Heating Systems

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. 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 will therefore change in the future.

On this page you'll learn:

✓ Why heating systems can have one of the biggest impacts on an EPC rating

✓ Why choosing the wrong replacement heating system can actually reduce your EPC rating

✓ How the EPC assessment treats main and secondary heating

✓ Why the number of heated habitable rooms can affect your EPC

✓ Why correctly identifying your boiler or heat pump is important

✓ Why you should model a proposed heating change before spending money

Heating systems can make a very significant difference to the EPC rating of a property, but they can also be expensive to replace or upgrade.

In an EPC assessment, the systems used to provide space heating are identified, together with their associated heating controls.

Domestic hot water is assessed separately, although it may sometimes be produced by the same system that provides space heating.

Under the current methodology for existing dwellings, the EPC rating is an entirely cost-based metric, based on the modelled cost of running the property. This means the type of fuel used by a heating system, together with its efficiency and the way the system is represented within the assessment, can have a significant effect on the rating.

This can produce some surprisingly large differences between heating systems.

It also means the heating system which produces the best EPC rating isn't necessarily the system which uses the least energy or produces the lowest carbon emissions.

EPC calculation methodology uses a standardised occupancy model

An EPC needs to provide a meaningful comparison between different properties. It therefore cannot simply be based on how the current occupants happen to use their heating.

Imagine two identical houses. One is occupied by a single person who heats only a couple of rooms and keeps the heating on for as little time as possible. The other is occupied by a family who heats every room and likes to keep the house warm.

If the EPC rating were based on actual occupancy behaviour, the first property would appear to be much cheaper to run even though the two buildings were identical.

To avoid this, RdSAP uses standardised assumptions so that EPC ratings can be compared with each other.

For space heating, the methodology assumes a standard heating pattern, including heating the living room to 21°C and the remainder of the dwelling to 18°C, for a defined period during the year.

Heating controls however can affect the way these assumptions are applied.

This is important when considering heating improvements: the EPC assessment does not simply reflect how you personally use your heating.

EPC rating isn't the same as energy efficiency

One of the most important things to understand is that the current headline EPC rating for existing dwellings is calculted using the modelled cost of running the property.

Fuel cost is therefore an important factor.

For example, a resistive electric heater can convert virtually all of the electricity it consumes into heat, but that doesn't necessarily make it a good choice for the current EPC methodology (or running cost) because standard-rate electricity is relatively expensive compared with mains gas.

Heat pumps are different because they don't simply convert electricity into heat. They use electricity to move heat from one place to another and can therefore deliver several units of heat for each unit of electricity consumed.

The result is that a heating system which looks "100% efficient" in advertising terms may produce a poorer EPC rating than a system with a lower nominal efficiency but a cheaper fuel.

This is one reason why you shouldn't choose a replacement heating system based on efficiency figures alone.

The EPC assessment records several aspects of the heating arrangements in a property, including:

• The presence of up to two 'Main Heating' Systems
• The presence of one form of 'Secondary Heating'
• Heating Controls for the Main and Secondary heating systems.
• The count of 'Habitable' rooms in the property
• The count of 'Heated Habitable' rooms in the property
• How Domestic Hot Water (DHW) is heated

Habitable and Heated Habitable Rooms

Habitable rooms can include living rooms, sitting rooms, dining rooms, bedrooms, studies and similar rooms. A kitchen/diner can also be regarded as a habitable room where it is capable of accommodating a table and four chairs.

Hallways, stairs, utility rooms, landings, bathrooms, cloakrooms, ensuites and rooms without access to natural light are not normally counted as habitable rooms.

Open-plan areas can also affect the way the number of habitable rooms is counted.

A conservatory separated by an internal quality door from a dwelling is also regarded as a habitable room, but a conservatory separated by external quality doors from the dwelling is not a habitable room

Once the number of habitable rooms has been established, the assessor also determines how many of them are heated by a fixed heating source.

This could be the Main Heating system or a fixed Secondary Heating source.

