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EPC software seems unable to handle eco improvements

Posted by Martin Hodnett on 2 August 2012 at 10:44 am

Recently, we had to get an energy performance certificate (EPC) done to qualify for the feed-in tariff (FiT) for solar panels.

We live in an old Devon longhouse constructed of cob. An Esse wood burning range cooker, a solar thermal panel system, and an oil-fired boiler (as backup for very cold weather) heat an 850l thermal store, which runs the central heating (radiators), with weather compensation, and provides mains pressure hot water. There are also two wood burning stoves, a 2kW solar PV system and a 6 kW wind turbine. The attic space is well insulated, the cob walls are very thick and there is some double glazing.

The first run with the EPC software gave the house a rating of 46, which is the middle of band E. This meant that we would not be eligible for the FIT for the new PV system. The EPC assessor and I were more than a bit surprised! The first assessment was based on the house being entirely heated by wood.

We then told the software that the house was heated using oil, which took the score to 58 and band D. It was only then that I realised several things:
- the EPC rating is based on heating cost – not carbon emissions,
- the EPC software costs the wood-fuel based on open market prices
- the EPC software uses an assumed efficiency for a wood-fired boiler

Burning wood for fuel keeps your carbon emissions down, but increases the EPC rating because wood is an expensive fuel, and the software assumes a relatively low efficiency for the boiler. In our case, we use wood for fuel because it comes from the maintenance of our own hedgerows and costs us our own labour, and fuel for the chainsaws and tractor to transport it.

The EPC figure still seemed remarkably low, so the assessor and I went through the various components of the software to ensure that the more unusual “eco” items had been entered correctly. He was at his PC in his office and I was at home on the phone to him. There were various items that the software could not handle. The particular issues that we found were:- the largest size of thermal store in the software was 170 l – some stores (or accumulator tanks) for large properties can be up to 2,000 l in size.

- For the wind turbine, the hub height and the rotor diameter had to be entered, but these were limited to a maximum of 5 m and 2m respectively. The hub height was the height above the roof ridge. A 6kW “domestic” wind turbine has a typical hub height of 15 m and a rotor diameter of 5.7 m, and is also not mounted on the roof of the house! As a result of the assumptions about the mounting height and diameter of a “domestic” turbine, the software cannot correctly assess their contribution to running costs.
- There was no option to select a range cooker as the source of heat to the central heating system, and no efficiency data for it either
- When specifying the solar thermal system, there was no option to select compound parabolic collector (CPC) panels. The only options were flat panels (glazed or unglazed) and evacuated tubes. CPC panels have a higher performance than flat panels. The collector area that could be selected was also limited, and much less than the area of panels that we have. As a result, the incorrect data about the solar thermal system made no difference to the EPC rating.

These limitations meant that it was not possible for the EPC software to correctly assess the EPC rating of this property. In order to get a reasonable figure out of it, we had to pick the highest values that the software would let us enter, but these would have considerably underestimated the contribution of these technologies to reducing heating costs.

The rating did improve, but the factor that made the biggest difference was specifying the oil fired boiler type (a Grant Multipass), rather than a generic oil boiler. The EPC then jumped to 86, which is a much more satisfactory band B, but it would appear that, had the software been able to correctly handle the “eco” technology that we have installed , we might have achieved an A rating!

The EPC software probably works extremely well for the majority of properties, but it is of concern that it cannot handle eco technology properly.

About the author: Martin Hodnett is a retired research scientist, with an interest in green building.

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

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8 comments - read them below or add one


surp1Comment left on: 1 September 2020 at 7:58 am

These limitations meant that it was not possible for the EPC software to correctly assess the EPC rating of this property. In order to get a reasonable figure out of it, we had to pick the highest values that the software would let us enter, but these would have considerably underestimated the contribution of these technologies to reducing heating matcha powder wholesale


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muneebqadarComment left on: 21 August 2020 at 1:10 pm

I think the software they got developed is filled with bugs and needs to be looked at by a professional software developer.

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tony nicholson

tony nicholsonComment left on: 13 March 2015 at 8:02 pm

AH -ha the EPC house only just scrapped through because we do not have central heating and we have balanced flue heating which is maually operated.

Now my daughters house is cenrtral heated with an energy efficient condensor boiler and the damn thing costs an absolute fortune to run!

After fitting the solar and having a constant reading for the room tempatures it became very clear why, the central heating kept a beautifully constant tempature while our system warmed up over the day and cooled down at night, if its cold we switch in more heat (gas or solar powered oil radiator) when its hot we turn it off, when its evening we bugger off to bed!

So old fashioned and quaint but by golly so cheap are we so as humans can shift about and choose their comfort zones why is it the inefficient is actually more fuel efficient??

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Energy Friend

Energy FriendComment left on: 6 August 2012 at 6:42 pm


 I think many of the major points have been answered. But I would like to add that having done Green Deal Advisor Training today.  You can access Green Deal Finance if you want to improve your home even If the Golden Rule is NOT met.  It is a loan subject to credit scoring, therefore if you're assessed as being able to afford the repayments you can install whatever you want under the green deal. 

(I know - we were surprised too!)

I guess the point is that the function of the EPC is to give prospective homeowners an indication of the running costs of the house they intend to buy or rent.  I meet people every day who look at the costs on the EPC and say "my energy bills are nowhere near that high" but on drilling down this is typically because they are using less energy in their house or heating it through some other way than using the central heating.  

But the EPC isnt allowed to include your patterns, it has to give a typical scenario.  So if a retired couple who use the log fire all day tell me they only spend £200 a year on oil and I move in there with my family of 5 and have the central heating on I wont be happy when my oil bill is over £2000. So the EPC is the best way to get this impartial view of the running costs.

