This is actually quite an interesting problem and highlights the difference between efficiency and cost. 

Heat pumps are most efficient at low flow temperatures and a 24/7 usage takes advantage of this. 

However if a house Is unoccupied for a substantial period it may make more financial sense to run the heat pump hotter for a shorter period of time. 

if you can achieve a SCOP of 3.5-4 then it’s likely you are beating oil. My house has  the heat pump on for 5-14 hours per day and achieves this making it cheaper than the gas combi boiler it replaced. 

I hope that helps answer your question. 

@potatoman @bontwoody The 'like for like' Q is one not asked frequently enough in my opinion. I am not a big fan of the 24/7 on model for heat pumps and obsessing on 'efficiency' (COP) - in my case it is a far more expensive way to heat my property to my needs. Rather, like you, I run my GSHP/UFH for 12h a day - hotter for 6h in the night (where the COP is theoretically at its worst), and 6h in the afternoon. Allowing for some lag, this way my home is warm first thing in the morning and when we get home from after work & evening (aka the conventional gas/oil model) . Other times (night, when we are out at work) the temp drops a couple of degrees, no big problem. The night heating is all done on an e-tariff which is 1/4 the cost of daytime, and the 12h of the rest of the day when the GSHP is off is a big saver. Overall my heating/DHW costs are comparable to gas/oil. Interested to hear of others that run this way

@delt I think also the levels of insulation in a house come into play. I corresponded with one user who used the same amount of kWh with a heat pump as he did with his previous system. His house was so poorly insulated that his COP gains were being totally wiped out by heating at night!

Very interesting question! Unfortunately, there is no clear answer that is true for all situations. It depends on thermal mass, insulation, comfort needs, emitters (rad vs UFH), tariffs, etc. Even the temperature evolution during the day matters, as it might be better to preheat a bit when it's warmer outside if you expect a temperature drop versus running fully open loop, because heat pump runs more efficient when it's warmer outside.

@delt Fully agree with your statement that the industry should focus on cost for the end user, rather than COP!

This got me thinking. We have solar PV so the direct comparison between boiler and ASHP for us was offset by the fact we can create some of the energy needed to run the house heating system and so the savings were rather obvious for us. However....

It seems pretty obvious that, with a reasonably well insulated house, running a modern and efficient gas boiler like an ASHP (i.e. relatively low flow temperature and with nice big heat emitters like large radiators and some underfloor heating, with the boiler kicking in whenever needed to maintain a pretty constant ambient room temperature) will use about 3 times as much actual energy to heat the house compared with an ASHP, so will be less cost effective if the leccy is less than three times the price of gas and more cost effective otherwise. All well and good, since currently for us (Octopus Agile) we're currently getting our leccy at an average of about 17-18p per kWh whilst gas in the same area supplied by Octopus is just under 6p per kWh, so by that reasoning both should be about as cost effective as each other.

Of course, the big benefit of a gas boiler is that it can chuck out a lot of heat quickly so if you come back from holiday into a cold house the boiler can get the rooms feeling toasty very quickly; far quicker, in fact, than the fabric of the house can absorb, so you can easily have cold walls and a warm room. Certainly not one of the ASHP's strengths. On the other hand, once you have the building's fabric up to an ambient temperature and are basically only heating enough to balance heat loss to the outside world, the building's fabric becomes effectively one great big storage heater, meaning you could turn off the boiler or ASHP for an hour and not really feel the difference at all.

I've looked around t'interweb and haven't yet found an answer to the one missing piece of information that would allow a practical calculation to be made for comparison purposes. That piece of information is how much energy it takes to heat the fabric of a given house by one degree celsius. Given the original scenario (a house empty between about 8:30 am and 4:30 pm), it all hinges on how much the building fabric would drop in temperature between those times and how much energy would be needed to bring it back up to temperature again come 4:30 in the afternoon. If it's more energy than an ASHP would use to keep the house at a constant temperature that set-back is going to make the boiler more expensive both in financial terms and also in environmental terms. If not, the always-on approach of an ASHP will not be the most efficient for people at work all day.

