Case Study 1 - 1950s Terraced House - Improving a C Rating at Minimum Cost

At the time of writing, the methodology for EPC assessments of existing dwellings is RDSAP 10. The initial work on this page was conducted under RDSAP 9.94 and then partially re-written following the introduction of RDSAP 10 on 15th June 2025. Note that a significant methodology overhaul from RDSAP to the Home Energy Model (HEM) is due sometime after 2026, and government consulations indicate future changes to EPC metrics. Approaches to EPC rating improvement will therefore change in the future.

This is a 1950s terraced house marketed for sale with a fresh EPC rated at 69C, which is a very reasonable rating.

This is only just in the C band though, right against the lower threshold, where just one point lower and it would have been in the D band.

I think it can be raised to more of a mid-C band rating fairly easily by making some simple and cheap changes. Low cost on this project is our priority.

A slightly higher rating could look better to prospective purchasers and may help the property achieve future MEES requirements, if a purchaser wanted to rent it out, although we don't yet know what those MEES requirements will be yet.

I live in an identical house to this, and I know that my house could achieve a 69C or 70C with a little effort, and that’s without cavity wall insulation (CWI). This property here already has CWI, so I think that the 69C rating has some potential for improvement.

I’ve spotted something that has reduced the current EPC rating and once that's fixed, there are three other things I have in mind which could improve it a little bit further at minimum cost.

Once we gain full access to the propery we'll model further options too.

Let’s take a look at the Features List on the existing EPC certificate…

We can see the cavity wall being declared at the top, with retrofitted cavity wall insulation. We can tell it has been installed since the property was originally constructed due to absence of the word 'assumed' in the description.

All the other items appear: 'Good' or 'Very Good'.

The one item that stands out to me however is the last line that reads: "Secondary Heating: Room heaters, electric".

I’ve viewed the property twice and I've confirmed it has a modern gas condensing boiler fitted in 2018 with radiators in all habitable rooms, TRVs (on at least 50% of the radiators), a programmer and a thermostat.

This boiler and the heating controls provided with it are clearly sufficient to heat the house cost effectively.

During the EPC assessment though, an additional form of heating, in this case one or more peak-rate electric heaters, were identified and recorded by the energy assessor. Again, this is indicated by absence of the word 'assumed' in the description. See our Heating Systems page for further information.

Secondary Heating can affect the EPC rating, and the amount by which this occurs depends on two things in particular. One is the relative cost (pence per kWh) of the fuel used by the secondary heating compared to that of the fuel used by the main heating system (but also factoring in efficiencies of both 'systems'). The second is the proportion of the floor area that the RDSAP methodology assumes is heated by the secondary heating. That area is between 10 and 20 percent, depends on the fuel type, and is regardless of how the current occupants actually use the heating in the property.

For secondary heating, peak rate electric heaters are the worst in both cases because the cost is about the worst of the mainstream fuel types in the methodology, and at 20% carries the highest floor area percentage too. (For reference, these relative fuel prices and area percentages are detailed in Tables 11 and 12 of SAP 2012 appendix S  documentation available on the BRE website (for RDSAP 9.94)).

The gas condensing boiler in this property, recorded as the Main Heating system, is at the cheaper end of the cost spectrum, creating a large cost disparity to the secondary heating.

Did you know, there is no Recommendation available on EPCs to suggest removing secondary heating from the property.

Later, we’ll confirm how much this has affected the EPC rating by modelling, once we get the keys to the property.

We've identified four initial paths to follow to improve the EPC rating of this property. They are:

• Remove Secondary Heating
• Replace boiler thermostat with a compensating controller
• Investigate the boiler's heating pump age
• Add time & temperature zone control

Once we have full access to the property and can conduct our own EPC assessment, we can then look at other options as well.

Secondary Heating

For the Secondary Heating issue all we need to do, asuming the EPC assessment is accurate, is locate any fixed electric heating that is present in the property, remove it, and double check that all habitable rooms are heated by the gas boiler and we would then expect the EPC rating to rise, likey by a small number of SAP points..

Compensating Heating Controls

We're going to swap the existing 'On/Off' thermostat/programmer of the gas boiler out for a 'Compensating' thermostat & programmer.

We'll do this when we have the boiler moved as part of the renovation of the property because access to the inside of the boiler is required and I'm not GasSafe qualified.

The boiler is a Worcester Greenstar 30i ErP condensing gas combi-boiler. The label on the boiler indicates the Gas Council (GC) number is: 47-406-062. This number categorically identifices the make and model of boiler.

