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How SAP ratings are used to measure the annual efficiency of a biomass boiler

Posted by Laurence Jones on 23 September 2011 at 10:15 am

Q: I understand that biomass boilers come with SAP ratings to measure annual efficiency. Please can you explain what an SAP rating is and how effective it is as a measure of efficiency.

A: What is SAP?

Standard Assessment Procedure (more commonly known as SAP), is a home energy rating that is recognised by the British government.

In brief, SAP provides an indexed rating of the overall energy efficiency of a building and is based on consideration of space and water heating, ventilation and lighting. These are calculated taking account of the shape and fabric of the building, its thermal insulation, the fuel used, the performance of the heating system and other energy using devices.

The SAP rating is expressed on a scale of one to 100. The higher the number, the better the energy rating. SAP also delivers an Environmental Impact rating on a scale of 1 to 100 based on carbon dioxide emissions.

How does SAP work for a biomass boiler?

Biomass boilers will run at various loads throughout the course of the year. A typical UK household will run up to “full load” throughout the winter months and “part load” during warmer seasons. The combination of these two figures (x0.5) is known as the seasonal efficiency.

Using this seasonal efficiency of a boiler, you can add it to the other SAP considerations such as insulation, the thermal conductivity of the walls etc to calculate your households rating.

Why SAP?

SAP is, by its very nature, designed to consider all of the energy that a property consumes. Accordingly, it is regarded by many as the definitive procedure for measuring home energy ratings.

You can find more information on SAP here.

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About the author: Laurence Jones was marketing support officer at HETAS

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

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4 comments - read them below or add one

Linn Rafferty

Linn Rafferty from JTec Energy PerformanceComment left on: 25 September 2011 at 2:14 pm

By the way, readers interested in SAP and how it's used to rate the energy efficiency of homes may want to read some of my earlier blogs on this site...

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Linn Rafferty

Linn Rafferty from JTec Energy PerformanceComment left on: 25 September 2011 at 2:06 pm

I often hear complaints about SAP that really should be complaints about the way SAP has been used by a particular assessor, rather than a genuine problem with the SAP method. Unfortunately, many EPCs are bought from online panel providers for a very cheap price.  Assessors who carry out these EPCs are generally only paid a pittance for their work, and therefore don't have the time to respond adequately to homeowners' concerns. I would always suggest going to a local DEA rather than buying online, as you are likely to receive a better quality of service, and possibly a more accurate EPC. See my website (link below) for more information, and a warning about how to protect yourself if you do buy online.

With regard to Pigasus' experience, I rather agree with Adrian. Although without inspecting your home I can’t be sure, it does sound like it is mainly heated by the heat pump, rather than the storage heaters, ie the heat pump is the primary heat source.  You are fully entitled to get a second opinion from another assessor who will carry out another inspection to determine this.

You mention triple glazing; if your home's glazing is to a higher standard than RDSAP (Reduced Data SAP) assumes, it is possible to enter these details explicitly into the calculation, so long as there is evidence.  In the case of triple glazing, this could be as simple as a detailed photograph of the glazing.  The assessor has to collect and input much more data than would otherwise be required, so it would be likely to cost more than a standard assessment, but worth considering. I am surprised that your assessor didn't make you aware of this option.

Another issue for your home is that the insulation is likely to be to a better standard than the system RDSAP assumes, which is something that your assessor was not allowed to change until recently – you don’t say when your EPC was carried out. RDSAP is always being updated to extend is usefulness and has just been amended to allow the assessor to increase the assumed insulation level, so long as evidence is available to support doing this. However this increase is by a standard amount and may still not fully reflect a very highly insulated element.

I hope this is helpful.

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@adrianenactComment left on: 25 September 2011 at 11:34 am

If I was you I would ask the provider of your SAP to recalculate the rating based on the heat pump being the main source of heating.  It has been a long time since I was an NHER trainer but having reviewed the SAP appendix A again it would seem that as long as your heat pump heats the lounge and at least as many rooms as the storage heaters this should selected as the main heating system, the default would also typically be the heating systems which heats at lowest cost which should be the heat pump.

Linn Rafferty is the expert so hopefully she will confirm my thoughts?

(1) If there is a central system that provides both space and water heating and it is capable of heating at least 30% of the dwelling, select that system as the main heating system. If there is no system that provides both space and water heating, then select the system that has the capability of heating the greatest part of the dwelling. For this purpose only habitable rooms should be considered (i.e. ignore heaters in non-habitable rooms).

(2) If there is still doubt about which system should be selected as the main system, select the system that supplies useful heat to the dwelling at lowest cost (obtained by dividing fuel cost by conversion efficiency) 

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pigasusComment left on: 23 September 2011 at 3:38 pm

Whenever I see the word SAP, my blood boils. We are selling our house which we built 12 years ago. It is insulated to within an inch of its life and all the windows are triple glazed. We have a mechanical ventilation system that is tied into a forced air heating system driven by an air source heat pump. We also have storage heating which comes into use only during the coldest winter months. The storage heating is supplied by off-peak electricity and controlled by an external weather watcher.

I have been able to determine our average energy consumption over the past 3 years and I think its pretty good. Our house is 274 square meters internally. In a year we use about 17,000 kWh for space heating, which is only 62 kWh per square metre. We use an additional 3400 kWh of off-peak electricity for domestic hot water.

When the assessor came to do the SAP, I explained the whole system in detail and showed her my figures. So you can imagine our surprise when we got an energy effeciency rating of only 45. I had expected a rating somewhere in the green area. I immediately contacted the assessor to question why the rating was so low. Her explanation was: we have storage heaters, we are all electric and our house is large and detached. She had discuused our unique system with her superviisor who told her that she must follow the rules.

We had been planning on making the energy efficiency of our house a strong selling feature. Needless to say we can't do that given our rating. So please don't mention SAP to me.

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