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Why is my EPC rating so poor with my new biomass boiler?

Posted by Tasha Kosviner on 12 September 2013 at 9:58 am

Q: Following the installation of a super-efficient biomass boiler, my energy bills dropped but my energy performance certificate (EPC) rating plummeted 26 points to a G. I’m convinced that the report is incorrect but where do I turn to get this corrected?

We removed a 20-year-old boiler which, at peak efficiency cost around 6.8p/kWh to run. We replaced it with a 92 per cent efficient biomass boiler. As a result we have seen a massive drop in heating bills without any changes to the fabric of the house. Imagine our shock then, when this reduced our EPC rating by 26 points to a G. Upon further digging, we discovered that the explanation for this is that when it comes to assessing biomass pellet stoves for the EPC, they are given a generic efficiency rating of 65 per cent.

A: Your EPC is prepared using a ‘SAP rating’ standing for standard assessment procedure. This is the government’s recommended system for assessing the energy efficiency of your home. The SAP charts are divided into seven bands ranging from A to G with each band having a set amount of SAP points. If your home scores 1-20 points, it is rated as G, 21-38 points qualifies you for an F rating and so on, with 92-100 points netting you the highest rating, A. You would rightly expect that by replacing an out-of-date, expensive and inefficient boiler with a brand new, energy efficient biomass boiler, your rating should go up, not down.

However, two things are at play here that are contributing to the drop in your energy efficiency rating. Firstly, you should be aware that the EPC rating is based on the cost of heating a home, so that while taking into account the efficiency of the boiler itself, it also makes adjustments according to the price of the fuel used in the boiler.

Wood pellets are more expensive per kWh of heat delivered than some other fuels, like mains gas and heating oil, so when this is taken into account the overall rating is lowered. Of course when looking at the environmental rating, the wood pellet boiler would achieve a higher score than either of those two fuels, but it's the cost-based score that appears on the front of the EPC and on which the rating band is based.

Secondly, and more worrying, is the way in which the energy efficiency of your boiler is being calculated under SAP. The 65 per cent rating that you refer to in your question is the generic figure that SAP software assigns to biomass boilers. The software can only accept a higher efficiency figure for your biomass boiler if that specific make and model has been added to the SAP product characteristics database

To go into the database, the manufacturer has to provide evidence that the efficiency they claim is supported by formal test results. It has to have been tested by an appropriately qualified test lab, as required under the construction products directive. For boilers that are included, the software is able to use the manufacturer's declared efficiency, rather than that default you mention, but otherwise the default must be used.

As our energy expert Linn Rafferty points out, the solution is clear: “Your boiler needs to be in the database,” she says. “However, this can’t happen unless your manufacturer applies for it to be included. Therefore the organisation you should turn to is the manufacturer to suggest that they apply for inclusion in the SAP product  characteristic database.”

Manufacturers who wish to submit data about their products for inclusion in the database should contact Kiwa GASTEC at CRE, The Orchard Business Centre, Stoke Orchard, Cheltenham, Gloucester, GL52 7RZ (phone 01242 677877). 

If you are considering installing a biomass boiler with a view to upping your EPC ratings, you should check whether the boiler’s listed on the database first. There are currently only 13 woodchip and wood pellet boilers listed although HETAS, the trade body representing biomass and solid fuel heating domestic heating appliances are encouraging manufacturers to get listed. 

More information about biomass boilers from YouGen

The YouGen guide to biomass boilers

Find a biomass boiler installer

From the blog

Introduction tobiomass boilers for heating and hot water

What to expect from your biomass boiler

How SAP ratings are used with a biomass boiler

Photo Credit: wwarby via Compfight cc

If you have a question about anything in the above blog, please ask it in the comments section below.

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11 comments - read them below or add one


SbchiltonComment left on: 9 November 2013 at 10:32 am

Tasha's blog was based around a question I had asked in September. For clarity, we installed a MCZ Suite Hydro 22kw. A couple of months on the boiler continues to deliver cheap heating to our 250 year old stone barn conversion. I'm a little frustrated when I read that the EPC drop is anything other than the boiler not being on the SEDBUK database. I agree on the comments of pellet responsiveness but to say oil is a cheaper fuel is just wrong.Here are some figures to back this up - if I am missing something here then please correct me: 1 tonne of bagged pellets = £240 delivered (67 15kg bags)  at peak load my boiler uses 4.9 kg per hour to deliver 22kw 0.24 x 4.9 = 1.176 (£1.176 per hour) 1.76 / 22 = 0.0535 or 5.34p per kW  Current oil prices are 54p / ltr   1ltr of oil holds 10kw 1ltr at 92% =  9.2kw  (my oil boiler was no where near 92%) 54 / 9.2 = 0.0587 or 5.87p per kW  The napkin maths above tells me that pellets  are cheaper than oil until oil prices dip below 50p/ltr  For me the EPC should be a way of informing people the true cost of running a house and using generic data does not do this.  Regardless of fuel or boiler type I have replaced an old inneficcient system with a more efficient one and have seen a huge drop in bills which I would expect to see reflected in the EPC.

