Heating Boilers

Heating Boilers

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The boiler must be installed on a fire-proof floor and it must be accessible for inspection and maintenance, and in front of it, there must be sufficient space for air. In a room with limited area, ventilation holes must be made. When the air comes from outside, the inlet hole should have a size of 600 square millimeters for each kilowatt of boiler power. The boiler’s chimney outlet is connected by a removable chimney pipe with a hole in the wall of the room, which goes into the chimney of the building.

Depending on the type of fuel used, boilers are categorized as: gas, liquid fuel, electrical and solid fuel. There are boilers which are able to work using two or three of fuel types, and they are called «combined» boilers.

The first step in choosing the type of fuel is governed by the choice of heating regime. If you build a country house for temporary accommodation, then it is possible to use solid fuel and use the boiler only while you are there. In this case, it is necessary to provide the possibility of draining the water from the heating system and water supply as well as avoid expensive decorations. Otherwise, constant temperature changes and condensation will spoil the appearance of the walls and ceilings. In cases of liquid fuel, gas, and electricity, in order to save money, you can use boilers with programmed heating regimes. For example, for a whole week the temperature in the house can be kept at +5 — +10 degrees C, but just before the arrival of the owners, automatic equipment puts the boiler on to preprogrammed power and heats the house to +20 degrees C.

Solid fuel boilers run on wood, coal, peat, pellets and so on. This type of heating is the cheapest, but at the same time more problematic than the others. The main disadvantage is that you must provide the fuel manually and when heating with coal, coal dust appears with resulting mud and dirt. The frequency of putting fuel depends on the automatic features of the boiler. To operate the boiler, one needs to provide it with fuel almost constantly and the fuel needs to be stored somewhere. Also, while using a solid fuel boiler, you should clean it and the chimney frequently. This type of fuel is more suitable as an extra option, or in combination with another type of fuel, for example, electricity. The most promising, to date, are pellet boilers (see Figure 76). Pellets are wood granules, made from sawmill waste. When burned, they emit a sufficient quantity of heat and they leave almost no ash. Pellets, which are sometimes sold in hardware stores, are loaded into a container and automatically fed into the boiler by conveyor. This process is automated; you only need to fill the container with pellets when needed, and sometimes remove ash from the boiler.

Solid fuel pellet boiler
Figure 76. Solid fuel pellet boiler

Electric boilers (see Figure 77) are the easiest to maintain: no furnace, no chimney, no fuel in the usual sense. All you need to install them is to have a good electrical supply. If you spend only weekends in your suburban house, then this variant may be the preferred one. With this type of heating, the main expenses for purchase and installation of the boiler are minimal. The boiler room doesn’t need to be too large and doesn’t consume much electricity in stand-by mode. All you need is to get permission from the electric company to receive the needed power, but if there are problems in your region with the amount of electricity you can have, then you should avoid this variant. But not entirely electricity for heating can be used in «warm cable floors», «warm moldings», etc. Direct electric heaters are also a good option for heating. Really, why heat water and then force it to run in pipes when it’s easier to just flip a switch?

Electric boiler</figcaption
Figure 77. Electric boiler

If there is no gas in your area, and it is not expected, and you can only dream about a good electricity supply, then, other than solid fuel, there is one more option — a boiler which works on liquid fuel. In this case, the expense for diesel fuel is 2 — 3 times higher than for gas, but there are obvious advantages. For heating with liquid fuel, you do not need approval from any organization. The main task is getting the diesel fuel to the boiler when it is needed. That is, there is a need for special containers and a fuel pump for delivering fuel to the boiler. When planning your landscape, it is necessary to provide the ability for a fuel truck to come to the containers. The bigger the containers, the less often you will need to refill them. If the furnace of the boiler is well regulated, then you will see almost no smoke. But at least once a year, you have to clean the soot from the chimney and from the boiler itself. For uninterruptable functioning of the boiler, the quality of the fuel is very important. All boilers working on liquid fuel have a «blowing» burner installed because diesel fuel ignites and burns only with a certain combination of air and fuel. In addition, domestic diesel fuel often requires the removal of sand, water, tar and other impurities. The use of poor quality fuel will lead to disruptions in the boiler, its becoming clogged with soot, and damage to the burner’s injectors. So try not to skimp on the filters and avoid low-quality diesel fuel.

