Just how good are heat pumps?

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.