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Gas Boiler Energy Efficiency


Energy efficiency how does it affect me?

Under Part L of the building regulations all new gas boiler installations or replacement boilers need to be high efficiency Condensing boilers. Where necessary Heating controls and systems will also have to be updated, such as converting from a gravity fed system to a fully pumped system, controls fitted such as TRV's fitted to Radiators and an Auto by pass valve fitted where specified. Any property with floor area exceeding 150M will require Zone controls. Boiler selection will be based on the whole house boiler sizing method as specified under SAP ratings.

What is a condensing boiler?

A condensing boiler is a high efficiency modern boiler that incorporates an extra heat exchanger. This allows the hot exhaust gases to lose much of their energy pre-heating the water in the boiler system. When working in condensing mode, the water vapour produced in the combustion process condenses back into liquid form releasing the 'latent heat' of vaporisation into warming the incoming water supply. A side effect of this process is known as condensate, which is usually mildly acidic, has to be piped away to a drain or soak away. Another side effect is known as pluming.
It is important to note that condensing boilers do not operate in the condensing mode all the time. However even when not in the condensing mode they are significantly more efficient than conventional boilers.

Who makes condensing boilers?

All the Major manufacturers make Condensing boilers, even a majority of the oil fired boiler manufacturers produce condensing boilers even though mandatory condensing boilers are required until 2007.All boilers are rated using SEDBUK, a useful guide to the efficiency of boilers called the 'little blue boiler book' is available.

What are TRV's?

Thermostatic radiator valves (or TRVs for short) will add to the effectiveness of any control system. TRVs will allow control of individual room temperature by gradually closing down a radiator if a room starts to overheat due to adventitious heat gains (e.g. sunshine). A TRV should not be fitted to the radiator located in the same room as the controlling room thermostat.

Programmable room thermostats

Provides greater control and energy savings ove a normal room thermostat. Being programmable allows it to meet the use of the room with heating on off or low dependant on the room usage.

What is SAP?

SAP is the government's Standard Assessment Procedure for energy rating a dwelling.

  • lt is a basic energy rating which assumes average factors for shelter, shade and orientation and considers the space and water heating only.
  • The SAP scale currently runs from 1 (extremely poor) to 120 (extremely good), with a score of 80 or more considered to represent an energy efficient home.
  • The average UK home (ie. not brand new) has a SAP rating of 40-50.
    Most new homes achieve ratings of 80 or above, and there are many that reach top scores of 100-120.
  • In 1995, Building Regulations changed to require a SAP rating on every new home and from 2001 builders had to to post a notice in every finished dwelling stating its rating.


('Seasonal Efficiency of Domestic Boilers in the UK') is the preferred measure of the seasonal efficiency of a boiler installed in typical domestic conditions in the UK, and is used in SAP assessments and the Building Regulations.

Efficiency ratings

The SEDBUK efficiency of most current and obsolete boilers can be found on the website

Although SEDBUK is expressed as a percentage, an A to G scale of percentage bands has also been defined in the table above. The table bands are colour coded green A band most efficient band through to yellow then on to red band G the least efficient.

Efficiency Ratings

Why are boilers targeted?

Boilers are amongst the least energy efficient appliances in your home, they alone account for up to a third of all domestic CO2 emissions. As the current lifespan of a boiler is 10-15 years, making the wrong decision about what boiler to have really could waste considerable amounts of your money and contribute to climate change.

A heating system that uses a high efficiency condensing boiler with the correct heating controls can save you as much a 40% on your heating bills. It really is time to consider whether you have the right boiler.

All homeowners will be responsible for ensuring central heating work in their home complies with the relevant Building Regulations. Your local authority must be told when you have a new or replacement boiler fitted. Your installer should do this for you and will arrange for you to receive a certificate to confirm that the new boilers conform to building regulations.

More information?

Gas Safe Central Heating Ltd.

Highfield House, 1562, Stratford Road, Birmingham, West Midlands. B28 9HA
Telephone: 0800 834 773 or 07970 655513 - Email

Gas Safety Registration Number: 531934
Company VAT Registration Number: 134195618
Registered Limited Company Number: 06929869 (UK)

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