Posted by: @cookie197

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JamesPa commented on this thread about what he calls "grant harvesters" - I guess they're doing the "easy" properties.

That would be a reasonable and morally justifiable strategy but, based on my own experience and that of others here and elsewhere, they are in fact doing over the easy to fool clients and the taxpayer.

@cathoderay 

Thank you - this is really interesting. Seeing the recovery time like that really puts it in perspective! My question would be, if I'm using the woodburner as another heat source when the HP is off, presumably that would reduce the recovery time, but would it reduce it enough to be feasible? Last winter we got by almost all the time just using the woodburner, and only putting the heating on for a few hours each day when the outside temperature remained very cold. The house is well-insulated already, and we have outer and inner doors front and back, so not much heat escapes that way. The biggest drawback to the woodburner is the time spent fetching logs and feeding the thing, because the temptation is always to loiter in front of it for a bit! We only burn dry, well-seasoned wood, and we're tree planting as well in an effort to offset the woodsmoke issue, but we are low income, and the choice last winter between free heating and very expensive heating was a no-brainer! This is one of the main reasons why I'm hesitating - I already know that I can make this system work relatively cheaply, but I don't yet know enough about HPs to be convinced that I could achieve the same end, and I can't afford to make a mistake!

Would it be feasible to run the HP during daylight hours, to make use of the free solar energy, and keep the house warm with the woodburner overnight? I doubt this is what I would actually do, but I'm just curious to know if this sort of combination approach would work.

In response to your second reply, I think the problem is that properties benefitting from the ECO4 deal must be shown to have moved at least two steps up the energy rating ladder, and as soon as these companies encounter something with leaky windows, or that are less than straightforward in some way, they simply say no and move on to the next "easy" one. There must be companies out there who could help my friends, but I think they're waiting to see what I do, so they can have a nosy around mine, if I put one in! (We're like penguins on an ice-shelf - none of us wants to be the first to take the plunge!)

Thank you!

@cookie197 

The topping up strategy with the woodburner will work just fine. Since you will want most of your heat in the living room, a wood burner in there means that you could be toasty there and use a lower weather compensation setting for the rest of the house to keep your bills down. If your insulation is good enough you might well be able to get away with just using the heat pump in the day as the walls will store a fair amount of heat.

@jamespa 

From what I'm learning here, I'm very much inclined to move away from the "cherry-pickers" and try Octopus, and then see what grants are available to support whatever they suggest, and in the meantime also continue to try to get solar panels installed (separately from "the deal"). Now that I understand a bit more about HPs, I'm more confident to go forward and have a discussion ... as I've said before, when you're coming from a position of ignorance, it's so difficult not to be bamboozled by snake-oil salesmen who aren't always entirely honest. I'm so grateful for your advice!

Posted by: @cathoderay

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On balance, I am still pretty sure that the laws of thermodynamics mean that it must take more energy to keep an entity, in this case a home, at say 20 degrees all the time, than to keep it at 20 degrees for 12 hours a day, and allow the temp to fall a bit in the other 12 hours, but I am stumped to give a mathematical proof for this.

I can say with absolute confidence that you are correct.  The house must lose more energy and thus must be supplied with more energy if it is hotter.  The laws of thermodynamics do guarantee it.

However you don't pay for energy lost from the house, you pay for energy supplied to your heat pump.   If, by allowing the house to lose a bit more energy, the heat pump works more efficiently because it doesn't have to strain itself, that may, and in some circumstances will, outweigh the extra energy lost.  Heat Geek have a worked example of this phenomenon here

  The example is for spatial zoning, but the same principal applies to temporal zoning.

Now obviously this has limits and at some point its better to let the house cool/zone it.  With spatial zoning shape of house is clearly significant and one might take a guess at where the crossover point lies.  With temporal zoning its not at all obvious, clearly 'thermal mass' (how much energy is held in the fabric) is a factor as is rate of loss.  It really needs someone to model it and ideally test the model, to the best of my knowledge nobody has, of if they have then they haven't made the results well known.  There are quite a few variables and an infinite number of house types and patterns of occupation, and so its a subject capable of considerable exploration.  Maybe not a PhD, but possibly a dissertation for a masters!

I did spend a few hours a few months ago in an attempt to start a crude model, but got distracted into the challenges of actually getting a heat pump installed rather than pontificating about it!

Posted by: @cookie197

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@jamespa 

From what I'm learning here, I'm very much inclined to move away from the "cherry-pickers" and try Octopus, and then see what grants are available to support whatever they suggest, and in the meantime also continue to try to get solar panels installed (separately from "the deal"). Now that I understand a bit more about HPs, I'm more confident to go forward and have a discussion ... as I've said before, when you're coming from a position of ignorance, it's so difficult not to be bamboozled by snake-oil salesmen who aren't always entirely honest. I'm so grateful for your advice!

Defo give Octopus a go but they might not touch your house at present if it doesn't fit their 'model'. I think they are trying to perfect some processes on relatively similar properties before trying to tackle the generalised retrofit problem.  British Gas are doing something similar - if you trust them.

