The principle of operation and features of double-circuit solid fuel boilers


A mod­ern res­i­den­tial build­ing is a whole com­plex com­plex, the task of which is to pro­vide the domes­tic needs of its res­i­dents and res­i­dents. The com­fort of the inhab­i­tants and the lev­el of liv­abil­i­ty depend on the lev­el of tech­ni­cal equip­ment of a res­i­den­tial build­ing. One of the lead­ing roles in this regard is played by the heat­ing sys­tem, which should not only pro­vide heat­ing of the inte­ri­or dur­ing the cold sea­son, but also guar­an­tee the oper­a­tion of the hot water sup­ply sys­tem. For a coun­try house, in the absence of a cen­tral gas sup­ply, issues with heat­ing and hot water sup­ply can be solved by a dou­ble-cir­cuit sol­id fuel boil­er.

Let’s take a clos­er look at what this tech­nique is.

The feasibility of installing a solid fuel boiler in a residential building

This type of heat­ing equip­ment is very con­ve­nient for those res­i­den­tial build­ings where there is no cen­tral­ized gas sup­ply sys­tem. The option with elec­tric heat­ing is rec­og­nized today as not effec­tive due to the high cost of elec­tric­i­ty, there­fore it is often used only as an aid. Sol­id fuel units, on the con­trary, are able to oper­ate on fuel, which is some­times at our fin­ger­tips. It can be fire­wood, wood­work­ing waste, coal. Sol­id fuel, avail­able in suf­fi­cient quan­ti­ties, will not only pro­vide a work­ing boil­er with a con­stant load, but will also bring sig­nif­i­cant sav­ings to the home bud­get.

Sin­gle-cir­cuit and dou­ble-cir­cuit sol­id fuel boil­ers pre­sent­ed for sale today are devices with high tech­no­log­i­cal char­ac­ter­is­tics. This tech­nique has now been brought to per­fec­tion by man­u­fac­tur­ing com­pa­nies. A num­ber of design inno­va­tions have made it pos­si­ble to sig­nif­i­cant­ly increase the effi­cien­cy of boil­ers, and the con­di­tions for ser­vic­ing sol­id fuel heat­ing devices have changed for the bet­ter. The dou­ble-cir­cuit sol­id fuel boil­er is quite capa­ble of heat­ing a res­i­den­tial build­ing dur­ing the heat­ing sea­son, at the same time pro­vid­ing res­i­dents with hot water for domes­tic needs.

Thanks to tech­no­log­i­cal progress, mod­ern tech­nolo­gies do not stand still. Demand cre­ates sup­ply. Just yes­ter­day, unpop­u­lar wood-fired or coal-fired heat­ing boil­ers are becom­ing pop­u­lar again. The inten­si­ty of sub­ur­ban hous­ing con­struc­tion has become one of the rea­sons that sol­id fuel heaters are back in price. The peo­ple mas­sive­ly install heat­ing units for the home heat­ing sys­tem, using organ­ic and fos­sil raw mate­ri­als as fuel.


A dou­ble-cir­cuit sol­id fuel boil­er of long burn­ing is the best solu­tion for heat­ing a pri­vate house. A pow­er­ful unit that pro­vides all liv­ing spaces with a suf­fi­cient amount of ther­mal ener­gy, is at the same time a very eco­nom­i­cal and easy-to-main­tain heater. More­over, such heat­ing devices require min­i­mal human inter­ven­tion.

The com­bus­tion of the fuel placed in the boil­er takes a long time, free­ing the res­i­dents of the house from the need to con­stant­ly throw up fuel mate­r­i­al. It is the long burn­ing process that makes this tech­nique as attrac­tive as pos­si­ble to the con­sumer. Not every­one wants to be a stok­er and stand near the fire­box on cold win­ter nights, throw­ing fire­wood or coal in the boil­er. The very prin­ci­ple of oper­a­tion, which is the basis of the oper­a­tion of the devices, gave them the name — long-burn­ing boil­ers, sol­id fuel.

The principle of operation of heating equipment of this type

In var­i­ous mate­ri­als, much has already been said about what the prin­ci­ple of oper­a­tion of heat­ing sol­id fuel tech­nol­o­gy for long-term com­bus­tion is based on. The achieved effect is due to a fun­da­men­tal­ly dif­fer­ent way of fuel com­bus­tion in the com­bus­tion cham­ber. Due to the mul­ti-stage com­bus­tion process, com­plete burnout of the fuel mass loaded into the boil­er is ensured. At the same time, wood gas is released in large quan­ti­ties, which per­forms the main work of heat­ing the coolant.

For ref­er­ence: units oper­ate on the basis of pyrol­y­sis, the process of gen­er­at­ing volatile com­bustible com­pounds obtained from wood as a result of exother­mic reac­tions. In the process of fuel smol­der­ing, wood (pyrol­y­sis) gas is formed, which, when inter­act­ing with hot air, ignites. This tech­nique is also called gas gen­er­a­tors.

