The Trouble with Heat Pumps Part 3

The last couple of posts I have looked at Ground Source Heats Pumps (GSHP). (See HERE)

This post deals with Air Source Heat Pumps (ASHP). Like their more expensive (and more capable) GSHP cousins, ASHPs are the subject of the most incredible wishful thinking, promotion and propaganda. This is what I hope to address here.

An ASHP (as the name suggests) takes its heat source from the air rather than the ground. Consequently instead of having a pipe system buried under the lawn there is a large out-door box with a fan that pumps air past a heat exchanger. It is significantly less expensive to install than a GSHP and requires minimal ground work. i.e. Maybe the laying of a concrete pad for the system to stand on.

There are in fact two types of ASHP. The most common is a wet system which I highlight in this post. This delivers the heat to water which is then used to transfer the heat to the house like a traditional central heating system does. 

The second type is a dry system that delivers the heat to the house as heated air. This system does not usually provide hot water. As I understand it, due to the additional problems a dry ASHP has when heating both home and hot water the government do not provide access to the  bribe Renewable Heat Incentive for a dry system. 

With an ASHP you still need a garden, or at the very least a suitable outside wall. As the things have an intermittently rotating fan (and so make noise) their location has to be in such a position it won’t annoy the neighbours. Or annoy the owner for that matter.

The installation costs are well below that of a GSHP.  This table from Tradesmencosts.co.uk HERE suggests the cost is about half that of a GSHP. 

To make the system effective you will also need to install underfloor heating or (at least) increase the size of your radiators. In essence, you would need to completely rebuild your central heating system. Then I'd suggest you would need to redecorate. Most sites also recommend you upgrade your household insulation. None of these extra costs are included in the figures in the table above.

The big issue with ASHPs is that their heat source (the air) is not always at a positive temperature. When ASHPs are required to work the hardest (cold weather) their efficiency is compromised. Even so, you will often find ASHP COP values, obtained during almost ideal conditions being used to promote their use year round. (like HERE)

With all heat pumps we really (really!) should NOT assume the COP value is a good indicator of their overall efficiency or capability. A far better figure is the SPF (Seasonal Performance Factor). 

Further-more we need to ensure we use an SPF that is calculated using the full system (H4) NOT simply for the heat pump itself (H2 or even H1) which ignores energy used by backup/boost heaters, immersion and the fan. Here is a diagram (from HERE) that itemises the difference between SPF vales for H1, H2, H3 and H4

I suspect that even if we use (as we should) SPF(H4) this still does not take into consideration the energy that is sometimes needed to de-ice ASHPs in sub-zero temperatures.

A realistic value for the SPF(H4) for a ASHP is 2.44. 

This is the median value obtained over a large 2017 DECC  sponsored analysis  (contractors RAPID-HPC) of many hundreds of of ASHP installations. The data was collected between 2013 and 2015. (Report HERE)

Notice that this is actually below the current minimum value Ofgem (SPF - 2.5) stipulate for claiming the bribe Renewable Heat Incentive! 

From the graph below it looks like the majority of ASHPs analysed by DECC are operating below and in some cases well below an SPF of 2.5. I'd put money on it though that they all successfully claim the RHI subsidy (or the then RHPP equivalent).

ASHPs are on the left and GSHPs on the right right. Notice the ASHP results are significantly weighted to the lower performance values. 

"cropped" means these graphs above have excluded very low and high values. 32 sites (25 ASHP 7 GSHP) were excluded. Of those, 29 had an SPF(H4) less than 1.5. Only 3 had an SPF above 4.5

Like GSHPs, ASHPs typically provide output hot water to the central heating system at less than 40 degrees C. (The Centre for Alternative Energy (HERE) suggests 35 degrees C is best.). 

For most installations domestic hot water needs supplementary heating. There are reasons beyond simple comfort for this.  (Note: gas hot water is usually 65+ degrees).  

Temperatures around 35 - 40 degrees C are the ideal temperature for growing Legionnaire bacteria. So additional water tank heating to 65 degrees C (using an immersion heater) to kill them off is an absolute necessity once a week even if you are happy with luke-warm hot water most of the time. (This applies to GSHPs too)

If you are changing your heating system to use a heat pump, changes have to be made to accommodate the low temperature output to the heating system. The user is recommended (HERE) to install underfloor heating. Or if they cannot afford that, then maybe they can install bigger radiators. DECC found the cheaper heat pump upgrade option (larger radiators) reduced the system efficiency by about 10%. compared to underfloor heating. 

Empirically (based on a sample size of nearly 400 units) the DECC study found costs associated with running a ASHP system were significantly more than that of a GSHP. They only draw on parr when the external temperature is around 10 degrees C or more. Which is, of course, at times when central heating is less likely to be needed. 

Both ASHPs and GSHPs are more expensive to run than a condensing gas boiler.

Incredibly, Greenmatch (HERE) even using a COP of 4.3 (!!) still found condensing gas boilers are cheaper to run! Imagine how the figures come out if you use DECC's most commonly achieved SPF(H4) value of 2.44.

Carbon Savings

ASHPs are less effective in cold weather than GSHPs. This inevitably will be reflected in their potential Carbon Dioxide savings against condensing gas boilers.  As a GSHP will save less than 2 tonnes of Carbon Dioxide per household per year compared to a condensing gas boiler it is difficult to see how an ASHP would save more than around 1.2 - 1.5 Tonnes.  

The average expenditure needed to prevent the emission of Carbon Dioxide (per tonne) gets compensated somewhat because ASHPs are cheaper to install than GSHPs. But I would suggest the gains and losses more-or-less average out and the cost will again be round £500 per tonne.

However there is the opportunity to  participate in the governments bribe Renewable Heat Incentive! So other people in essence will pay in part for your heating system. 

All of this for both GSHPs and ASHPs pre-supposes that the National electricity Grid will simply ramp up output to take over from gas if there is mass adoption of heat pumps.

Unfortunately most people do not realise how little energy the electrical grid provides to domestic consumers compared to that provided by the gas grid. Neither do they appreciate how a mass adoption of heat pumps (let alone EVs) will impact it. Or how much in total it will cost the country

That’s the subject of my last post in this series on Heat Pumps. (Here)