Wood-fired boiler for heating a private house

Recommendation points

• The main types of wood-burning boilers
• Equipment power
• Chimney device
• System inertia
• Automation and piping of wood-fired boilers
• Installation, connection

With a limited choice of energy sources, wood-fired heating may not have an alternative at all. It is important to purchase a suitable wood-burning boiler and understand the features of its operation. We will tell you about these heating units, starting with the choice of a suitable type and ending with installation questions.

The main types of wood-burning boilers

The arsenal of heating equipment is constantly being replenished with new, more advanced products, however, nothing fundamentally new has been invented among solid fuel boilers. In an attempt to increase efficiency, the designers somehow improved the design, which is why several varieties appeared with a number of characteristic features:

1. Complex Z-shape of the furnace or its division into 2 combustion chambers. Such boilers are designed for long-term combustion and the most complete combustion of the emitted pyrolysis gases..

An example of the construction of a long-burning solid fuel boiler: 1 – boiler automation; 2 – loading door; 3 – primary air supply; 4 – secondary air supply; 5 – ash pan; 6 – combustion of pyrolysis gases; 7 – pyrolysis of solid fuel; 8 – heat exchanger; 9 – fan-exhauster

2. The presence of water-filled grates and / or a jacket. It is believed that in this way the coolant takes up to 90% of the heat of combustion, but in practice, good wood-fired boilers with an efficiency higher than 80% are already a rarity..

Water-filled grates of a solid fuel boiler

3. Form of organization of blowing. By reducing the oxygen supplied, the duration of combustion can be greatly extended, maintaining the temperature of the water within specified limits. Usually it is realized by means of a manual or controlled damper; if the draft is not strong enough, a blower fan can be used. The damper is usually controlled by digital automation that controls the temperature of the water in the supply pipe.

Boiler pressurization fan

4. Type and device of the heat exchanger. On the one hand, there is a race to maximize surface area for more efficient heat dissipation, and on the other, for durability and reliability. In the first case, we have boiler-type honeycomb heat exchangers, highly efficient, but problematic in cleaning and repair / replacement. On the other hand, simple tubular coils made of cold-drawn seamless pipe are hard to argue with the classics..

Honeycomb heat exchanger

5. The presence of insulation. The need for a mineral filled casing is determined by the installation site. If it makes no sense to waste heat on the street, then for a garage or workshop an uninsulated boiler can be a good heating method..

There are also differences in the type of fuel, the horizontal and vertical arrangement of the shaft, and the material of the furnace. The chambers and coils, made of cold rolled mild steel, are simple and unpretentious, but prone to dross formation. Cast iron coils and water-filled jackets are the most durable and not prone to scale formation, but require special piping due to their high sensitivity to thermal shock.

Pellet boiler

Equipment power

Ultimately, it is not the details of its internal structure that are important for the boiler, but the basic performance indicators that have been achieved using a number of engineering innovations. The most important and defining parameter of the boiler is its rated instantaneous power. The heat capacity for different types of fuel is different, as well as for different types of firewood, the manufacturer cannot accurately predict it and calculate the rated power in a particular operating mode.

Determination of power occurs according to two indicators. By the volume of the firebox, one can judge the weight of the bookmark. On average, it is believed that for every kilowatt of power of a wood-fired boiler there is 2.5-3 liters of the volume of the combustion chamber. Having received the instantaneous value of the power released during combustion, it is possible to predict how much of it can be absorbed by the heat exchanger at different operating temperatures. Usually absorption is deliberately made higher than the calculated heat release in combustion conditions close to optimal, but this approach is fraught with trouble.

Condensate in a solid fuel boiler

The fact is that all wood-burning boilers suffer from a common disease – the formation of tar due to the high moisture content of the burned wood. Condensation only occurs when the temperature difference is high enough. So, the formation of build-ups on the economizer or secondary heat exchanger can stop when the heat carrier is heated to 40–45? С. The bottom line is that if you load the boiler by half, its power will not be enough for normal heating of the walls and heat exchangers, because of which the equipment will simply “flow”.

Chimney device

The main blow from the condensation of water vapor from the wood falls on the chimney system. The pipe is long, the temperature difference (especially outside) is very high. There are two approaches to solving this problem..

The first one consists in installing revision taps for draining condensed moisture. Such bends should be located at the lowest point of each swivel segment, starting from the last vertical section and ending with the connection point of the boiler. The disadvantage is that the pipes for draining water periodically need to be cleaned and cannot have a significant length with a small diameter.