The prin­ci­ple of oper­a­tion of boil­er equip­ment is rel­a­tive­ly clear, let’s fig­ure out how a dou­ble-cir­cuit autonomous sol­id fuel boil­er works at home. Let’s con­sid­er its main func­tions. The pres­ence of two cir­cuits in the sys­tem pro­vides heat­ing in the house and hot water sup­ply. In the pri­ma­ry cir­cuit, boil­er water cir­cu­lates, which diverges through­out the pipeline, sup­ply­ing heat to the heat­ing radi­a­tors. The sec­ond cir­cuit is designed to remove the waste coolant back to the boil­er for sub­se­quent heat­ing. Sen­sors installed on both cir­cuits — deter­mi­nants, sen­si­tive­ly react to the slight­est change in the set tem­per­a­ture of the coolant, giv­ing sig­nals to the con­trol pan­el. Accord­ing­ly, the automa­tion actu­ates a mechan­i­cal device that reg­u­lates the air flow in the com­bus­tion cham­ber of the heat­ing device.

Design features of the two-circuit model

Hav­ing a dou­ble load, heat­ing and hot water sup­ply, sol­id fuel units of this type are equipped with addi­tion­al devices. There are two types of such devices:

  • stor­age boil­er (heat accu­mu­la­tor);
  • addi­tion­al coil.

The dif­fer­ence in both devices lies in the prin­ci­ple of heat­ing water. In the first case, we are talk­ing about a stor­age tank that accu­mu­lates hot water and from there the hot water diverges to the con­nec­tion points. In the sec­ond case, a cast-iron hot water coil is installed inside the boil­er. Heat­ing of water occurs in the mode of con­stant cir­cu­la­tion of hot water. Both options are equal­ly suc­cess­ful and effi­cient, pro­vid­ing a sta­ble sup­ply of hot water to the res­i­dents of the house. Togeth­er with the boil­er, the heater will not take up much space, how­ev­er, the pres­ence of two cir­cuits requires cer­tain rules to be observed dur­ing the oper­a­tion of these devices.

The con­nec­tion dia­gram for a dou­ble-cir­cuit sol­id fuel boil­er looks some­thing like this. By their design, all heat­ing devices of this type are almost the same, with the excep­tion of some dif­fer­ences.

The only con­di­tion: when con­nect­ed to a heat stor­age sys­tem, it is nec­es­sary to have strict con­trol over the use of hot water. Installing more than two hot water con­nec­tion points in the house can adverse­ly affect the per­for­mance of the DHW sys­tem. The flow-through method of heat­ing hot water is more dynam­ic in terms of oper­a­tion, how­ev­er, at the same time, the boil­er pow­er should be much high­er.


Long-burn­ing dou­ble-cir­cuit boil­ers have a stan­dard design, which is almost the same for many mod­els, with the excep­tion of some dif­fer­ences.

The fig­ure shows a typ­i­cal design of a sol­id fuel unit oper­at­ing for a heat­ing sys­tem and hot water sup­ply.

Sol­id fuel boil­ers with two cir­cuits con­sist of:

  • main, heat-insu­lat­ed body;
  • com­bus­tion cham­ber (fur­nace);
  • pod­zol cham­ber (ash­pit);
  • the main heat exchang­er of the heat­ing sys­tem;
  • addi­tion­al heat exchang­er (coil) for hot water sup­ply;
  • automa­tion and con­trol devices;
  • damper and chim­ney.

In some cas­es, an addi­tion­al cir­cu­la­tion pump is installed, which pro­vides forced cir­cu­la­tion of the coolant in the main heat­ing cir­cuit. For bet­ter oper­a­tion of a long-burn­ing heater, fans are used that force air into the com­bus­tion cham­ber and con­tribute to a bet­ter out­flow of com­bus­tion prod­ucts through the chim­ney.

Some nuances in the operation of double-circuit solid fuel boilers


An obvi­ous plus that sol­id fuel boil­ers of this type have is long-term oper­a­tion with a sin­gle fuel load.

For exam­ple: 30–50 kg. fire­wood laid in the fire­box can burn up to 12 hours. Dur­ing this time, the unit is able to pro­vide the max­i­mum pos­si­ble heat to heat the liv­ing quar­ters in the house and pro­vide nor­mal domes­tic hot water needs. Reduc­ing the con­sump­tion of hot water will increase the dura­tion of fuel burn­ing up to 24 hours. The boil­er will oper­ate in nom­i­nal mode, main­tain­ing a cer­tain tem­per­a­ture in the house.

It should be remem­bered! A sol­id fuel unit extin­guished for a long peri­od dur­ing the win­ter can cause the entire heat­ing sys­tem to freeze. To avoid such trou­bles, antifreeze can be used for the main heat­ing cir­cuit.

It is also impor­tant that long-burn­ing dou­ble-cir­cuit boil­ers do not allow high accu­ra­cy to reg­u­late the tem­per­a­ture of the coolant in the heat­ing sys­tem. The rea­son for the dif­fi­cul­ty lies in the iner­tia of the oper­at­ing heat­ing equip­ment. The built-in boil­er will solve this prob­lem by cre­at­ing opti­mal con­di­tions for con­trol­ling the heat­ing unit.

In con­clu­sion, a few words should be said about where and how it is bet­ter to install this heat­ing equip­ment in the house. Long-burn­ing sol­id fuel boil­ers are pro­duced only in a floor ver­sion. Due to the bulk­i­ness of the design itself and the need to install a stor­age tank, the best option would be to equip a spe­cial boil­er room.


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