When installing or replacing a heating device, it is necessary to evaluate the draught of flue and the temperature of effluent flue gasses. Draught must be strong enough to overcome the resistance created by the heating device and connecting elements. With decrease in temperature of flue gasses flowing from the heating device also draught in the flue decreases. It is important for necessary draught in the flue to remain the same when the temperature of flue gasses decreases. In such a case, thermal insulation of the flue must be sufficient, and the diameter and height of the flue must be optimum. Therefore, in brick and concrete flues to which liquid or solid fuel heating devices with a high coefficient of efficiency, higher than 85%, are attached it is necessary to provide stainless steel lining. For gas heating devices this kind of lining is mandatory.
Dew point temperature
Each type of fuel has different dew point temperature. It depends on the CO2 amount in combustion products. When using various types of fuel (solid, liquid, gaseous), the temperature of combustion products on the flue surface may not be lower than the dew point temperature. The dew point temperature is about 56°C. If the temperature of the flue surface is lower than the dew point temperature condensate will form on the flue surface that will penetrate into the flue walls and facilitates intensive damaging of internal surfaces of the flue. Such a flue can collapse completely in 1-3 years. Depending on the heating device and the fuel used the effluent temperature of flue gasses reaches 80°C to 250°C.
Modern heating devices are constructed to give maximum heat output to the space, with high coefficient of efficiency (above 85 %). Therefore the temperature of effluent flue gasses is lower. As a result, the internal surface of the flue is not heated sufficiently and formation of condensate on the flue walls is inevitable, especially in cold weather. Placing of lining of appropriate diameter in the flue may solve the problem, as stainless steel lining warms up quickly and steadily, but if the due point is not reached, condensate is lightly drained across the even surface.
However, there are situations where the height or diameter of the flue does not meet the necessary requirements. The solution is a double-insulated stainless steel flue that can be made to the necessary height according to the necessary diameter and dimensions. It is easy to assemble in any place – along an outer wall or in inside a space.
Recommendations for determining the flue height and diameter.
To choose the right diameter, the following data are necessary:
1. Heating device power (kW);
2. Flue height (m).
Recommended flue height in relation to roof
Starting the work
- One flue channel for one heating device.
- The height of flue and lining may not be lower than 4 m for gaseous and 5 m for solid fuel.
- The horizontal connection from boiler to flue must be as short as possible and with as few elbows as possible. The length of the connection may not exceed ¼ of the flue’s length. Nominal connection length is 50 cm.
- Heating device must be connected to flue using flue phases that are interconnected in such a way that each connection is on the previous one (in the direction of the flow of flue gasses, “in the direction of hair growth”).
- Vertical phases of flue or lining must be interconnected in such a way that the connection of each phase is inside the previous one (against the direction of the flow of flue gasses, “against the direction of hair growth”). See Fig. 1
Metal flues are designed and installed according to standards LVS EN 1856-1 “Chimneys - Requirements for metal chimneys – Part 1”, LVS EN 1856-2 “Chimneys – Performance requirements for metal chimneys”, the Latvian Building Code LBN 231-03 “Heating and ventilation of residential and public buildings”.
1. At the bottom of the flue a channel of length around 1 m shall be hammered out for easy putting in of inspection in the flue base and its connection to the Tee-phase of the connection and the other phases.
2. On the roof the lengths of the lining shall be interconnected using locking bands as many as necessary for the total length.
3. The connected lining shall be inserted vertically in the flue by lowering it slowly. (To facilitate the work, a rope or a cord may be used connected to the first phase of the lining using assembly bands.)
4. When the lining has reached the Tee-piece already installed at the bottom, the two should be assembled together using a locking band.
5. At the top of the flue, between the lining and the inner wall of the flue, distancers shall be inserted that in future exploitation will prevent the lining from diverting to sides. The length of the lining above the upper rim of the brick flue may not exceed 5 cm.
6. The hole hammered out previously at the bottom of the flue shall be bricked up leaving a space for the condensate discharge tube outwards and access to the connection with the Tee-piece.
7. The end of the brick flue shall be closed with a cover plate.
Insulated flue assembly
There are several types of assembly including along the outer wall or through the inner space. However, the following rules must be observed in each case:
1. The distance from wood and other highly combustible materials (beams, trusses) to the wall of the outer skin of the flue must not be less than 13 cm (Fig. 2). If the structure of the building does not ensure such a distance (the pitch between trusses is too small), then an interconnector (in trusses) (Fig. 3) must be made or this distance must be ensured using a 45° elbow (Fig. 4), as well as additional insulation must be provided using 50 mm thick rock wool with foil.
2. The distance from concrete (walls) and other non-combustible materials to the wall of the outer skin of the flue must not be less than 5 cm.
Assembly through wall
Connection through a wall must be made in accordance with the above rules. If the wall is flammable (wood, etc.), then additional insulation must be provided along the perimeter (Fig. 5). The hole in the wall shall be covered with a decorative cover plate (Fig. 6) made from stainless steel or a Minerit sheet.
Metal flues are designed and installed according to standards LVS prEN 1856-1 “Chimneys - Requirements for metal chimneys – Part 1”, LVS prEN 1856-2 “Chimneys – Performance requirements for metal chimneys”, the Latvian Building Code LBN 231-03 “Heating and ventilation of residential and public buildings”.
Flues are provided both for internal and external assembly
Flues with outer skin made from galvanised iron are less corrosion resistant that flues with outer skin made from stainless steel. We recommend using flues with outer skin made of galvanised iron for internal assembly. A flue must be placed on a stable base made from fire-resistant materials. (Assembly scheme with and without the base – Fig. 6). Flue elements shall be interconnected using locking bands for fixing the connections. Locking bands are necessary for additional fixing and sealing of flue connections.
Flues shall be located so as to ensure their easy cleaning, maintenance and inspection. In flues cleaning hatches shall be provided with diameter not less than that of the diameter of the flue. In short, straight flues not longer than 7 m cleaning hatches may be omitted.
All assembly materials used must be fire safe and corrosion resistant.
Flue connections may not be provided in walls and coverings.
During the assembly flue modules must be clean, dry and undamaged.
Flue components shall be made separately and connected to one another from bottom to top or connected on the ground and then made in one assembly; a piece to be assembled may not be longer that 4000 mm.
A flue shall be attached to a wall at each 2000 mm, horizontally at each1500 mm.
The horizontal phase of a flue may not exceed 2000 mm; the minimum necessary deviation of the flue is 20 mm per 1000 mm.
Access to the flue-cleaning place must be accessible and accommodated. If the flue is to be cleaned from above, it must be easily and securely accessible from the roof.
TV and radio antennas may not be attached to the flue.
Maximum length above the roof without fixtures is 2000 mm.
The bottom part of the flue or the Tee-piece with condensate discharge or damper must be freely accessible for maintenance, as well as it may not endanger the surrounding persons.
Flue inspection and cleaning
Flue inspection and cleaning must be carried out by chimney-sweepers.
Mechanical cleaning of flues must be carried out at least:
a) Once in two months for heating devices using solid fuel;
b) Three times per heating season for heating devices using liquid fuel;
c) Twice a year for heating devices using gas.
During inspection the state of the flue and its connections shall be inspected visually; in case any damages are detected the damaged parts must be replaced.
Using the flue cleaning hatch the cleanliness and draught of the channel shall be checked. If soot and clog is visible in the internal smoke duct of the flue or if there is no draught in the flue, the flue must be cleaned from above.
For cleaning non-metallic cleaning devices and sponges (plastic or nickel-chromium) provided for stainless steel flues must be used. The inspection or cleaning results shall be documented in a deed on two copies.