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

We've had the tech to switch on the heating before we get home for a long time now, automatically.

Posted by: @deltona

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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. 

Posted by: @temperature_gradient

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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?

Before I got a heat pump I turned down my gas boiler flow temperature from 70 to 50 which was the lowest it would go.  The result was a much more comfortable house because of reduced temperature gradients in both time and space and reduced gas bills because the boiler (just like a heat pump) operates more efficiently when run at a lower temperature for longer.  Getting a heat pump took the comfort improvement  to another level, why would I go back to the previous mode of operation when it involves spending more money for reduced comfort?  

Posted by: @temperature_gradient

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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. 

If you go on holiday it of course makes sense to switch it off.  If you are out for the day it probably doesn't, unless your house gets cold very quickly.  What most people forget (and the heating aftermarket controls industry doesn't bother to remind us because it more or less invalidates their business model) is that  houses continue to lose heat at almost the same rate even if the heating is off, and that heat which was lost while the heating was switched off has to be replaced when you turn it back on. The amount saved therefore is actually quite small, unless you live in a house that has the characteristics of a tent (ie one that cools very quickly and has low thermal mass).  Turning the flow temperature down by a couple of degrees (a setback) may make sense, turning it off altogether is unlikely to do so.  If you are out during the daytime thats also generally the most efficient time to produce heat, because its warmer outside so COP will be higher.  Producing all your heat when its coldest outside usually doesn't make sense, unless of course you have an aggressive ToU tariff that fits with this model.  If you are out at night the argument for a setback is stronger.

Honestly when you get a heat pump you need to forget more less everything you learned about running a boiler.  Much of it was incorrect or overstated for boilers (but it suits the controls industry to tell us otherwise), even more of is incorrect for heat pumps.  

Feel free to explore further, as ever there are exceptions but in most cases there would be no sense at all in running short and hot.

Posted by: @jamespa

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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 ?

Posted by: @temperature_gradient

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Posted by: @jamespa

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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.

Posted by: @temperature_gradient

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Posted by: @deltona

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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.

Posted by: @jamespa

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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?

Posted by: @temperature_gradient

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Posted by: @jamespa

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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.

Reading through the whole of this thread, it doesn’t feel as if @temperature_gradient is trying to challenge received wisdom. The point, so far as I can see is that there are times when you want to stop heating the house for an extended period and then bring it back to an inhabitable temperature, and the initial question was whether a short period of high flow temp (i.e. us the heat pump like a boiler) is a good way to achieve this.

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.

Posted by: @deltona

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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.

Posted by: @majordennisbloodnok

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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?

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.

@temperature_gradient 

Posted by: @temperature_gradient

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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 😀

Regards