Backdraft in the ventilation of a private house: why ventilation works in the opposite direction and how to eliminate it

To create comfortable conditions, a sufficient supply of fresh air. In case of incorrect operation of the ventilation system may be a reverse thrust in the air of a private house, which is bad for its microclimate.

In this case it is necessary to understand and eliminate the causes of change in the flow direction.

The content of the article:

  • The causes of back draft
    • Law of conservation of volume in ventilation
    • Unaccounted ventilation to the street
  • Typical problems in the organization of air
    • Reducing the volume of incoming air
    • Faults hoods and their elimination
  • The reverse in the duct system
  • Useful videos on the topic

The causes of back draft

Backdraft occurs when there are errors in the design of the ventilation system or insufficiently qualified for her performance. But since the air has a well-studied physical properties, it is easy to identify the causes of the origin of the problem and to find ways to fix it.

Law of conservation of volume in ventilation

In the simulation of air for small volumes that are typical of private homes can be ignored air quality parameters such as compression ratio and uneven density. In this case, the basic law of physics ventilation can be summarized as follows: the difference in volume of incoming and outgoing air for any period of time is zero.

Exhaust ventilation is installed in areas of maximum accumulation of contaminating aerosols. It's a bathroom, kitchen and offices. Some hoods provided with a fan for forced air exhausted increase.

Ventilation is installed in the living room for the fresh air to the places of their permanent stay of people. It normally forced type with predetermined volumes of the incoming stream. This is necessary to accurately maintain the climate parameters such as temperature, humidity and oxygen regime.

Law of conservation of volume in ventilation
Before designing an air circulation system or introducing a new ventilation equipment based on the need to such a scheme to perform the calculation and analysis capabilities, and hoods pritochek miss a volume air

Thus, most of the volume of air into the different parts of the house are fixed, and the distribution of the air coming out of the holes is variable. If at a certain time of the exhaust system passes a greater flow than provide forced ventilation, there is a compensation for the missing volume through openings intended for release air.

EXAMPLE occurrence of reverse thrust
Before installing the hood it is necessary to carry out simple arithmetic calculations, which show the air balance in various combinations of the devices

To prevent such a scenario it is necessary in the design of the ventilation system to perform the following set of actions:

  • at each exhaust opening to establish a check valve to prevent the flow of air therethrough;
  • to organize one or more backup air supply holes to compensate for the natural type of the missing volume.

The cross section of the holes in the back-up air inlets must be calculated in such a way that they totally could compensate for the maximum amount of missing air. Each of them should be provided with a check valve to prevent outflow of air. It is fraught with icing and blockage pritochek winter.

Unaccounted ventilation to the street

Its role in the air balance of the house makes filtering - the uncontrolled passage of air through cracks and leaks. If this phenomenon is intense, it is impossible to make accurate calculations of the parameters of the ventilation system.

Moreover, infiltration (filtering inward) allows air to penetrate into the room without treating it with dust filters and without heating to the desired temperature in winter. Therefore, it is necessary to completely eliminate the phenomenon or to make its impact on overall air circulation negligible.

Shooting imager to determine the air leakage
Filtration through air leakages easily detected in winter with a thermal imager. Shooting indoors will cool infiltration place, and the outside - a warm place exfiltration

Open windows, balconies and roof doors provide additional ventilation. Advance reliably calculate it is not possible, as the speed and even the flow vector is dependent on a plurality of factors, primarily the external and internal temperature, and also the direction and wind intensity. The only solution upon detection of occurrence of reverse thrust duct may be check valves.

In the cold drawing device further serves as a variable power chimney. Thereon with hot air of combustion products escapes, making a significant contribution to the total value of the exhaust flow volume.

The volume of air exiting through a chimney depends on the power of the boiler. For plant products it is easy to calculate, since the instructions have tables that specify the minimum cross-section of the chimney and the rate of release of the products of combustion for a particular model. If the furnace or boiler is made independently, the approximate value of the exhaust air volume need to be taken from the technical documentation for a similar device parameters.

The volume of air removed during heating boiler
The technical documentation for boilers there may be mentioned the need for the amount of air consumed. If no such information, it is necessary to multiply the speed of the flue gases at the expiration of the chimney cross-section

If viewed as a chimney exhaust channel, its characteristics is the inability to install a check valve on it. An alternative could be integrated induced draft fan, but only if its installation is technically possible. Then the heating system can be incorporated into the model as a hood with a fan.

Calculation of the air balance with the chimney must be approached with responsibility. Even a single reverse flow is able to completely ruin the interior of the premises. In addition, when a large flame there is a risk of fire and the possibility of poisoning people combustion products.

Typical problems in the organization of air

Reducing the flow rate calculated with respect to even one pritochke or extract air circulation leads to an imbalance in the building. This affects the operation of other devices in which there is a compensatory change in the flow parameters, including the reverse may occur. Therefore, all of the income and outflow of air space should be tested and then get to find out why the general building ventilation works by directing the flow in the opposite direction.