This boiler is currently fitted with a standard Worcester DT20 RF programmer and thermostat which provides basic On/Off control of the boiler.

This operates as follows: As the room temperature falls below the target temperature on the thermostat, the boiler fires up and heats water through the radiators up to a fixed temperature, determined by a dial on the boiler, and maintaints the water at that temperature.

When the room temperature reaches the target temperature, the boiler stops heating the water. The room temperature however usually overshoots the target.

After a while, the temperature in the room falls again, and the loop repeats.

We're going to fit a thermostat/programmer that provides a form of 'Compensation' control instead. This causes the boiler to modulate (i.e. vary) the flow temperature of the water through the radiators based on either or both:

• Load Compensation – taking into consideration the difference between the actual room temperature and the target room temperature
• Weather compensation – similarly taking into consideration the temperature outside the house compared to inside the house.

Load compensation reduces the flow temperature of the water through the radiators, particularly as the target room temperature is approched. This should prevent overshoot of the target temperature.

Weather compensation reduces the flow temperature of the water through the radiators for warmer temperatures outside the property.

Both of these approaches contribute to ensuring the gas boiler operates in condensing mode as much as possible, thus further increasing the efficiency of the boiler.

You'll notice the radiators feel cooler to the touch and the boiler runs for longer periods of time. The room temperature in the property will also be more constant. 

If we can find a product that provides both of these forms of compensation for our boiler at a reasonable price then we will do so, otherwise a product that just provides load compensation on its own would also be fine.

Our particular boiler does not support 'OpenTherm' and instead uses the proprietary Worcester Greenstar EMS control bus so we will have to choose a Worcester device.

Importantly, the compensating controller we choose must be listed in the Product Characteristics Database (PCDB) held by the Building Research Establishment (BRE). It must also be listed as compatible with our boiler, and it must be accessible from within the EPC assessment software.

The first step then is to take a look in the Product Characteristics Database on the BRE website. It's located at:

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

For our Worcester 30i ErP boiler, we click on the big button for 'Compensating Boiler Controls' . On the next page we select the Fuel as 'mains gas' and the Brand Name as 'Worcester'. Leave the other two options as they are (i.e. don't select anything), and click the 'Search' button. We then get the following results:

For your own boiler, you need to research the advanced controls listed by your manufacturer, to work out which ones interest you the most.

Importantly, you must go into each listing (click the corresponding Details link in the right hand column) and make sure your boiler is present in the list of supported boilers for that particular device. (I've had an issue with an Alpha climatic controller not being listed as compatible with my Alpha boiler even though it actually is - see our page on resolving PCDB issues for more on that).

Also cross-reference this information with your own research online for each device to make sure it is actually compatible with you boiler, and check the features of each device. You can't rely on the accuracy of the information within the PCDB.

Once you have a particular device in mind, check with a local Energy Assessor to make sure the control is available from within the EPC assessment software, and whether it would make a difference to your rating.

For this house I chose the Worcester Comfort I RF thermostat and programmer which provides load compensation only. It has a listing in the PCDB and our boiler is present within the list of boilers that are compatible with it (in the PCDB). The device is also available to select from within the EPC assessment software.

The thermostat also has a very simple appearance which should keep non-technical future owners comfortable.

I’ve purchased a second hand one on eBay for £44 and we’ll have that fitted when we have the boiler moved during the renovation.

Next we consider the age of the heating pump.

Heating Pump Age

During an EPC assessment, the Domestic Energy Assessor can select from the following options to describe the age of a central heating pump:

• Unknown
• Prior to 2013
• 2013 or later

This option relates to the introduction of the EU’s Energy-Related Products (ErP) Directive which obliged manufacturers and importers to provide consumers with products that were more energy and resource efficient.

This directive included heating pumps for central heating systems. These pumps move the water through a boiler and around the radiators or underfloor heating pipes in your house, and they can be operating for considerable periods of time.

Although there were stages in the applied legislation, the effect is that if an energy assessor can identify a heating pump in a central heating system to have been manufactured to the required modern efficiency requirement, and they indicate this within the EPC assessment by declaring the age of the heating pump is '2013 or later', for a house like our 1950 teraced property, it is possible the EPC rating might be uplifted by 1 point.

What I need to do is check that the pump fulfils the criteria and then I need to record sufficient evidence to support that claim in case my EPC is ever audited by the accrediation scheme I subscribe to.

Here is the convention, ie the rule, that we need to follow:

Our boiler is a combi, and the heating pump is located within the casing of the boiler. According to the Convention, we must ensure the pump complies with the EEI standard, so we need to look for markings on it that show the EEI specification.