The most advantageous fuel is gas. If you have gas in your area, then you should look at getting a gas boiler. Gas is relatively cheap, it is not necessary to be constantly delivering it, there is little soot when it is burned (therefore less frequent cleaning of chimney and boiler) and so on. But the biggest problem is getting the supply of gas to your house. The fact that the gas pipeline goes near your land doesn’t mean that you can connect to it. Installation of a gas boiler can be done only by a specialist from the gas service after infinitely long and almost always expensive permits and approvals. However all this paperwork is justified by the high reliability of gas heating. There is almost never a failure in the delivery of natural gas, in contrast to all other forms of energy. When using gas heating, water can be poured into the system.

In boilers operating with gaseous fuel, there are two types of burner which can be installed: atmospheric — where the process of mixing gas with air occurs naturally; and with a fan (or blower) in which the mechanical mixing of gas with air happens under pressure and this mixture is fed to the burner. (See Figure 78.) Both burners have their advantages. The atmospheric burner has no mechanical parts, and therefore its lifetime is theoretically more and it is not subject to power outages; the boiler will work in any case. But it can go out if the gas pressure falls in the supply pipeline (and this happens sometimes). In this case, the automatic shut-off will stop the flow of gas completely. The blow burner is less susceptible to falling gas pressure and more economical, but for its effective operation, a constant supply of electricity is needed. To reduce fuel consumption, the burners can be two or three levels, or with continuously variable power (modulating).

Boilers for individual boiler rooms (from left to right: universal boiler with blower gas burner; boiler with atmospheric burner; wall-mounted boiler)
Figure 78. Boilers for individual boiler rooms (from left to right: universal boiler with blower gas burner; boiler with atmospheric burner; wall-mounted boiler)

A wall-mounted gas boiler is a special case. It is like a mini boiler room — each boiler includes in it the circulating pumps, the safety group, and expansion tank. Typically, such boilers are combi-boilers: they serve the heating system and provide hot water for the hot water supply. It seems that everything is wonderful: you installed this boiler and don’t have to worry about designing the piping for the boiler. But you should not forget one thing: if there is too much equipment collected in a small space, you can be sure that something must be sacrificed. And indeed it is. Nowadays different anti-freeze liquids are poured into heating systems as warmth carriers. On one hand, this desire is understandable. The heating system will not freeze under any frost. You come to the dacha once a week, turn the heating on, then you leave and turn everything off. In general, almost maximum comfort with a minimum of trouble. However many companies which make boilers and especially the wall-mounted type, in their manuals, strictly forbid putting anti-freeze in their equipment. The reason is that in a running heating system the warmth carrier gets hot while it flows through the heat-exchanger. The heat exchanger is usually made of copper, which provides high thermal conductivity. This system is designed only for water. Anti-freeze which has a high viscosity, greater fluidity, and a different boiling point, very quickly makes the wall-mounted gas boiler stop working. Besides, in wall-mounted gas boilers, the electronic brains installed are very sensitive to electric voltage changes. If you do not have reliable electric power, then you must buy a voltage regulator in addition to the boiler.

Quite recently condensing boilers have appeared on the market. These use water steam released during the burning of gas. In conventional boilers, water steams together with other gasses and goes up the chimney; but condensing boilers collect water steam and direct its heat into the heating system. Therefore the usual efficiency of the condensing boiler is 107–109%. People who do not know too much about boiler equipment but who learned at school that efficiency can not be more than 100% and in general never comes close to this figure will be absolutely right. The point here is that the efficiency of combustion systems is calculated from the lowest heat of combustion — the amount of heat released by the complete combustion of fuel. It doesn’t take into account which went up the chimney along with the waste products of combustion. If you add the heat of the combustion of waste to the lowest heat of combustion of gas, then it is called the highest heat of combustion. Since everywhere in the world, the efficiency of boilers is counted from the lowest heat of combustion, therefore the manufacturers of condensing boilers decided not to violate the established rules, and added the efficiency of water steam to the efficiency of the boiler calculated under the lowest heat combustion of the fuel. In the end, it turned out to be higher than 100%, but if you calculate it correctly — i.e. regarding the highest heat combustion of fuel — then of course it will be less than 100%.

