heat sources

Probably the most cost effective way of reducing the emissions in most of the houses we have come across is to change the boiler. While a definite choice if the boiler is 15-20 years old this can work on younger boilers too.  However if you want to wean yourself off fossil fuels you need to look at renewable heat sources.


For most of the retrofits, this is wood and it comes in 3 different versions. 

wood log

The easiest to use and most easily locally sourced but least automated, you will become intimately acquainted with every piece of wood you burn. This can work well for behaviour change. The equipment in which you can burn it is considerably cheaper than the other options.


wood pellet

Compressed sawdust, it can be treated almost as a liquid fuel, it can be piped into a store from a waggon outside, boilers can have auto-ignition systems so you don't need to light them yourself as you do with almost all log burners. However it is costly, the burners are costly and it is more carbon intensive in production than the other options needing manufacture and rarely so in this country. Maintenance can be an issue and some burners can be temperamental about chip quality


wood chip

Readily available locally as it is an arising from tree and forestry maintenance, the quality can vary and it can have bits of inert material in it so requires a heavier duty burner, often not small enough for the average home but may suit larger ones. or non-domestic retrofits. Noise and maintenance can be issues

In all cases space will need to be made for wood storage, an average UK home would burn 10 cubic metres of logs in a year - hence the need for radical demand reduction first.

Some burners have interchangeable grates so that more than one of the forms above can be used. Burners come in two basic forms: boilers and stoves. Boilers then subdivide into on show versions and ones that need a plant space to hide them in. If the demand reduction is high enough (<£2,500kWh/A) then some of the on-show versions can provide enough heat for space heating and hot water. We’ve only managed to specify this on one house so far but it  helped make it the best performing houses among 120 other deep retrofits from the Retrofit for the Future

Stoves only provide heat no hot water. If the building needing heating is in a smoke control area then the burner will need to have a DEFRA exemption.

solar thermal

Evacuated tubes or panels on the roof are heated by the sun, in turn heating a water tank. Most systems are only designed to produce hot water and sized accordingly. This is quite expensive for the amount of energy saved as the average UK hot water usage is about 2,000kWh, with gas heating this is only £80-90pA, assuming 50% saving on hot water this is a long payback. If your house is heated by more expensive energy then this can improve radically. 

Another consideration is labour; if your primary heat source is logs for example then not having to light the boiler can be a relief. Here also the costs can be shared as log burners need a large heat store so that they can burn efficiently and store the heat produced for later, this heat storage can be used by the solar thermal system in summer. This in turn allows a larger system to be installed which then allows some level of space heating assistance to provided from the sun too - the best we’ve managed to get to so far is 15%.

heat pumps

There are various types but there are 2 basic types for use in most retrofit situations - Ground Source (GSHP) and Air source (ASHP). Water source is a bit specialised and unusual. Air source then divide into air to air and air to water, there is also the Exhaust Air Source Heat Pump (EASHP) that can achieve a slightly better performance by retrieving heat from outgoing ventilation.

They all work on the same principle using the way heat is pulled from or produced by liquids turning to gases and back, called state change. They all use a compressor that does these changes so heat is pulled from the outside and then the state change goes the other way inside taking the heat previously absorbed and emitting it. The same as a fridge but the bit in the back of the fridge is inside the house, the inside of the fridge being outside.

The key figure when working out whether a heat pump will work for you is the Coefficient of Performance (CoP). This is the ratio of electricity put in driving the compressor to the amount of heat in the house. Crucially too, this is not fixed for any given machine it depends on the temperature difference needed. So if it is very cold and hot water for washing is needed the CoP will be nearly 1 for many heat pumps, if however it is designed with a low temperature heating system such as underfloor then the CoP rises 3-4 for air source and 4-5. This means that between a thrid and a fifth of the amount of energy put in is available for use. Despite the use of electricity this is how heat pumps get to be called renewable heat. Manufacturer’s CoPs however need to be looked at in a similar way to car testing, heat pumps can produce very good CoP’s in controlled conditions that many have found difficult to realise in real life so it is worth seeking out independent test data. The more nuanced figure of available is the Seasonal Performance Factor which is the same measure as CoP but in use not from a test certificate. In the 2011 Heat Efficiency test from the well respected Fraunhofer Institute, GSHP’s came out averaging 3.88 and ASHP at 2.88.