The Trouble with Heat Pumps Part 2

Last post (HERE) I looked at the overall cost of installing ground source heat pumps (GSHPs) and how much land was needed. I touched upon other issues too which I will expand on here.

The first and most obvious issue with domestic GSHPs is simply their incapability. A typical domestic GSHP has an heat output of 4KW - 13KW. The most powerful have an heat output of about 16KW.

Compare that to a standard modern condensing gas boiler that has a heat output of somewhere between 20 – 50KW.

This low output from GSHPs has a number of consequences.

    • The typical water output temperature fed into the heating system is less than 40degC. You can increase that. But if you do the efficiency rapidly decreases. ( a gas system typically produces hot water at 65 degC or more)

    • With a GSHP it is difficult to get domestic hot water for the bath/sink/shower up to an acceptable temperature. The usual fix is that you use a standard electric immersion heater to raise the temperature to an acceptable level. 

    • Because the output temperature is so low, the recommended usage for central heating is in under-floor heating. So factor installing that into the installation costs as well.

    • If you decide to ignore the under-floor heating recommendation and use radiators it is recommended you increase their size. Either way, underfloor or larger radiators - you trash your decor and need to redecorate. Factor the cost of that in too.

    • The system has to be running 24/7. You cannot allow the house to cool down when empty as it takes so long to heat up due to the low heat output.

   • When the system does reach temperature, as the hot water heating the underfloor system/radiators is at a lower temperature than a gas system, the system cools more rapidly and so tends to cycle on/off/on/off more frequently than a conventional system.

    • Every site (I have seen) that recommends GSHPs always encourages the owner to increase household insulation during installation (more cost). Meanwhile any cost comparisons they perform are with obsolete gas/oil/electric systems like this one taken from  (HERE)

heatin cost comparison GSHP and old systems
So - Wheres the comparison with a new condensing gas boiler?

I have never seen a headline comparison of GSHP with (say) a condensing gas boiler system in an equally well insulated house. It is always old obsolete systems. I suspect the reason for this is because it is actually cheaper to run a modern condensing gas boiler system than a GSHP. (more later)

So, the take out from this is that:

    • On a like for like basis GSHPs are not as effective as gas boilers when it comes to heating your home. They can do it. But in reality, condensing gas boilers are more capable (and cheaper to run).

    • To get acceptable hot water temperatures while using GSHP you either compromise the lauded efficiency of the GSHP or use an immersion heater. Either way the overall system efficiency takes a nose dive.

     • You need to factor in a complete ground-up rebuild of you houses heating system with all the decorating and associated building/plumbing work. Your garden will be trashed and need to be re-layed. This will be on top of the cost of the GSHP installation. 

    • The system runs 24/7. It needs to do this as it will take a long time to heat up your home again if you let it cool down. So if you plan to solely rely on cheap Economy 7 electricity to drive the compressor – dream on.

    • There’s mountains of deception and propaganda being peddled by people and organisations promoting heat pumps. Make sure you get the all the figures and make sure their comparisons are honest ones (they won’t be).

Running Costs

OK let’s assume you have overcome the installation restrictions as outlined in the first post in this series. 

You’ve sold the kids into slavery and spent your £20,000 - £30,000+ on a GSHP plus associated rebuild of your heating system/redecorating/relaying garden.  Let’s say it works satisfactorily (some don't).

How much are you going to save?

Here’s a table from Evergreen Energy (HERE)

So from this rather optimistic table we find KW for KW a GSHP has just short of 4 times the efficiency of gas. So far so good you may say. What’s not to like about that? For one KWh of electricity used by the GSHP provides 4KWh of heating. If its gas we need 4KWh of gas. 

(As I'll show later this efficiency of 350% for a GSHP is wildly optimistic and does not stand up to scrutiny when compared to actual real data from a large installation base - but never mind for now. We'll run with it as-is)

The trouble comes when look at a typical gas/electricity tariff like this one. (Its mine by the way)

Notice the electricity price for one KWh is over six times that of gas. True you need a second standing charge which will slightly increase the gas price. But not by much.

So, let us assume the choice is between replacing an old gas boiler with a £2000 new condensing gas boiler or paying out £20,000 for a GSHP system and another £10,000 for the associated rebuild of your heating system, garden and decor.

