Air Source - why do some people pay high running costs?

simpjd

Hi all,

Appreciating are already lots of posts and advice on Air Source on here, however I'm trying to make sense of them all, and more importantly trying to understand why it seems to work for some, whereas others face huge electricity bill.

From what I can see, best way to compare is by comparing £/kWh against kWh heat demand, from the EPC or heating bills.

So if a house has a 10,000 kWh heat demand per annum, this is irrespective of the heat source.

Talking simply round numbers here:

- Gas is say around 5p per kWh. So £500/year based on 10,000 kWh house.

- Electricity is say around 15p per kWh. And Air Source will produce around 3 kWh of heat per 1 kWh of electricity (or better). So 5p per kWh. So £500/year based on 10,000 kWh house.

So on this maths there will be no cost saving, but equally its not costing any more?

I understand Samsung may have had an issue whereby the electric immersion heater was being used more than expected (so costing the full 15p per kWh)......however is this the only reason why some people pay more? Or is it something is do with the lower temperature output or air source? Could it take more kWh to heat a house to the same temperature using a lower heating temperature?

I'm concerned as I've head quite a few horror stories where people's energy bill has significantly shot up following the installation of air source, however aside from the specific Samsung issue (which in knowing this should be avoidable), I can't understand why it costs some people a lot more...?? Some people talk of £800/month electricity bills!!

Any help will be appreciated!

Flt._Lt._Biggles

simpjd said:

Hi all,

Appreciating are already lots of posts and advice on Air Source on here, however I'm trying to make sense of them all, and more importantly trying to understand why it seems to work for some, whereas others face huge electricity bill.

From what I can see, best way to compare is by comparing £/kWh against kWh heat demand, from the EPC or heating bills.

So if a house has a 10,000 kWh heat demand per annum, this is irrespective of the heat source.

Talking simply round numbers here:

- Gas is say around 5p per kWh. So £500/year based on 10,000 kWh house.

- Electricity is say around 15p per kWh. And Air Source will produce around 3 kWh of heat per 1 kWh of electricity (or better). So 5p per kWh. So £500/year based on 10,000 kWh house.

So on this maths there will be no cost saving, but equally its not costing any more?

I understand Samsung may have had an issue whereby the electric immersion heater was being used more than expected (so costing the full 15p per kWh)......however is this the only reason why some people pay more? Or is it something is do with the lower temperature output or air source? Could it take more kWh to heat a house to the same temperature using a lower heating temperature?

I'm concerned as I've head quite a few horror stories where people's energy bill has significantly shot up following the installation of air source, however aside from the specific Samsung issue (which in knowing this should be avoidable), I can't understand why it costs some people a lot more...?? Some people talk of £800/month electricity bills!!

Any help will be appreciated!

We pay less than 2.5p/kWh for gas.
Our gas boiler is not 100% efficient, unlike an electrical heater will be (almost).
I don't know what the efficiency of an air source pump heating systems is as I cannot afford one - but I do know they typically cost £6k-£8k!!! Will take a lot of savings to get paid back.

An average household energy cost is about £1127 per year on a suppliers most expensive, standard variable tariff, and you can save about £350 on that by switching:
https://www.moneysavingexpert.com/utilities/you-switch-gas-electricity/

So for the second time today, regretably, I do not agree with the basis of your argument.

Highland76

simpjd said:

- Gas is say around 5p per kWh. So £500/year based on 10,000 kWh house.

Ermmm..I think you'll find gas costs nowhere near that. Its between 2-3p kWh on many providers' cheapest tariffs. 

Mouldymeat3

We were obliged to fit air-source by the planners in 2013, I don't think greens get past the front page of the brochure to the spec at the back.  The quoted output is 5 kW (the epc guess was 3.6kW to maintain room temperatures). Because the rooms are so small we opted for underfloor heating. For kitchen use we have an induction hob ,combi-microwave and dishwasher, the conventional oven still has its wrapping. All LED lighting. For hot water a small under-sink unit and in the bathroom an instant shower. The commissioning engineers parting words were "don't treat it like a boiler" so we let it do its own thing. Room-stats are set to 22deg. Installation cost just over £2k at that time.

