Jason Orme explains how to choose the right one and why their cost-effectiveness is really all about the fuel
Choosing the Right Size Stove
A stove’s output, like a boiler, is measured in heat output (kW). Depending on your home’s insulation levels, a large 5x5m room will require between 3-7kW of heat.
A good rule of thumb is to calculate the room size in cubic metres and divide by 15 (based on average insulation levels). So a room of 4x5x2.5m (ceiling height) – 50m³ – would require 3-4kW of output. You can subtract the output of any radiators from this figure. In most cases it’s best to get a HETAS engineer in.
Performance – or the actual heat output – varies hugely. Beware manufacturers’ headline output figures — they are the best case.
The cost-effectiveness of logs depends not just on how much you’re paying but also what you’re buying. For a start, hardwood logs are denser than softwoods and you’ll receive more kWh from a cubic metre bag of hardwoods than softwood. But softwoods are usually cheaper.
By far the biggest impact, however, is moisture content. Table 1 is a very general guide to how moisture content affects kWh.
Table 1: How moisture content affects heat output
|Moisture Content||Approx kWh/kg|
|10% (kiln dried/briquettes)||5|
Most merchants sell in cubic metre bags but the weight of these bags will vary depending largely on the moisture content.
- Kiln-dried logs (where the moisture is below 10%) will cost £120-140 per cubic metre bag. But the weight of these will be just 250kg.
- A bag of ‘seasoned’ logs where the moisture content can still be up to as much as 30% even after 12 months of air drying will weigh on average 300kg per cubic metre per bagful and cost you between £70 and £90.
- A cubic metre of freshly cut logs would come in at 500kg with a moisture content of 50% and be cheapest at £50 per bag.
Table 2 shows the relative values of those different options based on the above assumptions. Perhaps somewhat controversially, the figures show that the differences in price between dried and freshly cut logs are not usually reflected in the improved outputs you’ll receive (by paying more for dried). The best option is to store as much wood as you can and dry it out over two to three years.
Table 2: Cost per output of different moisture contents*
|Type of Log||Cost/kg||Cost/kWh|
|Fresh (50%)||10p||4p (at 2.5kWh/kg)|
|Seasoned (25%)||26p||6.5p (at 4kWh/kg)|
|Dried (10%)||52p||10.4p (at 5kWh/kg)|
|*output will also vary by type of wood — see below|
The trick is to buy cheap cubic metre bags of freshly cut logs (at £50-70 per bag) and then let it dry out to boost its output to 4-5kWh/kg. That way you’ll be enjoying costs per kWh of around 2-3p, making it a fantastically cheap source of heat.
It’s also worth checking out briquettes to supplement your log usage. They give a guaranteed output of around 4.8kWh/kg, with a stable moisture content often below 10%. They’re easier to handle and produce less ash and while expensive on a cost/kg measure, are undeniably effective. Try talu.co.uk.
In terms of your annual heat demand figure, heating our typical room to say 4kW all day (24 hours) would have used up 96kWh of energy — the equivalent of £4.32 at current gas prices (that’s assuming, of course, that the heating isn’t very well controlled and you lose a lot of heat).
The cost for a woodburner to do the same (based on the above figures) would be £3.84-9.60 — or as little as £2 if you can dry logs yourself over two to three years.
C-Four Woodburning Stove by Charnwood
Woodburners with Back Boilers
A smart way to maximise the benefits of a woodburner is to specify one with a back boiler. This, as the name implies, combines a small boiler function to provide a space and domestic hot water function as well as traditional spot heating.
- These products, known also as boiler stoves, can typically provide between 5-20kW of water heating as well as the equivalent in output to the room. For instance, Charnwood’s Island IIIb model (around £1,950) has a 4-9kW room output and can heat domestic hot water and approximately six to nine radiators.
- In terms of RHI (Renewable Heat Incentive), stove boilers that burn logs are not permitted. Under the scheme, those that can only burn wood pellets do qualify (in any case, wood pellets do have a cleaner and quicker burn, and they often have integral hoppers which can easily store up to two days worth of fuel, unlike the log models). For the purposes of the scheme they’re known as biomass stoves. Try Ecofire’s range of stoves, such as the Palazzetti Ecofire Martina Hydro.
- Pellet stoves with back boilers in particular tend to be larger than woodburners and cannot easily be recessed into an existing chimney breast as they are quite tall (around 1,600mm with the hopper lid open) and have to be loaded with fuel from the top.
- A HETAS-registered installer will be able to advise you on the best way to link it into your system, which will probably be done by plumbing it into a hot water cylinder.
So the conclusion?
Given the huge variance in price and output of the fuel itself, the success of a woodburner depends on being a good shopper and asking the right questions of your supplier.
The Worst Burning Woods
- Douglas fir