Manufacturers of condensing boilers show the efficiency on the lowest heat of combustion only because the consumer can objectively compare the performance of conventional and condensing boilers.

Condensing boilers, like conventional ones, are available in two versions: floor and wall-mounted. However the wall-mounted condensing boilers, which are now preferred worldwide, allow an output of the heating system of more than 100 kilowatts to be achieved, unlike conventional wall-mounted boilers the output of which usually does not exceed 32–40 kilowatts. The output of conventional wall-mounted boilers is more than enough for apartment heating, but not enough for a big cottage. Installation of a condensing wall-mounted boiler solves this problem. It can be installed for heating a large cottage and allows for a smaller boiler room at the same time.

Piping of condensing boilers is slightly different from piping of conventional boilers. In this case, one more contour (see Figure 79) is added into the heating system, which takes away water steam’s warmth. The operation of condensing boilers is based on a simple principle: the combustion products come to the condensational recuperator. where the cooled water from the return pipeline of the heating system comes; here it cools the steam, converting it into water. Upon cooling, the steam gives warmth to the supply water and the water goes into the heating system and the resulting condensate from the steam drains to the sewer system and to a special container. In general, the recuperator is in essence a conventional distillation apparatus. The temperature of the heated warmth carrier in it is not very high, so it is used for mixing with the supply of warmth carrier for the main heating contours.

Heating schemes with condensing boilers
Figure 79. Heating schemes with condensing boilers

It is due to the fact that the heat transfer of steam while condensing is not very high that it was ignored for a long time and the conventional boilers were made. But for the same reason the companies which produce condensing boilers managed to increase the efficiency of their equipment up to 107–109, compared with conventional boilers. The more effective the recuperator, the better performance the boiler has, and it is more economical, because more useful warmth is taken from the same quantity of fuel during burning.

All is well with condensing boilers, but there is a hard task at hand. Where to drain the condensate? Up to 4–6 litres per hour are formed and it is not water, but a weak acid. Draining it into a stand-alone or local sewer system is strictly forbidden because all the microflora of the tank will be killed, and purification in the sewer system will stop. Neutralization as in Europe is troublesome and expensive. Therefore condensing boilers can be used in our country only with difficulties. Some people are scared by the fact that during the operation of the boiler the condensate appears but very few remember that while the conventional modern boiler works, condensate appears also which needs to be disposed of. The chimney pipe, after the boiler stops working, gets cool from the top to the bottom and condensate forming on the walls of the chimney starts to flow towards the boiler where it must be collected and removed.

Sometimes during modern construction, architectural-constructional plans are made which are based on old regulations, in which the chimneys are designed in intra-house walls with a thickness of 38 cm (1.5 bricks). A chimney for a modern boiler should not be made in a wall with such a thickness.  The efficiency of modern boilers is greater than 90% and therefore the exhaust gas temperature is low. The water steam condenses during a temperature decrease and the temperature of this condensate (dew-point temperature) for natural gas is about 55 degrees C. The exhaust gases will be cooled to this temperature at a height of 4 or 5 metres from the exit of the boiler. Therefore if the height of the chimney is more than four metres, it should not be constructed in the brick wall, but if you want to do so, then the cross-section should be not less than 250×250 mm. In this case, a chimney made of stainless steel can be put in this channel. A boiler is not a stove and it is not a fireplace and it is necessary to think about the sealed duct. During the combustion of any hydrocarbon fuel, carbon dioxide and water in the form of water steam are formed. Besides, each fuel has some impurities, including sulfur, which, burning, creates anhydride and it, in its turn, mixing with the water, forms acid. A mixture of acids in the chimney is so aggressive, it can destroy a brick wall completely within several months.

Two-contour smoke tube
Figure 80. Two-contour smoke tube

Construction of chimneys for condensing boilers is not very different from that for conventional gas boilers with a closed combustion chamber. Due to the construction of the condensing boiler, the combustion products are removed by force, and this makes it possible to connect the boiler to the smoke removal systems, such as coaxial chimney or two-contour tube (see Figure 80), as well as to the system which takes air from the room and removes combustion products through the chimney.

 

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