The direc­tion­al move­ment of com­bus­tion prod­ucts along the flue ducts is a fun­da­men­tal fac­tor for the effi­cient and safe oper­a­tion of almost all heat­ing devices. There is a move­ment of air mass­es in the chim­ney with a dif­fer­ence in pres­sure, tem­per­a­ture and den­si­ty between the air in the chim­ney and the atmos­phere. The warm air in the chim­ney has a low­er den­si­ty and tends to rise, and cold­er air mass­es take its place.

But there is such a thing as The man­i­fes­ta­tion of this effect is imme­di­ate­ly vis­i­ble to the naked eye — the smoke from the com­bus­tion of fuel does not come out as it should be in the chim­ney, but moves into the room, which cre­ates the dan­ger of poi­son­ing a per­son with com­bus­tion prod­ucts and car­bon monox­ide. Why there is a reverse draft in the chim­ney and how to deal with this phe­nom­e­non, we will con­sid­er in this pub­li­ca­tion.


Reasons for this phenomenon

Reverse thrust, as it became clear from the above, there is a con­se­quence, a . And above all, the main rea­son for poor draft in the chim­ney, and, accord­ing­ly, the appear­ance of this effect, is the incor­rect design of the heat­ing sys­tem and non-com­pli­ance with the rules for the use of build­ing mate­ri­als for arrang­ing the smoke exhaust sys­tem. Among oth­er rea­sons, the most com­mon are:

  • Banal clog­ging of the chim­ney.
  • Incor­rect cal­cu­la­tion of the cross sec­tion of the chim­ney.
  • Fail­ure to com­ply with the height of the smoke exhaust sys­tem.
  • The pres­ence of hor­i­zon­tal and nar­row sec­tions in the smoke ducts.
  • The loca­tion of the chim­ney in the zone of “wind back­wa­ter”.
  • Improp­er­ly orga­nized and insuf­fi­cient sup­ply air.

But it often hap­pens that the phe­nom­e­non of reverse thrust occurs sud­den­ly, on pre­vi­ous­ly nor­mal­ly work­ing equip­ment. This may be due to adverse weath­er con­di­tions such as strong winds, fog, or high out­door tem­per­a­tures dur­ing the sum­mer. Each of these rea­sons is able to reverse the direc­tion of move­ment of com­bus­tion prod­ucts, there­by stop­ping the nor­mal process of smoke removal from heat­ing devices.

How to determine insufficient draft in the chimney system

If the reverse draft is vis­i­ble to the naked eye, then poor draft in the chim­ney should be deter­mined, and this must be done before each start of the heat­ing appli­ances. To do this, sev­er­al basic meth­ods are used.

  1. Mea­sur­ing the speed of air flow with a spe­cial device. Such a device is called an anemome­ter. Nor­mal, thrust is con­sid­ered when the device reads from 10 to 20 Pa. An excel­lent means of mea­sur­ing the move­ment of air flows, only the device is very expen­sive. In addi­tion, an anemome­ter can show the move­ment of air, but it is almost impos­si­ble to deter­mine the direc­tion by it, and even at a speed of less than 1 m / s.Portable anemometer for measuring the speed of air masses
  2. Banal obser­va­tion of the appear­ance of smoke in the room. If there is smoke, then there is no trac­tion or it is insuf­fi­cient. This method can be used in sol­id fuel heaters. In heat­ing sys­tems equipped with , it may not work. Although mod­ern sys­tems have many degrees of pro­tec­tion. Smoke sen­sors are one of the com­po­nents of the secu­ri­ty sys­tem on heat­ing boil­ers.
  3. If a stove or fire­place is used to heat the room, then the pres­ence of air move­ment and its direc­tion can be deter­mined using smoke from a smol­der­ing cig­a­rette or can­dle flame. If the smoke brought to the fire­box enters the chim­ney, then there is move­ment in the right direc­tion. Also with a can­dle flame, if the flame devi­ates towards the smoke out­let, then the sys­tem works, but how strong it can be deter­mined only with spe­cial devices.

