Heat pumps: just how good are they?
Over the past few weeks, the Independent has been carrying what it calls an Advertisement Promotion for Ice Energy heat pumps. It’s a full page and it appears in their Wednesday property supplement. The key feature of this promotion is a green boxed-out section which contains some data which is entitled Typical cost savings of a ground source heat pump (GSHP) against oil and gas. Here’s what it contains:• House type: 230m2 detached property in a rural location comprising 2 bathrooms, 4 bedrooms and 3 reception rooms, underfloor heating installed throughout
• Annual energy consumption: 32,400kWh, based on a heat requirement of 45W/m2 for central heating and domestic hot water
• Annual energy costs:
Oil 32,400 x 0.0357 x 1.25 = £1,446
Gas 32,400 x 0.02 x 1.25 = £810
GSHP: 8,120 x 0.07 = £568
Assumptions: heating oil costing 3.57p/kWh and boiler efficiency 75%, gas costing 2p/kWh and boiler efficiency 75%, electricity costing 7p/kWh and GSHP efficiency being 400%
There is some more stuff about how boilers only last 12 years and would need replacing before a GSHP system, which will last 25 years, but this is essentially a side issue. The central claim is that it is much cheaper to run a GSHP system than either oil or gas. It looks too good to be true. Is it?
First assumption. Will a 230m2 detached house really take 32,400kWh per annum to provide space heating and hot water? It could but if it was a newly built house and it took that much, you’d be very disappointed. We live in a 200m2 house with oil-fired heating: the house was built in 1992 to slightly above thermal envelope standards, which are much lower than those currently operating. We burn 27,700kWh/annum. Now this theoretical house is larger than ours by 15% - that makes 27,700 into 31,855kWh, very similar to Ice Energy’s figure. But, and it’s a BIG BUT, this is our consumption of oil, not our heating requirements. Ice Energy multiply this 32,400 by 1.25 to take account of a boiler running at 75% efficiency. I have an idea that our 14-year-old (and still going strong) Boulter boiler burns at around 75% efficiency, so our actual heating requirement is much less than our consumption figure.
Coupled to which, our energy bills could have been much lower still if we had built to 2002 standards. So, Ice Energy, you are over egging this particular pudding. A newish 230m2 house really shouldn’t need anything like 32,400kWh to keep warm. 20,000kWh would be much closer to the mark, and it could be much less if built green.
Second assumption. Energy costs. I think Ice Energy’s take on energy costs is pretty accurate as a snapshot of what is happening in the market as of now. What it will be like over a 25-year period is anyone’s guess but 2005 was marked by much higher oil prices, slightly higher gas prices and no change as yet in electricity prices. Logic would seem to suggest that the price ratios are currently out of equilibrium and that either oil will fall or electricity will rise.
Third assumption: the efficiencies of boilers v GSHP. They have suggested that boilers operate at around 75% efficiency. The new generation of condensing boilers are designed to operate at around 90%. They have also suggested that the efficiency of GSHP is 400% - i.e. that every unit of electricity fed into the system produces four units heat output. I think that’s high, at the top end of what we expect from GSHP. It might get to that sort of figure in spring or autumn when it’s not doing much work, but in the depths of winter it’s not going to get there. And as for heating domestic hot water, it’s never going to get there. In fact as regards hot water, GSHP is hardly any more efficient than using an immersion heater. I would have thought a more realistic assessment of GSHP efficiency would put it at between 2.5 and 3.0, say 2.8 for arguments sake.
So let’s replay the annual energy costs with my assumptions, rather than Ice Energy’s.
• House type: 230m2 detached property in a rural location comprising 2 bathrooms, 4 bedrooms and 3 reception rooms, underfloor heating installed throughout
• Annual energy consumption: 20,000kWh, based on a heat requirement of 25W/m2 for central heating and domestic hot water
• Annual energy costs:
Oil 20,000 x 0.0357 x 1.1 = £785
Gas 20,000 x 0.02 x 1.1 = £440
GSHP: 7,150 x 0.07 = £500
Assumptions: heating oil costing 3.57p/kWh and boiler efficiency 90%, gas costing 2p/kWh and boiler efficiency 90%, electricity costing 7p/kWh and GSHP efficiency being 280%
I think that’s a far more realistic appraisal of what Ice Energy and the whole GSHP industry are offering. It is cheap to run, but not phenomenally cheap. I am familiar with an Ice Energy installation where the fuel bills have been monitored and the outcome is around 36kWh/m2/annum, which would make their notional 230m2 house come in at 8,500kWh/annum. And you have to set against that much higher installation costs, typically around twice as much as an oil-fired boiler and maybe three times as much as a gas-fired one.
