Wood-burning boiler central heating stovesIt’s not always easy to ensure even distribution of heat through very large areas or rooms (over 100m3). The size and shape of the room and the positioning of wood-burner should be carefully considered before deciding to use a single appliance even if apparently large enough.
Most of us in France though will have a nice (usually large) open fire place crying out for a wood-burner. If you want to evenly heat a large area or indeed several rooms as well as provide for domestic hot water, then it is possible to do so with the right wood-burning stove or cooker. These are called ‘boiler’ model stoves & cookers, and there are primarily two kinds;
those with an integral (wraparound) boiler which is formed as part of the stove body itself,
those with ‘clip-in’ boilers which can be added to most makes of stove (even if you have a stove already, it’s likely that the manufacturer will have a ‘clip-in’ boiler that could be retro-fitted to your stove).
It’s worth noting that integral boiler model stoves have a far greater output capacity than the “clip-in” types, although a clip in type boiler stove will give more direct heat to the room.
Heat outputs of our stoves
All stoves manufacturers will quote the output of their stoves either in kilowatts or BTU’s (British Thermal Units). Always bear in mind that these outputs usually reflect the stoves MAXIMUM capability using well seasoned wood. This being said, all of our range of British HETAS (Heating Equipment Testing and Approval Scheme) approved stoves will have been independently tested in an approved laboratory using a standard test fuel. The actual test regime will vary depending on the appliance type, but in all cases the relevant British Standard Testing Procedure is followed. As a result you can be confident that any of our approved boiler and non boiler stoves will, when operated according to the manufacturers instructions, burning a suitable fuel and correctly installed, achieve the thermal performances quoted by our manufacturers.
When you buy a boiler model stove you have to consider the amount of heat that you will need to power the radiators as well as considering the demand for a domestic hot water supply if you plan to use your stove to heat a hot water cylinder. With this in mind, if you choose a roomheater with a high output boiler, careful consideration has to be given to the effect on direct room heating output when the total connected ‘heat load’ is below the rated output of the appliance. In other words, the ratio between direct room heating and the boiler output is proportional.
For example, a wood-burner with a rated output of say 2kW direct room heating and 11kW boiler output would not produce 2kW direct to the room if the ‘total connected heat load’ was only 8kW. In this case the direct room heating output would be reduced by about a 1/4 (25%) to a figure of 1.5kW.
If you need more heat to be generated in a large room where the stove is fitted, you may have to consider fitting radiators (or a supplementary means of heating such as convector heaters) in order maintain the room temperature to a comfortable level. These sort of calculations would normally be carried by an engineer in order to determine your requirements which include factoring in heat loss on pipe runs, etc. but it’s important to give careful consideration to these issues at the outset before buying a stove and having one installed..
Linking your boiler stove into your existing central heating.
A boiler stove can be linked to an existing or new central heating system, either on its own or in conjunction with an existing oil or gas central heating system.
· The existing central heating boiler continues to service the system as and when required.
· The hot water and radiators can be supplied by either or both of the appliances depending on the heat input needed.
· The system can be installed to link either the hot water only or the hot water and radiators.
The real advantage of this ‘link-up’ system is its flexibility. Whilst you enjoy the comfort of your wood-burning stove, the system can take over automatically, all or part of the heating load at no extra cost. When you are not using the stove the central heating operates as normal.
Some systems can be very simple and relatively inexpensive. In many cases a simple link-up is most cost effective with an existing system to which you wish to add a boiler stove whereby the boiler stove is connected directly to the hot water cylinder. Indeed, this type of system used may years ago with an open fire with backboiler for the production of hot water from solid fuel is as valid today as it ever was.
Using your boiler stove for central heating and/or domestic hot water supply
When you require the fitting of a new central heating system to be powered by your woodburner, it is advisable to consider the fitting of a fully pumped system. Despite the system being fully pumped it is still necessary to ensure some gravity fed circulation, preferably to the cylinder, in the event of a power failure. Under these circumstances some heat and hot water can be obtained during such failure.
To design and install a system requires a full understanding of solid fuel heating systems. However, there are at least two companies who provide fully constructed and pre-wired kits with full fitting instructions to make an installation more straight forward and efficient.
The above diagram shows a typical installation incorporating a manufacturers control “kit”.
Hot water & hot water cylinders
The domestic hot water used on solid fuel type installations should always be of the double feed indirect type. Don’t attempt to try to connect this to your chauffe eau!
In order to prevent the risk of “localised” boiling, it is necessary to position the hot water cylinder or radiator to enable gravity circulation to take place when the circulating pump is off or the power supply interrupted.
Some control packages require the installation of a motorised valve/s within the gravity circuit, and if this is the case the valve must return to the fully open position when the power is switched off or interrupted.
Even though the system will be safe if an open vent is provided, the provision of a gravity circuit will avoid the inconvenience of the system boiling and allow some use of the system in the event of a power failure.
Heat leak radiator
The continuous combustion of a wood-burner and the heat it produces, even at a minimum level, demands some means of dissipating the heat from the boiler. Traditionally this has been achieved by supplying a the domestic hot water via a gravity primary circuit.
However, due to the very high efficiency of modern wood-burners and other solid fuel appliances as well as cylinder insulation, some wood-burners with high output boilers require the installation of a “heat leak radiator”. The appliance manufacturers will specify the size of radiator required.
The function of this kind of radiator is two fold:
1) To dissipate the heat output from the boiler stove during slumbering conditions once the domestic hot water cylinder has been satisfied. This will prevent overheating of the domestic hot water supply.
2) In order to maintain combustion the appliance requires an adequate supply of air. Once the hot water cylinder (during slumbering periods) has been satisfied the appliance thermostat (if fitted to your wood-burner) will remain closed, and under these conditions the fire may “go out” due to the lack of combustion air. The heat leak radiator ensures the appliance thermostat is stimulated from time to time during any slumbering period to prevent this. Normally, the “heat leak” radiator should be installed in a bathroom as a towel rail as well as heating the room.
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