Posted by: @temperature_gradient

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Yes I get the whole plan in advance, remote control etc, I've got a Hive with the existing system, but even then there's odd occasions where you forget, but with the existing CH it can start getting the place warm in a few hours. If it's going to take 24+ hours (I've read some discussions with people suggesting even upto 48hr), that's too long and realistically you'd be using other heat sources (gas fire, electric heaters) to help get the house back to a tolerable temperature.

If you insist on this functionality then your principal options are, so far as I can see:

1.  Get an R290 heat pump which is about the right capacity and which has a controller that allows you to increase the FT by 20C+ above the design temp.  Design for say 45C, if you want a rapid reheat use the controller to increase it.  You can do this with several heat pumps from the user menu one way or another, and with all het pumps from the installer menu.  Accept that the trick will not work as well when its very cold, because you will hit the capacity limit of the heat pump

2. Get an R290 heat pump which is about double the right capacity, with a decent size volumiser.  Then as above but the trick should work even if its very cold because you have lots of reserve capacity.  Accept a performance degradation of 10-20% most of the time or use the extra capacity to take advantage of a ToU tarrif, provided your house has sufficient heat capacity to do the smoothing (eg because you have UFH in a slab).

3. Install a properly matched heat pump.  Buy some cheap fan heaters, about £25 for a 2kW one in Argos, for the rare occasion its needed.  These will heat the air quickly so the house will feel  warm quickly.  The fabric can then recover in a longer time.

I know which of these I would choose!

BTW a boiler will take exactly the same time to reheat the house unless the emitters are also well oversized (the boiler is certain to be oversized).  Often they are, but you cant assume that this is the case without doing the math. 

Furthermore, if you do go for this option, then you will definitely need either to arrange for a way to increase the flow temperature when you want a rapid reheat, which your system may or may not provide 'out of the box', or suffer the significantly reduced level of comfort (and reduced efficiency btw) that operating permanently at a higher flow temperature delivers.  Your boiler installer is unlikely to help you with this, because very few if any (other than those that also install heat pumps) understand it properly.  Having experienced the benefits of 'low temperature' heating and weather compensation, I once again know which I would choose!

Im happy to explain any of the above in more detail but the basic message is that there is no such thing as a free lunch with either a boiler or a heat pump, either way you will need to make some decisions which materially affect your comfort/convenience both from day to day and in the rare cases where you need a rapid reheat.

Actually, @temperature_gradient, here is where I disagree.

Since a heat pump can easily send out water at 50degC (and some can do 60degC+, as you say), the radiators can be heated easily enough and warm enough to do exactly what you say. It’s not so much that the heat pump is inflexible as that believing homes need to be subjected to big swings in temperature is cutting down options. Heat pumps can be run like boilers but that is very rarely necessary, and certainly dar less often than you’re seeming to expect.

And if, for a weekend break, it’s cheaper to set back the temperature a few degrees and keep the heating going rather than turn it off completely and then heat from scratch, why not operate  the most efficient and effective way?

This is back to the classic analogy of boy racers. A driving style that constantly accelerates hard from the traffic lights only to brake to a full stop at the next set is always going to put a premium on high performance cars, but a lighter touch on the accelerator will still get you to the same place at the same time. There are times when some good acceleration is desirable (for instance, safe overtaking) but what you need isn’t fast as much as fast enough. However, until we realise there’s more than one way to drive we’ll always be convinced only a Ferrari will do.

Several years ago we went away for a weekend in the middle of a cold snap and without giving any thought to the heat recovery time I turned the heating off. We had recently moved to a larger house with an oil boiler and it was a lot more expensive to heat than the previous house (that was better insulated and had a gas boiler).  I thought I would save some oil by turning the heating off but that was a mistake.  When we got back home the house felt very cold, about 10 or 12deg is what I remember, and it took 36-48 hours with the heating on continuously for the house to feel warm again. After that experience I didn't turn the heating off again during the winter, I just turned it down to about 15 and left the timer set to do the usual thing. If the system design isn't great there will be an issue with any heating system if you turn if off then want instant heat once the house is very cold. 

If there is some additional capacity in the heat pump and the rads are design to run at a low temperature then turning the flow temperature up will give more heat output.  If the heat pump is only just large enough and the outside temperature is at or below the design temperature then you will have a slow recovery. If it is very cold then leaving the heating on low is probably more efficient anyway, and better to keep the house slightly warm and dry.