Energy Saving Applications in Flue Gas Installations

Muammer Akgun
BACADER, Association of Chimney Manufacturers and Builders


Energy saving has been an important issue from the decreasing of the world’s energy resources, the pollution of the environment and the global warming. Because of the fact that energy saving is really important and the fuel in our country is uneconomical, the energy loss in flue gas installations should be considered and prevented.

If the external ambient temperature is colder than the boiler room the flue gas installation draws air from the heating device. This situation occurs more frequently if the burner stops or is in stand-by mode.  It draws heat energy from boiler or its heat exchanger. It decreases the temperature of the water in the boiler or heat exchanger. Depends on external ambient temperature. The density difference and the chimney draught increases as the external ambient  temperature is colder.

These kinds of losses can be reduced by correct design to type of fuel and special equipment. There are different systems designed for different devices such as secondary air appliances, thermal and motor-driven flue gas dampers.

In this paper, the equipments for energy saving in flue gas installations, their areas of utilization, advantages and applications are explained.


In this  essay,  the methods which are appropriate for energy conservation in the chimney and waste gas systems, the advantages and the disadvantages of utilising these equipments will be evaluated.


Waste gas regulator is connected to the waste gas exit or vertical connection part to increase the flow resistance on the waste gas way. Waste gas regulator does not cover the radius of the flue 100 percent. At least 3 percent of the area or 20 cm2 should be space. The regulator should be visible with the help of the adjusting machine that is located near the regulator.

The waste gas regulator can be utilitized with the natural gas using devices that works with fan if they are proved to be safe(with a type test or an expertise report).

Dust catcher is connected to the flue horizontally or vertically and makes full impereability in the flue. Dust catcher can be utilized with the devices that works with stiff or liquid fuel.


Flue gas dampers are connected to the vertical connection part of the flue or to the exit of the boiler, to close the gas way. Thus it hinders the device to cool down as it is in stand-by mode.

Multiple devices that are connected blocks the access of cold air to the devices that are excluded and by that, enhances the ejaculation of the waste gas. Moreover it contributes to the decrease of the sound and the vibrations that can occur through the flow direction from the boiler room.

These valves can be implemented with;

  • Liquid/Gas fuel using devices with fans
  • Gas fuel using devices without fans
  • Open fireplaces which burned wood or gas

The limitation of the air flow to the waste gas walves should only be enough to dry the wet parts which are due to the condensation that occurs when the device is off.

The thermal resistance of the flues or horizontal was gas system should be enduring to moist and 0.65 m2K/W. Also, the air supply should be made with the help of secondary air appliances if the was gaste valve is off.

There are two types of flue dampers:

  • Thermal flue dampers
  • Motorised flue dampers


Thermal flue dampers are used in line with the devices that are for DIN 3388-4. These dampers can be erected according to the flow resistance.  They work with bimetal. The flue damper takes action with the heating of the waste gas or the cooling of the environment that is around the bimetal. The damper turns on as the burner steps in and the valve actively starts to work as the temperature reaches to 80-100 C.

These dampers can pose danger if the annual maintenance is not made or if there is fatigue of materials.

Fig. 1- Thermal Flue Damper



The motor driven flue dampers are erected to fuel exit of the gas/fuel oil using heating systems. The lid that is controlled with the motor closes down the flue gas port as the heating systems gets into the standby mode. Thus the energy that is produced in the system is stored and the unnecessary coolings are prevented by hindering the gas waste with chimney draught  and entrance of cold air. As this system lowers the fuel outgoing by energy conservation (which varies from system to system but is between 5-12%), it also lowers the occurance of faults in store equipment by protracting the heating system.

Motor driven flue dampers start working with additional energy. The dampers start working with the electricity in a short amount of time (5-7 seconds) before the starting of the burner. The mechanism is controlled with a switch. Motor driven flue dampers are connected to the flue or should be connected to the front or the back of the working device if the device does not use a fan. If it is connected to the front, it is dependent to the internal design, the installation and the utilization of the device.

The document that is for the safety of the valves is DIN 388-2. This standart categorizes the devices according to three different installations:

  • The minimum gap with the damper (the released diameter)
  • Non-leaked dampers (the endurance to the metal)
  • Secondary-air appliances are non-leaked dampers, with the essential setting system or thermal flue dampers.

100% non-leaked damper is only used with burner devices.

Fig.2- Motorised Flue Dampers


Secondary-air appliances are valves that provides air with the mechanism that they have inside. They are used for the increase in the condensation speed of the flue gas with the entrance of additional air. The independent operating secondary-air appliances are also known as draft regulator. Draft regulator also keeps the burning in an average value for the differences in air intake.

Moreover, there should be an air circulation for the entrance of additional air to the flue since there can be a pressure difference between the boiler room and flue.

According to DIN 4795 the flues that are negative pressure designed can be utilized only if they implement these rules;

  • The waste gas road should be open
  • There should be no danger incase of an occlusion or leaking out in the flue
  • It should not handicap the cleaning or the maintenance of the waste gas system
  • The flue should be designed in a way that is enduring to the corrosion and able to block any leaking out due to the condensation.

According to DIN 4795 the flues that are negative pressure designed can be utilized only if they implement these rules;

  • The waste gas road should be open
  • There should be no danger incase of an occlusion or leaking out in the flue
  • It should not handicap the cleaning or the maintenance of the waste gas system
  • The flue should be designed in a way that is enduring to the corrosion and able to block any leaking out due to the condensation.

