The Trouble with Heat Pumps Part 1

Domestic heat pumps are being heavily promoted by energy companies and the Green lobby as an efficient way to cut carbon emissions while simultaneously reducing  consumer energy bills. 

But do the figures really stack up? In fact is either of the above claim true? Or are the promoted advantages of heat pumps based more on wishful thinking than reality?

There is so much hype going on around Heat Pumps it is difficult to separate the facts from propaganda. But as this topic has been bugging me for some time I thought I would have a go and maybe puncture a few of the propaganda bubbles and economies-of-truth surrounding heat pumps. 

So here goes.

There are two main types of domestic heat pumps. Ground source heat pumps (GSHP) and air source heat pumps.  (ASHP). 

The first two posts will deal with GSHP’s then I’ll cover ASHP’s and in a final post I’ll look at the often stated aim of nationally replacing domestic gas heating with heat pumps. I'll show what I think are some extremely serious financial and electrical issues society will have to deal with if a mass adoption of heat pumps were to be seriously attempted.

Ground Source Heat Pumps. (GSHP)

By any measure, for a normal family installing or updating  their domestic heating with a GSHP, it will be an enormously expensive operation. In addition to the basic installation costs there are also other significant costs that are, if not hidden, quite obfuscated.

The Energy Saving Trust estimated the cost of installing a GSHP in a typical family home at between £14,000 to £19,000. (HERE)

Selectra (HERE) give a slightly higher but more detailed cost break-down

Ground Source Heat pump installation costs

Notice from the above Selectra chart, a bore-hole system (which intuitively feels like the less costly of the two techniques) actually involves over twice the ground work cost of a horizontal system.

So the cheapest GSHP system will involve installing an underground pipe network under your garden. This means your home has to fit the following criteria:  

    • You have a garden (which will be trashed and need relaying)

    • It is accessible for heavy machinery

    • It is on a sufficiently pliable bedrock/soil

    • Your garden is big enough.

With regard to garden size The Ground Source Heat Pump Association (HERE) claim the following:

“As a general guide, for a newly built 3-bedroomed house of around 120 m with a heat loss of around 6kW, two trenches of 30-40 metres in length would typically be required.”

Note “with a heat loss of 6KW”. I assume it equates to the output from the heat pump required to maintain a modern 3 bed house at around 20degC. 

That’s a tiny heating system! Maybe suitable for a new build conforming to all the latest building regulations regarding insulation but hardly likely to be adequate for existing housing stock.

I’d suggest the vast majority of the current UK housing stock would have a lot of difficulty maintaining warmth all year round with a mere 6KW and two forty meter trenches.

EverGreen Energy (HERE) suggest as a rule of thumb you need a garden size of:

"..roughly twice the total floor area of your home from every storey.

The Centre for Alternative Energy (CAT) (HERE) suggests you need 10m of “slinky coil” per KW and that a typical 8KW heat pump requires 400 sqm (20m x 20m). According to the CAT you need 5 meters between the trenches. 

I suspect that both the EverGreen estimate and the CAT estimate assume unrealistically thermally efficient housing. But never mind. We will run with it anyway.

So, how big are typical houses in the UK? Savilles (HERE) tell us what the average floor area per house type is. So using the “twice floor area of your house” rule of thumb we can get a rough idea how big a garden you need for installing a GSHP into each type of house.

Typical Detached House:

  • Floor area 152 sqm. 
  • GSHP area (twice house floor area) 304 sqm.
  • Minimum required garden size: 18m x 18m or 57ft x 57ft

Typical semi-detached house:

  • Floor area 93 sqm. 
  • GSHP area (twice house floor area) 186 sqm.
  • Minimum required garden size: 14m x 14m or 45ft x 45ft

Typical Terraced house:

  • Floor area 83 sqm. 
  • GSHP area (twice house floor area) 166 sqm.
  • Minimum required garden size: 13m x 13m or 42ft x 42ft

What percentage of the UK homes have gardens that big? 

In  fact from (HERE) 1 in 8 UK homes have no garden at all! Of those who have a garden, the median garden size (i.e. the size where half are bigger and half are smaller) is 188 sqm. Which suggests that houses whose garden (if they have one and including front and back) would be big enough is about one third of the total number of UK homes.

Of those with big enough gardens most will be homes owned by the more well-off in society. Especially in London and the South. Notice the NONE of the above fulfil the CAT requirement of 400 sqm for an 8KW heat pump.

If you then assume that only one space (front or back garden) could be practically used for the piping, I would suspect you would eliminate all bar 10% of homes. Of those, almost all will be older and more expensive properties.

So what about bore holes? 

Look at the Selectra table (above) that gives ground work costs. Notice that vertical ground work costs are at least double (with most approaching three times) the cost for horizontal systems. So while there would be less stress on garden area when installing a vertical system, there would be considerably more stress on the bank account. 

Also notice that nasty little reality check paragraph underneath the Selectra table. It details some (not all) of the building work that is excluded from the price estimates.

Water temperature

GSHPs can only efficiently heat water to around 40 degC rather than the typical 65 -70 degC of a gas system.  In just about all domestic scenarios GSHPs are less capable than condensing gas boilers. 

As a result of the low output water temperature you need to install (recommended by most) underfloor heating with your GSHP. Alternatively you could significantly increase the size of all your radiators.

Either way you trash your house. 

To cap it off most companies selling these systems also highly recommend you upgrade your homes thermal insulation as well (assuming that is that there is anything left in the bank account to pay for it)

From installation issues alone I would suggest that GSHP’s are wholly inappropriate for most properties in the UK. There will be exceptions. But they will be exactly that: Exceptions. Not the rule.

Lets not forget, all I have covered at this stage is the impracticality for installing GSHPs in most UK properties. 

I haven’t covered the practicalities in actually running the things yet. (that's in the next post). 

There is also the potential impact on the electricity grid if the proposed scenario of GSHPs and ASHPs replacing gas boilers ever came to pass. ( last post in this series)

The next post (on the problems with the day-to-day running of GSHPs) is HERE

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