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juliaaComment left on: 25 October 2013 at 8:26 am

I have had exactly the same problem with my EPC. We have double glazing throughout, warm roof construction, insulated walls (stone so no cavity insulation possible) We spent five years reducing our oil usage by 56% installing radio controlled Honeywell controllers where each radiator talks directly to the boiler. We then installed a pellet boiler which is fantastic and we end up with an epc of E which means it is hard to install PV and get the FITS. The EPC doesn't seem to take account of anything out of the ordinary or innovative. I've looked up the SPA boiler database on line and they don't even have an option of biomass boilers. I know our Okafen boiler is over 90% efficient so it is crazy that we get no credit. If a boiler is MCS approved then it ought authomatically get on the SPA database. it appears to be a lack of joined up thinking??

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Linn Rafferty

Linn Rafferty from JTec Energy PerformanceComment left on: 18 October 2013 at 1:03 pm

hi JeffB

sorry I didn't see your question earlier.  The improvements you've made to the fabric of your house, and the additions of solar thermal and solar PV panels, will all have pushed up your EPC rating as they have made the home cheaper to heat and light. 

The one improvement that may have pushed down the rating is the change from oil to biomass heating. Of course, this change will have reduced the carbon emissions from your home, so on that aspect it's a real improvement. However, as biomass is currently more expensive than oil, and biomass boilers are slightly less efficient and less responsive than oil, the headline rating on the EPC (based on cost to run) is going to go down slightly.

I imagine that the overall effect of all your changes will have been to increase the EPC rating.  The impact on your RHI application will be that the fabric improvements you've made will have reduced the heat demand of your home, on which RHI is paid, as explained by Cathy in her response to LRolfs.


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Linn Rafferty

Linn Rafferty from JTec Energy PerformanceComment left on: 18 October 2013 at 12:54 pm

hi Robsmum

In response to your question about your glazing, the DEA has to record exactly what is in the home when he visits, and will have recorded the presence of both single glazed and double glazed openings and their relative proportions. The software then produces the statement in the EPC based on that. I think that however small the proportion of single glazed windows is, the EPC will report its presence with the comment 'part double glazing'. When you think about it, this is a true statement unless every opening is fully double glazed.

On the lofts, you've said that they are both boarded an insulated. Again, the DEA has to record exactly what he sees, and cannot take the word of the customer that there is insulation present when it can't be seen. The problem with boarded lofts is that any insulation, if present, is invisible under the boards. If you have written evidence that the lofts were insulated prior to being boarded (eg a certificate from the installer) then the DEA can take this as evidence and record the loft as insulated.  

In this instance, I assume you didn't have any written evidence, so the EPC has to infer a likely level of insulation in the two lofts, assuming that there has been no insulation added since the property was built. There are two different inferences here, 'no insulation' and 'limited insulation'. In the earlier part of the property there would have been no insulation as built, and in the more recently built addition, the assumption is that some insulation was present when built. All of these assumptions are underpinned by extensive research when the EPC methodology was developed, so that the EPC can make reasonably accurate assumptions about insulation levels where the insulation itself can't be seen.

You can always contact your DEA's accreditation scheme for more guidance if you think that there may have been any deficiencies in his work.


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RobsmumComment left on: 17 October 2013 at 5:47 pm

Can anyone help me with an issue ( or two ) that I have with a recently purchased EPC report please ? 

Before I go charging off and complaining ( maybe  ? ) to the company that provided the report I would appreciate your expert comments . 

In August 2010 I replaced an old conservatory with a new one + all the main windows ( 9 in total ) and a new back door . All A rated glass and with the usual 10 year guarantee . Even the garage window was replaced , with similar glass ! 

On the front of the house there are three ' ornamental ' glass panes , measuring 23 cm x 27 cm by the front door . The solid wood front door has a leaded light glass viewing panel in the centre measuring 21 cm x 47 cm . These were not replaced or double glazed . 

My EPC report has 3 stars for windows and states ' part double glazed ' . Whilst I appreciate that the EPC inspector cannot tell what rating the glass is just by looking , does having four small panels , not double glazed , warrant a comment of ' part double glazed ' ? 

Also , I have two lofts  - both boarded and insulated . My report is showing 1 star and inferring no insulation  , assumed and limited insulation , assumed . Only one loft was briefly looked at ! 

Ok , so is this acceptable in your expert opinions ............ many thanks 

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Cathy Debenham

Cathy DebenhamComment left on: 3 October 2013 at 2:23 pm

@LRolfs - From what you say it sounds as though you meet the criteria for being a self builder.

The RHI is calculated on the head demand for your home, as measured by the EPC. If you have a look at the bottom of your EPC document you'll find two lines:

Space heating
Water heating

with a figure for each in kWh per year. If you add them together, that is the sum on which you will be paid. 

For example; mine are as follows: 

space heating: 15,183, water heating 2,806; total 17,989 kWh (I'm going to use 18,000 for ease of calculation).

If I were to install a biomass boiler I would get 12.2p per kWh, per year, for 7 years.

18,000 x 0.122 = £2,196 per year x 7 = £15,372 over the life of the tariff

Then subtract the RHPP payment (£2,000 if you installed since 1 April, less if you installed before that) = £13,372.