For EPC purposes, it can be very important whether all of the habitable rooms are heated.

Main Heating Systems

A Main Heating system might include:

• A gas boiler
• An oil boiler
• An air or ground source heat pump
• Electric storage heaters
• Electric panel heaters (in the absence of other qualifying main heating systems)

The system can provide heat through radiators, underfloor heating or, in some cases, directly into the room.

Most properties have one principal heating system, although the assessment can record up to two Main Heating systems where appropriate.

Identifying your boiler or heat pump

Where a boiler or heat pump is installed, the exact make and model are particularly important.

The assessor must try and locate the device in the Product Characteristics Database (PCDB) to identify the specific appliance:

https://www.ncm-pcdb.org.uk/sap/searchpod.jsp?id=17

Alternatively, a generic heating code may have to be used where the appliance cannot be identified.

Using the specific PCDB entry provides more accurate information about the appliance's performance than relying on a generic heating code.

If the model number or Gas Council number isn't visible on the appliance, it is definitely worthwhile gathering documentary evidence before the EPC assessment.

For example, a recent gas safety certificate may identify the make and model of a boiler.

If you're replacing a heating system, it's worth making sure that the exact appliance can subsequently be identified in the PCDB for any EPC assessment.

Because EPC ratings are calculated on the modelled running costs of the property, the fuel used by the heating system can have a major effect on the rating.

RdSAP 10 uses a set of fixed fuel-cost values when calculating the rating. These are not the same as current household energy prices and should not be interpreted as today's electricity or gas tariffs. The figures used in the calculation are just values used by the methodology. 

These fuel costs are available to view in the RDSAP 10 Specification document on the Building Research Establishment (BRE website):

https://bregroup.com/expertise/energy/sap/sap10

The fuel prices used are located in Table 32 and I beliee these were last defined in 2014.

Note that these values are different to the fuel costs which are used in the formulation of financial figures on EPCs. Those fuel costs are updated periodically.

The complete list is quite large, and I have only reproduced the most common fuel types. Some fuels also have additional standing charges:

These fuel costs help explain why different heating systems can produce very different EPC ratings.

• Resistive electric heating uses expensive standard-rate electricity
• Storage heaters can make use of cheaper-rate electricity.
• Mains gas has a relatively low unit cost within the EPC methodology.
• Heat pumps are different again because their high efficiency means that they can make effective use of electricity despite its higher unit cost.

The result is that the fuel used and the efficiency of the heating system need to be considered together when considering the effect on the EPC rating from a heating upgrade.

Heat pumps are an important exception to the simple relationship between fuel cost and EPC rating.

Unlike a resistive electric heater, a heat pump does not convert electricity directly into an equivalent amount of heat. Instead, it uses electricity to move heat from one location to another.

A heat pump can therefore deliver several units of heat for each unit of electricity consumed. Its actual performance depends on the design of the system, the installation and its operating conditions.

For EPC purposes, the specific performance of a heat pump is obtained from the relevant PCDB entry for the appliance.

This entry needs to be chosen carefully as there are multiple entries for each model, with different options.

Important: The performance of a heat pump should not be assumed simply from a headline efficiency figure. The actual system and the data used in the EPC assessment are important and very sensitive.

If the appliance cannot be located in the PCDB then a  generic heating code is used instead, and that uses basic low efficiency figures.

A 30F Property That Could Reach 71C - By Just Changing The Heating System

One of our case studies involves a property with no Main Heating system and no Secondary Heating.

Its EPC rating was just 30F, despite other aspects of the property being relatively good.

Under the assessment methodology, where there is no heating system the heating requirement still has to be accounted for. In this case, the methodology has to assume a worst case scenario using resistive heating and peak-rate electricity.

The original EPC recommendations indicated that installing a central heating system based around a condensing gas boiler, together with solid-floor insulation, could take the rating to 74C.

Based on the modelling, we estimated that the heating system alone would take the rating to approximately 71C.