At the end of the day (day-to-day running cost-wise) you would still be better off with the condensing gas boiler. 


The government wants to entice you to spend your money on a GSHP. This is so the government can grand-stand about how “environmental” it is. As a consequence the government will issue you with a big fat subsidy if you install a GSHP. It’s called the Domestic Renewable Heat Incentive (RHI).

Now subsidy is a funny word. 

Many people think the word "subsidy" is actually an abbreviation for “somebody else’s money”

I couldn't possibly comment.

Even so, the subsidy, (which will be payable for 7 years) won’t cover the initial cost of the GSHP let alone the extra money you’ll pay out on rebuilding your heating system, or on enhanced energy bills. It's considerably cheaper to leave the bribe with the government and buy a condensing gas boiler.

Carbon Emissions.

Ah – right! This must be a slam-dunk for GSHPs. Surely no government would ever propose a “green” solution that was (well…) less than it was made out to be? 

To be fair this is one area that GSHPs does win out over gas. But not by as much as some would have you believe.

First let's visit what the real efficiency value is rather than the promoted value. 

Usually, the value promoted by advocates of GSHPs is the COP (Coefficient of Performance) and usually they use a COP value obtained in more-or-less ideal conditions. They then use this idealised value to figure out both savings and emissions for the whole year. 

First of all, if we want a realistic figure we shouldn't use the COP at all. We should use what is known as the SPF (Seasonal Performance Factor) which is roughly the performance you would get over a year rather than just at ideal conditions. We then need to ensure the SPF also includes the bits that the COP ( and heat-pump protagonists) studiously avoid. Like hot water immersion heaters and the need for other in-built resistive heaters in cold conditions to boost the sagging room heating performance of the heat pump itself. 

DECC did a study of several hundred GSHPs and ASHPs (Air Source Heat Pumps) in 2017. (HERE) 

They found the typical (median) SPF for a GSHP was actually 2.71. 

Here's the DECC graph for the distribution for H4 (H4 is the full input/output from the GSHP system rather than just a sub-system of it). The data we are interested in this post is the right hand bar graph.

If you want to understand the differences H1,H2,H3 and H4 for heat-pumps read THIS paper

So now we find we need just under 3.0KW of gas (condensing gas boiler) to match  1KW electricity used by a GSHP. Surely that is still (nearly) a 3 - 1 win? 

Well, no. You still need to factor in the carbon emissions from the electricity generation – and transmission losses for both electric and gas.

From Carbon Independent (HERE) we find that, after accounting for transmission losses and leaks, UK electricity has a footprint of 0.309Kg/Kwh and gas 0.203Kg/Kwh. So now we are down to less than 2:1. Which, to be fair is still a gain. For today at least. 

If you work it out it comes out at less than 2 Tonnes of CO2 saved for the average house over a year if the comparison is with a new condensing gas boiler.

The problem here is that this still ignores the main inherent issue with GSHPs. Namely the low power output. If people start supplementing their meagre GSHP output with electric or calor gas heaters and regularly bump up their hot water temperature with immersion heaters, all bets are off.

So (maybe) a gain. But at what cost? Somebody has to pay the £20,000 even if some of the cost is spread across society (including the poor) with the government subsidy.

So let us  assume the carbon dioxide saving is (optimistically) 2 tonnes per household and the GSHP lasts for twenty years without needing repair and only costs £20,000 to install. 

(Be aware that ALL rotating machinery needs care, repair and maintenance over time. The suggestion from the Green lobby that heat pumps last “forever” is simply absurd propaganda. )

The initial capital cost over 20 years averages at least £1000 per year assuming zero interest. So each tonne of Carbon dioxide saved has cost a minimum of £500 just in up-front capital overhead alone.

Which must be not far short of a record. 

You need a big garden for a GSHP.  So basically poor people need not apply. But big gardens usually come with rich people attached, ready and willing to claim the big fat subsidies. Even then they'll be out of pocket.

This government subsidy (like most renewable bribes) is in fact a Dennis Moore tax. I am sure we all remember good 'ol Dennis Moore. 

Next I’ll look at Air Source Heat pumps. 

It doesn’t get better. (HERE)

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