The photo above is from an eBay listing for a heating pump from a Worcester 30i ErP combi boiler, and is the device I will be looking for inside my boiler.

The marking that shows this pump complies with the required EEI standard is the text 'EEI < 0.23'.

The date of manufacture is actually embeded in the Production Code: PC1550 - which  represents 2015, week 50. The convention however specifically requires the EEI rating for a pump located within a boiler.

See our Heating Pump Age page for more information.

For evidence requirements of the EPC assessment, I would suggest taking photos of the pump that include geo-location meta data at the property - any modern smartphone should be sufficient. I'll take photos both close up (showing the pump label) and from mid-distance (showing location context of the boiler) with the case still off. Check with your local energy assessor to confirm they are happy to accept evidence like this.

Time & Temperature Zone Control

We have more information about this on our Heating Controls page but essentially, we'll look to fit special TRVs (Thermostatic Radiator Valves) which will qualify us for Time & Temperatue Zone Control. This might give us 1 additional SAP point on the EPC rating, for a potential cost of £200 or slightly more. We won't be able to determine the success of this until we gain full access to the property and can run a full EPC assessment and model this change.

That's it for now. I'll add an update once we get full access to the property.

We've gained permanent access to the building, and I've run an EPC assessment.

After looking around the property I've confirmed there's no longer any sources of 'Secondary Heating' present at all.

I've looked back at photographs from the original property listing on Rightmove and I can see there was a fixed electric feature fireplace in the living room, but this was removed prior to completion of the sale of the property.

This is almost certainly the reason that electric room heaters are listed as Secondary Heating on the original EPC Features table:

I've also confirmed that all habitable rooms in the property are heated by the main gas fired central heating system.

So that's great news, and the EPC rating has increased by 4 SAP points due to this alone, to a 73C, without making any changes to the property.

Next, we've had the boiler moved, and I've fitted the Worcester Comfort I RF controller, and taken photographs of the heating pump within the boiler, using my mobile phone.

The label on the pump confirms it was manufactured in 2018 and more importantly for us has an EEI < 0.23. This means it can be declared as having an age of '2013 or later':

The new controller and heating pump age declaration individually would each add 1 SAP point to the EPC rating, but with them both together I also only get 1 SAP point increase overall.

This is likely due to fractional rounding, and therefore I didn't need to purchase that heating controller after all, but that's fine, the financial outlay has been small. See the section below for more about the floating point SAP rating before it was rounded to an integer.

So, the result has come out as a 74C, which is 5 points up on the previous EPC and I'm pleased with the improvement as the rating sits more firmly within the C band.

We can now look at further options for EPC rating improvement for this property.

Having full access to the property now, we can model EPC rating improvements from other measures too. This was conducted under RDSAP 9.94 which has since been replaced with RDSAP 10:

* = strictly not quite applicable for this property due to the bathroom being a wet room

The rating improvements indicated above are from the new 74C baseline for this property. The rating change displayed is for just that one single improvement listed, and is not in combination with any other change.

The EPC rating and any improvements are indicated using an integer value, or in other words a 'round number'.

Behind the scenes though the rating and any recommendations are actually calculated to a fractional number and then rounded to an integer.

It's possible therefore that combining a couple of measures in this list with zero improvement might result in a single point improvement. It would be best to model that before proceeding with any work however.

The fractional rounding of the EPC rating can be seen in the SAP Worksheets document which is a data file produced in the background by the assessment software when an EPC is calculated, and lodged to the national register.

Here is the part of that document where it rounds the final 74C rating for this property:

The rating was calculated as 73.7060 and rounded up to 74.

For any measures to improve this rating by a single point to 75C, the rating would need to improve to at least 74.50 which would then be rounded to 75.

This would need a raise of 74.50-73.7060=0.794 SAP points.

Let's consider each of the improvements in the table above:

Loft Insulation

When increasing loft insulation, the EPC rating initially rises quickly from an initial start of zero insulation depth, but then quickly plateaus. With an increase of 100mm making no integer difference to the EPC rating here, I won't bother with this option. I will need to maintain the 200mm level though as we renvate the property which includes clearing old water tanks from the loft. For more information, see our Loft Insulation page.

Solid Floor Insulation

This measure provides a very small benefit despite a considerable financial investment and a significant amount of physical work. I will skip this option.