That's a potential improvement of around:

30F → 71C = +41 SAP points

This is an unusually large improvement and should not be regarded as representative of a typical heating-system replacement.

It demonstrates, however, just how much influence the heating system can have on an EPC rating where the starting point is particularly poor.

Read more about this on our Case Study 3 page.

The EPC assessment can record one form of Secondary Heating.

Examples include (but are not limited to):

• a fixed electric panel heater
• an electric heated towel rail
• an electric plinth heater
• an electric feature fireplace
• electric ceiling heating
• electric underfloor heating
• an open fireplace with a grate or
• a fixed wood-burning stove

The heating source needs to meet the relevant criteria to be recorded. For example, a portable electric fan heater would not be recorded, but a fixed electric feature fireplace would be recorded even where it is plugged into a socket.

Secondary Heating Is Recorded Whether You Use It Or Not

One particularly important point is that the assessment doesn't simply ask: "Do you use this heater?"

If a qualifying Secondary Heating system is present, it will be recorded regardless of whether the occupants actually use it.

The methodology then usually assumes that a proportion of the property is heated by that Secondary Heating system.

This can have a surprisingly large effect on the EPC rating.

Secondary Heating Can Reduce Your EPC Rating

There are circumstances where Secondary Heating can work against the EPC rating.

For example, if a property has an otherwise good central heating system using a relatively inexpensive fuel, but also has fixed Secondary Heating using a more expensive fuel such as peak-rate electricity, presence of that Secondary Heating in the assessment can adversely affect the EPC rating.

This can happen even if the occupants never use the secondary heating.

In my own property, recording a fixed electric decorative fireplace reduced the EPC rating by 6 SAP points to 62D.

This is a particularly important example of why the EPC assessment doesn't necessarily behave in the way you might expect from your own day-to-day use of the property.

This is another interesting situation.

If a property contains habitable rooms which are not heated by the Main Heating system, and there is no qualifying Secondary Heating system recorded, the methodology makes an assumption about how the heating shortfall will be met.

In the example of my own property using a gas boiler, reducing the number of Heated Habitable Rooms by one resulted in the methodology assuming peak rate electric heating.

The EPC rating fell from: 68D → 62D

That's a 6 SAP-point reduction caused by an absence of heating in just one habitable room.

This is why it is important to understand the count of Habitable Rooms and Heated Habitable Rooms in relation to EPC assessments.

If you have electric storage heaters and are considering replacing them with electric panel heaters, check the consequences for your EPC rating before proceeding.

Some electric-heater advertising focuses heavily on the fact that resistive electric heating can convert almost all of the electricity it consumes into heat.

That is true, but it doesn't mean that replacing storage heaters with panel heaters will improve the EPC.

Storage heaters are able to make use of cheaper-rate electricity to charge their thermal store. A standard electric panel heater normally uses electricity at the prevailing rate when it operates.

Under the current EPC methodology, the difference in fuel cost can result in a lower EPC rating after replacing storage heaters with panel heaters.

The standardised occupancy models used by RdSAP also mean that simply saying "I only turn my panel heaters on for an hour a day" doesn't change anything about the EPC calculation.

The important lesson: Don't replace an existing heating system simply because a replacement system is advertised as "more efficient". Model the proposed change first.

This is particularly important where storage heating is involved.

My own 1950s terraced property provides some useful examples.

The baseline EPC rating is 68D.

The property has a condensing gas combi boiler and no qualifying Secondary Heating recorded.

It has five Habitable Rooms and five Heated Habitable Rooms.

If one of those five rooms is recorded as not being heated by the Main Heating system, the methodology assumes additional heating is required and the rating falls to:

62D

Similarly, if my decorative electric log burner had remained a fixed Secondary Heating appliance, the rating would also have fallen to 62D.

This demonstrates that the EPC effect of a heating system isn't simply about the boiler or heat pump. How the heating system is recorded and how it relates to the habitable rooms can also be important.

