In general diameters thru 18” can be readily joined via the solvent cement method. Beyond 18”, the actual cementing and drying times become so protracted that sections of large diameter duct cannot be maneuvered into position quickly enough to effect a good solvent fusion before the cement hardens and dries.
Systems using Duct above 18” should be joined via the hot-air welding method, using PVC welding rod and hot air welders. Click here to see the welders that we have available.
Solvent Cement welding is by far the most widely used process for joining PVC pipe and Duct. Properly assembled, it is the easiest way to make quality sealed joints.
Solvent Cementing PVC Duct
- Solvent cementing should not be attempted at temperatures below 40° F or much above 90° F. Joints should not be made in hot, direct sunlight.
- Remove all burrs and chips from any duct that has been cut. With a clean, dry, cotton rag wipe away any surface contamination on the surfaces that are to be joined. If the surfaces are wet i.e. condensation, DO NOT ATTEMPT TO JOIN THEM – they will fail.
- Using and applicator, approximately half the side of the duct diameter, apply primer. The function of primer, in making quality joints, is to penetrate and soften the hard surfaces of extruded PVC duct. This must be done on both surfaces to be joined. A simple rule of thumb, in checking the adequacy of surface priming, is the ability to scrape a few thousandths of the softened PVC surface.
- DO NOT ALLOW PRIMER TO DRY before applying the solvent. Once the two surfaces to be joined have been fitted together, give the joint a quarter twist to thoroughly mix the two solvent reduced surfaces.
Before starting an installation consult with the supplier of the primer and solvent cement for advice on the best product to use for your application and work environment.
Hot Air Welding
Welds on Duct pipe are usually made using 3 passes of 5/32” PVC welding rod.
Before welding, each joint should be prepared by cleaning the duct and fitting surfaces of any dirt, oil, or other contaminants. This will ensure good fusion conditions.
Once the duct and belled-end fitting or coupling are securely seated, the fit should be secured by “tacking” the joint with a hot-air welding gun. The process of “tacking” creates a PVC fusion between the two components, holding them in a position for the actual welding. Another benefit of “tacking” is that it seals the gap between the two components, so that during the actual hot-air welding, the PVC welding rod and the two surfaces being joined are adequately heated to the point of a strong weld. If the joint were not first tacked then the hot air from the welding gun would pass thru the gap resulting in the surfaces to be welded receiving heat inadequately for fusion with the welding rod. This would result in “cold joints” that are brittle and subject to failure under stress.
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