Cost of running a heat pump

[Matt Beard]

I have been planning to fit an oil boiler in my house, however I am not all that happy about the running costs. I have considered a heat pump, but I am not sure that the up-front outlay is worth the extra economy.

What I am very confused about is how much it would cost to run a heat pump - googling this group gives two conflicting opinions:

1) Oil is now so expensive that it costs about the same to run as electricity for CH + DHW.

2) Although a heat pump runs between 300% and 400% efficiency they use "expensive" electricity and so when you compare overall running costs it would be just as cheap to run oil.

How can both of these be true?

If my house requires 40,000kWh per year (rough stab) for CH + DHW and I get this all by off-peak electricity at 3.07p + VAT per kWh (ZEST4 online) it would cost =A31289.40 per year to run.

If I get the same amount of heat from a heat pump, and pay 7.23p + VAT per kWh (Scottish Hydro standard rate) the break-even point for efficiency is when the system uses 16,985kWh of electricity which is

235% efficiency. If I could manage an avaerage of 300% efficiency I would save about =A3277 per year.

Assuming an oil system at 33p per litre and about 10.5kWh per litre and an 80% efficient system... this gives =A31587 for 40,000kWh! (Yikes)

So, I guess the big question is how much would it cost to install a heat pump system? If it costs less than about =A33000 more than an oil system it looks like it would pay for itself in 5 years.

[Tony Bryer]

The oil-fired system has the extra overhead of servicing, also the possible inconvenience of dealing with oil deliveries. I don't know what's involved with heat pump systems but my last fridge was still working after 23 years.

[darren_forward]

Would it be possible to run your boiler on cooking oil, or has that one been covered. Not sure on the price of heating oil / with cooking oil.

Regards

Darren

[Tony Bryer]

The way BBC2 Working Lunch has been talking up waste cooking oil it will soon be more expensive than the real thing . Chip shop owners used to have to pay to get it taken away; now they get paid for it.

[Alan]

Hi Matt,

Answering your first question, I think the second opinion is out of date. The first one is true if you use off peak electricity but not on peak.

Next, you can do much better than 80% efficiency - try 95% and that means an oil cost of =A31323.

Also, I'm not sure how large or energy efficient your house is, but 40K kWh per year seems steep. I am only using 26K. I would get an accurate figure rather than a rough stab as this greatly affects the payback time.

That said, if I were doing my renovation again, I would probably go the heat pump route because oil looks like it will keep increasing in price faster than electricity.

Costs for a heat pump are hard to work out but =A33000.00 more than an oil boiler after grants is probably about right but the installation work is more expensive for a heat pump which would increase this difference. Unfortunately, I never worked out the cost of this as I would do all the work apart from the commissioning myself.

I'm not sure if you are building a new place or adding this to an old place but another factor to consider is that using under floor heating is more efficient for heat pumps because they don't have to heat the water up to very high temperatures. I've never been able to find out accurate figures for the differences in efficiency though...

Finally, I have thought about running a heat pump from off-peak electricity and storing the output in a thermal store to use during the day. I guess this might require a larger pump and collector and there will be inefficiencies in the thermal store but my gut feeling is that it would save money and the thermal store could be combined with solar thermal during the summer.

Cheers,

Alan.

[Matt Beard]

I'm assuming "real-world" figures - the 95% sedbuk figure is only achieved when running at certain flow rates with certain temperature differential (I don't know how fast it falls away though) and after 5 or 10 years I doubt that it will still run at 95% at any point.

The house is a detached granite building with not much insulation (and little scope for improving it short of totally stripping out and rebuilding) and it is in the north of Scotland. It takes a lot of heating! Last year I spent just short of =A32000 on electric heating and it still felt cold.

I hate solid floors - if I can't have a carpet under-foot I'm not comfortable! I have rented houses with laminate and have relatives with solid wood and even stone floors - I just can't wait to get back home! So underfloor is not that good an idea (especially with suspended wooden floors) so it's likely to be radiators.

[Matt Beard]

Isn't that illegal?

[Alan]

Hi,

You are probably right that after a few years, the new efficiencies will be hard to meet. I wonder if this is true of heat pumps?

If you know you spent =A32000 on electric heating then you can work out exactly how many KW you used to heat the house although I agree that

40K kWh is probably not far off. I live in a granite house myself and I know that they are cool in summer and freezing in winter. I am glad that I have at least some insulation. I feel for you!.

You can have UFH with suspended floors but unfortunately this isn't much use unless you are building new or completely redoing a place.

Please let us know what you eventually decide on and also what it costs you. I for one am very interested to hear how you get on.

All the best,

Alan.

[Doctor Drivel]

The best thing is to install cavity wall insulation, insulation around the slab perimiter underground (assuming a concrete slab) and over 1 foot in the loft. Triple glazed windows with low E glass and make the place as air tigt as possible too. Do a heat loss calc on that spec. You may find that a cheap oil boiler or even LPG will be more cost effcetive as you will use less energy.

For very cheap heat pumps, solar panel, and the rest:

[Doctor Drivel]

Hi Matt,

Answering your first question, I think the second opinion is out of date. The first one is true if you use off peak electricity but not on peak.

Next, you can do much better than 80% efficiency - try 95% and that means an oil cost of £1323.

Also, I'm not sure how large or energy efficient your house is, but 40K kWh per year seems steep. I am only using 26K. I would get an accurate figure rather than a rough stab as this greatly affects the payback time.

That said, if I were doing my renovation again, I would probably go the heat pump route because oil looks like it will keep increasing in price faster than electricity.

Finally, I have thought about running a heat pump from off-peak electricity and storing the output in a thermal store to use during the day. I guess this might require a larger pump and collector and there will be inefficiencies in the thermal store but my gut feeling is that it would save money and the thermal store could be combined with solar thermal during the summer.

[Doctor Drivel]

I'm assuming "real-world" figures - the 95% sedbuk figure is only achieved when running at certain flow rates with certain temperature differential (I don't know how fast it falls away though) and after 5 or 10 years I doubt that it will still run at 95% at any point.

Do a google on me on heat pumps.

[Doctor Drivel]

A heat pump is a waste of time then.

[Doctor Drivel]

No. Not paying tax on it is.

[Doctor Drivel]

Best he insulates under the floors.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Doctor Drivel" saying something like:

You can burn it as a heating fuel free as a bird.

[Doctor Drivel]

No problem then, but put it in your car and the C&E man will have you.

[The Natural Philosopher]

Mmm.

Couple of thoughts

(i) Electriciy generation apart from nuclear (25%) and non fossil (about

5%) is mainly tied to raw fossil fuel prices. We do use coal a bit but the bulk of electricity generation is gas., Gas is going up JUST as fast as oil..so you may excpect electricity to actally go up anyway.

(ii) Burning oil at 60% efficiency to make electrcity that is transported at 95% efficiency is not as efficient as burning oil in the home at 70% efficiency.

(iii) insulation is what you need. Dry line that granite pile mate!

(iv) U/F heating will work with carpets. ...BUT what is critical is the thermal resistance between the heat source and the room and the heat source and the cold ground underneath., With a ventilated raised wooden floor though, you are in trouble: You need massive underfloor insulation to ensure the bulk of the heat transfer is upwards, and a higher in-floor temperature to heat the room through the carpet. This may exceed sensible limits with a wooden floor and lead to high levels of wood movement.