What type of green boiler would best fit my needs?

A green boiler could be gas condensing, combi, micro-CHP, wood pellet, or log burning. So, what type, size and fuel best fits your needs? Whether you are looking to upgrade to a new heating system or just improve the one you’ve got, this article should clear up many questions.

Green Boiler Efficiency

Wood pellet boiler at Chalfont St Giles SuperHome

25kW wood pellet boiler. The manufacturer claims 86% efficiency. In practice it is less says Charles Ainger. Lower than optimum heat demand impacts efficiency.

First off, it’s worth noting that since October 2010 only boilers that are 88% or more efficient can be installed in homes, that’s SEDBUK (energy efficiency) rating of B or A.

For the UK, the SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) efficiency of most current and obsolete boilers can be found on BRE’s Product Characteristics Database.

Top A rated boilers are 90% plus efficient – under ideal conditions. This is the current benchmark of a really green boiler. It converts nearly all the fuel you put in it into useful heating. For comparison, it’s worth noting that old open fires (where a lot of the heat is lost up the chimney) are around 30% efficient!

Sizing your boiler

Boilers are chosen in a range of different heat outputs to suit a range of different-sized homes, typically ranging from 12 to 40kW. It’s important not to over or under-size what you need, as this affects efficiency and fuel use.

Here is a handy calculator to help you work out what size of boiler you need. Another, even more detailed one, is here (but you’ll need the U-values of your walls).

The amount of heat you need depends on your hot water use requirements and your heating system as well as the efficiency of the property. The delivery temperature required for the same degree of comfort, using different distribution systems are shown in the table below:

Distribution system Delivery temperature(Degrees C)
Table 1: Delivery temperature required for the same degree of comfort, using different distribution systems
Underfloor heating 30-45
Low temperature radiators 45-55
Conventional radiators 60-90
Air 30-50

Selecting a green fuel

You can burn oil, gas, LPG or wood. This table gives you an idea of the carbon dioxide emission factors by gross calorific value (heat output):

Energy source kgCO2/kWh KgCO2 per other units
Table 2: Carbon dioxide emission factors by gross calorific value (heat output) for various energy sources
Wood pellets 0.03895 183.9 per tonne
Natural gas 0.18523 5.3808 per therm
LPG 0.21445 6.2915 per therm
Burning oil 0.24683 3165 per tonne
Diesel 0.25301 3188 per tonne
Fuel oil 0.26592 3228 per tonne
Coal 0.32227 2383 per tonne

The greenhouse gas conversion factor comprises the effect of the CO2, CH4 and N2O combined, quoted as kgCO2e per unit of fuel consumed.

This table reveals the typical bulk, calorific value (CV) and energy densities of different biomass and fossil fuels. Ths gives you an idea of how much fuel you’d need to burn to get the same amount of heat:

Fuel CV kWh/kg Bulk density kg/m³ – Lower Bulk density kg/m³ – Upper Energy density by volume kWh/m³ – Lower Energy density by volume kWh/m³ – Upper
Table 3: The typical bulk, calorific value (CV) and energy densities of different biomass and fossil fuels. Source: Gastec at CRE Ltd. and Annex A, Digest of UK Energy Statistics 2007
Woodchips @ 30% 3.5 200 250 694 868
Log wood (stacked – air dried: 20%MC) 4.1 350 500 1,419 2,028
Wood – solid oven dried 5.2 400 600 2,067 3,100
Wood pellets 4.7 600 700 2,833 3,306
Miscanthus (bale – 25%MC) 3.4 140 180 471 605
Anthracite 8.9 1,100 1,100 9,808 9,808
Oil 11.5 865 865 9,972 9,972
Natural gas n/a n/a n/a 10.13 10.13

Boiler Controls

Boiler controls ensure the boiler is only on when needed

Boiler controls ensure the boiler is only on when needed

To reduce space heating requirements, If not already present, controls for the boiler and hot water cylinder should be upgraded with a boiler interlock: an arrangement of controls (room thermostats, programmable room thermostats, cylinder thermostats, programmers and time switches) that ensure the boiler doesn’t fire when there is no demand for heat.

Programmable room thermostats allow rooms to be independently managed. There should be additional timing capability for hot water. Fit TRVs (thermostatic radiator valves) on all radiators in rooms with no room thermostat. TRVs have an air temperature sensor used to control the heat output from the radiator by adjusting water flow.

Grants and finance for green boilers

Most of the heating and heat-saving measures discussed here were eligible for support from the Green Deal:

Modern Biomass Boiler at Shrewsbury SuperHome - a green boiler for a larger house

Modern Biomass Boiler at Shrewsbury SuperHome

• Air source heat pumps
• Biomass boilers
• Biomass room heaters (including with radiators)
• Cylinder thermostats
• Flue gas heat recovery devices
• Ground source heat pumps
• Heating controls (for wet central heating system and warm air system)
• Heating ventilation and airconditioning controls including zoning controls)
• Hot water controls (including timers and temperature control)
• Hot water cylinder insulation
• Mechanical ventilation with heat recovery
• Micro combined heat and power (Micro-CHP)
• Pipe-work insulation (external pipework only)
• Gas-fired condensing boilers
• Oil-fired condensing boilers
• Warm-air units
• Radiant heating
• Solar water heating
• Solar blinds, shutters and
• Under-floor heating
• Waste water heat recovery devices attached to showers
• Water source heat pumps.

