Pipe Freezing

The freezing itself does not take long, but the preparation decides whether it works. An ice plug only holds when the water in the pipe is not flowing and not hot. In practice the process looks roughly like this:

1. The engineer checks the material, diameter and condition of the pipe and picks a straight, undamaged section to freeze.
2. On a heating system the boiler and circulating pump are switched off, or the valve on that branch is closed. The water has to cool down, because warm water freezes much more slowly.
3. Freezing heads are clamped onto the pipe. Smaller pipework usually needs one, larger pipes sometimes two side by side.
4. The equipment runs until a full plug forms inside. On typical domestic pipe sizes this usually takes from a few minutes to a few tens of minutes, depending on the material.
5. The engineer carefully checks that the plug holds: a joint beyond the plug is loosened slightly, and once the trapped water has run out, nothing more should come through.
6. The repair itself follows. The freezing equipment keeps running the whole time to maintain the plug.
7. When the work is finished, the freezing is switched off, the plug is left to thaw slowly, and the system is restarted, topped up and bled.

It pays off most where draining down would mean a lot of work and unnecessary risk. Typical situations:

• Replacing or removing a radiator that has no isolating valves
• Replacing an old valve or tap that no longer seals
• Connecting a new branch, for example during a bathroom or kitchen refurbishment
• Repairing a leaking section of pipe
• Working in one flat in a building with a shared heating system

Why do people prefer to avoid draining down? In an older system, draining and refilling stirs up sludge and deposits, which then block valves and radiators. Air gets into the system more easily, bleeding takes longer, and in winter the rooms go cold in the meantime. On a larger system there is also a big volume of water to deal with, often with inhibitor or other additives in it.

Pipe freezing does not work everywhere, though. If the heating contains antifreeze (glycol), a plug forms poorly or not at all. Constant flow is another problem, for example in a mains water supply under full pressure that cannot be reduced. A badly corroded pipe may not cope with the sudden change in temperature either. In those cases the engineer will suggest a different approach, such as partly draining a single branch.

In a block of flats the riser is shared pipework. Before anyone works on it, even just by freezing, agree a date with the building management or owners' association. Often the pump in the plant room or the district heating substation has to be switched off for a while, and that cannot be done without them.

Three types of equipment are common. Which one the engineer chooses depends on the pipe diameter, the material and how long the plug needs to hold.

• Electric freezing units work much like a fridge. The heads are connected to a compressor and can hold a plug for a long time. They suit domestic heating and water pipework.
• Carbon dioxide from a pressurised cylinder is used for short jobs on smaller pipes. It is simple, but the plug only lasts as long as there is gas in the cylinder.
• Liquid nitrogen is used on larger diameters and more demanding pipework, for example in plant rooms or commercial buildings.

Steel and copper pipes are frozen routinely. Plastic and multilayer pipes conduct heat less well, so freezing takes longer and needs closer monitoring. Some pipe manufacturers set their own restrictions on freezing, so it is worth finding out what your pipework is made of.

How much work the job involves, and therefore what it ends up costing, depends mainly on these factors:

• Diameter and material: thin copper pipe freezes quickly, while thick-walled steel or larger diameters need more powerful equipment and more time
• