to the demand, switch on the immersion during a cheap rate period.  That will only be necessary for a handful of days per year.

Sorry I think you lost the point. The main question was about the CH turning off when DHW was being called for. 

So having two HP, that person can have one looking after DHW & CH, while the other one just deals with CH. That way the house will always receive heat and the DHW request will not effect it.

Allows the best of both 

You can (just like you can have two boilers)  but unless you use vast amounts of dhw to the extent that the dhw is a material proportion of the total daily energy demand, there is not much point.  Better just to tweak your settings to do whatever it is you want to do.

Most of the time your ashp has plenty of excess capacity assuming it's designed to cope with the standard requirements.  If it is getting challenged on the coldest days because it is well matched to the demand, switch on the immersion during a cheap rate period.  That will only be necessary for a handful of days per year.

Allows the best of both 

I also don't understand why the heating is switched off when heating the HW cylinder and would much rather heat the cylinder via immersion at TOU tariff overnight to give consistent heating but it was never set up that way.

This is simple.  Heat pumps and boilers can only supply water at one flow temperature at any one time.   You need a higher ft for the dhw than you need for space heating, so it's inevitably one or the other.  The only way to avoid this is to operate permanently at the much higher temp needed for dhw, which makes space heating inefficient both for boilers and heat pumps. 

In the UK we have historically suffered this inefficiency for boilers, costing us for the past 20.years an additional 10% on our gas consumption as well as a reduction in comfort.  Other countries with more advanced heating industries have operated in either-or mode for decades. You could still heat both dhw and space simultaneously with a heat pump, but efficiency has become more important because of rising energy prices, so the general choice is not to.

There is nothing at all to stop you turning off the dhw cycle on your heat pump and heating overnight or whenever you have cheap electricity via the immersion.  It's entirely your choice.

Some heat pump controllers (eg ideal, Vaillant, possibly others) are designed to do this (they 'disguise' it as changing the room temperature

Thank you for that, on those HP that have this facility then setback will work.

The Grant doesn't have this, but it may be included in the new controller.

It's actually quite a revelation to see how Vaillant and ideal have designed the UI to WC.  I suspect this because they have 20 plus years of experience of compulsory WC (for boilers) in some mainland european markets, so have worked out how to present it to the 'punter' in a way they have some chance of understanding 

What they have done is actually quite simple, but goes a long way to bridging the gap between the human desire to control based on IAT, and the fact, dominated by unavoidable physics and the dynamics of control loops, that control based on OAT works better.

I have WC on and have long pipe runs in a dormer so the furthest bedrooms are always on the cooler side especially my sons which has also has 3 external walls which doesn't help.

I've never got my head around why 2 HP were installed rather than a 17kw single unit but that was the installers perogative at the time. I also don't understand why the heating is switched off when heating the HW cylinder and would much rather heat the cylinder via immersion at TOU tariff overnight to give consistent heating but it was never set up that way.

@allyfish has provided invaluable resources and I have adjusted flows etc which has helped with running costs but is still hellishly expensive November to March to run only to be offset in the warmer months.

I'm hoping that when the HP (?20 year life span) do need replacing the technology then will at least half the running costs.

The Grant does have an eco and nightime mode

I just looking into those myself. From what I gather you just put a wire between the two contacts to turn it on. You could use a Grant EP001 (edit should read EP002) and a mains switch inside the house and just run a 12v wire outside, connect to each side of the terminal. Then you can turn on/off those settings when required.

Not sure if you can switch them on via modbus, without the wires.

I did find a note somewhere that one will slow it down and the other will effect the compressor by 50%, but it also seemed to indicate the the compressor may run at this setting all the time. At present my compressor runs below 40%

Some heat pump controllers (eg ideal, Vaillant, possibly others) are designed to do this (they 'disguise' it as changing the room temperature

Thank you for that, on those HP that have this facility then setback will work.

The Grant doesn't have this, but it may be included in the new controller.

The pump is cycling a lot but I guess it is not short cycling so that is a good thing

Is it cycling or defrost. More likely the latter at current overnight temperature