One approach is to ensure all your rooms that are classified as 'Habitable' rooms are heated by the Main Heating system in the house, and remove any forms of secondary heating (particularly electric) if your main heating system runs on a cheaper fuel.

This is just one strategy, but you won't see changes like this listed as a recommendation on your EPC certificate.

Many properties have a boiler as their main heating system and if this is the case, then this can be recorded in the EPC assessment either by:

• Referring to the exact make and model of boiler in the Product Characteristics Database (PCDB) from within the EPC assessment software, or
• Using a generic heating code to describe the type of boiler instead

This also applies to heat pumps, and is also important for High Heat Retention Storage Heaters.
 

The best EPC rating outcome is obtained when the PCDB record is used, because this imports the most accurate information about the heating system.

If the boiler cannot be identified, or cannot be found in the PCDB then the generic heating code will result in significantly lower figures being used for efficiency and performance, when calculating the EPC rating.

The PCDB can be accessed here, and you can lookup your make and model of boiler or HHRSH or heat pump to make sure it is listed:

https://www.ncm-pcdb.org.uk/sap/searchpod.jsp?id=17

If your boiler doesn't have a model number or Gas Council (GC) number visible on the casing, prepare as much documentary evidence as you can.

A suitable document for example could be a recent gas safety certificate where the boiler model is listed.

It is worth doing this before an EPC assessment rather than discovering afterwards that the assessor couldn't identify the appliance.

Also, check to see if you can upgrade your Heating Controls to try and improve your EPC rating.

Perhaps also check to see if you are able to help identify the Heating Pump Age of your boiler for your energy assessor.

There are also Conventions (rules) about what must be recorded when there is a mixture of heating systems present.

I've modelled my own property with a range of different heating systems to see how the EPC rating changes.

These figures were originally calculated using RdSAP 9.94 and are included as an illustration of the effect different heating arrangements can have.

They should not be treated as predictions for other properties:

In the above table, all references simply for 'gas' means 'mains gas' rather than LPG.

The results are quite striking. In this particular property, the difference between the various heating systems is substantial.

However, these figures should not be used to predict what will happen in another property. The EPC rating depends upon the whole dwelling, including its size, construction, insulation, hot water arrangements, heating controls and the particular heating appliance.

Use this table to understand the principle, not to predict your own EPC.

Changing a heating system can be an expensive decision, and the system which looks most attractive on paper isn't necessarily the one which will produce the best result for your particular property.

Before committing to an installation:

Check Your Existing EPC

Make sure the existing heating system, boiler/heat pump and heating controls have been recorded correctly.

Model The Proposed System

Ask a Domestic Energy Assessor to model the proposed change if your main objective is to improve the EPC rating.

Consider The Whole Property

The effect of a heating system depends upon the property as a whole. A heating system which produces a particular EPC rating in one property may produce a different result in another.

Don't assume that a more expensive system produces a better EPC

Heat pumps, storage heaters, gas boilers, oil boilers and resistive electric heating are treated differently by the current methodology.

The most appropriate choice also depends upon factors beyond the EPC, including installation cost, running costs, carbon emissions, comfort and the future direction of energy policy.

If improving your EPC rating is your objective, model the change before you spend the money.

Even where you already have a relatively good heating system, improvements to the heating controls may provide an opportunity to improve the EPC rating.

For more information, see our Heating Controls page to see what might be possible. 

✓ Heating systems can have a very large effect on an EPC rating.

✓ The cheapest or most efficient-looking heating system isn't necessarily the one which will produce the best EPC rating.

✓ The EPC assessment uses standardised assumptions rather than your actual heating habits.

✓ Secondary heating can affect the EPC even if you never use it.

✓ Unheated habitable rooms can also affect the EPC.
Correctly identifying your boiler or heat pump can make a significant difference to the assessment.

✓ Don't assume that a heating improvement which works well in one property will produce the same result in another.

✓ If you're considering changing your heating system to improve your EPC, model the proposed change before spending the money.

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