Does anyone else here have any knowledge of that part of the heat loss calculations?

Posted by: @potatoman

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Hi, there is a lot of talk about COP/SCOP, efficiencys of heat pumps etc but non of this answers my question, which is; given two identical houses in the same location experiencing the same weather. One of these houses has a well designed heat pump runing 24/7, as I am told this is the most efficent way, the other house has an efficent  oil boiler which runs for 2.5Hrs in the morning and 6hrs at night, no need to run any longer as nobody is in any of the houses between 8.30 am and 4 pm. Then you pick the same day to compare running cost for heating, which house would cost the most to run the heating  on a typical winters day of about 5 degrees C at night and 10 deegrees C during the day.

hi @Potatoman I've tried to answer some of this question in my post on “Creative Trials” but the results are limited to a radiators only setup and an air source HP system.

I don’t know much about Ground Source and so I was wondering; does it matter whether it is day or night when a setback would be more cost effective when using a ground source or geothermal system?

Posted by: @potatoman

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Hi, there is a lot of talk about COP/SCOP, efficiencys of heat pumps etc but non of this answers my question, which is; given two identical houses in the same location experiencing the same weather. One of these houses has a well designed heat pump runing 24/7, as I am told this is the most efficent way, the other house has an efficent  oil boiler which runs for 2.5Hrs in the morning and 6hrs at night, no need to run any longer as nobody is in any of the houses between 8.30 am and 4 pm. Then you pick the same day to compare running cost for heating, which house would cost the most to run the heating  on a typical winters day of about 5 degrees C at night and 10 deegrees C during the day.

To do a fairly rough calculation isn't difficult, but of course the answer may fluctuate given the fluctuating costs of oil and electricity (particularly oil).  Frankly a fairly rough calculation is good enough given how much oil prices fluctuate.

What price do you want to assume for oil per kWh and what efficiency for an oil boiler (I have no knowledge of oil so Im asking someone who presumably does).  Also what temp do you want to assume the house drops to when the oil heating is off? 

If you would like to answer these questions I will do a little spreadsheet over the next few days.  The above parameters will be adjustable but I would like to start somewhere that makes some sense to you.

@jamespa, I don't have oil any more but do keep a close eye on prices for the smug factor it provides me. Currently the place I used to buy it from is quoting 66.19 pence per litre. As for efficiency, assume 91%.

From what I have found on the Internet, it reckons the house's fabric takes about 8 hours to cool to ambient temperature so by that reckoning can we assume the house to be a degree or two warmer than outside when people get back home?

OK I found some prices etc.  Attached a very crude comparison.  In both cases I assume that the system is well set up - ie the Oil boiler actually condenses and the HP is running only just below manufacturer specified COP.  The methodology is to work out the total heat loss from the house over the 24 hr period and then work out how much it costs to supply that energy to the house.

It shows oil cheaper by about 13% than HP if you compare oil with setback with HP no setback, reducing to 1% difference if you apply setback in both cases.  CO2 emissions from oil are 4x those from the electricity consumed by the heat pump (using the govt 2023 emission factors).  HP with setback may be a bit optimistic because there may (or may not) be some penalty for the fact that the HP has to work harder to supply the total energy requirement in a shorter time.  I would say 1% is well within the noise and 13% marginal relative to noise, but indicative of a slightly greater cost

There is also a plot of relative prices of oil vs electricity from ONS showing the shocking change in the market which has occurred, driven almost entirely by the political choice to link electricity prices to gas prices (this was once sensible from a market perspective but no longer is).  This is something which has to change and which there are moves afoot to change but these things take a long time.  Historically also gas has been much cheaper than oil but at present they are essentially the same price!

You can fiddle with the cells that are not shaded, but the COP has to be input manually from the tables in the second tab, it wont change with the temperatures.