How to assemble plastic ventilation pipe systems

Solvent Weld - Manufacturers recommendations are that sizes over 250mm should be hot air welded, however many fitters still use solvent cement for larger sizes. The pipe should be cut square and a small chamfer of between 22o and 45o applied to the end of the pipe. This prevents the end of the pipe acting like a knife edge and pushing all the glue in front of it. The pipe and socket should next be cleaned using a solvent cleaner. The solvent cement should be applied to both the pipe and socket using an appropriately sized brush (1 1/2” is usually a good choice) and the joint assembled. Due to the lightweight and thin walled nature of ventilation pipe, it is recommended that supports and clips are in place and tightened as soon as a joint is made.

Hot Air Welding - Manufacturers recommendations are that sizes over 250mm should be hot air welded. This process uses a hot air gun and nozzle, to melt PVC weld rod into the joint. Some experience and skill are required to perform this, as it is easy to overheat and either burn the plastic or deform the pipe / fitting. Before welding, the pipe and the face of the fitting, where the weld rod will melt on to, should be scraped to remove the thin outer oxidized layer (FAILURE TO DO THIS WILL RESULT IN A VERY POOR, WEAK WELD). The pipe should then be pushed into the socket and all clamps / supports tightened. It is often best to use a tacking weld nozzle first, before applying the main weld.

Silicon / Mastic - Sealing a system with silicone / mastic can only be done if the pipe system is very well clamped and supported. It must also be ensured that the silicone / mastic used will not be degraded by the fumes being extracted.

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