Did you ever get to the bottom of this my setup sounds similar with similar problems ?

My thinking has certainly moved on since then but I’m far from a resolution.

One thing that is now very clear is that the system is grossly oversized. In the actual heat loss calculations from the vendor they think the heat loss is 5.1kW, in contrast with the 6.1kW figure they had in their performance estimate. Either of those indicate that the 9kW heat pump is absurdly oversized for the house. In fact it's even worse than that since, as I suspected, they haven't taken into account some of the insulation improvements we've made. So, even following conservative MCS heat loss methodology my calculations suggest a heat loss of around 4.2kW. I'm in the throes of putting together quotes to replace the heat pump with something smaller in order to put together a legal claim against the installer since this shows they really dropped the ball with the sizing and they're being very slow to admit that self-evident fact.

What makes the situation more complicated is that the DHW performance is still extremely poor. I'm only heating the cylinder to 47C (either when the temperature drops to 37C or daily - it doesn't make a difference) and I'm only getting a DHW SCOP of 2 in East Anglia. Given that the heat pump is heating a cylinder from the same manufacturer which is a mere 2 metres away. I can't conceive of a situation where system design could possibly impact that, so I think there also must be something wrong with the heat pump. Daikin have been worse than useless in trying to find a solution to that. The first two engineers they sent out didn't know anything. A more recent one was fine but didn't find a root cause.

That leaves me with an installer who is responsible for the system design and sizing and the manufacturer who is responsible for the heat pump itself. It's a pain but hopefully legal action will raise the temperature enough so I can get some movement.

My heat pump isn't Daikin but same rules apply, it's 6kW, it could be 3kW for heat loss.

Zones, make into a single zone, any zoning can lead to small area to output the heat - leading to short cycling, poor defrost, reduced circulation - more short cycling.

Big heat pump can work but you need to work on commissioning yourself. Small heating areas don't work, you need to get loads of flow through the system and lots of engaged water volume - 9kW in the region of 90L to stop short cycling.

Don't do set backs. The heat pump plays catch up running flat out until it gets back into balance. Two set backs, one out at work and night means it never gets to steady state = big consumption.

All the thermostats set to 3 Deg above target room temp, then balance the rads or floor loops to get room temp correct. One big zone, run 24/7.

If I leave the heat pump to manage the cylinder it does it 4 or 5 times a day, each time the thermal current stir up the whole contents of the cylinder and heating takes 45 mins. If I set times (X2) it still only takes 45 minutes each time, we never run out of water. Set to timed operation, to get better use of your energy. The trouble is the temperature probe is near the bottom of the cylinder so you run the tap a few times the bottom few inches is cold, but the rest of the tank is hot. But you get a demand to heat cylinder. Really good you may get 2.5 CoP in good weather, most the time it's worse.

Posted by: @johnmo

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Don't do set backs. The heat pump plays catch up running flat out until it gets back into balance. Two set backs, one out at work and night means it never gets to steady state = big consumption.

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Whilst you're correct in your technical thinking, @johnmo, I'd still inject a word of caution.

Any means of heating one's house and domestic water is for one's comfort. Any solution that is a significant compromise on comfort is not a workable solution. If you are happy with a house at a constant temperature then that's fine and dandy. If, like my wife and me, you can't sleep when it's too warm and are too cold if a comfortable night-time temperature is maintained during the day then setbacks are simply necessary. If so, it's a process of finding out the least inefficient compromise that still provides the home environment deemed necessary.

As a result, I wouldn't say "don't do set backs"; I'd prefer the message "think carefully before implementing set backs". In other words, never say never but go in eyes wide open.

Posted by: @majordennisbloodnok

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I'd prefer the message "think carefully before implementing set backs

Sounds fair. But the OP needs to move away from many thermostats and multiple setbacks and implement a steady state setup assess energy consumption and comfort compared to now. Then move forward from there. Comprising comes after understanding what does or doesn't work.

I didn't do any of the above last year I did batch heating - using UFH and slab as a big storage heater. Cost less than gas, due to mostly cheap rate electric and battery, but I did use a lot of kWh's. More than I am comfortable with. This year I will be practicing what I preach, if only to understand the whole benefit . So zero setbacks - in fact no thermostat or timers, just straight WC, across a 195m2 bungalow and summer house. Been doing cooling that way for the last month or so and it's worked really well.

…and there I can agree, @johnmo. You’re describing a textbook case of simplifying a problem to understand it better and it’s sound advice. Mine was only a note of caution because we can all get wrapped up in improvements whilst forgetting what we’re actually trying to achieve, and as long as that end goal is understood and kept in sight real progress will be made.

@mark-h did you eventually get this matter resolved with the installer?