Water Heating

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 9.94. This updates to RDSAP 10 on 15th June 2025 and a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is proposed for 2026. In addition, government consulations indicate future changes to the format of EPCs and minimum rating levels required for priate rental properties. Methods for improving EPC ratings may therefore change in the future.

'Water Heating' here refers to the heating of 'Domestic Hot Water' (DHW) which we use for filling the bath, for use in mixer showers, and which supplies the hot taps in kitchen sinks & bathroom basins etc.

This contrasts to 'Space Heating' which is heating of the air in the rooms within the property, although often one central heating system is the source of heat for both.

When an energy assessor looks around an existing property to carry out an EPC assessment, they will look to see how the domestic hot water is heated.

Currently, under RDSAP 9.94, the final EPC rating is a cost based rating. The cost of the energy required in heating the domestic hot water over a year is calculated and this is combined with space heating costs and lighting costs for calculation of the final EPC rating. 

As with Space Heating, as a generalisation, the cost of the fuel used for Water Heating is probably the most important factor currently when looking to improve your EPC rating.

The fuel costs used in the calculation behind the scenes are available to view in a document on the Building Research Establishment (BRE) website:

https://bregroup.com/expertise/energy/sap/standard-assessment-procedure-2012

Scroll down to the link for the document named: 'SAP 2012 document' and click on it.

The fuel costs used in the calculation of the EPC rating are listed in Table 12 on page 225 (at the time of writing). The unit costs is expressed in pence per kWh.

(Note that these prices have not been updated for quite some time, and are different to the fuel costs used to calculate the cost of running the home and the finacial improvement from carrying out any Recommendations - these are located in a separate document, acessed via the same link above, called 'SAP2012 Prices used by SAP and RdSAP for calculation of fuel costs' and these are updated every few months)

Clearly, the more expensive the fuel, the worse the EPC rating will be, and conversly the cheaper the fuel, the better the EPC rating will be.

In the future, things will change. The goverment have issued consulations in 2024 and 2025 which include the suggested future form of EPC certificates, and it is likely that the single cost based metric that EPCs currently have will be replaced with perhaps four or five metrics instead, and the importance for MEES targets for example shifting to 'building fabric' as a primary metric and one other metric to be met also. At that point, approaches for EPC rating improvement may change due to different criteria being introduced.

At the time of writing however, under RDSAP 9.94, financial cost is still the important thing to consider when looking to improve your EPC rating.

Here is an extract of information from Table 12 of the above document listing the costs of the most typical fuels, but ignoring standing charges and fuel factors (see Table 12 for the full information):

The complete list is quite large, and I have only reproduced the most common fuel types and the unit price information here.

Of the common fuel types listed above, mains gas is the cheapest and is therefore likely to be the best choice for a good EPC rating.

The most expensive in this list is peak rate electricity on a standard tariff (and also on the 7 hour tariff although water heating would normally use the Low rate where a 7 hour tariff is in use). This as a fuel would likely lead to a poor EPC rating - Heat Pumps being an exception however.

Often, domestic hot water is heated by the same central heating system that provides heat for the 'Space Heating'. These systems could include:

• Gas or Oil Combi boiler
• Gas or Oil System boiler with Hot Water Cylinder
• Heat Pump with Hot Water Cylinder

These systems usually support a decent EPC rating because the fuel costs of those systems are pretty good compared to other options - see the table above to compare.

Heat Pumps do use electricity but their efficiency (often expressed as Seasonal Coefficient of Performance - SCOP) can be as high as 400 to 500% on a well designed and installed system so they would be considered a bit of an exception to the previous section about Fuel Costs.

In comparison, other Space Heating systems that operate on electricity as a fuel type (such as an electric boiler) will be much more expensive if they use peak rate electricity for the required heating, as they will only ever be a maximum of 100% efficient.

Hot water can be heated via other methods such as:

• Instantaneous hot water heater
• Immersion heater(s) in a hot water cylinder

Instananeous heating is where the hot water is heated at the time that it is needed, rather than being heated in a cylinder and stored until it is required.

The fuel type of course will be the main driver towards EPC rating improvement.

If the instantaneous heating of hot water uses peak rate electricity which is expensive as a fuel, then this will affect the EPC rating.

Some properties, particularly those without gas, may rely on electric immersion heaters in a hot water cylinder for domestic hot water provision. The space heating for such properties may also be electric in nature such as storage heaters.