In short, GSHP is currently, at today’s fuel prices, a compelling option for home heating in a newly built house. But not nearly as compelling as Ice Energy would have us believe.
Labels: Renewables
posted by Mark Brinkley @ 2:22 PM
7 Comments:
This is very useful as was the original detail in the sixth edition.
Do you or anyone else have experience with the Veismann heat pump?
A heatloss from a current building regs house is in the region of 50w/m². A super efficent european house struggles to achieve 25w/m² heating load.
Running cost at 17-2-2006
Gas 80%Eff £725
Oil 80%Eff £1087
HP 400%Eff £362
HP should be designed to meet 80% of the heating load for the building. Ice Energy design at an unrealistic 45w/m² and 60% of heat demand
Question
Why?
Answere
UK houses are 95% single phase electric supply, Ice Energy only have upto 6kW heatpumps on singe phase supply. I've seen quotes by these guys for 350m² properties using a 6kW heat pump.
Don't get caught out by small print in contract of sale, no design liability. If you want to know the heat load of your new build it is in your SAP calculation for your building regs.
Any heat pump is only as good as the design that has been done for the heating requirement of your building and the ground structure in your area.
My favourite two gurus, Mark Brinkley and David Snell certainly pour some doses of necessary cold water. Every time I get over-enthusiastic about some new technology, I reconnect to these two to get feet back on the ground.
I read Ice Energy's original article in the Independent and spoke to them on the phone as well as half a dozen others. I have spent over two weeks on the internet, on the phone, and getting written estimates. Ye gods, it is hard to prise consistent facts out of them. So much guff about saving the planet (fair enough but it is mainly flannel, and not actually useful for decision making). Still, I've done the sums as best I can, extrapolating common threads from each of them (and other green suppliers), and conclude photovoltaics achieve payback in about 65 years, solar panels 30 years and heatpumps, with a bit of luck, depending what proportion you can load into Economy 7, possibly 10. That's beginning to make economic sense. But it does depend on having enough land not to need boreholes, and not to incur costs of installing 3 phase.
But some nitpicking:
1. I agree with gec who suggests that the heatpump should cope with 80% of the heating load but am surprised that he thinks 45w/m2 is unreasonable in a newbuild well insulated house - that's only slightly better than meeting building regs.
2. Mark costs HP at peak elec prices. Surely as much as possible should be off-peak, maybe even 40-50%? The price (in my area) is 3.087p as opposed to 8.736p peak.
The contradictions/variations from the suppliers are in themselves fascinating:
1. You definitely should/should not have solar panels as well.
2. You definitely should/should not keep DHW separate from UFH.
3. You definitely should/should not use thermal storage.
4. You should size HP to 100% heat load (Kensa) or 60% (Ice Energy).
5. Some say, use boreholes only when you must, others that they're better anyway.
Finally, a question. Much of my economics depends on use of Economy 7 type tariffs. Somewhere I spotted something about a quantitative limit to its usage. I might need 5kw for HW, 5.2kw for night storage Aga (OK, Mark/David despite your very pragmatic views on these devices...) and perhaps 2.7kw immersion heaters. Is 13kw too much? Not technically, but are we rationed?
Yes....very good....if (as you have) only the current financial aspect is considered. But you have totally ignored the ecological advantages of GSHp against fuel burning heating systems. When (and it is a when, not an IF) the government introduces a domestic carbon tax the differences in running costs will improve even further in favour of GSHP. Relying on fuel costs nowadays to argue for or against the installation of heat pumps at the build stage is incredibly short-sited, and even arrogant.
Having read Mark Brankley's comments it is plain to see that his knowledge of heat pumps is limited at best. I think it should be noted that he has several 'assumptions' i.e. he has not bothered to research the subject properly and has just assumed that he knows all about the product. Stupid boy! Has he never heard the phrase that 'you should never assume, as it makes an ass of you and me'?
Heat pumps will always find it hard to replace gas in todays price regime and we don't recommend you try. Against oil or electric space heatings is another matter, but yields can be disappointing. Its great technology when carefully deployed
Gec
Not true, Ice Energy supply 11kW output heat pump in single phase.
They would never recommend 6kW for a 350m house, that would be more like a 14kW heat pump.
We generally design for 80% max load, not 60%, please check your facts.
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