Change to triple glazing (RDSAP 9.94)

I've not modelled this in RDSAP 10 yet, and the way it treats triple glazing is a little different to RDSAP 9.94. However, considering the signficant cost of replacing windows, and that we can evidence the double glazing in this property was only fitted in 2015, I'll skip this option as well.

Solar Thermal Water Heating

This has flallen out of favour in recent years to Solar PV instead. This relies on the presence of a hot water cylinder of the right type for use with solar thermal. The expense for this property would not be worth the 1 SAP point improvement. See our Solar Thermal page for more information. 

Time & Temperature Zone Control

This is something that I am interested in. This might only cost around £200 or so, and could give us 1 additional SAP point on the rating. I would go for the swap-out TRV heads that allow independant time and temperature control of individual rooms. See our Heating Controls page for more details. This is the most likely option I would follow next, and it would be at the last minute before we sell the property.

Flue Gas Heat Recovery

We've already moved the boiler and decided not to go for this option due to there being no integer improvement in the EPC rating, and bearing in mind the considerable financial cost this measure would involve. For more information, see our Flue Gas Heat Recovery Page.

Waste Water Heat Recovery

I would love to fit one of these devices one day, and the easy option would be to fit a Recoup Easyfit+ to an existing bath. This house currently has a wet room, but we will rip that out and fit a proper bath shortly. The Easyfit+ however costs about £600 just for the device, and I'm currently not willing to pay that much money to gain just one SAP point. For more information, see our Waste Water Heat Recovery Page.

Heat Pump

We could have looked into this further, but the gas boiler was only fitted in 2018 and our gas installer advised it looked very clean inside so we just had it moved and a significant amount of pipework pulled out and re-laid properly. We're fitting replacement radiators to a very low dt so in the future if a heat pump was fitted there shouldn't be any that need replacing. For the model of heat pump I chose to model here, and whatever the water cylinder options were, there was no gain in EPC rating, so not much incentive based on the EPC rating on its own.

Wind Turbine

Not really viable as the terrain type at this property is sub-urban. For more information see our Wind Turbine page.

Solar PV

This is a very productive option to make significant improvements in the currently cost-based EPC rating metric on an existing dwelling. We want to keep the costs low on this renovation project so unfortunately we will not be installing solar PV. Had the situation been different and our rating been difficult to raise to a C then solar is probably a path we would have followed.

Measure All Windows

This is an interesting one. This option has always been available in RDSAP, most recently in RDSAP 9.94, but it was much easier and quicker for an energy assessor to simply declare the glazing area to be 'Typical', 'More than typical', 'Less than typical' etc. See our Glazing page for more information. At the time of assessing this property myself, I chose the option for a 'Typical' quantity of glazing, so I am unable to say whether it would have made any difference to the rating at the time by measuring all the windows. All I can say is that I have reassessed the property fully under RDSAP 10 since, under which it is  mandatory to measure all the windows, and the rating is exactly the same at 74C. From this, you could potentially infer that measuring all the windows would have made no difference to this property.

Re-assess Under RDSAP 10

I've re-assessed the property under RDSAP 10, measuring all windows, counting all the individual light bulbs and determining their type etc. Lucky for me there was no room-in-roof. For this property, the rating still comes out as a 74C with the heating pump age, and load compensation heating control. That's the same rating as under RDSAP 9.94 so there was no difference. Other properties might be different, particularly if you have a high proportion of South-facing windows, or a room-in-roof.

Home Battery (RDSAP 10)

With a single rate, smart, export-capable electricity meter and a 15kWh home battery, the improvement to the EPC rating under RDSAP 10 is zero, either with or without solar PV present. For more information about this disappointing situation, see our Solar PV and Home Battery page.

Commission an Air Pressure Test (RDSAP 10)

This would be a bit of a gamble. RDSAP 10 now allows the entry of an air pressure test result for existing dwellings. For this particular property we know from modelling that a gain of 1 SAP point would be possible if we had a semi-decent air pressure test result of 6 m3/m2/hr @50Pa or better. The trouble is that we can't tell in advance how good or bad a test result would be. At a cost of perhaps £200 to commission an air pressure test, this would be a bit of a gamble. For more information, see our Air Pressure Testing page.

For this property, we've improved the EPC rating from 69C to 74C, and might also go down the TTZC route to get a 75C.

So far we have spent:

If we were to go down the TTZC route that might cost us an additional £200+ for 1 SAP point.

If I was wanting to improve the rating even further then Solar PV would be the next option I would look at, but that would come at a higher cost.

The information here is relevant to this particular property. Always check with a local energy assessor to see what improvements could be gained for your property before proceeding with any works or spending any money.