How to make a coaxial chimney for a gas boiler

Recommendation points

• The principle of the coaxial chimney
• equipment requirements
• Making a pipe pass
• External and internal installation
• Differences in the installation of vertical chimneys
• Fire safety standards and installation requirements

Chimney systems of coaxial type have a number of advantages, but at the same time they are distinguished by increased complexity of the device and certain requirements for the materials and equipment used. Therefore, the issue of installing this type of communications should be considered in detail..

The principle of the coaxial chimney

The coaxial pipe includes two channels located one inside the other. If we are talking about a chimney, then the combustion products are removed through the inner channel, and the supply air necessary for the operation of the boiler is supplied through the outer channel. Of course, the nominal passage of both channels is carefully calculated so that the volume of the inflow is excessive..

Thanks to this scheme, an increase in the efficiency of the heating system is achieved. Firstly, room air is not consumed for combustion, for heating which energy was expended. At the same time, there is practically no decrease in productivity due to the low temperature of the incoming gases: the hot inner channel heats them effectively.

The device and principle of operation of the coaxial chimney

The safety of coaxial chimneys is higher than that of classic ones. On the one hand, due to the constant cooling of the outer shell with street air, heating of the building structure through which the pipe is led to the smoldering temperature is excluded. Also, due to the lack of connection between the firebox and the room atmosphere, it becomes impossible for carbon monoxide to enter the living quarters. Therefore, the device for additional ventilation in the room where the gas heating equipment is installed is not required.

equipment requirements

In a coaxial chimney, the end of the exhaust and supply channels are at approximately the same height, which does not contribute to the formation of natural aerodynamic thrust. In parapet boilers, the correct operation of the chimney system is ensured by the pressure of the expanding gases and the special design of the deflector. However, in this case, the first condition for installing a coaxial chimney must be observed – an insulated type firebox.

The second condition – a device for forced movement of the gas flow – is required when installing the supply and exhaust system of boilers in which the outlet of combustion products is located on top, and not on the rear wall. In such cases, the length of the chimney is much longer, moreover, the channel has turns, and therefore the built-in fan is the only way to overcome the increased aerodynamic resistance..

Coaxial flues can only be installed on gas or oil boilers with a capacity not exceeding 30 kW. The main requirement is the coincidence of the diameter and configuration of the internal and external channels on the boiler outlet and the chimney itself. A socket system must also be provided to ensure a tight connection of both pipes in accordance with the directions of flow..

Making a pipe pass

The chimneys of parapet boilers have a large channel diameter, as a rule, from 150-200 mm, which is necessary to exclude the slightest signs of aerodynamic drag. Therefore, a hole in the wall is usually made by contour drilling and fragmentary removal of the central part. For these purposes, it is very convenient to first glue a paper template copied from the back of the boiler to the wall in order to correctly align the passage with the fastening points. The circle on which the drilling points are located must be determined so that the entire chimney section is inside the hole.

Pipes of coaxial flues, designed for forced draft, have a smaller diameter, usually up to 130 mm. Therefore, it is more reasonable to make a passage in the wall using a crown with victorious solders, or by the method of shockless diamond drilling. The contour method is not acceptable here due to the difficulties in coring.

Also, when drilling a channel, it is necessary to take into account the slope of the chimney towards the street of the order of 3-5 °. This is necessary to prevent condensation from entering the combustion chamber. If the passage is made by diamond drilling, it is only necessary to set the required angle of inclination of the conductor shaft. If drilling was carried out along the contour, the passage hole is simply expanded vertically upward by 3-4 cm from the inside and down the same value from the outside. Do not oversize the passage: this will cause difficulties with sealing, and there may simply not be a wall body left for attaching the diffuser.

External and internal installation

For the device of a coaxial chimney, special pipes, fittings and transition couplings are used. It is impossible to underestimate the nominal bore of the channels, therefore, throughout the entire length, the nominal diameter of the pipe must be no less than the boiler chimney. If the system is mounted in conditions of constant subzero temperatures, it is better to give preference to metal products, it will be easier to warm them from ice without the risk of damage.

A prerequisite for the construction of chimneys is the absence of joints between the elements located in the section of the wall. Therefore, initially the enclosing structure is cased, and then a middle segment of the corresponding length is installed in the hole, which will protrude from the outside at least 50–70 mm, and from the inside – according to the indent of the boiler flue pipe from the wall. The middle part of the coaxial channel is immediately sealed with mineral material along the contour, observing the required slope. After sealing the channel in the sleeve, it is necessary to seal the gaps on both sides, for which they are filled with sealant or cement mortar, and then decorative overlays are installed.

The inner part of the chimney, as a rule, is represented only by a swivel elbow installed directly on the protrusion of the middle segment, as well as a compensating insert, the length of which is selected taking into account the height between the installed boiler and the rotary elbow.

A deflector is installed from the outside. It is designed in such a way that the middle channel protrudes slightly above the edge of the outer channel to avoid short-circuiting the flow. Also, the design of the tip can provide for a system of fairings, and in the case of turbocharged versions – installation of a ducted axial fan.

Coaxial chimney systems are usually sold in sets of pipes and fittings of the appropriate cross-section, so no difficulties in assembling the channel on standard joints are expected. The manufacturer’s instructions may provide for a certain order of making connections, prescribing the use of sealants or crimping clamps, but in general, the assembly scheme itself is quite simple and straightforward.

Installation is complicated only in cases where the point of gas outlet from the boiler and the passage of the duct through the wall do not coincide along one or two axes, which requires the addition of one or two swivel elbows and inserts of appropriate length. However, such a difference in location appears only due to the need to observe certain rules for removing the coaxial chimney from combustible surfaces and bases..

Differences in the installation of vertical chimneys

Vertical coaxial chimneys are less common, because one of the advantages of horizontal systems is the elimination of the need to cut the roof and ceilings with a technical channel. However, in colder climates where boilers are severely affected by condensation, vertical placement is preferred..

A characteristic feature of such a system is the presence of a sump for collecting condensed moisture. This is a special adapter that has a threaded fitting for connecting a drain hose to the outside or to the sewer system..

The passage through the ceilings is carried out in the same way as through the walls: making a hole, rounding, laying a single segment and connecting the entire stave together. However, the passage through the roof must be carried out using special terminals, which have a built-in deflector and a roof cutting flange, and also provide the required difference in the height of the supply and exhaust ducts..

Fire safety standards and installation requirements

Coaxial chimney installation standards are not even formally regulated by current standards. In particular, none of the standards applicable to the field of civil engineering allows horizontal chimneys to be installed without a booster section, which is why cases of refusal to approve a gasification project are common..

However, a coaxial chimney is an integral structural element of the boiler, therefore, the fact of state certification of the equipment is sufficient, as well as compliance with the general requirements set forth in Appendix D to SP 42-101-2003 and the design standards for heating and ventilation systems (SNIP 2.04.05-91). If there is no desire to study the regulatory documentation, it is enough to use the installation recommendations set forth in the boiler user manual. But only on condition that the used equipment model is approved by the technical supervision service for domestic use..

In general, the installation rules are not very complicated. The distance from the outer edge of the inner channel of the coaxial pipe to the combustible bases must be at least 25 cm, the distance between the chimney and the combustible horizontal surfaces must not be less than 20 cm.The edge of the protruding inner channel must be 60 cm or more away from any objects opposite. The emission of cooled gases must be carried out at a level from the ground not lower than 2 m. The exit point should not have windows located closer than 40 cm in the horizontal and 1 m in the vertical directions. If a gas pipeline is laid in parallel, a gap must remain between it and the chimney of at least half the diameter of the coaxial pipe in the widest part.