@jamespa We have a floor area of 97 sq. metres and are fairly well insulated but… we keep our whole semi det. 4 bed at 22.5 with a slight setback from 00:00 to 06:00 (the temperature drops approx. 1 degree during this time and the pump rarely runs during those hours. We have had OAT’s  of ~10 degrees for the last week at night and our consumption has been ~9-12 kW per 24 hours (controlled by Homely). Just thought I would mention this for comparison. Toodles.

Posted by: @toodles

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@jamespa We have a floor area of 97 sq. metres and are fairly well insulated but… we keep our whole semi det. 4 bed at 22.5 with a slight setback from 00:00 to 06:00 (the temperature drops approx. 1 degree during this time and the pump rarely runs during those hours. We have had OAT’s  of ~10 degrees for the last week at night and our consumption has been ~9-12 kW per 24 hours (controlled by Homely). Just thought I would mention this for comparison. Toodles.

Thanks.  A consumption of 9-12kWh (corresponding to energy to the house perhaps 30-40kWh) seems reasonable to me.  If you mean that you are managing on 9-12kWh/day heat delivered to the house, I will be wondering how!

@jamespa Definitely the former! The Homely summary indicates approx. COP of ~4.6 - 4.8 so 50 kWh or so of heat is keeping us cosy each day. As to the accuracy of the consumption metering … Toodles.

I believe that my floor area (excluding the unheated conservatory and downstairs lavatory, and excluding the shed which has an independent heating system) is 59.9 square meters.

It may well be correct that my neighbours are contributing to my heating - I have yet to discover whether I shall need to increase the heat curve or the set point if my neighbours should decide to go on holiday!

As to leaving the heating on being beneficial, I do not quite follow that. The only benefit would be less need to add more heat to the house when I return - but that is already taken account of by looking at overall space heating energy consumption for the day, including the energy required to re-heat after the absence period, which shows a net reduction in energy demand as already documented - so leaving the heat running is indeed like leaving a 100w lightbulb on for 10 hours with nobody in the house.

@jamespetts Consideration might be given to the efficiency gained through ‘low and slow’ heating as this can sometimes reduce overall energy requirement compared to faster heat up performance. but … YMMV! Toodles.

Posted by: @toodles

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@jamespetts Consideration might be given to the efficiency gained through ‘low and slow’ heating as this can sometimes reduce overall energy requirement compared to faster heat up performance. but … YMMV! Toodles.

That is what I am trying to compare - I am trying to get my heat curve right for "low and slow" heating when I am present (0.5 so far and still enough heat), and then also to turn the heating off when I am not present to see whether that reduces usage even more. Heat pumps generally are low temperature heating - in current weather, my flow temperatures are rarely above about 35C and often as low as 25C (as inferred with the Vaillant heat curve of 0.5 and a minimum flow temperature of 23C). 28-32C seems to be the commonest range of flow temperatures at present, although that might increase a little as the weather gets colder.

@jamespetts Some of us (well, me at least) are envious of your low consumption figures! Regards, Toodles.

Posted by: @jamespetts

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As to leaving the heating on being beneficial, I do not quite follow that.

I presume that you heat your hot water at times you are not taking a shower.  Just like your hot water tank your house stores energy.

Posted by: @jamespetts

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The only benefit would be less need to add more heat to the house

Not quite.  Because you would be heating over a longer period you could do so at a lower flow temperature which makes your heat pump more efficient, so you can deliver more energy to the house for less money.  The trick is to balance the time and the efficiency.

Posted by: @jamespetts

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28-32C seems to be the commonest range of flow temperatures at present, although that might increase a little as the weather gets colder.

This, and your consumption, is low that most of the assumptions dont apply because the basic thermodynamics (which argues unequivocally for low and slow) is dominated by the engineering factors (which we cant currently calculate).  This means you have to do the experiment.

However what I would say is this.  At present your electricity consumption for heating is very likely one half to one third of your total electricity consumption and you may well be better off by concentrating on the other two thirds rather than getting too obsessive over heating.  Obviously I don't know your lifestyle but typically a house consumes 3500kWh/year of electricity without heating.  Thats your electronics, refrigeration, cleaning, lighting.  This equates to a UK average 450W per household, whereas your heating is currently 150W.  If you want to reduce energy consumption (which is definitely a good idea) you may want to look also at the things you leave on overnight.  Several 'low power' devices each consuming 25W may well be more than your heating!  Obviously this balance will change as it gets colder but in most of the UK its actually quite mild for most days.

Please dont take this as a criticism, many on here (including me) admire the low energy use of your heating.  All Im doing is reminding you that you use (and pay for) energy for other things also and, once you get to the admirably low figure you have for heating, its worth taking stock of what else matters.

My purpose on creating this thread was simply to document my tests of the oft-made claim that leaving the heat pump running all day is ultimately more energy efficient than turning it off when one is out for an extended period.

So far, those tests appear to show that that claim is not correct, at least in the case of the conditions in which my heat pump operates. This, in turn, suggests that it is not wise for anyone to assume that this claim is true for them without also testing it. This may be reason at least to be somewhat sceptical of other, similar claims, such as that it is better not use TRVs and also test those claims before acting on them.

I will continue to document my findings here lest my findings so far cannot be replicated over the long term. I should encourage others alsoyto test this claim with their own setups.

Accidental multi-post.

Accidental multi post.

Posted by: @jamespetts

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My purpose on creating this thread was simply to document my tests of the oft-made claim that leaving the heat pump running all day is ultimately more energy efficient than turning it off when one is out for an extended period.

So far, those tests appear to show that that claim is not correct, at least in the case of the conditions in which my heat pump operates. This, in turn, suggests that it is not wise for anyone to assume that this claim is true for them without also testing it. This may be reason at least to be somewhat sceptical of other, similar claims, such as that it is better not use TRVs and also test those claims before acting on them.

I will continue to document my findings here lest my findings so far cannot be replicated over the long term. I should encourage others alsoyto test this claim with their own setups.

Thanks for doing so.  We do need more data on this and many other things because there are lots of variables only some of which theory can currently take into account.  This week is a particularly good one for such tests (in the south of England anyway) because we have several days in a row when weather conditions are very stable,  something that happens only rarely.

That said, until we have robust data to the contrary, the guidance based on theory is, i would suggest, still the best starting point for most that we currently have.  Many won't be bothered (or won't know how) to run proper experiments and so we do need a way for those, perhaps the majority, who aren't very interested in the detail of heating, to procede.

Keep posting!