Back in the 80's I fitted a Main 7 boiler for my domestic hot water so I could move some inertia walls and make the airing cupboard into a third bedroom. This was before CH and DHW boilers were combined. It worked well for 40 years, on selling the house the two boilers were swapped for a combi by new owners (My Son).
It was done to gain room, but it did have it's down side, same with my late mothers house, Combi fitted because the water tank was leaking.
However in both cases the problem was size of pipes, to get domestic hot water there were some large diameter pipes which need filling, so turn on a tap and one gets a bowl full of cold water before the hot water arrives, had the house been designed to use a combi, then pipes would have been 15 mm not 32 mm. Combi in mothers house was in kitchen, so kitchen taps were OK, my old house it was in garage, so all points slow to get hot water.
There was also a delay with the boiler, it takes time before the output water is hot, there was a option with mothers boiler for a small reservoir which could be selected to reduce heat up time, but using it resulted in shower going hot, cold then hot again as the reservoir ran out before the boiler warmed up.
There old cistern was low pressure with a hot coil to heat it from the central heating, the hot coil was not very fast transferring the heat, I know with this house the boiler only runs 20 minutes to heat DHW before the return becomes too hot, and the tank is no where near hot enough, the hot coil simply can't sink 20 kW of heat. But the new cisterns heat exchangers can sink enough heat so the hot coil can supply the domestic hot water at mains pressure and the cistern is still header tank fed, so no annual tests required.
The main advantage of using stored hot water is one can combine multi heating methods, and use off peak power, my brother-in-law had this system, when he moved he looked into having it in his new house, but cost was around the £20k mark, and at 72 not worth the money it will never pay back.
So we move to the cost side, tanks of water are heavy, and if using solid fuel they need to be high up so thermo syphon will still work with a power cut, we don't want a back boiler over heating.
So we start to look at the whole plan, not simply the heating, I with electric solar panels use the excess in the summer to heat my domestic hot water, that is cheaper than fitting extra batteries, so I want a storage tank as it saves me money. My tank is not pressurised, and the DHW is at low pressure, I can still have a power shower, but my problem is getting the hot water supply to the shower, at the moment they only have a cold water supply. Also cold water would need to come from same header tank as hot water. This could be done, but the point is cost of doing it.
And that is the major problem, it is easy to say best system is xyz so fitting it into my new build, but to convert an old house costs a lot of money, so we still use instant heat electric showers, OK if we shower at right time of day cheap to run, solar and battery provide some of the power, but using the DHW would be better.
I have a big problem, my floors have been repaired with plywood over the original MDF so to access the pipes between the beams means drop the ceiling. Not impossible but expensive. And with an oil fired boiler, the combi option is really a hot water storage tank built into the boiler, it is not like the gas combi. If I was using gas then combi would be possible, but then I would have a massive storage tank in the garden as would not be allowed where my oil tank is.
Some people do have mains gas, if I had that then would use gas, but the main point is one has to plan as a whole, not just how am I going to get DHW, it is how can I best do it with this house. Are my floors strong enough to take weight of a water tank, can I route new pipe work, will I be adding solar panels, will I want EV charging, how big is the electric supply, can I have electric showers on this supply, if I do can I still charge an EV, even can the EV be used as a reservoir to run the electric shower.
I have solar panels which can still charge my battery in a power cut, and I have the oil fired central heating supplied from the battery back up system so with a power cut my central heating still works. Which is why to my mind heat pumps electrical powered are non starters. An oil powered heat pump would be good, but not seen them marketed, other than for refrigerated vehicles. Likely it would need a generator to supply the electric to an electric heat pump, and the government has changed the tax laws so generators must run on diesel engine road vehicle (DERV) fuel, so now rather expensive to run a heat pump which can still work in a power cut.
