Problem of sizing a wood-fired boiler

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

The problem of oversizing a wood-fired boiler with logs or pellets

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!

Understanding the thermal needs and the regulation 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.

The heating of a house is a system in constant equilibrium between a flow of heat which escapes from the house (these are the heat losses) and an incoming heat flow (heating obviously but also sun, occupants ... each person brings between 80 and 120W depending on 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.

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

But this is not the case with a wood fire. Here is all the problem linked to the correct sizing of a wood boiler.

Very little control with a log wood boiler

The power regulation by "chopping" (on / off cycles) is very poorly suited to the combustion of wood, not to say impossible, since the embers would have to be extinguished at each stop.

So on a wood-fired boiler, the only way to "regulate" the power is to play on the draft (by smothering the fire more or less effectively).

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 calculation of a thermal buffer and here the presentation our wood boiler installation with thermal buffer.

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limited control over a pellet boiler

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

Manufacturers of pellet boilers have therefore sought and found means of modulating the power of the boilers by various more or less efficient devices. We will not detail these devices here but these systems essentially vary 2 factors: the fuel supply (not possible with logs) and the air supply (as with logs).

The modulation always has a minimum threshold, and the efficiency at this minimum threshold is not very good. In other words, and this is valid for almost all combustion systems - engines included - the efficiency is (the) best when the operation is close to the full power of the device. To consume less, you must therefore operate your wood-fired boiler, most often close to its maximum power, in other words: less often at this minimum threshold.

In addition, the automatic re-ignition of the pellets is also quite difficult to achieve technologically. Indeed; it is much more difficult to automatically ignite a fuel when it is solid than liquid and, obviously, gaseous.

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.

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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 if it means having to resort to auxiliary heating in very cold weather. For example: a pellet stove which will be fully paid for by the price difference between the 2 boiler models.

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

Read more: properly size a pellet boiler

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