All of the above are eligible for ECO support for low income and vulnerable households.

Wood, or biomass, boilers is eligible for the Renewable Heat Incentive, which will pay you for every kilowatt hour of heat you generate, when it comes in.

Selecting a green boiler

Now you can think about the best type of boiler or stove for your needs.

Condensing boilers

Condensing boilers will be the choice for many buildings, provided they have space for a hot water tank.

These boilers contain a combustion chamber that the hot gases created by burning gas or oil pass through. This is surrounded by a heat exchanger which transfers heat from the gases into the water inside, heating it up.

Then, a second heat exchanger removes even more of the heat from the flue gases. It pre-warms water coming back into the boiler from a heating system, so less gas is required to heat the water up.

By using this otherwise wasted heat, condensing boilers use around 90% of the heat they generate.

Since the waste gas loses some of its heat, it cools down into an acidic water called condensate and water vapour, giving the boiler its name. The cooler, heavier flue gases are blown out of the boiler by a fan.

Manufacturers claim that up to 98% thermal efficiency can be achieved; but a field trial conducted by the Energy Saving Trust in the UK found an average efficiency of 85.3%.

The energy source could be electricity (a renewable tariff of course), gas, oil (renewable or fossil) or biomass (wood, in one form or another).

Because the water they heat is stored in a tank, they can be used in combination with other heat sources such as a heat pump, solar water panels, or electric immersion.

Always choose an A-rated boiler.

If you have a non-condensing boiler, it’s possible to upgrade it cheaply by fitting a passive flue gas heat recovery system to the flue above the existing boiler to capture some of the lost energy and use it to heat your water. Up to 30% of the total heat can be reclaimed this way.

A hot water tank or cylinder and all the pipework should have as much insulation around it as possible. Pipes should also be well lagged with no gaps. Most now come pre-insulated. Some have the option for a supplementary coil to be inserted, e.g. from a solar panel.

Combi boilers

Modern efficient gas combi boiler - a green boiler choice for the typical on-gas home

Combi boiler. Controls are simple. Provided water pressure is occasionally topped up, these boilers perform reliably on demand.

Combination or combi boilers give on-demand hot water without the need for a cylinder. They may or may not be of the condensing variety and combine the central heating with (tankless) hot water supplies in one unit.

They are used in smaller properties where there is not room for a tank.

In the same EST trial, these recorded an average efficiency of 81.5%. The size (output) is chosen on the basis of hot water requirements.

Combi boilers can produce between 9 and 18 litres of water a minute (when the cold water is heated up by 35°C).

Combi boilers can provide high water pressure to just one tap at a time. They’re not suitable for big homes where more than one use for hot water might be required simultaneously.

There’s also a delay between turning the tap on and the water heating up. But you can run a powerful shower off the bath taps with a combi boiler.

There is a trend towards increasing the electricity use of these boilers by fans, pumps and control systems. Average combi boilers use around 30% more electricity to supply 10,000 kWh of heat the regular boilers, and around 50% more to surprise 20,000 kWh of heat. This consideration should be a factor in the choice of heating supply.

Always choose an A-rated boiler.


Schematic diagram of how a micro-CHP unit works to produce heat and electricity.

Schematic diagram of how a micro-CHP unit works to produce heat and electricity. Click to enlarge.

Micro-CHP – combined heat and power – units are fairly new. The size of domestic fridges, they run on gas and produce up to 1 kW of electricity per hour as well as up to about 10kW of heat, making them in theory more efficient than a boiler producing just heat.

The best home for them is a medium-to-large, moderately well-insulated one, maybe with solid walls, solid floors and no loft space, that is harder to insulate any better and has a relatively large heat demand.

Units need a consistent heat load to supply for a minimum of 11 hours per day throughout the year, for 17 hours per day for two thirds of the year in order to be economic.

A Baxi micro-CHP unit in situ - a green boiler which also generates electricity

A Baxi micro-CHP unit in situ

The output of the micro-CHP is chosen to match the base heating load so that they operate not intermittently but for many hours at a time, making the value of electricity generated pay for the marginal investment in as little as three years.

The property’s needs for the heat and electricity must coincide throughout the year, and the electrical load in the property must exceed the supply of the plant. It therefore works best with a buffer storage tank to save the surplus heat for later, and with a grid connection for electricity export.

Electricity output is around 1kW, enough to maintain back-up power in the event of a power cut or boil a kettle.