And that's the nub of the issue.  Design for retrofit is a very different beast than design for new build.  It requires genuine understanding, flexibility and out of the box thinking to make use of as many existing components as reasonably possible in order to minimise cost and disruption. 

Furthermore you have to grapple with the fact that you don't really know what the fabric construction is, so you cant possibly calculate the heat losses accurately.  Of course you can still have fancy spreadsheets which give the impression of being scientific, but GIGO applies.  Add in the fact that hardly anyone ever bothered sizing a gas boiler, they just fitted (say) a 28kW one and left it to its own devices, and you have a sound basis for at least some installs presenting problems.   

Currently the most pervasive practice is to overdesign, sometimes by a large margin, to account for the uncertainty, which is bad for heat pump efficiency and frequently introduces additional complication which, in reality, may be unnecessary.  However (unless the plumbing is particularly bad - of which there is an example in another thread) it avoids 'im cold' call outs which of course is about the only thing that matters to most installers. 

There is actually some evidence that mild undersizing (and making up the difference, which of course only manifests on the coldest days, using other means, even resistance electric), is beneficial from an efficiency/cost PoV, but few installers will even countenance that and the current MCS and grant rules make it almost impossible unless the installer is very creative.  

Solar panels went through a similar stage (but not so problematic because its fundamentally simpler).  They are now much more mature, so much so that here in the South East its nigh-on impossible to get a solar installer to touch them unless they also supply a battery system, because there isn't the margin. 

Batteries are a whole other ball game.  The thing to remember that they inherently have (almost) no environmental benefits, so a decision should (IMHO) be based on a pure financial business case.  The business case is made more difficult by the better export tariff that Octopus offers, but if you are installing a new PV system from scratch can definitely work.

Don't be put off entirely, there are clearly good installers out there because many report a positive experience.  The key is to maximise the probability you find one.  I will personally discover if I have succeeded over this coming heating season!

@jamespa I've been in contact with Octopus, but they can't help me with a heat pump as they currently don't have installers in our area, so I'll have to start hunting for installers locally. I'm thinking I might wait until I get the info back from the company that did the survey, as it might give me some useful facts and figures to use as a basis.

Thank you so much for all your advice and guidance - it has been incredibly helpful. I wish you had been my physics teacher back in the day - you explain things so clearly that I might have understood some of it! The only thing I actually remember is learning to measure the length of an oleic acid molecule - which hasn't come in handy once since I left school ...

Thanks again

Posted by: @cathoderay

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Posted by: @cookie197

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It still feels oddly wrong to run a heating system all the time

Indeed it does, and it is counterintuitive. If we imagine heating a house to be like keeping a bath with a leaky plug hole full, then it stands to reason that keeping the bath constantly full takes more water than letting the level drop for a while, until we realise that bringing the level up to the top again takes more water than just keeping up with the leak. This is the complicating factor: how much extra water/energy is needed to raise the level/temp, over and above that needed to maintain the level/temp. I should add that I suspect this analogy is not ideal, since I am pretty sure the recovery period (getting back up to level) will use exactly the same amont of water as keeping it at top level all the time - unless, perhaps, the higher hydrostatic pressure from being full means the rate of loss is higher when the bath is fuller (and it is just possible the same applies to heat energy, when the temp is higher, the rate of loss is greater).  

I have never managed to get to the bottom of this question, though I do have some thoughts, One key factor has to be the length of the setback period (setback being the somewhat obscure term for the period when the heating is off - presumably derived from idea that the heating has had its thermostat dialed down, ie setback, when in reality it is a timer that has turned the heating off). Given a long off period - say a week - then clearly one saves energy. But what happens over shorter periods, say over night for six hours? Clearly one saves six hours worth of energy use, but that has to be set against the extra output needed during the recovery period. On balance, I am still pretty sure that the laws of thermodynamics mean that it must take more energy to keep an entity, in this case a home, at say 20 degrees all the time, than to keep it at 20 degrees for 12 hours a day, and allow the temp to fall a bit in the other 12 hours, but I am stumped to give a mathematical proof for this.

But in reality, we are overtaken by the characteristics of a heat pump, and its inability to run really fast and furious. It has to by and large rely on 'heat momentum' to keep on top of things, hence the general advice to keep them running 24/7. On normal settings, my heat pump takes hours if not days to recover from a setback, be it a deliberate one set by me, or an external event, like a power cut. Here's a chart of my kitchen room temp during a trial period of setback last December (on this occasion, this was a setback, from 19 to 16 degrees, rather than a binary on/off, though in effect it behaved as if it was a binary on/off, ie the heat pump didn't run at all during the set back period). As you can see, it took all day to recover from the setback. In fact it only really got back to design temp in time for the next setback, had I not turned the setback off:      

-- Attachment is not available --

I was intrigued by the analogy between heating a house and a bath with a leaky plug hole, so I created a spreadsheet simulating a bath 100cm long and 100cm wide filled to 20cm depth. In this scenario, I set the leak to be 120cc/min (when depth of water = 20cm) and calculated the change in bath volume, depth of water and leak rate every 10 minutes. After 5 hours, the water depth had fallen to 16.7cm. (The numbers were chosen to be similar to the fall in temperature, which might occur during a setback in a house heating scenario). The volume of the bath water had fallen by 33,000 cc and the leak was reduced to just over 100 cc/min.