If the draft in the chim­ney sud­den­ly dis­ap­pears, then, first of all, the cause of what is hap­pen­ing should be deter­mined. A visu­al inspec­tion of the flue duct is the first thing to do. Often, you just need to clean the chim­ney, and the effect of reverse draft in it will dis­ap­pear.

Back draft and ventilation

Many users of stoves and fire­places, espe­cial­ly after repairs in the house, say that: “There was a reverse draft in the chim­ney, what to do about it.” Every­one is imme­di­ate­ly inter­est­ed in solv­ing the prob­lem, but few peo­ple thought why it was after the repair in the room that this phe­nom­e­non appeared.

Very often, the effect reverse thrust appears pre­cise­ly in hous­es with installed plas­tic win­dows and high-qual­i­ty doors. Most often, when replac­ing win­dows and doors, the own­ers of the premis­es do not think that after such alter­ations

Odd­ly enough, the appear­ance of this effect is direct­ly relat­ed to ven­ti­la­tion, or rather, to its absence. Warm air has a low­er den­si­ty and there­fore tends to rise. Warmed up in the heater, air mass­es rise through the chim­ney, and cold­er air rush­es to its place. But in the process of work, it may hap­pen that a low-pres­sure area will grad­u­al­ly appear in the sealed room and the vac­u­um will become so strong that it will begin to draw fresh air from the chim­ney, “over­turn­ing the draft”, and turn­ing the move­ment of smoke in the oppo­site direc­tion. The bet­ter the draft in the chim­ney, the longer the effect of the reverse move­ment of com­bus­tion prod­ucts will not be, but with­out a suf­fi­cient amount of sup­ply air, this process is inevitable.

Supply valves.  Left - wall, right - for plastic windowsIn order to solve this prob­lem, it is nec­es­sary to orga­nize the flow of air mass­es from the street. This can be solved in sev­er­al ways:

Each of these mea­sures will con­tribute to the pen­e­tra­tion of out­door air mass­es into the house, pre­vent­ing a decrease in pres­sure in the room and the for­ma­tion of a reverse draft effect. Which method to choose, it is best to con­sult a spe­cial­ist.

Ways to improve the removal of combustion products

Keep in mind that even with the best ven­ti­la­tion sys­tem, smoke extrac­tion will be inef­fec­tive if the flue ducts are clogged or improp­er­ly designed. With the wrong project, there is only one way to deal with it, dis­as­sem­bling the chim­ney and com­plete­ly rework­ing it.

If there is a defect in the flue ducts that reduces the removal of com­bus­tion prod­ucts, sev­er­al devices can be used to improve this process.

  • . The prin­ci­ple of its oper­a­tion is to cre­ate a low pres­sure area above the chim­ney, due to the flow of air mass­es around the device. There are many types of such devices. They should be select­ed based on the diam­e­ter of the chim­ney and wind load. It should be under­stood that the deflec­tor is effec­tive only in the pres­ence of wind. In calm weath­er, this device is prac­ti­cal­ly use­less.A deflector and a rotary turbine will help increase the draft in the chimney
  • Insuf­fi­cient height of the flue pipe. It is pos­si­ble to sig­nif­i­cant­ly increase the exhaust of com­bus­tion prod­ucts in the smoke exhaust chan­nel by increas­ing it in such a way that its height from the grate to the head is 5–7 m.
  • Hav­ing mount­ed a rotary tur­bine on the chim­ney. This device great­ly enhances the extrac­tion of smoke, cre­at­ing a vac­u­um by spin­ning the tur­bine, which uses wind ener­gy for this.

Back­draft is a very dan­ger­ous phe­nom­e­non, and not only by the appear­ance of car­bon monox­ide in the liv­ing room, but also by spark­ing direct­ed into the room. At the first man­i­fes­ta­tions of this effect, you should stop using the stove or fire­place, try to deter­mine and elim­i­nate the cause your­self, and if this does not give a pos­i­tive result, imme­di­ate­ly con­tact the spe­cial­ists.

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