Woodburning stoves pre-date the mass availability of coal, and the early part of their evolution was based upon the shape and burning characteristics of split logs.
These factors dictate a typically wide rectangular firebox shape, maximising the length of log that can be inserted, and allowing efficient loading.
A flat base with no moving parts and no air intake from below allows a bed of ash to be created and maintained.
Warm air is applied from above to supply the fire.
Coal however calls for a different approach - it needs air to rise up from below through the coals, and a solid ash bed prevents that.
You do not want to spread the coal thinly over the large firebox of a woodburner, and so furnaces designed for coal firing are circular in plan, tall and cylindrical overall, with at their base a movable 'riddling' grate which can be agitated to shake ash into the tray underneath.
Of course in today's market there is a multitude of shapes and sizes but in many you can see the lineage of the original design: either it was a woodburner or a coal burner.
The former, more squat and likely to be some form of box.
The latter, taller and narrower, maybe cylindrical.
This imposes design compromises if you try and convert them.
It is often stated that wood does not burn as well in a multi-fuel device.
Depending on the design, this may be true: woodburners are more or less sealed units while multi-fuel ones may be less hermetically sealed.
And the presence of the grate, even if it can be blanked off, means that unwanted air can seep upwards into the firebox.
A taller design may be less ideal for creating the desired downward warm airflow for log burning.
You may however see some very impressive efficiency figures quoted for modern multi-fuel designs, and clearly they have come a long way since the early days of coal.
But just because a fire burns efficiently does not mean it is economical.
If you try to use coal thinly across a large stove's bed it will be burned up very quickly and you may not like the consumption per hour and its effect on your wallet.
Conversely, some argue that for use in a smoke control zone you need a multi-fuel boiler that will primarily run on smokeless coal.
With modern all-wood designs meeting and beating the legal requirements, that is no longer true.
What may be more pertinent is the lower cost of coal per BTU in an urban area where wood is scarce and expensive to procure.
What to do then? With many manufacturers offering identical designs with multi-fuel grates or simpler flatbeds at a slight saving, it may seem that multi-fuel is better because it gives you options.
And if you want to admix your wood with some coal, then that makes sense.
However, if you want to use other fuels including peat, straw, wood waste and compressed paper, then they all burn well, and probably best, when mixed into the wood supply of a traditional woodburning stove.
So do your homework; view and check the figures of the various designs.
This will help you to weed out the ones that impose compromises in performance to offer fuel flexibility, from the ones that are genuinely smart and multi-tasking (probably those designed by women?).
Online there ia an abundance (as well as a wide range of pure woodburners) of clever multi-fuel stoves from the makers Wolverton and Pevez, as well as a built-in unit from inStove.
They come in three categories: 3-5 kW (below the level at which a permanent external air supply has to be provided in the installation): 6-8kW: and 9-11kW.
Your choice will be influenced by room and fireplace opening sizes.
These days, when in wood burning mode, the units have the 'airwash' flow that keeps the glass clean as well as being efficient.
Keep it clean But it is not all about flexibility.
One has also to compare the nature of the fuels themselves.
Coal, a fossil fuel, is ultimately not a renewable resource.
It imposes a heavy environmental burden in its extraction and it is a major cause of emissions of sulphur dioxide (acid rain), CO2, Nitrogen Oxide that forms smog, and mercury that builds up in the food chain.
Even smokeless fuel causes harmful emissions in its creation.
Wood by contrast is carbon neutral when it is being continuously replaced in managed forests where it absorbs CO2.
Apart from some cutting and transport costs, it has one of the least impacts of any combustible fuel.
So long, that is, as you religiously season your wood, ideally for 3 years, to reduce the moisture to below 20% before burning.
For country dwellers, wood is a sensible choice when compared with expensive and less green oil or propane tanks or coal.
The other factor to consider is that wood and coal are not happy firebed-fellows.
The sulphur of the coal mixed with the moisture given off by the wood creates a highly corrosive vapour that will shorten the life of your flue, even if it is stainless steel.
And of course you should not be using an unlined chimney but if you were to, it would eat its way out from in, with dangerous consequences.
The solution for multi-fuel users is to concentrate on one or the other at a time, rather than mix them, if at all possible.
And tempting though it may be, never burn up plastics or household waste.
You do not have the technology of a combined heat and power plant: what you would be creating would be a fug of toxic waste.
And that's not really the idea of these lean, green heating machines.