I then filled the bath with varying inflow rates. At 120cc/min it took 1.38 days to get the water depth to 19.0 cm and 4 days to get to 19.9 cm. I was quite surprised how much more inflow I had to use to get the depth of water back to 20cm in a shorter time. I needed to increase the inflow by a factor of 4 to restore the depth of water to 20cm in 1.5 hours. To restore it in 5 hours, required a factor of 1.86.

In virtually every case using different inflows, the saving in bath water volume (analogous to kWh of energy) was around 8%. With a gas boiler, this would probably translate into energy and financial savings. However, with a heat pump, one would expect the reduced efficiency from higher flow temperatures would negate any savings.

And yet, my own experience of not running the heat pump between 10pm and 4am is that I think that I save money. I believe the answer to this apparent paradox is that I never allow the house heating to achieve a steady state and that if I did do so, then I may find that I was using between one half and one third of the power that I needed between 4am and 7am to restore the house temperature. In January the power use was around 2kW with an outside temperature of 10 deg C and 4kW when -3 deg C outside. If I left the ASHP on all the time, once the house heating was in equilibrium with the heat loss, would the power needed have been 0.7 to 1.5 kW? Some would say 'Yes!'

@mike-h This is an intriguing calculation - thank you. My inclination (as a total novice to HPs) is still to not run the HP 24/7, but rather do something like you do and leave it off overnight. Most of the general advice I've seen is to run the thing 24/7 as that is the most efficient way, so I'm pleased that I'm not the only one considering the way you run yours! We have the advantage that our woodburner can heat the house quite rapidly, even on very cold mornings, which would, I think (hope?), negate to an extent the extra effort the HP would have to make to bring the house back up to temp. As I've said before, we've never had a HP, so we're having to learn a whole new approach to heating. We're going to see a complete system "in action" next week, which I hope will make things clearer ...

Thanks again!

@mike-h

I find a better analogy to represent a home, is a smaller vessel containing equidistant pins holes (heat loss) located inside a larger vessel with a drain, the level of which can be varied to denote changes in outside temperature.

So if the water level in the outer vessel is low indicating cold outside temperature, there will be more pin holes uncovered through which water can flow from the inner vessel to the outer vessel. To keep the level in the inner vessel constant it will therefore be necessary to feed more water into this vessel to replace the losses.

If the level of the drain on the outer vessel is now raised, indicating an increase in outside temperature, the loss from the inner tank to the outer tank will reduce, so less water will need to be fed into the inner tank to maintain a constant level.

Obviously in the real World things are a little more complex.

On sunny days, even when the outside temperature has not been too high, solar gain may have heated the outer fabric of some of the building, which has the same effect as having a higher outside air temperature. This not only causes a reduction in heat loss during sunlight, but can also continue to have an effect after the sunlight is no longer hitting the building.

At other times higher winds can have a cooling effect, hence increasing the heat loss although the outside temperature remains constant. The same can be true of rain, which will not only cool the roof, but also possibly some of the outer walls of the building.

With sufficient measurements, and data, it may be possible to create a model of your home's heat loss under varying conditions.

Posted by: @cookie197

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@mike-h This is an intriguing calculation - thank you. My inclination (as a total novice to HPs) is still to not run the HP 24/7, but rather do something like you do and leave it off overnight. Most of the general advice I've seen is to run the thing 24/7 as that is the most efficient way, so I'm pleased that I'm not the only one considering the way you run yours! We have the advantage that our woodburner can heat the house quite rapidly, even on very cold mornings, which would, I think (hope?), negate to an extent the extra effort the HP would have to make to bring the house back up to temp. As I've said before, we've never had a HP, so we're having to learn a whole new approach to heating. We're going to see a complete system "in action" next week, which I hope will make things clearer ...

Thanks again!

Take a hypothetical home, with a calculated heat loss of 12 kW at an outside temperature of -3C, when maintaining an indoor temperature of 21C.

So for each 1C difference between the indoor temperature and the outside temperature there is a heat loss of 500 W.

If the outside temperature is 9C, then to keep the indoor temperature at 21C, it will be necessary to supply approximately 6 kW of thermal energy into the building. If the desired indoor temperature is now set at 20C, the required supply of thermal energy will now be reduced to 5.5 kW. The reduction in thermal energy demand will therefore be approximately 8%.

Obviously this is very simplistic, since many other factors can come into play, but it can be used as a good starting point in assessing heat losses and how best to optimise overall thermal efficiency.

@derek-m I take your point that adding more variables will make the resulting model much more complicated. However, the purpose of my simpler model was to suggest that there may be indeed a very good scientific explanation to support running your heat pump 24/7. Up until now, I have been far from convinced. Now I am not so sure.