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

To create a comfortable environment requires an adequate flow of fresh air. In case of improper functioning of the air exchange system, a back draft can occur in the ventilation of a private house, which is bad for its microclimate.

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

Causes of reverse thrust

Reverse traction occurs when there are gaps in the design of the ventilation system or its insufficiently qualified performance. But since the air has well-studied physical properties, it is easy to identify the causes of the origin of the problem and find ways to eliminate it.

The law of conservation of volume during ventilation

When simulating air exchange for small volumes, which are typical for private houses, it is possible to neglect such parameters of the air environment as compression ratio and uneven density. In this case, the basic physical law of supply and exhaust ventilation can be formulated as follows: the difference between the volume of incoming and outgoing air for any period of time is zero.

Exhaust ventilation is installed in places of the largest accumulation of polluting aerosols. This is a bathroom, kitchen and office space. Some hoods provide fans to forcibly increase the volume of exhaust air.

Supply ventilation is installed in the living rooms for fresh air to the places of permanent residence of people. As a rule, it is of forced type with predetermined volumes of incoming flow. It is necessary for accurate maintenance of microclimate parameters, such as temperature, humidity and oxygen regime.

The law of conservation of volume during ventilation
Before designing an air circulation system or introducing a new ventilation equipment, it is necessary, based on a similar scheme, to perform a calculation and analyze the possibilities of the air inlets and extracts to skip the obtained volumes

Thus, most often the volumes of air intake in different places of the house are fixed, and the distribution of exhaust air through the holes is variable. If at a certain point in time a part of the exhaust system passes a stream larger than the supply ventilation provides, then the missing volume is compensated through the holes intended for air release.

Example of reverse thrust
Before installing the hoods, it is necessary to perform simple arithmetic calculations that will show the air balance with various combinations of device operation.

To prevent such a scenario, when designing a ventilation system, it is necessary to perform the following set of actions:

  • install a check valve on each exhaust port to prevent air flow through it;
  • arrange one or more natural-type backup inlets to compensate for the missing volume.

The cross-section of the holes in the backup supply devices must be calculated so that they can totally compensate for the maximum amount of missing air. Each of them must be equipped with a check valve to prevent the outflow of air.It is fraught with icing and blockage of the inflows in winter.

Unaccounted air exchange with the street

Filtering contributes to the air balance of the house - the uncontrolled passage of air through cracks and leaks. If this phenomenon is intense, then it is impossible to make accurate calculations of the parameters of the ventilation system.

In addition, infiltration (filtration directed inward) allows air to enter the room without being treated with dust filters and without heating to the desired temperature in winter. Therefore, this phenomenon must be completely eliminated or make its impact on the overall air circulation insignificant.

Imaging with a thermal imager to detect air leakage
Air filtration through leaks is easy to detect in the winter with a thermal imager. Indoor filming will show cold areas of infiltration, and outside - warm exfiltration areas.

Open windows, balconies and attic doors create additional air exchange. It cannot be reliably calculated in advance, since the speed and even the flow vector depend on many factors, primarily external and internal temperature, as well as the direction and intensity of the wind. The only solution for detecting the occurrence of reverse thrust in ducts is to install check valves.

In cold weather, an additional exhaust device of variable power is the chimney. According to it, hot air with combustion products comes out, making a significant contribution to the total value of the exhaust flow volume.

The volume of air discharged through the chimney depends on the power of the boiler. For factory products, it is easy to calculate, since the instructions contain tables that indicate the minimum cross section of the chimney and the rate of emission of combustion products for a particular model. If the stove or boiler is made by itself, then the approximate value of the volume of exhaust air should be taken from the technical documentation similar to the device parameters.

The amount of air removed by heating the boiler
The technical documentation for the boilers may indicate the need for the volume of air consumed. If there is no such information, then it is necessary to multiply the flue gas flow rate by the cross section of the chimney.

If we consider the chimney as an exhaust duct, then its specificity is the impossibility of installing a check valve on it. An alternative could be an integrated exhaust fan, but only if its installation is technically possible.Then the heating can be included in the model as a hood with a fan.

The calculation of the air balance in the presence of the chimney must be approached responsibly. Even a single reverse flow can completely ruin the interior of a room. In addition, with a large flame there is a risk of fire, as well as the possibility of people being poisoned by combustion products.

Typical problems in the organization of air exchange

Reducing the flow rate relative to the calculated even in one inflow or exhaust leads to an imbalance of air circulation in the house. This affects the operation of other devices in which a compensatory change in flow parameters occurs, including a reverse. Therefore, it is necessary to examine all places of entry and outflow of air, and then it will turn out to find out why general house ventilation works by directing the flow in the opposite direction.

The emergence of reverse thrust with problematic inflow
This example clearly shows that even with the establishment of dependence of the occurrence of reverse thrust (in the bathroom) on any condition (the inclusion of a kitchen hood), the reason may be different. Therefore it is necessary to check all ventilation.

