Choosing an Energy Efficient Boiler

The current lifespan of a boiler is around 12 years [1]. Currently, there is no obligation for householders to replace inefficient boilers in existing properties however it is highly recommended. Steps have been taken by the government to regulate new boiler installations. Since October 2010, any new boiler installed must be A-rated, or 88% efficient [2].

In general, the boiler efficiency is determined by measuring how well an appliance turns fuel into useful heat. Boilers are rated on the SEDBUK 2005 or the updated, SEBBUK 2009 rating systems (Table 1) [2]. If your boiler is more than 15 years old, it is likely to be SEDBUK G-rated [1]. By replacing an old G-rated boiler with new condensing one that is A-rated, you can save potentially 25-30 % on your fuel bills and up to 1,2000 kg of CO₂ emissions [1].

Table 1. SEDBUK 2005 and SEDBUK 2009 rating systems [2]. Under the newer 2009 system a SEDBUK 2005 90% efficient A-rated boiler is regarded as 88% efficient. The 2% difference between SEDBUK 2005 and 2009 is mainly down to differences in efficiencies between using a boiler for space heating in the winter months and only using it for water heating in the summer months.

1. What kind of boiler do you have?

The main difference between a non-condensing boiler and a condensing one is the type of flue [1]. A flue is a pipe that sticks outside the external wall adjacent to the boiler. It allows boiler exhaust gases like carbon dioxide and water vapour to escape from the property.

If your boiler has a plastic flue pipe, which emits visible steam (i.e. water vapour) then it is a condensing boiler. Alternatively, an older non-condensing boiler will have a metal flue instead of a plastic one because the exhaust gases are hotter– hot enough to melt plastic [1].

“Back” boilers situated behind main fireplaces are usually old enough (+20 years) to be non-condensing and inefficient. These types of boilers are usually decommissioned or bypassed if a new condensing boiler is installed [3]. So in this case, another suitable space (e.g. ventilated cupboard) would be identified for the system, before installation.

If you are interested to know your boiler’s efficiency and SEDBUK rating have a look at the national boiler efficiency database at www.sedbuk.com. Use Table 2 to see how much you could potentially save off your fuel bills!

Table 2. Approximate fuel cost savings for replacing current boiler with SEDBUK (2005) A-rated boiler (88 % heating efficiency with regards to the updated SEDBUK 2009 rating system). Calculated by subtracting boiler efficiency for each SEDBUK rating (B-G) from SEDBUK A-rated boiler efficiency (88%) [2].

*SEDBUK 2005 Boiler Rating*	*Corresponding SEDBUK 2009 Boiler Efficiency* (%)	*Approx. Fuel Cost Savings* from switching to SEDBUK 2005 A-rated* boiler*
B	84 – 88 %	0 – 4 %
C	80 – 84 %	4 – 8 %
D	76 – 80%	8 – 12 %
E	72 – 76 %	12 – 16 %
F	68 – 72%	16 – 20 %
G	Below 68 %	20 % or more

2. Why change to a condensing boiler?

Condensing boilers are more efficient than non-condensing boilers because they have a ‘condensing’ mode that kicks in at regular intervals during their operation [1]. During this operation mode, they extract heat from the exhaust gases (i.e. products of combustion including water vapour), which turns steam back into liquid water. This is achieved through an additional ‘heat exchanger’ (i.e. a good heat conductor with a large surface area) which works by transferring heat from one substance to another without them making contact. As water vapour cools and condenses to form water droplets, the boiler will utilise this extra ‘latent’ heat to warm pipes and radiators [1]. As a result the ‘condensing’ mode increases boiler efficiency by up to 12 %, compared to a non-condensing boiler.

3. Types of Condensing Boiler and Suitability

If you decide to replace your boiler, the type of condensing boiler you choose to have installed will depend on its suitability for the property and your lifestyle or needs. A qualified installer should be able to provide advice on the cheapest, most efficient option however a few guidelines are outlined below.

