Heating of the coolant of a central heating installation can be done in many ways. With regard to wood heating three systems are available:
- the air / water heat recovery unit
- the boiler fireplace integrated in a classic fireplace
- the boiler with wood, logs or wood waste (platelets, pellets also called pellets.
- Air / Water recuperator
- Pellet boiler
Air / Water recuperator
For installation an Air / Water recuperator, see the notice Heat transfer heat recovery unit.
The boiler hearth
It is a true central heating unit, designed to supply hot water to a network of heat emitters (radiators, ceiling or underfloor heating), either alone or in combination with a conventional boiler. These devices are designed to be masked by integration into a chimney. They therefore fundamentally differ from the air / water recuperators since they are closed hearths, in the walls of which there is a circulation of water.
A boiler fireplace of this type (photo opposite) can develop a power of 19 kW, of which 15 are transmitted to the water. Some models can go up to 35 kWh.
The average consumption is 7 to 10 kg of wood per hour. Like the recuperator, it can operate independently or in boiler backup.
The log boiler is a little bit more bulky than a fossil fuel boiler.
It requires a regular replenishment, every 5 to 12 hours depending on the devices.
The log boiler
Before the generalization of oil and gas to supply the central heating boilers, it ran on coal in town and very often in the woods in the countryside, where this type of heating, it is true, was not widespread. The wood stove was often used as a boiler in large homes. In the years 1970-1980, mixed boilers appeared, marking the timid return of the wood, generally associated with the fuel oil. Recently, the boiler exclusively logs returns.
The devices are distinguished by:
- their mode of combustion;
- the way air is admitted into the home (natural or forced draft).
The modes of combustion have a significant influence on the efficiency of the appliance:
- in a rising combustion boilerthe logs are simply stacked on the rack and they all ignite at the same time. It is therefore difficult to control this combustion, which is rather mediocre and never complete;
- in a horizontal combustion boilerthe arrangement of the primary and secondary air inlets provides better control of the combustion which is therefore more complete;
- in a reverse combustion boiler, the draw is from the bottom, the secondary air inlet allows a re-ignition of imperfectly burned gas, resulting in better performance and less pollution.
The granules are mainly derived from sawmill waste. It contains no glue or additives.
This pellet boiler (MB Solo HS France) with a power of 15 to 23 kW has a "balloon" (silo) storage 750 to 1000 l ensuring a long battery life. It also heats the sanitary water.
Platelet or pellet boilers
Logs have a double disadvantage:
- a large size which requires to have large storage possibilities;
- the near impossibility of an automatic reloading and therefore a limited autonomy.
Inspired by industrial boiler rooms who used sawdust, it was thought possible to burn more solid wood "waste" from logging operations, sawmills and wood processing companies:
- the platelets are made up shredded wood, obtained by grinding branches or forest or fruit trees. They are 2 to 5 cm long;
- the pellets or pellets result sawdust compression. They contain no additives and are in the form of small cylinders 6 to 10 mm in diameter and 10 to 50 mm in length, the first for stoves the second for boilers. They are delivered in bags of 10 kg or more, which facilitates their transport and storage.
Platelets and granules can be delivered automatically (by gravity or auger) to the boiler from a silo more and more like a tank (which can even be buried like a tank of oil or gas).To note the existence of boilers accepting wood waste, pellets and cereals.
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