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.

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.

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.

Is anyone doing this? Are the newer models more flexible if suitably sized, can they get a house warm quickly within several hours from cold?

I do not normally run my heat pump like this but I did indirectly run it one night like this when the heat pump went into DHW mode but due to a configuration issue the diverter valve failed to open. I woke up in the middle of the night with a very hot house only to discover the DHW cycle had been driving the heat pump at 60C+ for a couple of hours and feeding the radiators not the storage tank :-D

Regards

That's it - when the situation demands it, how quickly can a good and well configured A2W system, driven hard, get the house back up to temperature?

I'm in the position of chewing over what to do about my heating as the existing boiler reaches end of life, and one of the big concerns I have about converting the existing gas central heating to an A2W heat pump is whether these systems are responsive / dynamic enough to be convenient. The impression I've got from all of my research, and this thread is reinforcing it, is that these systems are not flexible or dynamic, getting a typical A2W heat pump installation is basically locking yourself into heating the house 24/7, with minimal flexibility for set-back of upto a few degrees and needing to plan 24+ hours ahead to switch the heating back on. 

This is much more inflexible than existing gas central heating (or indeed any other form of heating) but also much more inflexible than alternatives like A2A heat pumps, which have outputs that can get rooms warm pretty quicky, acting much like a fan heater.

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.

Most of the answer has been addressed with the ability to remotely control settings and therefore to pre-empt the return to the house. However, the remaining exceptions (had to cut holiday short, hurried home and have only just remembered about the heating as I’ve walked through the door) are still valid and haven’t been addressed. My personal feeling is that temporarily switching the heat pump to fixed flow temperature and running at 50 degrees and more before switching back later to weather compensation again is obviously not the most efficient but hardly likely to cause any issues and will result in only a small blip in the long term heating costs. What price being warm, eh?

Obviously, it’s better if one can remember in advance to let the heating build up before you walk through the front door, and if one has the luxury of a bit of forward thinking then, as @jamespa says, it’s worth exploring whether turning the heating off altogether during the holiday is the best strategy in the first place. Only the homeowner can decide that.

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Is it such a trauma to arrive home to a cold house? Come on now, really?

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Spoken like someone with no medical issues. We’re not all the same and if a homeowner deems it necessary to get the house warm again quickly, none of the rest of us are in a position to argue and even less to trivialise or dismiss something just because it’s not important to us.

I dont, because I dont turn it off unless Im going away for several days, and in that case the technology to schedule a timed restart (and if necessary to change the time of the restart remotely) is built into the controller.

So how far in advance of returning home, do you need to turn it back on to recover the temperature - what sort of temperature rise can it achieve, what set-back do you use and how long does it take to recover if you were going away on holiday?

I don't know exactly how long it takes to heat up (and to specify I would need to specify conditions) but in very general terms its 24 hours+ for the fabric (walls, floors) properly to heat if its got down to say 10C air temperature inside, although the inside air heats up in perhaps half that time or less.  I know that because my heat pump was fitted when it was pretty cold and, despite using a 4kW log burner as back up (my house needs 7kW at design temperature), the house got pretty chilly.  

If we go away for a couple of days I might turn the temperature down by 2-3 degrees and turn it back up half a day before we come home.  We typically don't do long winter holidays and obviously the question doesn't arise in summer, but if we did I would probably turn the house temperature down to about 10C.  As I say the holiday function on the controller (which almost every heat pump and many boilers feature) allows you to schedule and remote control allows you to override the schedule.  Its just a non-problem and certainly not worth compromising performance the rest of the time for (which I think is the key point here).

That said nothing particularly wrong with accelerating the recovery after a holiday by increasing the flow temp above the normal value for whatever the OAT is, so long as you don't rely on being able to do that when its very cold outside and thus the heat pump may already be working at or near capacity.  This probably will cost about the same or bit  more than it would to raise it slowly.

Returning to the title if you do want to heat up rapidly then 4 components need to be in place so that all parts of the 'chain' work

1.The emitters need to be sized for a lower flow temperature than the 'rapid heat' temperature, so that their output can be increased for reheat.  Thus you would 'normally' operate them at the lower flow temp and only use the higher one for reheat

2. The heat pump needs to have the additional capacity needed

3. The heat pump needs to be capable of operating at the higher flow temperature

4. The heat pump needs to have controls that allow this to be initiated easily

(1) is what you naturally do anyway, ie design the emitters for (say) 45 C flow temp.  Thus if you can run at 65 for reheat, in principle the emitters can output a little more than double the normal amount

(2) can be done subject to space and noise requirements but will incur a penalty because a heat pump with (say) double the capacity needed is likely to perform relatively poorly at a 'typical' OAT because its forced to cycle

(3) now happens naturally, particularly with R290

(4) depends on the model.  With most heat pumps there is an easy way to adjust the flow temp by 5-10C, but fewer feature an easy way to adjust it by say 20C which is what you need to approx halve the reheat time.  Of course its possible but perhaps not so user friendly

Overall it seems easier and more cost effective simply to use the holiday/remote control function but that is of course an individual choice.

So how far in advance of returning home, do you need to turn it back on to recover the temperature - what sort of temperature rise can it achieve, what set-back do you use and how long does it take to recover if you were going away on holiday?

Why do something which isn't needed, is uneconomical and shortens the life of the ASHP/boiler?

Because there's situations where it is needed. You do want to get the house warm quickly when your plans change and you come home early or unexpectedly, or the heating has been off for some reason - maintenance, break-downs, power-cuts and you want to get the house warm. 

That isn't every day, or even every week, but having a heating system which can heat the house up reasonably quickly when needed is quite an important feature. 

No it isn't, these occasions are very rare, it's like buying a fire engine 'just in case'.

Is it such a trauma to arrive home to a cold house? Come on now, really?

You've been able to get Geofencing for a long time now, it automatically detects when you leave the house and lowers the temp, lifts it up when you're coming home. That and being able to turn the heating on manually, remotely mean those days of coming home to a cold house are a long time gone.

Getting a heat pump took the comfort improvement  to another level, why would I go back when it involves spending more money for reduced comfort?  

But if you got home and found the house cold, for whatever reason, how long does it take for your heat pump to heat your house up to 20 deg C from cold, say +10 deg C, from 10 deg C upto 20 deg C ?

I dont, because I dont turn it off unless Im going away for several days, and in that case the technology to schedule a timed restart (and if necessary to change the time of the restart remotely) is built into the controller.

Also dont forget its not only flow temperature that matters its also capacity.  So if I wanted to be able to do what I dont actually need to do even when its very cold outside, I would need a heat pump that has a much higher capacity as well as a high maximum flow temperature, which would compromise its performance the rest of the time.