There are two types of condensing boiler:

‘System’ or ‘Open Vent’ [1]: This is a more common/standard heating system that is programmed to heat and store hot water within a separate hot water tank, for later use. Therefore, a large amount of hot water can be made available to multiple taps at any one time. However, not all properties are compatible with this type of system as they may lack space for a hot water cylinder (e.g. ground floor flats). Also, to avoid wasting energy and money, it requires more consideration for your lifestyle and habits, as water has to be heated in advance, using a time programmer.
Combination or ‘Combi’ [1]’: This type of system provides instant hot water without the need for a hot water cylinder, or hot water programme timer. When a tap is turned on, the boiler ignites and allows mains water to be heated as it flows through a tray situated above a heat exchanger. As a result, they may be a slight delay before hot water flows through the tap because it is being heated on demand within the boiler. To allow more efficient operation, the pressure gauge displayed on the ‘combi’ boiler should read between 0 and 1 Bar [3].

The main benefits of using a ‘combi’ are that it increases hot water efficiency and saves space within the home. By heating water on demand, it reduces the amount of fuel needed for hot water heating and reduces the space needed for the system. So it is ideal if you don’t need a lot of hot water each day (e.g. occasional washing up and showering as it takes longer to run a bath) or you want to save money and space. However, potential problems could arise if more than one tap in the house is being used at the same time, as the water pressure will drop significantly. Therefore, combi boilers are not recommended if the property has a low mains pressure or multiple baths or showers (that are not electric).

4. Biomass Boilers

A stove burning logs or pellets can heat a single room or it can be fitted with a back boiler to provide space heating to radiators or just hot water heating. A more expensive, larger scale alternative is a Biomass boiler central heating system that burns pellets, logs or chips. Most pellet and chip biomass boilers have automatic fuel feeders which refill them at regular intervals from fuel storage units called hoppers [4]. Log burning boilers and stoves are fed by hand [4].

Factors to consider before deciding to switch to biomass fuel [4] are:

Space: These boilers need a large, dry area close to the boiler site for wood storage. Wood pellets have to be stored in purpose built silos.
Flue: Biomass boilers and stoves need a vent which is specifically designed for proper operation. Most existing chimneys can be fitted with a lined flue, which is relatively inexpensive.
Smokeless Zones: You will need to check if your property is within a smokeless zone (See http://www.uksmokecontrolareas.co.uk/locations.php). Wood can only be burned within exempted appliances.
Planning Permission: You need to talk to your local authority if your flue will extend 1m or more above the height of your roof. If the property is in a Conservation Area or World Heritage Site, you need to speak to the local authority if you plan to install a flue on the principal elevation visible from a road.

5. Types of Boiler Fuel and Suitability

Selecting the fuel best suited for heating your property depends on many factors which include:

Fuel cost and availability
Efficiency of system used to convert that fuel to heat
Environmental impacts associated with the heating fuel

In general, natural gas is the cheapest and ‘cleanest’ of the common fossil fuels, including oil and LPG (Table 3). However, all fossil fuels have a greater impact on the environment than wood logs or pellets, which are considered to be carbon ‘neutral’. This means that the higher CO₂ emissions (Table 3) emitted from wood fuel boilers are ‘balanced’ or ‘zeroed’ out by tree/plant intake of CO₂ from air during photosynthesis [4]. However, the emissions associated with wood fuel transport to the property have to be negligible for this to be true (i.e. fuel has to be sourced locally). Wood fuel is generally more expensive than natural gas due to economic ‘supply and demand’ factors, however, if bought in bulk loads (e.g. per tonne) it can significantly reduce your potential costs [4].

Heating system efficiency varies depending on fuel type. Typically natural gas and oil condensing boilers are very efficient, reaching efficiencies over 90 %. However, wood pellet systems can also reach similar fuel efficiencies (85-90 %) and have the added advantage of being carbon neutral [5].

Table 3: Cost and CO₂ emissions per unit energy (kWh) for each fuel type [5,6, 7]

Fuel Type	Cost (p/kWh)	Carbon Emissions (kg/kWh)
Natural Gas	4 – 5	0.23
Oil	5 – 6	0.26
LPG (Liquid Petroleum Gas)	7.5 – 8.5	0.24
Wood Chips	3 – 3.5	0.39
Wood Pellets	4.5 -5.5	0.39