Green Deal occupancy assessments DO take into account your heating patterns and use so give a more accurate representation of where you can make savings on your bills.

The focus here is to reduce bills, we hope that carbon reduction is a by-product of bill reduction, which mostly it is, we just have to put our common sense hats on when the software tells us to use oil over wood.  

However if everything was re-designed to be based on carbon reduction rather than bill reduction then the Green Deal would not appeal to the mass market and would fail.  We at Energy Friend are really excited about Green Deal and think it will genuinely help a lot of people sort out the houses and live in warm comfortable homes without costing the Earth.



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Cathy Debenham

Cathy DebenhamComment left on: 6 August 2012 at 8:34 am

Posted by Linn Raffety on LinkedIn:

I like to explain the EPC rating by saying that it reflects how efficiently the home turns pounds into what you are paying for - heat, light, hot water, comfort etc. Yes it is based on fuel cost (not just heating cost), but this is for the very good reason that it allows buyers or renters to compare homes they are considering on the basis of the likely fuel bills. This is particularly important for renters, who need to know that they can afford the monthly bills - and it is very strange that prospective tenants seem not to realise that they are entitled to receive this valuable information before they decide to rent.

Now, back to your question about Green Deal. For an EPC, the recommendations have to be made without any discussion with the customer, because when a home is changing hands, the DEA's immediate customer (who paid for the EPC) is about to move out. You don't get the chance to talk to the new occupier about their preferences - you don't even know who they are yet.

So, the EPC has to make recommendations following a prescriptive decision tree, & it's a requirement of the European legislation that this is based on cost effectiveness, not carbon effectiveness. However, in Green Deal, there is an opportunity to discuss other options with the customer.

If the customer don't want cost effectiveness, if instead they want to choose a carbon saving technology even though it will not save as much money as the other option, they can. However, they can only use Green Deal finance for it if the Golden Rule is met.

Now the Golden Rule is based on cost, so some of the more extreme carbon saving measures won't meet it. If the customer wants a carbon saving measure, even knowing that it won't save them money, they can have it, but they won't be able to pay the whole cost using Green Deal finance. The Green Deal charge has to be covered by the money savings that the improvement brings to the fuel bills.

I see this as an important protection for future occupiers, who may not want to pay through the nose for the previous owner's green principles and convictions. 

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Cathy Debenham

Cathy DebenhamComment left on: 6 August 2012 at 8:32 am

Posted by Chris Holmes on LinkedIn:

Hmm! If there was an oil heating system and also a wood burning range that was piped into the same system then these would normally be input as two main systems on a 50/50 split unless there was good solid evidence of a different % split.
A log burner would be entered as secondary heating. If this was for a FITS application the professional and financial implications of an "Adjusted" EPC are huge.

The EPC cannot be produced just recording the oil system as the main heating and not inputting the wood burning range.

I may of course have misinterpreted what happened in the end and stand to be corrected if I have. 

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NorthGlosEPCComment left on: 5 August 2012 at 10:07 pm

I'm an energy assessor and I too have sometimes wondered about the way what might appear to be a rating increasing measure can often be turned by the EPC software into something that actually reduces the rating.

The examples given in the above blog are something of an extreme. The number of cob houses with outsize thermal stores, large wind turbines, several wood stoves and oil fired central heating are going to be a tiny minority of UK housing stock. It would be very difficult to devise a complete catch all system, and to be fair the current EPC methodology and software gives a fair rating for 99% of UK housing. I think Martin would soon be complaining if a far more in depth survey to cover every possible eventuality cost many times the modest sum he paid for his EPC

However it does sometimes seem there are anomalies within the current methodology and software that appear to defy logic. An example I often come across is where a house has mains gas powering a 90% efficient condensing boiler and a wood stove for secondary heating. Include the wood stove and the rating is lower even though wood is a renewable fuel, often sourced for free.

But if you think about it the software assumes a percentage of heating will come from a 65% efficient wood stove. Take a weighted average efficiency and the result must be lower. It is also perhaps unrealistic to assume wood to burn will always be available for free, but it does all seem a little counter intuative at first glance.

As for Mike's comment on the EPC "it still appears to focus too much on cost and not enough on carbon". Well if you consider what matters most to most people it should focus even more on cost. That is if it is to have any credibility at all with the public at large.

Of course we must try to reduce carbon but please realise to most people their energy cost is most definately number one priority. People ask me about which improvement measures give the biggest energy cost reduction for the lowest investment. No one ever asks which gives the biggest carbon reduction.

It is also becoming clear to me that "energy efficiency" and "energy cost efficiency" are not exactly the same. You can be super efficent if you throw enough money at it, but it might in the extreme take over twenty years before you start saving on your bills. So I advise people most of them living in normal average housing (you know the vast majority) to insulate as much as possible and go for a good efficient well controlled heating system soon as practical. More exotic renewables are great and by all means go for them if you can afford them but get the basics right first.

Martin, sounds like you have an efficient house and you only really wanted an EPC so you could qualify for the FiT. Your accomodating energy assessor was able, within the rules, to provide your "D" rating. It shouldn't really worry you that your unusual home didn't make the "A" or "B" rating you'd like. You can sit back in the confidence that you have achieved something a little special that does not conform to normal generic perameters.



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mike's carbon revolution

mike's carbon revolutionComment left on: 2 August 2012 at 3:05 pm

hi Martin, I've had a similar experience and I live in a more 'standard' Victorian cottage. I had an EPC done after installing an air source heat pump and was shocked to find out that I would have scored much better on the EPC if I'd just upgraded my oil-fired boiler. Although the EPC presentation has greatly improved since then, it still appears to focus too much on cost and not enough on carbon.


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