Water Heating

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 9.94. This updated 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' means the heating of 'Domestic Hot Water' (DHW) which we use for filling the bath, for use in mixer showers, and supplies hot taps in kitchen sinks & bathroom basins etc.

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

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

Currently, under RDSAP, the final EPC rating is a cost based metric. 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.

If you can change your hot water heating to one that is effectively cheaper to run this can help improve your EPC rating.

In the future, EPC reform will lead to certificates with multiple metrics and approaches to improving ratings will change.

As with Space Heating, as a generalisation, the fuel type used for Water Heating is probably the first factor to look at.

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 a more expensive fuel could lead to a lower EPC rating, and a cheaper fuel to a better rating, but there's another aspect considered by the calculation.

Efficiency varies from one system type to another, and this also affects the overall cost.

Modern gas boilers are perhaps around 90% efficient.

Direct electric heating is perhaps ~100% efficient, and examples include immersion heaters, electric boilers and instantaneous electric water heating.

Air source heat pumps also run on electricity but are perhaps 350% efficient, so when this is factored in their running costs end up being comparable to gas boilers because electricity costs about 3.5 times that of gas (p/kWh).

With electricity there is also another aspect to consider, single or dual rate. Immersion  heaters are ~100% efficient but if your water is heated at night using cheaper electricity the running costs are much lower than using a single rate tariff.

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 this table only reproduces the most common fuel types, and only the unit price information.

Of the common fuel types 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 due to their high efficiency.

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, and close to the point of use, rather than being heated in a cylinder and stored until it is required.

The fuel type of course will be a major driver towards EPC rating improvement.

If the instantaneous heating of hot water uses peak rate electricity, which is expensive as a fuel, then this could lead to a lower 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 this 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 are often 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 you might get a better rating with dual immersion heaters and dual rate electricity.

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 (55 to 130 litres)
  • Medium (131 to 170 litres)
  • Large (170+ litres)
• 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 (this is always recorded but has significance with immersion heaters)

Plus either:

• Insulation Type: No Insulation/Jacket/Foam
• Insulation Thickness: None, 12mm, 25mm, 38mm, 50mm, 80mm, 120mm, 160mm

or:

• The cylinder heat loss in kW over a 24 hour period (if documentary evidence is available)

When heated by a boiler, a thermostat would normally be 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 would expect 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 heating costs in the calculation will likely be greater and the EPC rating will be affected. This is also supported 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.

With the update to RDSAP 10 in 2025, it is now possible to enter a cylinder heat loss figure in kW/24hr instead of selecting the type and depth of insulation. Documentary evidence is required to support this data entry but this field was introduced along with others to 'improve the accuracy' of EPC assessments.

In all cases, 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.

The Dimplex Edel is a hot water cylinder with an air source heat pump built into it.

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

The Edel uses ducting to draw air in from outside the property, and exhaust air from the unit. It's powered by electricity, but with the efficiency of a heat pump is cheaper to run.

I've included it in the next section showing indicative EPC ratings for my house, and it produces a better rating than my baseline with a 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 58D 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 6 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.

Showers are recorded in EPC assessments as either:

• Electric - where water is heated instantaneously in the shower unit using electricity at the time of use
• Mixer - where the hot water is usually sourced from a main heating system, and perhaps stored in the interim time in a hot water cylinder.
   

The methodology considers the cost of the fuel used (p/kWh), efficiency of the heating system and potential heat losses of water stored in a cylinder for a period of time. It also considers any WWHRS, if installed.

It's likely one type of shower will be cheaper to run than the other, and if this is the case it's usually the mixer shower compared to the electric shower.

In this case, by changing the shower, it's possible to improve the EPC rating.

There's no EPC recommendation to change shower type.

Here's an example of the property we cover in our Case Study 4 property. This is a 1 bed ground floor flat with electric HHRSHs for space heating, dual immersion heaters in a hot water cylinder for water heating, an electric meter on a dual rate tariff, and currently an electric shower.

Here is the current rating with the electric shower and the rating that we would obtain with a mixer shower instead:

Behind the scenes, EPC ratings are calculated to four decimal places and rounded to the nearest integer.

In this example, and expressing to 2 decimal places for brevity, the rating rose from 66.14 to 67.81. This is an increase of 1.67 points.
 

In this particular property the water cylinder is a fortic design, which provides a low hot water pressure. We would have to replace this with an unvented hot water cylinder to support a mixer shower. This in turn would need a pressure relief pipe adding to the outside. This all needs to be considered when making the choice of changing the type of shower.

Click here to return to our 'Improve Your EPC' main page, and see if there is another way you could improve your EPC rating.