Fig.3- Operating principle of secondary air appliance

Secondary air appliances can be utilized in boiler rooms or the rooms which has air flow with the room next to it. All devices should be in the same boiler room if there are multiple connections in the flue gas system.

Seconder air valves can be grouped into three:

  • Independent seconder air appliances
  • Seconder air appliances that mechanicly adjusted
  • Combinated seconder air appliances

The upwards pressure of the waste gas in draught limiter which is an regulations made specifies adjustment weight. The weights of openings and the closing cause an equilibrium. Because of this equilibrium, the hot waste gases of the waste gas installations gets mixed with the cold air automatically. Inflowing air through transpiration so the formation of dew point tend to be reduced. If the combustion unit stops additional to the upwards pressure above the adjustment valve of draught regulator, the ventilation of the system is made with the air in the room. However this drying effect decreases with upward pressure and gets cut out completely if the adjustment value of the draught regulator is not reached.

The air flap is opened in an appropriate amount with the help of the excitation current running motor when the adjustable working secondary air flap is deactivated.

Motorised damper is placed on the mounting or installation of the exhaust side connection part or applications. Motorised dampers need more pressure for larger flue gas installations. This requires additional air assembly.

Fig. 4- Self-employed secondary air appliance.

When the burner stops in combined systems, it makes the complete ventilation.  The ventilation duration can be limited to 10 minutes according to the choice engine control.  If the heater should be on first the motor control starts. Than the setting weight releases the adjustable disk. Thus the upwards pressure can be adjusted. However the ignition switch is release when motor control reaches final position.

Forced control and combined systems are not suitable for solid fuels burning devices.

The waste gas temperature between the device and installation because of secondary air appliances. Also condensation temperature decreases and the speed of waste gas increases. This dries the chimney.

If the draught regulator works in an inappropriate situation there too much waste gas lose and there may be some difficulties with the optimal adjustment of the burner with fan.The draught regulator provides optimum working conditions for burning by increasing pressure.

Figure 5. The secondary air appliance with adjustable mechanical flue gas damper.


The sizing of the secondary air appliance by the manual which is determined by the manufacturer and according to height,  diameter, structure of the chimney and capacity of the device.

The amount of additional air (m3/h)=A x H Qmax                             [kW]

A: Chimney cross-section                                                                         [m2]

H: Height                                                                                                     [m]

Qmax: Chimney thermal capacity                                                           [kW]

The draught regulator should have at least 40 cm from the floor when it is attached to the chimney. It should not endanger the fire safety of chimney. There should be at least 40 cm. between the secondary air appliance which are 2,5 meters above the ground and combustible materials.

Secondary air appliance should be positioned differently according to the requirements. Issues that must be considered according to DIN4795;

Fig. 6-. Types of draught regulator assembly
(Installation of additional air assembly facilities).

Position 1: Very good regulation, ventilation effect limited in long flue gas pipes or small flue gas pipe cross-section Q1 in relation to chimney cross-section Q2.

Position 2: +3: Very good ventilation effect, good regulation, only retrofitable at installation place 3 in brick-built chimneys.

Position 4: and ventilation limited. However, due to the low accumulation of soot this is a good place of installation for solid fuel boilers and lined chimneys.


in large installations draught limiter with pressure flap and hydraulic damper for the setting weight which are for chimneys that are higher than 20 meters must be used for operation of the installation (Fig.7).  The speed of waste gas occurs importance during the selection of secondary air appliance. When the ignition stops, the flow of waste gas stops and the sub pressure at the waste gas installation increases quickly.  As a result, draught limiter works with pulse. This may cause deformation on chimney.  The draught limiter should have minimum waste gas speed.

In the summer and in the transition period is upwards pressure is low and adjustment disc is close because of that. However, when the burner gets operated for long time it reaches the set value of the draught limiter, thus the adjustment disc opens. Released cross section grows with the steadily rising up the pressure.

Fig.7- Draught limiter for large installations (Hydraulic Shock Absorber).


The equipment for flue gas systems are used to prevent deformation at the chimney and recover the losses which may occur in the flue. The selection and the utilization of these systems for solution are important. When the payback period is considered, thermal valves pay off their prices in two heating seasons. Motorized dampers provide fuel savings of 5-12% per annum based on the applied system. Draught regulators vary according to the amount of annual fuel savings chimney pressure. Also the draught regulators are essential for the prevention of deformation which may occur in chimneys if the burners in the central heating system suddenly close.

[1] POSTRENRIEDER, E. and SCHLEE, G., Abgasanlagen, Gentner Verlag, Stuttgart. 1992.
[2] SCHAFER, W., Schornsteinfragen in der heizungstechnik, Kramer Verlag, Düseldorf. 1994.
[3] MILTON Keynes., Mechanical venting of flues and chimneys, Exhausto Ltd., 2000.
[4] Schiedel GmbH & Co., Schornsteintechnik, München. 2001.
[5] DIN-Taschenbuch 146., Schornsteine planung berechnung ausführung, Beuth. 1993.
[6] Kutzner Weber GmbH & Co., Abgastechnische produkte geratebau, Maisach. 2000.

This article presented at the XIIth International Hvac+R+Sanitary Technology Symposium