Heat Pump - where to start?

They would not give me the details or calculations once our house had been deemed unsuitable.

Probably just not attractive to their business model.  I'm certain it could be done, but you'd have to find someone willing to do all the design and remediation work and it wouldn't be cheap.

What size space have you got for the cylinder, and have you got a loft space? Horizontal ones are available, granted not as optimal as vertical ones. There are also vented versions that can be used as others have said, slimline too and a few square options that also may work as they also often have everything you need inside them so less space required for pipework etc.

Octopus just pick the low hanging fruit, because they claim install is only £XXXX anything that is outside of that boundary or the products they want to use is a no.

In terms of the hot water tank, it comes down to whether the installers (or the equipment suppliers) are willing to warranty the overall system. I had a good sized 250litre tank that enough coil surface area to "do the job" and the tank was more than fine fir our actual usage. The overall system supplier (Midea) wouldn't warranty the overall system without installing a 300litre tank based on their assumption of out usage cycle as it could result in the system "not performing" - either the heat pump cycling too often or future users having a higher hot water demand.

Most installers will want a hot water tank and another buffer tank in the system yo reduce the number of starts of the heat pump as it gives the system the opportunity to smooth out the stop starts by heating water that you don't strictly need it to.

All a bit too Conservative in my view, but they are the ones backing up the performance/warranty, do you just have to go with the flow.

Set the heat pump to run at reduced output on hot water..... problem solved

The hot water tank was an issue for me as the existing one is in an airing cupboard in the eaves and couldn't cope with the bigger tank without extending into the bedroom.

It got relocated to the garage and I'm planning to box it in with some studwork and insulated plasterboard, but tanks are so well insulated it's not a big deal.

Longer pipe run but that's about it. Needed to replace the shower with one that doesn't have a flow restrictor in it.

Currently trying the low and slow method with our gas boiler, i.e. heating 'on' essentially all the time. All of the main rooms have Smart TRVs which are set to 19C in the day then 18C upstairs or 17C downstairs at night. Flow temp is set to 'e', not sure what that translates to on a Logic Heat 18. House is definitely warm enough (unless it's windy, but that can be fixed). I can see that the heating comes on briefly at times overnight, but this has all but eliminated the large period of constant running in the morning to get the house back up from cooling overnight.

I'm wondering about dropping the flow temp lower, but this will also drop our hot water temp. This lead me to wondering about hot water safety, doesn't your hot water not need to be hot enough to kill any nasties in the tank? If you have a heat pump with a flow temp of 50C, do you risk hot water that's unsafe?

I’m wondering about dropping the flow temp lower, but this will also drop our hot water temp. This lead me to wondering about hot water safety, doesn’t your hot water not need to be hot enough to kill any nasties in the tank?

My boiler has two temperature settings, one for hot water and one for heating

doesn’t your hot water not need to be hot enough to kill any nasties in the tank?

Yes and no:

https://www.heatgeek.com/hot-water-temperature-scalding-and-legionella/

The problem is though that even if you want your hot water heated to 50C you may well need a flow temp of more than that.  How much more depends on the size of the heat exchanger in your tank and how well your gas boiler can modulate.

I have an EV electricity tariff, so I can use the immersion heater to heat as much as I like overnight for less than the cost of gas; but if I want a lot of water later in the day I manually turn the gas on and for that I whack the temperature up.  Some boilers as above do have two temp settings so you can do this automatically.

Heat pumps run different flow temps for hot water. Modern R290 ones flow temp can get to 70deg and they neatly all can run an anti legionnaire cycle, others use a back up heater.
The number of deaths from legionella from domestic hot water systems is very small possibly non existent.

For info, my HW cylinder, expansion tank, controller and all the gubbins fits in my kitchen cupboard with plenty room to spare which is 800 wide, 600 deep and floor-to-ceiling. The installer originally wanted to use the whole understairs which is a metre by a metre.