Air exhaust system

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In the case of the air-exhaust system or air-exhaust chimney (LAS) , the warm exhaust gases and the cool supply air of a heating system are routed through two flat, connected but separate lines. This system corresponds to the state of the art, since the residual heat can be extracted from the warm exhaust gases.

origin

Description of the combustion air

Originally, LAS systems only referred to fireplaces that worked independently of the ambient air, where the combustion air could not be extracted from the room air due to technical conditions. Today, however, the focus is on energy gain.

Executions

These can on the one hand (old building, modernization) lie within a shaft, chimney or old chimney , but on the other hand they can be completely rebuilt as a system chimney (new building). Three variants are technically possible:

  • The exhaust pipe is routed as a concentric one-pipe system in a chimney, with the supply air required for the combustion being sucked in through the remaining cross-section of the shaft against the flow of the exhaust gas.
  • The exhaust pipe is discharged as a concentric pipe-in-pipe system, with the exhaust gases being directed in the inner pipe and the supply air in the outer pipe.
  • The exhaust pipe is designed as a normal three-shell chimney and the combustion air is supplied through an adjacent air shaft.

advantages and disadvantages

In the first two cases, part of the heat of the exhaust gases discharged through the exhaust pipe is given off to the colder supply air that is passed through the external flow or the external pipe of the LAS system to the burner . Depending on the length of the pipe and the temperature difference, heat is not only achieved through direct heat transfer from the warm exhaust gases to the cold supply air, but the latent heat released by condensation of the exhaust gases also increases the efficiency of the system (see condensing technology ). If there is an adjacent air shaft, the preheating is lower and the supply air is therefore colder.

The intake air of the heater is through a LAS of z. B. warmed outside air temperature of -10 ° C to around 20 to 40 ° C (depending on pipe length and exhaust gas temperature). This preheating of the intake air results in energy savings. As the outside temperature drops, the efficiency of the system increases, as a greater temperature gradient (between the combustion air drawn in and the exhaust gas) results in greater heat transfer.

The disadvantage of fresh air supply in the chimney is that it cools the chimney. Then it is no longer a warming component in the building (with a noticeable lack of heat radiation) and the chimney acts as a thermal bridge . At the point where the concentric LAS chimney enters the warm area of ​​the building, mold can form when the air humidity is high . This problem is much greater with the adjacent air shaft, here the cold bridge is routed through the entire building due to the lower heat exchanger function, the risk of mold forming over the entire length. However, the formation of mold occurs only very rarely, as the high humidity necessary for mold formation usually only exists in the early stages, due to the moisture in the plaster and concrete of the new building.

See also

Guidelines and information sheets (selection)

  • Sample administrative regulation for technical building regulations (MVV TB).
  • Pattern Combustion (MFeuV).
  • Model guideline on fire protection requirements for ventilation systems (model ventilation system guideline - M-LüAR).
  • DVGW worksheet G 600 - Technical rules for gas installations (TRGI).
  • DVGW Merkblatt G 635 - Gas devices for connection to an air / exhaust system for overpressure operation (standardized procedure).
  • DVGW Merkblatt G 636 - Gas devices for connection to an air-exhaust system for negative pressure operation (standardized procedure).
  • DVGW worksheet G 637 - Connection of gas fireplaces with mechanical flue gas routing without flow protection to house chimneys.
  • Institute for Structural Engineering: DVGW Inventory LAS, approval Z-7.1.230 from July 28, 1993 (not extended)
  • DIN V 18160-1 exhaust systems - Part 1: Planning and execution.
  • DIN EN 13384-2 Exhaust systems - Thermal and fluidic calculation methods.

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