Problem of the design of a wood boiler

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Heating with wood pellets: the question to ask before choosing this method of heating?

The problem of over sizing a log wood boiler or pellet

A common mistake is very common and on the boiler sizing.

On an oil or gas boiler, it's no big deal although consumption increases but on a wood pellet boiler is more annoying, and a log wood boiler is a disaster.

Furthermore this overconsumption is often the case posteriori discontent and bad publicity of these systems as well sized, a wood boiler is really efficient and economical and environmentally friendly!

Understand the thermal requirements and the control of a boiler

To understand the problem of design, we must understand the heating techniques in wood and fossil technologies as well as the thermal equilibrium problem.

Heating a home is a system in constant balance between heat flux escaping from the house (it is the heat loss) and a heat inflow (of course heating but also sun, occupants ... each person brings between 80 and 120W following its morphology).

Throughout the year the need for heating a house varies, but most reduce in scale (a few hours), there is a heat demand that can be defined as almost fixed. This required heating value is always less than the maximum output of the boiler.

How the boiler is it really suits the power required? By its regulation system but mainly operating intermittently.

When the burner of your oil boiler 30kW works, it delivers about 30kW! If the need in the house is only 10kW, then the burner stops 2 / 3 time average indeed kW = 10 1 / 3 30 * kW.

An oil or gas burner starts up in seconds and stops instantly, it has been studied for this and it is one of the advantages of fossil fuels.

But this is not the case of a fire. That is all the problems related to proper sizing of a wood boiler.

Very little control with a log wood boiler

The regulation of the power by "hashing" (cycles on / off) is very badly adapted to the combustion of wood, if not impossible, since it would be necessary to extinguish the embers at each stop.

Thus on a wood boiler, the only way to "regulate" the power is to play on the draft (by stifling the fire more or less effectively so).

This solution is quite limited in effectiveness and therefore, properly discharge a permanently heat thermal buffer timber is almost always required on all boiler mounting logs! Here is a method of first approximation calculating a thermal buffer and here the presentation our wood boiler installation with thermal buffer.

limited control over a pellet boiler

If the regulation is different (less worse) if the pellets but not solved.

pellet boiler manufacturers have sought and found ways to modulate the power boilers by various more or less effective devices. We will not detail these devices here but these systems vary substantially 2 factors: fuel supply (not with logs) and the air supply (as with logs).

The modulation is always a minimum threshold, and performance that minium threshold is not very good. In other words, and this is true for almost all combustion systems - including motors - performance is (the) best when the operation is close to the full power of the device. To consume less, so you have to run his wood boiler, usually close to its maximum power, in other words: the less often than this minimum threshold.

In addition, the re Automatic ignition of pellets is also quite difficult to do technologically. Indeed; it is much more difficult to automatically ignite fuel when liquid and solid that, of course, gas.

A practical case

So if your pellet boiler has a max power 30kW like your old boiler fuel oil, it has a reduced modulation at minimum power may 8 or 9kW so when the need for heating is 4 or 5 kW, it will s stop 50% of the time.

During each start-up phase, it will consume more electricity (to turn the pellets), and goes to waste pellets that go wrong or do not burn and it will have a deplorable performance.

When stopping, it will also waste energy by sweeping the foyer with an air stream (for security reasons) sending the passage calories in the fireplace.

A good sized pellet boiler with a finely tuned regulation has to operate over long periods to be effective. To give an order of magnitude, I would say that periods of operation below 30 or 40 min are unfavorable (when a boiler fuel oil sometimes works 5mn). In addition to the freeze period had better be short, because in this case the reboot is better (presence of embers). It is better pellet boiler works 40mn and stops 10mn than the reverse.

Conclusion on the choice of power

a) Do not hesitate to professional pressures and especially in view for several absolutely avoid over sizing your installation, Even to use a backup heating by extreme cold. For example: a pellet stove which will be fully paid by the price difference between the 2 boiler models.

b) Precise heat balance will pay for itself in a few months of use in bad sizing your installation. Think about it before because after it's too late!

Read more: properly size a pellet boiler

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