A couple of thoughts:

What size are the flow/return pipes pipes ?

What are you currently using for DHW heating? Which part of this is at the EOL?

Do you have an EV or are thinking of getting one?

Is improving your house insulation possible?

@bontwoody I think flow / return are 1 inch. I should probably double check that.

EV - yes, 3 currently! Old Leaf going to be sold. New Kia Niro. And my DIY EV conversion of my old VW campervan. Plenty of solar PV on the roof. But cars are pretty low mileage.

How much detail I go into on the failed DHW probably needs to wait until the visit on Tuesday. Not trying to be mysterious, just want to be fair to the supplier. But as it stands it’s seems it’s the part that heats the water and likely unrepairable - tbc. But might still leave me with a viable 300l water tank for the next system to make use of.

House insulation is pretty good, although don’t ask me for numbers. Amazing how quick all that stuff evaporates when you move on to new projects. It’s a new build timber frame. Not passiv haus, but good enough.

What's failing more precisely?  I think the way forward rather depends on this.

I have several ideas but without knowing what must be replaced it's difficult to advise.

My instinct is to delete the buffer at which point you can likely reduce the flow temp from the ashp by a few degrees, thus createing some more headroom on output, and this combine the systems into one.  But without more detail this is nothing more than a wild guess.

Do you have any data from the last year or two which might help inform the way forward.

But the key question remains, What's failing more precisely.

Posted by: @cleardene-ashp

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mileage.

How much detail I go into on the failed DHW probably needs to wait until the visit on Tuesday. Not trying to be mysterious, just want to be fair to the supplier. But as it stands it’s seems it’s the part that heats the water and likely unrepairable - tbc. But might still leave me with a viable 300l water tank for the next system to make use of.

Ok just read this.

So basically dhw is failing.  First of all I'd say fix that, but if not delete buffer on main system, gain perhaps 10% output capacity because you can run at a lower flow temp, add diverter valve, use main system to do everything.  Tank may not have the coil you need in which case fit phe and circulator pump.  This setup will likely cost less to run because of buffer deletion.  Then when main heat pump reaches eol in perhaps 10 years time, update.

Problem may be that nobody will want to guarantee that, people often want to guarantee only completely new systems or ripping everything out.  Personally I'd accept the guarantee limitation because it's fair.

Is it in or out of warranty?

@cleardene-ashp I'm particularly interested in DHW because you mentioned it was smart. Is it a mixergy cylinder? 

I use one to heat my hot water overnight on an EV tariff very cheaply. 

@jamespa thanks for your thoughts.

Whether DHW is fixable will be found out on Tuesday, but they suggest not so far.

I’m sure it’s obvious, but what’s ‘phe’ please?

One thing I will need to find out is whether the current ashp can deliver the heat for DHW aswell. As I said in the intro, I kind of doubt it based on it only just delivering enough UFH heating in winter when set to 38C. But that temp was based on guidance from the supplier when there was a separate DHW system in place. Maybe it can work, albeit less efficiently.

Out of warranty.

Thanks. It’s all useful to start my grey cells to start processing my possible options.

@bontwoody no, not mixergy.

@cleardene-ashp OK. Well the same logic still applies. If it is able heat water with an immersion heater and you can access octopuses 7p per kWh overnight then that's the equivalent financially to a 3.5 ish COP for hot water. That just leaves heating your house to worry about.

Something we use to boost our heating in a cold room was an air conditioning unit. Inexpensive and would reduce the load on an ASHP unit. 

Posted by: @cleardene-ashp

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Whether DHW is fixable will be found out on Tuesday, but they suggest not so far.

OK.  It will help if they tell you why and whats actually broken

Posted by: @cleardene-ashp

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I’m sure it’s obvious, but what’s ‘phe’ please?

Plate heat exchanger., basically like a coil in water but a different geometry.

There are two ways to heat a DHW tank,  The first is a coil within the tank through which water from the heat pump/boiler is passed.  I dont know what coil you have.  If its suitable that's the simplest way.  The second way is to fit a PHE, pump water from the tank through the PHE, and use the heat pump to pump water from the heat pump through the other side of the PH,E thus transferring heat to the water from the tank.  This can actually be more effective if you use the right PHE.

Posted by: @cleardene-ashp

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One thing I will need to find out is whether the current ashp can deliver the heat for DHW aswell. As I said in the intro, I kind of doubt it based on it only just delivering enough UFH heating in winter when set to 38C. But that temp was based on guidance from the supplier when there was a separate DHW system in place. Maybe it can work, albeit less efficiently.

If you remove the buffer which is probably unnecessary unless your HP is very old you will almost certainly eliminate a likely temperature drop across the buffer.  This means that you could reduce the flow temp from the HP whilst still achieving the same FT at the UFH.  This will very likely increase the max output of the HP providing headroom for the DHW.  All of this is a bit dependent on the HP, but the basic effects described are pure thermodynamics, so most HPs will follow them unless the firmware deliberately restricts output (some do).  If you can do this your efficiency will increase because you will be operating the HP at a lower FT.

You may have enough capacity anyway without removing the buffer (you definitely will on most days).  Need to do some calcs to verify that based on your DHW usage and heat pump output on the coldest days. 

Are you certain that its the heat pump that's currently limiting the heating?  Do you have any data from the heat pump?  What model is the HP.  Is it modulating or fixed output?

Posted by: @cleardene-ashp

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Out of warranty.

Well that helps, you have nothing to lose by changing things and moving them in a more modern direction (subject to compatibility with your equipment)

If they declare the DHW unfixable I would spend a bit of time thinking about options, if you can.  You want to be heading in the right long term direction for any investment you make in repairing the failed system.

It would be helpful to know a bit more about the central heating.  You say that it struggles to keep the house warm in winter:

- Are you confident that the UFH has been designed adequately?  

- How are you controlling the primary and secondary heating circuits?  Are you using weather compensation, or room thermostat, or a combination of the two?  

- Are you running it continuously 24 hours per day? Do you set back the temperature at night?

I am wondering whether the heating might be constrained by the heat pump itself, the emitters (UFH), distortion due to the buffer tank, or possibly the way it is being operated.