Because your house is highly energy efficient, the heat demand will be smaller and so your costs and the amount you get paid will be less. However, it's definitely worth applying. You'll need an EPC and an MCS certificate to apply. The subtraction of the RHPP payments will be spread over the seven year life of the tariff.

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LRolfsComment left on: 3 October 2013 at 12:00 pm

Am I right in thinking that those with a high efficieny construction, requiring a smaller boiler, will we get less than a less efficient building?  I am trying to calculate whether it is going to be worth us applying for RHI since we have already obtained RHPP.  If the returns are going to be small, based on our efficiency plus the repayment of RHPP,  then will it be worth it? 

Can you please repeat how to calculate the expected returns?  We have a 9KwH biomass boiler and our Code for Sustainable Homess assessment suggested a requirement of 6KwH.  That was probably a bit too low and in hindsight a 15KwH boiler would have been better, but we manage.  Not sure how to do the maths.

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Jeff B

Jeff BComment left on: 2 October 2013 at 4:23 pm

Can't quite believe what I'm reading here! In terms of heating efficiency our dormer bungalow has been transformed since we moved in 6 years ago. We have replaced all the double glazing with better units, removed one set of patio doors and replaced with a double glazed window (and bricked up the opening below), made secondary glazing units for the Velux windows upstairs, installed IWI along the sloping ceilings and dwarf walls upstairs, increased the insulation in the attic, got rid of an old oil fired boiler and installed a wood pellet boiler, and installed solar (thermal) panels and solar (PV) panels. At the end of all this you are telling me that our EPC rating could well go down as a result!!! As Sam Weller famously said, if that's the case then the law is an ass, sir.

  Presumably this will have an impact on my MCS application for RHI?

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Linn Rafferty

Linn Rafferty from JTec Energy PerformanceComment left on: 28 September 2013 at 7:09 pm

hi David and Tomagdsl

My earlier answers to Tasha were something of a simplification, for ease of explaining this complex subject to a consumer reading the YouGen blog.  The biomass boiler treatment in SAP (and RdSAP) takes account of the boiler efficiency, the fuel price and the system's responsiveness.  Like all solid fuel, biomass scores a little lower on responsiveness, as it can't respond to its controls as quickly as a gas or oil fired appliance.  This is in the nature of burning a solid fuel that can't be turned on or off as quickly as the other two fuels.

The massive drop in heating bills will be partly due to the better efficiency of the boiler, and possibly also to a cheaper unit price and slightly better responsiveness than the SAP assumption. SAP uses an average price over three years within the rating, although it does use a more recent 'spot price' for the running costs and savings, but this price might still be higher than the price paid by the customer.

If the 20 year old oil boiler had been included in SAP's product database it might indeed have been credited with a much higher efficiency than 65%.  This also applies to older gas boilers - for example, the economy plus 40f by Glow-worm has a seasonal efficiency of 77%, and was first manufactured in 1992. There was quite a bit of difference between efficiencies in the boilers available in the early 90s, prior to the EU Directive on boiler efficiency (1998) introducing a minimum efficiency for gas boilers.

Lastly, oil fired boilers are always slightly more efficient than a like-for-like gas or biomass boiler, due to the physics of how oil is combusted.

So in this case, I do believe that the major drop in the EPC rating could have been simply due to the way biomass boilers are treated in SAP, rather than a difference between DEAs - although it's always a possibility that this could contribute to the difference.

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TomagdslComment left on: 24 September 2013 at 6:32 pm


I think the boiler will have a great efficiency but the problem is the sap calculations within the epc software takes into account the high cost of the fuel pellets and so reduces the over all scoring within the EPC . I think they are working on the software to resolve the problem 

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David Hoole

David HooleComment left on: 20 September 2013 at 12:18 pm

This doesn't sound like the full picture to me. The resident says the new boiler has given 'a massive drop in heating bills', so the explanation of the new fuel (pellets) being more expensive than the old fuel (which type wasn't stated) doesn't sound right to me. Secondly, the 65% efficiency which you say the RdSAP software has assumed for the biomass boiler, i would think is broadly similar to the efficiency of the 20 year old boiler. So although the software isn't giving the new boiler full credit for it's actual high performance, this doesn't explain why the EPC rating of the property has dropped, and massively so.

Linn and others have discussed on LinkedIn this issue of second EPC's done post-installation of renewable technologies, being much worse than the initial assessment EPC. There seems to be some feeling that further investigation may find fault with differing methodologies, individual assessors (the choices they make when inputting to the software), or the software itself.

Am i right in observing that the common issue in these complaints is a discrepancy between an initial EPC done by a Domestic Energy Assessor or a Green Deal Advisor on the one hand, and an 'estimated' or actual second EPC calculation done by the 'MCS approved' installer of a Renewables system for the purposes of qualifying the property for the Renewable Heat Incentive payments? If so, are these different assessors, working for different purposes, producing inconsistent results for some reason? And if so, is the net effect that the RHI payments, based on the calculation of the MCS installer, will be much lower than they would / should be? 

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