Combined heating boilers: gas-electric or gas-firewood

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Why choose between different types of energy when you can use several at once? We will introduce you to hybrid heating technology: the bases for installing boilers for different types of fuel, the advantages of different combinations, installation and use features.

Combined heating boilers

The two main varieties of hybrid boilers

Manufacturers periodically expand the range of technical means designed to increase the efficiency of heating equipment. However, there are only two main schemes of the device for operation on combined fuel:

  • boilers with a combined combustion chamber;
  • boilers with separate combustion chambers.

In total, for hybrid boilers, the same design features are true as for conventional ones: material of manufacture, shape of the furnace and chimney ducts, types of heat exchangers. But the very possibility of using different types of fuel makes its own adjustments..

Combined heating boilers

The difference between boilers with combined and separate fireboxes is obvious. On the one hand, the production of single-chamber units implies less material consumption, respectively, the weight and dimensions are relatively low as a result. The other side of the coin is negative – it is very difficult to achieve an equally high efficiency of combustion of different types of fuel, in this matter unification is not beneficial.

The problem is that the heat and composition of the combustion products for solid fuel and gas are very different. If using natural gas it is preferable to increase the area of ​​the heat exchanger and lengthen the path of the flame for the most efficient heat absorption, then in the case of wood and coal, the situation is somewhat different. The combustion temperature of solid fuel is slightly lower, while the flame length is higher. The power cannot be flexibly adjusted, and therefore the heat exchanger is designed for increased reliability instead of energy efficiency. In addition, there is no way to avoid the formation of condensate and soot in large quantities, which greatly and rapidly reduce the rate of heat transfer..

Combined heating boilers

The presence of separate combustion chambers entails an increase in the cost of equipment of the order of 30% in comparison with single-furnace designs. However, such a design allows not only to increase the efficiency of fuel combustion, but also makes it possible to operate two furnaces simultaneously. If, when operating a single-chamber bi-fuel boiler on coal or wood, the gas burner must be removed in order to exclude its contamination and damage, then in two-combustion versions the gas chamber can be turned on even without the participation of the operator..

Operating modes

An electric heater for hybrid boilers is optional. Such an additional source of heating is not intended to smooth out interruptions in the supply of fuel or to reduce the cost of the generated heat power. However, heating elements are very ergonomic in terms of temperature control, which means they can be used for accurate heating of the coolant without a complex automation scheme, controlled dampers and pressurization.

Combined heating boilers

In most cases, it is important for the consumer to level the disadvantages associated with low gas pressure and high cost. The main heating occurs due to the combustion of solid fuel, while the gas burner is turned on only to reach the set temperature. This may be required both due to the low energy consumption of wood, and at moments when the next fuel filling is exhausted.

You should know that the most effective scheme of work is when water from the “return” enters the gas combustion chamber first. There, it is heated exactly to the extent that, after passing through the solid fuel furnace, it acquires an exactly set temperature value. In this case, the most complete absorption of heat from the most expensive type of fuel occurs. At the same time, water enters the heat exchanger of the solid fuel chamber already warmed up, which makes the formation of tar and soot not so intense.

Combined heating boilers

The heating elements are switched on according to a slightly different principle. Naturally, if necessary, electric heaters can play the same role as a gas burner. However, electricity is usually just a safety net for solid fuel boilers: electric heaters only maintain the coolant temperature above the freezing point if the entire fuel load is used up.

Safety requirements

Due to the difficulties in organizing the joint operation of furnaces, many boiler units do not meet safety requirements. This mainly applies to single-burner bi-fuel boilers “gas-wood”, for which there are two risk factors.

Combined heating boilers

One of them is associated with the need to manually remove the gas equipment. Without proper qualifications, the risk of error is great, and the consequences of careless handling of natural gas are well known to all. Due to the risk of freezing of the coolant, the reconnection of the equipment should be carried out as quickly as possible, and after all, intelligent specialists in boiler equipment cannot be found in a couple of hours.

The second danger lies in the need to periodically change the air supply scheme, which differs for different types of fuel. Failure to operate the boost can lead to the formation of carbon monoxide in the case of solid fuel or incomplete combustion of natural gas.

Combined heating boilers

There is only one way to eliminate such dangers – by installing protective automatics. These are, first of all, household gas analyzers with control of the concentration of carbon dioxide and carbon monoxide, as well as methane, connected to the cut-off valve. Additionally, it is recommended to install a draft stabilizer on the chimney; it will also not be superfluous to install a fire alarm in the boiler room.

Differences in strapping

The hydraulic piping of the boiler equipment and the wiring diagram of the heating system depend on the type and organization of the heating circuits. The simplest connection scheme is characterized by radiator heating systems with a closed expansion tank. The installation of a circulation pump and a safety group at the outlet of the heat exchanger of the solid fuel chamber is mandatory.

Combined heating boilers

When operating a hybrid boiler for underfloor heating, the risk of too high a water temperature must be excluded. The two simplest ways to ensure the correct mode of operation for circuits with one distribution manifold is to include a three-way valve in the supply pipe that regulates the outflow of excess hot water to the “return”.

For systems with several collector units for underfloor heating, it is recommended to install individual mixing modules. Built-in forced circulation devices provide the required flow with the ability to adjust, while all loops and collector groups work independently of each other and of the radiators included in the system.

Combined heating boilers

DHW preparation in bi-fuel boilers can be carried out both in a coil and in an internal boiler. In the latter case, the water supply has a reserve of hot water of the order of 30–40 liters, which is heated indirectly from the heat carrier of the heating system, that is, it is absolutely safe. When organizing a second circuit on a direct heating coil, there is a high risk of liquid boiling, especially when working on solid fuel. It is recommended to install a storage tank above the boiler of the order of 4–5 liters for each kilowatt of boiler power with the expectation of gravitational exchange of liquid between the storage tank and the heat exchanger. It is also possible to organize circulation in the water supply system with a sufficiently long pipe length.

Installation features

Combined fuel boilers belong to the class of massive and dimensional heating devices. The weight of boilers for heating residential buildings with an area of ​​250–300 m2 can reach 500–700 kg. Because of this, a reinforced concrete pedestal and lining with refractory materials of walls located closer than one meter to the boiler body is required.

Combined heating boilers

There are also difficulties with the organization of the chimney. Hybrid boilers, including those with separate fireboxes, have a common exhaust of combustion products with rather stringent requirements for the rate of natural draft in the chimney. Problems with the formation of carbon monoxide can be guaranteed to be avoided by installing smoke exhaust fans, or by observing all regulations when installing a vertical chimney segment. Basically, this concerns a sufficient overhang (about 2-2.5 meters) above the roof ridge of adjacent buildings, compliance with the required throughput and the use of composite insulated pipes..

Combined heating boilers

The rules for the ventilation of the boiler room are regulated separately. Bi-fuel boilers are installed only in non-residential premises, mainly in attached boiler rooms without insulation of the enclosing structure. Equipment from leading European manufacturers can be designed for complete containment from the internal environment with a permit for installation in non-attached premises without its own supply of general ventilation. In this case, air is taken from the street, which has a good effect on saving energy resources..

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