Occurrence of reverse thrust at the problem pritochke
This example demonstrates that even when setting of feedback depending thrust (in the bath) from any of the conditions (inclusion kitchen hood), the reason may be different. It is therefore necessary to check the entire ventilation

Reducing the volume of incoming air

Reduced air velocity in the plenum to the fan installation may be caused by the following reasons:

  • Loss of engine power, the fan vibration and other signs of wear and tear. It is necessary to carry out repairs and should eliminate the problem.
  • Failure to set up the system parameters. You need to reconfigure the system or repair the control unit.
  • filter pollution. The most common cause of reduced power ventilation. Necessary to clean or replace the liner.
  • fans pollution. As a result, adhesion of contaminants varies the geometry of the blades. Then, with the same number of turns decreases the pressure characteristic of the device.
  • Reduction live channel section. It can occur at any node of the system supply air through which the air: tube heat exchanger, a heater, etc. It is necessary to carry out their regular service cleaning.
  • Partial or complete closure of the external grid. The filter array may be packed with insects, leaves and other debris. In winter it is possible to sweeping snow. Therefore it is necessary to periodically check its capacity.

When natural ventilation frequent cause, resulting in insufficient inflow and sometimes occurrence reverse thrust is the effect of the discharged air from the outdoor side. This phenomenon may occur on the leeward side of the building. It can be eliminated by using a special enclosure near the air inlet.

Protection against wind supply openings method
To prevent the creation of low pressure zones near the supply hole you need to put a small fence. However, the winter will have to clean up the most snow

Faults hoods and their elimination

The reasons for the poor performance forced the type of extraction is the same as that of the air inlet. But more often the problem occurs not because of dirty filters (which are not always installed on the hood), and due to the narrowing of the effective cross-section of the ventilation duct.

In contrast to the fresh air stream carries the moisture removed, couples smaller fat droplets which adhere to the walls of the ventilation duct. Over time, the buildup formed on the inside, creating aerodynamic drag, resulting in a decrease in the volume of air discharged. When this extractor device itself can operate properly.

In private homes vent is designed so that you can effortlessly carry out a visual inspection. This should be done 2-3 times a year and when it detects a narrowing to clean their own or with the help of workers' organizations that provide such services.

There are rules concerning the position of pipes of the exhaust system on the roof. If you ignore them, depending on weather conditions can occur wind backwater, which leads to slower fan speed or even overturning thrust. Display, showing exactly this reason is a variable, not a constant problem with air circulation.

Regulation chimneys relatively roof elements regulated n. 6.6.12 SNiP 41-01-2003 "Heating, ventilation and air-conditioning." It is spelled out in the document on the basis of calculations of wind backwater, so it can be used for pipes and ventilation.

Terms of installation of flue and ventilation pipes
Terms pipe arrangement on the roof are simple enough, however can be output through the roof ventilation virtually anywhere. You should only have to comply with the desired height

The situation arises frequently when building a house with all the rules on placement of ventilation outlets were We met, but later there was a change of the landscape, which led to the formation of the wind zones overpressure:

  • modifying the geometry of the roof (including the installation of a satellite dish);
  • the construction of high buildings in the immediate vicinity;
  • I grew up near the tree.

In this case it is possible to transfer vent outlet to another place, or increase the height of the tube to try to remove the cause of the wind creates a zone of backwater.

You also need to check the quality of the work of the deflector. If it is damaged or as a result of poor attachment has changed its position with respect to the pipe, it may also be the cause of problems. Significantly increase traction can be replaced with an ordinary rotary deflector on.

The reverse in the duct system

In private homes was extended duct system connecting multiple exhaust vents with a ventilation shaft. Some points may be provided with forced air induction devices, such as cooker hood or channel fan.

Occurrence of reverse thrust in one of the branches of the duct may be caused by not only the above-described problems with total extract, but also the flux vector redistribution inside the ventilation system channels.

EXAMPLE occurrence of reversal in the ducting
Work powerful extracts creates a high flow and increases the pressure in the duct. Therefore, the air rushes to all open vents

To prevent such a scenario, the installation of check valves are used for each of the branches of the duct. These devices stream is passed in one direction and closed section channel with its reverse.

As a general rule, the installation of the duct professionals install check valves. It may not be the case for additional sidebar branches, if the installation was carried out later. In this case, it is necessary to install such a device. You can do it yourself without rebuilding the entire duct system. To do this:

  1. buy valve suitable size;
  2. disassemble the duct system in the place where it will be installed;
  3. cut off part of the rectangular channel or flexible tube size length of the valve;
  4. assemble the system with the valve;
  5. check for leaks.

If a check valve is installed, but the occurrence of reverse thrust occurs, it means that the device is faulty.

May be several reasons:

  • occurrence of cracks in the membrane, or envelope flap valve;
  • wear of moving parts, whereby there is no flap or closure membrane;
  • getting dense debris (e.g., a large piece of paper or beetle), which prevents full closing of the device.

In any case, you need to disassemble part of the air duct and remove the valve. When it can not repair must purchase a device of the same size and raise the duct again.

Useful videos on the topic

How to make a forced ventilation to prevent back-draft hoods:

Proper operation of the exhaust, and especially ventilation is a guarantee absence of reverse thrust. This requires periodic inspection and carry devices for their flow rate measurements.

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