In such a situation, you should aim to have a cylinder with two immersion heaters, one wired for night time electricity, and the other for daytime peak rate electricity. Both would normally be wired with a manual switch, the off-peak one switched on all the time, and the peak rate one switched off all the time unless you end up using all the hot water during the day and urgently need to heat more.

If you instead have a hot water cylinder with only one immersion heater and are on single rate electricity then your EPC rating will be affected. 

Hot water cylinders are used to create and store hot water, from where it can be drawn off at a later time, when required.

Currently, when an energy assessor locates a hot water cylinder they will record:

• The size of the cylinder: Normal, Medium, Large
• Insulation Type: No Insulation, Jacket, Foam
• Insulation Thickness: None, 12mm, 25mm, 38mm, 50mm, 80mm, 120mm, 160mm
• Heatings Immersion Heater: Single, Dual (If not heated by the main Space Heating system) 
• Cylinder Thermostat?: Yes, No (If heated by the main Space Heating system which is a system boiler etc.)
• Electricity Meter Type: Single, Dual

(The last item, Electricity Meter Type, is always recorded whether a Hot Water Cylinder is present or not)

When heated by a boiler, you should aim to have a thermostat fitted to the side of the hot water cylinder. Presence of a thermostat will lead to a better EPC rating than without a thermostat.

When heated only by electric immersion heaters, you should aim to have two immersion heaters, and the eletricity meter configured on a dual rate electricity tariff. One immersion heater should be connected to off-peak rate electricty and the other connected to peak rate electricity. If you only have one immersion heater, the EPC rating will be affected, and so too if you only have a single rate electricity meter.

In all cases, the insulation around the cylinder should be as good as you can get it. Aim ideally for factory fitted foam insulation and if possible 50mm or more insulation. Foam is better than mineral wool jacket insulation which of course is better than no insulation at all.

In all cases however, always check with a local domestic energy assessor before making chages. This will ensure you know whether the costs of the changes you are making will produce an acceptable change in EPC rating.   

As I was researching for this page, I came across a device I hadn't heard of before. It was described in a Dimplex document advising energy assessors how certain Dimplex products should be entered in the RdSAP EPC assessment software. It's the Dimplex Edel:

I've stolen the above image from the Dimplex website. Dimplex, I hope you don't mind.

The Dimplex Edel is a hot water cylinder with a bult-in Heat Pump which uses ducting to draw in air from outside the property, and exhaust air from the unit.

The smaller unit uses a single, balanced duct for both inlet and exhaust whereas the larger units use separate inlet and exhaust ducts.

I've included it in the next section showing indicative EPC ratings for my house, and I was surprised to see it produce a better rating than the baseline using my gas combi boiler. The raw cost of one of these cylinders is over 2,000 GBP however.

Anyhow, this might be an option if you have a property without gas or in the example in the Dimplex case study in Jersey where at the time, the cost of gas was three times that in the UK.

In the following table, I model my house with various forms of Water Heating. For more information about this house, see our Case Study 2 page. In all cases, the space heating continues to be provided by my Alpha Intec2 28X gas combi boiler: 

This shows a potential improvement from 44E to 64D for my property if I were to move from a single immersion heater on single rate electric, to dual immersion heaters on dual rate electric (50mm foam insulation in both cases). This is an increase of 20 SAP points, and indicates the importance of using low cost night time electricity for heating a hot water cylinder.

The rating for the example Vaillant Heat Pump is disappointing but likely limited by the methodology.

The Dimlex EDL200 produces an impressive rating, and could be useful if you don't have access to gas in the property, although the cost of this unit is high, and ducting is required.

The rating for the gas combi boiler being so good is due largely to the lower cost of the fuel type.

In the complete absence of a hot water heating system, the EPC rating is badly affected. I've not been able to locate exact information for the current RdSAP 9.94, but looking at the RdSAP 10 documentation due to be implemented shortly, this would be because the methodology falls back on assuming one electric immersion heater if a single rate electric meter is present, or dual immersion heaters if a dual rate electric meter is present. Also a normal cylinder size would be assumed, but with an insulation level dependant on the dateband of the property. For my property, dated 1950-1966 the insulation level would be vey low.

I hope this section has been useful, particularly if you use immersion heaters as the source of hot water in your property. 

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