Reduced air intake

The decrease in air velocity in the air handling unit with a fan can be caused by the following reasons:

  • Loss of engine power, fan vibration and other signs of equipment wear. It is necessary to repair and eliminate the identified faults.
  • Failed to configure the system settings. It is necessary to reconfigure the system or repair the control unit.
  • Filter fouling. The most common cause of reduced power intake ventilation. Cleaning or replacing the liner is required.
  • Contaminated fans. As a result of sticking of pollutants, the geometry of the blades changes. Then, with the same number of turns, the pressure characteristic of the device decreases.
  • Reducing the living section of the channel. It can occur in any node of the supply system through which the air passes: pipe, recuperator, heater, etc. It is necessary to carry out their regular service cleaning.
  • Partial or total closure of the external grille. The filter screen can be clogged with insects, foliage and other debris. Winter is possible sweeping it with snow. Therefore, it is necessary to periodically check its throughput.

With natural ventilation, a common cause, leading to insufficient inflow, and sometimes the emergence of reverse thrust, is the effect of discharged air from the street side. Such a phenomenon may occur on the leeward side of the building. It can be eliminated using special fences near the air intake.

The way to protect the inlet from the wind
To prevent the creation of a discharge zone near the inlet, a small fence should be installed. However, in winter it is necessary to remove snow more often.

Faults hood and their removal

The reasons for the poor performance of the forced draft hood are the same as those of the inlet device. But most often the problem arises not because of contamination of the filters (which are not always installed on the hood), but due to the narrowing of the living section of the ventilation duct.

Unlike fresh air, the removed stream carries moisture, vapors, and small drops of fat that adhere to the walls of the ventilation duct. Over time, growths are formed on the inside, creating aerodynamic drag, which leads to a decrease in the volume of exhaust air. In this case, the exhaust device itself can work properly.

In private homes, the ventilation duct is designed so that it can be visually inspected with little effort. This should be done 2-3 times a year and, when detecting the resulting contractions, clean them alone or with the help of employees of organizations providing such services.

There are rules for the location of pipes exhaust system on the roof. If they are ignored, depending on the weather conditions, wind pressure may occur, which leads to a slower ventilation rate or even thrusting. Indicators that indicate precisely this cause are variable, rather than permanent problems with air circulation.

The position of the chimneys relative to the roof elements is regulated by clause 6.6.12 of SNiP 41-01-2003 “Heating, ventilation and air conditioning”. It is written in this document on the basis of calculations of the wind backwater, so it can also be used for ventilation pipes.

Rules for the installation of flue and ventilation pipes
The rules for the location of pipes on the roof are quite simple, so you can remove the ventilation through the roof at almost any point. You will only need to comply with the desired height.

Often there is a situationwhen during the construction of the house all the norms for the placement of ventilation outlets were observed, but later there was a change in the landscape, which led to the formation of zones of wind backwater:

  • modification of the roof geometry (including the installation of a satellite dish);
  • construction in the immediate vicinity of a tall building;
  • near tree

In this case, you can move the exit of the ventilation duct to another place, increase the height of the pipe, or try to remove the cause creating the zone of the wind backwater.

You also need to check the quality of the deflector. If it is mechanically damaged or as a result of poor attachment it has changed its position relative to the pipe, then this can also cause problems. Considerably to increase draft it is possible having replaced the ordinary deflector with the rotational.

Reverse in the duct system

In private homes, the air duct system became widespread, connecting several exhaust openings with one ventilation shaft. Some outlets may be equipped with forced air intake devices, such as a kitchen hood or duct fan.

The emergence of reverse thrust in one of the branches of the duct can be caused not only by the problems described above with a common exhaust, but also by redistributing the flow vector within the system of ventilation ducts.

An example of the occurrence of the reverse in the duct system
The work of powerful hoods creates a high-speed flow and increases the pressure in the duct. Therefore, air rushes to all open air vents.

To prevent such a scenario, the installation of check valves for each of the duct branches is used. These devices pass the flow in one direction and close the channel section when it is reversed.

As a rule, when installing the duct experts install check valves. It may not be in the case of additional tie-in branches, if the installation was made later. In this case, you need to install such a device. You can do it yourself without rebuilding the entire duct system. For this you need:

  1. purchase a valve of suitable size;
  2. disassemble the duct system in the place where it will be installed;
  3. cut off a portion of a rectangular bore or flexible pipe the size of a valve length;
  4. assemble a system with a valve;
  5. check its tightness.

If a non-return valve is installed, but a reverse thrust occurs, this means that the device is faulty.

There can be several reasons:

  • the occurrence of a gap in the membrane, sash or valve shell;
  • wear of moving parts, as a result of which the sash or membrane does not close;
  • the entry of heavy debris (for example, a piece of paper or a large beetle) that prevents the device from completely closing.

In any case, it is necessary to disassemble part of the duct and remove the valve. If it is impossible to repair it, you should purchase a device of the same size and reassemble the air duct.

Conclusions and useful video on the topic

How to make forced ventilation to prevent reverse thrust in the hoods:

Proper operation of exhaust and, especially, ventilation is the key to the absence of reverse thrust. For this, it is necessary to periodically inspect the devices and measure the flow velocity for them.

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