The Baxi Ecogen model supplied by British Gas, above, works on a Stirling engine for the basic 1kW electricity & 1KW heat output and a supplementary heater for extra heat.

The micro-CHP is the only boiler that makes electricity and qualifies for the Feed-in Tariff. British Gas is offering them and says they will earn around £245 per year compared to a standard A-rated boiler. You’d get paid for all the electricity you generate and extra for what you don’t use but is fed back into the National Grid.

Micro-CHP units can potentially deliver carbon savings of 5–10 per cent – fewer than a condensing boiler, since capacity is likely to be best matched to demand, for both heat and power. Payback can be around five years. Not really suitable for most smaller and newer dwellings.

Wood pellet and woodchip boilers

According to the Energy Saving Trust, if you replaced electric heating with a wood-burning system you could save as much as £580 a year.

It is vital to have sufficient space and a system for the delivery, storage, and loading of the fuel. Storage must allow the fuel to dry, but not get wet when it rains. The wood’s moisture content also affects is calorific value, so it must be dried and ‘seasoned’ for at least a year.

Biomass is assumed to be carbon neutral, because the vegetation burnt is replaced by a new plantation which, in turn, will recapture the carbon dioxide from the atmosphere. However, it will take the new trees up to 20-30 years to do this, meanwhile the CO2 is heating the planet. The global warming impact also depends on how far away it is sourced, and whether it is in the form of pellets, which are energy-intensive to produce, basic timber, or wood chips. Therefore the fuel should be available, reliable, locally sourced, accessible and appropriate.

As an example, a 15kW woodchip boiler might use up to 20 tonnes of seasoned woodchip a year. This would require around 330 to 430 acres of nearby woodlands from which the pollarded or coppiced wood could be taken and a service agreement to do so.

Burning wood also emits dioxins and persistent organic pollutants such as PAHs and PCBs, as well as particulates such as DM10, all of which can kill.

The level of these pollutants is at its highest in the just after the burner has been lit, before it approaches its optimum operating temperature of 200ºC. Even at this point, it only reduces the pollutants by about 50%. Incinerators are legally impelled to operate at 800ºC, more effectively to break down these pollutants. Unseasoned wood produces even more pollution. Seasoning takes at least a year in a well-ventilated, dry environment.

Biomass stoves

12kW wood pellet boiler

12kW wood pellet boiler

These generally have to be large consumers of fuel to heat a heating system as well as hot water. They need a back boiler to do this as well as to provide heat for the room.

A wood-pellet stove with back boiler is more convenient and easier to control than a log stove with a back boiler, as wood-pellet stoves have automatic ignition, require less storage space to store the fuel, need fuelling less often, produce less ash and can have temperature sensors.

Some models can be programmed to come on automatically like a conventional boiler and be operated by remote control.

However, the fuel may be more expensive and cause more global warming than locally-sourced timber.

Larger models can also sit in a dedicated boiler/utility room and come in versions of 5–50kW. They use electricity and can have up to three motors, including an augur motor, convection fan and combustion blower, plus a microprocessor, maybe electronic ignition, and a back-up power supply. So check their electricity consumption before buying.

Log stoves

These vary hugely in price and efficiency. The Solid Fuel Technology Institute publishes a register of over 3,000 solid fuel appliances, including their efficiency percentage ratings.

Chimneys are important for the optimum working of any stove. It’s easy to take them for granted. Learn how they work here.

Log boilers

These produce space and water heating. They are loaded by hand, on average once a day. They need to be in a dedicated boiler/utility room and are available in versions of 5–50kW. They can be an ideal solution for the dedicated householder.

Log burner and wood pellet stove prices compared

Log burners are about half the price of wood pellet systems, and automatically fed pellet systems are over twice the price of manual ones:

Type Cost
Table 4: Typical biomass heating costs
Pellet Stove about £4,300
Log Stove about £2,000
Automatically-fed pellet boiler about £11,500
Manually-fed log boilers up to £11,500

On the other hand, they will save you money in running costs. The exact amount depends upon what fuel they replace:

Fuel replaced Expected annual saving Expected annual CO2 saving
Table 5: Savings from a wood pellet boiler used for central heating. Source: Energy Saving Trust
Electricity £580 7.5 tonnes
Oil £280 4 tonnes
LPG £720 3.5 tonnes
Coal £300 7.5 tonnes
Gas £100 3 tonnes

Using recycled oil in an oil boiler

There might also be a local source of use vegetable oil, for example from catering businesses, or of biodiesel, which is chemically almost identical. Some existing oil burning boilers can be converted to consume such fuels.


Remember to draughtproof your home and properly insulate it before buying a new boiler! Every home is different, but follow these guidelines and you’ll be able to cut your heating bills and carbon emissions while staying warm and safe.

eco renovation - talk to homeowners

Updated Feb 2017. © David Thorpe Jan 2013. David is the author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency.

Also See:
Biomass boilers, combi boilers, condensing boilers at free eco open house events in September
Are biomass boiler costs justified?