Friday, 8 January 2021

Energy Ratings for heat pumps - what do they mean?

Example energy label from [1]
Do you check the energy labels when you buy appliances such as washing machines or refrigerators? I hope you do. To make comparisons easy each product has  a rating, which is generally between A and G except for some appliances it goes up to A+++. This is helpful only if you know what the best or typical rating is. In any case, the rating does not actually tell you what the efficiency is or typical energy use would be - hence my advice has always been to look for these on the energy label. I would love to be able to say the same for heat pumps but there is nothing like that on the energy label for heating appliances. The rating is the best we have to go on. Here is a picture of the design of the energy label for a heat pump with integrated hot water cylinder, so it supplies hot water for the taps as well as for the central heating system. To understand what this means I have been reading the legislation behind it [2].

The scale varies depending on the product efficiency.

There are separate ratings for space heating and hot water so we have two scales: the top grade for space heating (on the left) is A++ and for hot water is A. You would think that in each case the top one was the best grade possible but actually it is not. For a heat pump rated A+++ you have to show A+++ to F, but for a lower rated heat pump you can show A++ to G (as in the picture) . So an A++ product can look just as good as A+++.

The efficiency ratings for low temperature heat pumps are different.

The space heating rating is based on the seasonal performance factor (SPF) under certain conditions. For most heat pumps this will be for providing heat at 55°C - around the upper limit for a normal heat pump. A lower temperature of 35°C - suitable for underfloor heating - is used for low temperature heat pumps or heat pumps used in 'low temperature applications'. Many heat pumps will have a label showing ratings for both 55°C and 35°C - the latter shown on the right of the label instead of the hot water ratings. This applies to heat pumps that can supply at the higher temperature and do not have integrated hot water. However, the definition of an A at 35°C is different from an A at 55°C - as shown in this chart. A 'C' rating at 35°C is better efficiency than an A+ rating at 55°C.

SPF requirements for heat pumps derived from [2]

This is not ridiculous, because heat pumps are more efficient at lower supply temperatures. However, it is something to be aware of when comparing heat pumps by energy rating.

Gas boilers are rated in the same system - with a handicap advantage.

The same rating scheme (at 55°C) is used for gas boilers. However, there is a correction factor applied for electric heaters: the SPF is divided by 2.5. This is supposed to reflect the ratio of primary energy (the energy used to generate it) to delivered energy for electricity, averaged across the EU. The value 2.5 is not too far off the UK current average - about 2.3 last year [3] (taking into account all losses from the network as well as conversion losses). This figure changes over time. It decreases with more power from wind and solar and increases with more use of thermal power stations using nuclear power, gas, biomass or other fuels.

This means an SPF of 90% for a gas boiler is equivalent to an SPF of 2.25 ( 0.9*2.5 = 2.25) for a heat pump. They both rate an 'A'. A more typical gas boiler SPF of 85% would be rated 'B'. 

Perhaps this does not matter, because no knowledgable and sane person would compare the energy rating of a gas boiler with that of a heat pump. However, it is unfortunate that the comparison is based on primary energy and nothing to do with carbon emissions.

Hot water supply ratings are based on one of 8 different demand profiles.
The rating for hot water supply is also non-trivial. This is for heating systems which supply hot water to the taps - equivalent to a combi boiler. There are eight different demand profiles labeled 3XS to XXL. The label above, has an 'L' by the icon of a tap which indicates the load profile this efficiency is based on. The 'L' profile includes periods delivering up to 10 litres/minute at a peak temperature of 40°C. The next load down, 'M', has nothing more demanding than 6 litres/minute - not quite enough for a standard flow shower but plenty for a low flow shower. So far I have only seen 'L' and 'XL' used.

Other things on the label - mainly noise and peak power.
There are other useful facts on the label including:
  • How noisy the heat pump will be, with separate noise levels inside and out for split systems
  • The rated heat output in three different climate conditions, as indicated on the map. Generally the rated heat output is lower in colder conditions.
  • Whether the system can be configured to avoid running at off-peak times (the little clock and pile of coins pictogram in the bottom right corner indicates this). 
Possible reasons why the SPF is not shown.
Perhaps the reason that the SPF is not shown is because this will vary depending on your climate conditions (there is much variation even within the UK) and on aspects of the heating demand in your particular home. However, the same logic could be applied to washing machines and other appliances; the energy use depends on how you use it such as the length and temperature of washing cycles and how full the load. That does not preclude the energy label showing energy use for a 'typical' cycle.

I cannot help thinking that clearer information would go at least a little way towards helping a typical consumer navigate the complexities of procuring a heat pump, and be slightly less reliant on blind trust in the sales people. I do appreciate that efficiency is not the only difference between heat pumps and the best product in one case may do less well in another. However, if we are to trust the sales people implicitly, why bother with labels at all?

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