The Differences Between a Wood Boiler, a Wood Burning Stove and a Gas Boiler
How does a wood burning boiler stove (or more commonly, a wood boiler) differ from a regular wood burning stove (or wood burner) or a conventional oil or gas boiler?
The central and obvious difference between a wood burning stove and a wood boiler is the addition of a metal box containing water that uses heat from the wood burner to heat that water, which is then piped to household radiators and/or stored in a hot water cylinder for washing and so on. A wood boiler differs from a normal oil or gas boiler simply by virtue of the fact that it burns wood to produce heat with which to heat water.
For the record, the term boiler has its origins in the Steam Age of the Industrial Revolution, where water was indeed heated to boiling point in order to produce steam to power pistons and the like. Modern boilers do not in fact boil water and typically have safety devices to prevent this happening. Water heated in a “boiler” is typically in the range 50 to 70 degrees centigrade.
As ever in the world of wood burning appliances, there is a bewildering variety of styles, sizes and makes of wood burner to confuse you choose from. Some look very much like the traditional woodburner – black metal with a glass window – we’re all familiar with while others more closely resemble a conventional gas boiler.
The differences in aesthetic styling are usually a function of the intended purpose and location of the wood boiler. If it is to be installed in a main room, say the lounge, then it is probable that it will also be required to provide space heating and to look like a typical wood burner stove. If on the other hand the wood boiler is purely for the utilitarian purpose of heating water for radiators and bathing then most people prefer that it be fitted in a kitchen or utility room and that the wood boiler conforms to the expected outward appearance of a normal gas boiler.
How a Wood Boiler Works
Regardless of the apparently endless permutations in looks though, all wood boiler stoves perform the same function the same way by transferring heat from the wood burning stove to water in the “boiler”, which is then pumped to heat radiators or stored as domestic hot water.
The boiler is, as noted above, nothing more complicated than a water-filled metal box with a number of what are called “tappings” that permit water pipes to be attached. Obviously there is at least one input, providing cool water and one output for the heated water to flow away once it becomes hot enough (but before it actually boils). A common design of wood boiler involves replacing the firebricks that insulate the back of the firebox with the boiler itself – these being termed wood burning back boilers for self-evident reasons.
Connecting wood burner stoves to the household heating system is little different to installing conventional gas boiler systems, but even if you are capable of installing a conventional heating system you would still be well advised to consult a heating engineer experienced with wood boiler systems. But in principle, connect an output tapping to the domestic radiator feed pipe and connect the radiator return pipe to an input tapping. If using under floor heating then you need to connect the wood boiler to a hot water tank (quite commonly an “accumulator” tank capable of keeping a large quantity of water hot for extended periods) and connect the underfloor heating system to the tank.
Points to Consider Before Installing a Wood Boiler
A wood boiler stove can run the central heating system and provide plentiful hot water for houses of all sizes, but there are a number of points worth bearing in mind if you are thinking of installing a wood boiler. Most of these are the exact same set of things to consider when installing a woodburner of any kind, but there are a couple of considerations specific to wood boilers.
A key difference between a modern oil or gas boiler and a wood boiler is the length of time needed for the water output from the wood boiler to reach the required temperature. Many modern gas boilers deliver hot water on demand, but a wood boiler takes time (not unlike filling a large kettle). The solution to having constantly available hot water has been around for a very long time and is simply to have the wood boiler store the heated water in a hot water tank (an accumulator is best) and draw the water from that as and when needed.
The other point to pay attention to is the heat output rating for a wood boiler. This is quoted in BTUs (British Thermal Units) and wood boiler stoves can deliver heat output from 5,000 to 90,000 or more BTUs. To calculate roughly how many BTU’s heat you would need simply work out how many square feet of space you want to heat and do the sums as follows.
BTUs = square feet * 40
So if your house covers 1200 square feet in total (easiest is to count each room separately then add it all up) you would need a wood boiler that could output 48,000 BTU. The weighting factor of 40 used for this example will vary according to where you live. A value of 40 is good for latitudes on a par with New York, so much of Northern Europe up to and including the UK. If you live nearer to Canada or in Scandinavia then consider a value of 50-55; Southern United States and Mediterranean Europe could drop to 30. This simple wood stove size calculator can give you a rough guesstimate, but as ever, if you don’t know – ask someone who does, like a professional heating engineer.
However, we’re not quite done with BTU ratings. Always check with a wood boiler BTU rating whether the figure quoted is the maximum or nominal rated output. In general you want the nominal BTU rating, as this states the output for the boiler’s normal operation. Like most things in life, a wood boiler will not benefit from being run at maximum output all the time, so ensure you install a wood boiler whose “nominal” output matches your heating requirements.
To figure out how this also translates into how much hot water you can heat, you need a bit of simple maths.
The first thing is to convert your BTU figure into kilowatts (kWH) which you do multiplying by 0.000293 (if your heat output is already expressed in kWh then you’re good to go, but for reference to go the other way multiply kWh by 3414 to get to BTU).
Now for a couple of useful factoids: 4,200 Joules are needed to heat 1 litre of water by 1 degree centigrade, and 1 kiloWatt equates to 1,000 Joules per second, therefore you need 4.2 kW to heat 1 litre by 1 degree in 1 second.
From this you can figure out whichever permutations you want, for example to heat 400 litres to 60 degrees over 2 hours would require 14 kW, but using 2kW of heat it would take 14 hours instead. You can find more than you probably want to know from this overview of energy and power but if math really isn’t your thing then just use this handy dandy Energy Calculator to get the answers on a plate.
