I did some research when writing a website, it seems to stop particular emissions the temperature has to exceed a set point, and only way to get it hot enough is to add air after the fuel, and to control the burn until at least all the wood is charcoal to a set level, too high and energy wasted out of the flue, too low and particular emissions.
This means the unit has a set output until at least all wood has turned into charcoal, so it needs a means to store the energy, so for example you can light the fire in the evening, and then allow it to burn at set rate adding wood as required, until room warm enough and then let if burn out, and store any excess as hot water, then use the hot water to heat the home when the fire is not lit.
Problem is this storage system is expensive,
the water jacket is an integral part of the fire,
some even condense the flue gases gaining latent heat, there is a DIY version
however I can see insurance problems building your own. I have seen just one, installed when the house was built, so cost was on the mortgage, and also the floor was reinforced to take the weight of the twin tanks which stored the energy, mounted high enough so with a power failure thermo syphon would stop the water boiling.
I inquired about what happens with the Hughes design with a power cut, there was no reply, with a plastic horizontal flue there would clearly be a problem if there was a power failure, so it would need some battery back-up, same with using pumps for the water.
The one I looked at would have an installation cost of around the £30k mark, which is clearly not going to be got back with the reduced use of fuel.
The big problem with all wood burners is wood has about the same calorific value by weight, but some wood is far denser than other woods, so the area needs adjusting to suit the fuel being burnt, it may burn multi fuels, but no in an efficient way unless it can be altered to suit the fuel being burnt.
Also some fuels break up when being burnt, where I volunteer at a steam powered railway when the South Wales mine washing plant failed so no more steam coal, many alternatives were tried, the carriages were coated with coal dust ejected out the chimney un-burnt, the whole way of firing had to be altered, it was common when they first tried the new fuel to see the train stopped while they got up steam again. Seems new fuel had to be put on the fire little and often, where coal was put on when on the flat so burning well on the hills.
To redesign the engine to take new fuel would mean no longer a heritage engine, so not really an option. This is also a problem with stoves, as we source new fuels, they need modifying to burn them, a stove designed for oak will not work well with pine, so it is all very well if you have 10 acres of woodland so you know the type of wood, not so good when you need to buy in the wood.
