An open fire depending on the design is usually between 20% and 30% efficient when running. At 6am when its not running it pulls heat from your central heating system up the chimney so the overall efficiency is very very low. Some rate it as a negative figure.
A stove must be at least 65% efficient to achieve the legally required CE rating. However there are three ways of measuring a stoves efficiency, these depend on the moisture level of the wood used on test, in a nutshell the lower the MC level the higher the efficiency. Some manufacturers such as Morso are very conservative with their published efficiency ratings so don't take to much notice of stove efficiency figures, they should all be at 65% or more although I am aware of one or two that do fall below that. All will produce at least 4 times the amount of heat from the same amount of wood burnt. There are new regs coming in around 2018 time that will uprate the minimum energy efficiency required, while this will not effect stoves already installed older designs with lower efficiency are likely to be withdrawn. Parts will therefore become harder to obtain once the stove gets older, Morso excepted.
Some stoves can be fitted with direct air vents, this brings air direct into the stove from outside the room.
So installing a stove is a very good idea, most of my customers ( I retail stoves ) say its the best thing they have EVER bought. Just make sure you feed it DRY wood, maximum moisture level 18%.
There are currently schemes through energy companies like British Gas to uprate your properties heat insulation, typically you will be required to pay around 20% of the cost.
As far as I am aware there are no grants for disabled persons for the installation of stoves.
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