With brick chimneys or with asbestos-cement pipes in the lining, such problems do not arise. The difficulties of constructing such chimneys are obvious, but they can also be circumvented using more expensive materials. We are talking about insulated stainless steel sandwich pipes with mineral filler. For a chimney device today, this is the best option. To summarize: condensate does not form only in insulated chimneys.

System inertia

Another disadvantage of a solid fuel boiler is hidden in the cyclical nature of its operation and the inability to independently replenish fuel reserves. Because of this, either a high heat capacity of the house structure itself is required, or a deaf insulation, close to a passive house, or a way to store a supply of heat for the period of cooling of the system..

Since the volume of the heat exchanger itself in the boiler is small, the displacement of the system can be raised mainly by increasing the diameter of the pipes. Another alternative of this kind is the installation of a heat accumulator. With the right approach, you can realize the accumulation of heat only after heating the main system, which will help speed up the heating of the house and solve the problem with condensation.

Heat accumulator in the heating system

Underfloor heating systems have their own high inertia, which grows with the thickness of the accumulating screed. With an underfloor thermal cutoff, this can be a truly long-term source of heat. With a sufficiently thick layer, even the use of water of a higher temperature will not cause discomfort and a thermal zebra, but together with the inertia of the system, the time it takes to reach the regime increases. On the other hand, connecting a warm floor to a wood-burning boiler requires a rather complex piping, which excludes overheating of both the boiler and the pipes in the floor..

Water heated floor

Automation and piping of wood-fired boilers

As already mentioned, the boiler power can be regulated only by changing the amount of supplied oxygen. This system works most efficiently and accurately in mine pyrolysis-type boilers using wood or briquettes..

Solid fuel boiler control unit

The main automation complex includes an electronic control unit, a fan, an electrically driven draft regulator and a thermocouple tightly attached to the supply pipe. All these components are sold both individually and in sets for a specific boiler model. On average, all automation costs about 4-5 thousand rubles. Separately, we will mention the importance of having protection against falling damper and sealing grooves on loading and inspection hatches, chimney, draft stabilization system.

Air draft regulator

For solid fuel boilers, a number of protective devices of a different kind are provided, installed in the hydraulic piping. Of the absolutely obligatory ones, these are a circulation pump and protection of the furnace against overheating – a safety group for relieving excess pressure. Please note that the outlet of the relief valve must be facing the wall or fitted with a hose, so as not to injure those present when triggered. Safety requirements can provide for the installation of a chimney undercut valve to stabilize draft and combustion, it is mounted immediately at the outlet of the boiler or directly in front of the final vertical section.

Connection diagram of a solid fuel boiler with a heat accumulator: 1 – chimney; 2 – thermostat; 3 – security group; 4 – air separator; 5 – circulation pump; 6 – heat accumulator; 7 – three-way mixer; 8 – weather-dependent automation; 9 – heating radiator; 10 – circulation pump; 11 – check valve; 12 – expansion tank; 13 – protection against dry running; 14 – make-up valve; 15 – laid-on temperature sensor; 16 – solid fuel boiler

For cast iron boilers, it is practiced to install a two-way valve that mixes water from a hot outlet to prevent cold water from entering the heated firebox. When installing a heat accumulator, a three-way electric valve is added, which starts the supply of heat to the tank only after the main composition has warmed up. The control is carried out by the return temperature, the additional circuit is switched on 7–10? С before reaching the cutoff temperature. In this aspect, control units with a PID controller are most preferred..

Installation, connection

The boiler is installed on a concrete plinth, the height of which is determined by the balance of the hydraulic system. Ideally, the coolant in the boiler should be at the highest pressure compared to the rest of the pipeline. The position of the boiler is also determined by the direction and point of connection of the flue. Its connection can be either vertical (top) or horizontal (back or side).

It is recommended to use only steel or copper pipes in the boiler piping, for plastic there is too great a risk of overheating. The section of the supply pipeline 3–5 meters from the boiler is also traditionally made with metal. Connections can be welded, in the absence of the necessary equipment – threaded on tow and anaerobic sealant.

The boiler should be adjusted taking into account the low heating rate and high inertia. The operating temperature of the coolant is maintained, as a rule, within the range of 50–65? С. In this case, the temperature of the reverse hysteresis should be slightly higher than the maximum, at which the boost completely stops. This is necessary so that the ignition starts long before the water in the system cools down more than expected..