For those using air- or ground-source for domestic hot water it is necessary to periodically run a pasteurising cycle to destroy legionella. this is usually set in the controller. Some setups may have an accelerating heater too. If these are wrongly set up on commissioning, it can be very expensive.

We use about 3800kWh a year (tiny two bedroom bungalow 64 m2 ) with Neon Reef.

matelodave

Lets get back to heatpumps shall we.

I've had mine for ten years and it was installed as part of an integrated heating system - an overlay underfloor heating system, heatpump and hot water tank and I learned how to use it in the first year we had it by trial and error (our first years bills were higher than subsequent years until we'd worked out the best way to use it effectively)

However, a lot of people don't understand how they operate and so try to use them as a conventional boiler, they want boiling hot water and red hot radiators and frequently just install them to replace an existing boiler without upgrading the rest of their system.

Heat pumps work best with low flow temperatures, around 35-45 degrees, so radiators only get lukewarm rather than hot - this means they take a lot longer to heat the place so the conventional 2 hour blast first thing in the morning and four hours in the evening doesn't keep you warm. they need to be gently idling all all the time. Bunging a heat pump onto an existing system where the rads are designed to heat the place in 2-3 hours at a 70 degree flow temperature means that it could take at least three times longer. Unless you increase the radiator sizing you could end up not actually heating the place adequately at all, so you need supplementary heating

Our system takes about two days to get the house back up to temp if we let it get stone cold so it only reduced the temp by a couple of degrees overnight or if we go away it comes on 24-48 hours before we get home
.
The same with hot water - if you are used to really hot water then you'll need the immersion to get the temperature above about 50 degrees so some people have it set to 60 or even 70 and then wonder why the bills are so high.

As the OP points out the ASHP has a COP of around 3 so but if you use the immersion to get the rads or hot water hotter than 50 degrees then you will only have a COP of 1. My leccy costs me 11.5p/kwh but with a COP of around 3 = 4/p/kwh for heating & hot water.

We've also got half decent insulation, lots in the roof, dunno what it's in the walls (it was there when we bought the place) and 20+ year old double glazing. We have a flow temp of between 30-40 degrees (weather compensated) and only have our hot water at 45 degrees (with a boost to 60, using the immersion every Saturday for legionella sterilisation).I have disabled the 6kw boost heater.

I reckon our heating uses around 3500kwh of leccy a year and our hot water is just under 1000kwh a year. Our system is on 24/7 and the programmable stats control the temp. So thats about 4500kwh @ 11.5 = around £520, I'd guess that would equate to about 12,000-13,000kwh of gas at gas at around 3p/kwh = £375. but as we don't have the option of mains gas we'd have to use oil or lpg

Mouldymeat3

I should have added in my piece that we live mid-Cornwall, the worst we have had was the Beast from the East when the system grunted with frequent defrost cycles. I offered it as an example of how we tried to treat it all as a package. It must be correctly  commissioned otherwise any electric heaters can run all day as reportedly has happened in the past (no heaters in ours). As matelodave well describes in detail the thermal inertia means it must run 24/7.

When the ambient temperature falls, the performance falls sharply. The FIT gives a clue where it uses a performance factor of 2.5. The headline factor at 7 deg can be 4, hence my comment to read all the spec.

matelodave

Just to clarify Mouldy's comment.

The specified COP is usually 7 degrees with a flow temperature of 35 degrees. A lower outside temp or higher flow temp with reduce the COP, You need to check the COP profile of your selected heatpump to get a feel of how the COP changes. (however it isn't always ever so easy to find although some manufacturers will also quote the COP at 0 degrees)

It's reckoned as a rule of thumb that increasing the flow temp by 1 degree will increase the running cost by 2.5% so running it at 45 degrees could add 25% to your bills,