Oxy-fuel Welding of Metal Pipework

A gas flame suits thin-walled steel pipe of smaller diameters, the kind found in heating systems in many older houses and blocks of flats. The welder can control the heat finely, so the thin pipe wall is less likely to burn through. The torch also needs no power supply, and it reaches tight spots behind a radiator or under a boiler room ceiling.

In some cases a different method is the better choice:

• Thick-walled and large pipework in boiler rooms is usually arc welded, which is faster on heavy material
• Copper pipe, very common in heating systems in many countries, is almost always brazed or soldered rather than welded. A flame is used, but the copper itself does not melt
• Galvanised pipe, typical of old water supply runs, should not be welded. The zinc burns off and gives off harmful fumes, and the joint loses its corrosion protection. These pipes are joined with threaded fittings or adaptors
• Gas pipework is not ordinary heating work. Only a qualified, registered gas engineer may work on it, followed by testing and a tightness check

In older buildings welding has one more advantage. The original pipes often have sizes that modern press fittings do not match, or the walls are so thin that they would not survive pressing. With a weld, the new section connects straight onto a sound part of the old pipe.

The welding itself takes less time than you might expect. Most of the time goes into preparation and checking.

1. Switch off the boiler, let the system cool and drain at least the section being worked on.
2. Make the area safe: remove anything flammable, cover the floor and walls with a fire-resistant mat or metal sheet and have an extinguisher ready.
3. Cut out the damaged section and clean the pipe ends back to bright metal, removing rust, paint and grease. Bevel the edges if needed.
4. Fit the new piece accurately and fix it in place so the joint stays in line and does not shift during welding.
5. Set the flame to suit the wall thickness and heat the joint evenly all the way round.
6. Weld the joint with a steady movement around the pipe, adding filler wire as needed.
7. Let the weld cool naturally. Do not quench it with water.
8. Inspect the weld, refill and bleed the system and carry out a pressure test.
9. Keep an eye on the work area for a while afterwards in case dust or insulation is smouldering.

If the pipe is going to be chased into a wall or floor, do this only after a successful pressure test. A leaking weld hidden under plaster is much harder to find than one you can still see.

The effort, and therefore the cost, depends mostly on how easily the welder can reach the joint and what condition the pipe is in. A pipe running freely along a wall or through a boiler room is simple, because the torch can go right round it. These situations make the job harder:

• The joint sits tight against a wall, in a corner or under the ceiling, so the welder has to work in an awkward position
• The pipe is badly corroded and the welder has to find a spot where the wall is thick enough to weld
• Timber panelling, ceilings, cables or plastic pipes nearby need protecting
• In a block of flats the whole riser has to be drained, which needs agreement with the building management or owners' association
• Chasing, masonry repairs and replastering follow the welding

In a block of flats the heating riser is usually a shared part of the building. Do not touch it without permission from the building management or owners' association, even if it runs through your flat. For a larger heating rebuild, for example a new heat source or a changed layout, check with your local building authority whether any approval is needed under local building regulations. Repairing a section of pipe does not usually need a permit, but if you are not sure, ask.

Flame welding means open fire and pressurised gas cylinders, so it belongs in the hands of a trained, qualified welder. It is not a job for DIY. A qualified welder knows the technique and also the fire precautions for hot work in buildings: preparing the work area, keeping an extinguisher at hand and watching the area after the work is done. Some insurers also have their own hot work rules, so it is worth checking your policy.

Why does the watch afterwards matter so much? Sparks and hot metal can ignite dust in a chase, insulation or timber behind a wall, sometimes with a delay. A fire may only start after the welder has left. You can help by clearing anything flammable away from the pipework before the welder arrives and making sure the room can be ventilated.

If you smell gas during the work, stop immediately, open the windows, do not switch any electrical appliances on or off and call the local emergency number or 112.

These problems come up most often in practice:

• Poorly cleaned pipe ends: rust and paint cause porosity, and the weld starts to weep over time
• Burn-through on thin pipe when the flame is too strong or stays too long in one place
• Lack of root fusion: the joint looks neat from outside, but inside the metal is not fused through the full thickness
• Welding onto weakened pipe: the joint is sound, but the rusted pipe a little further along soon fails
• Covering up without a pressure test, after which a leak shows up as a damp patch on the wall

When you accept the work, check these points in particular:

• The weld is continuous all round the pipe, with no holes, cracks or visible pores
• The joined pipes are in line and the run is not bent or sagging
• Nothing around the work area is scorched and the floor and wall protection did its job
• The system has been refilled, bled and pressure tested
• With the heating running, the joints stay dry at normal operating temperature
• Radiators heat up evenly and there is no gurgling air in the system
• The pressure on the boiler gauge does not drop after a few days of use
• Any chased-in pipe was covered only after the tightness test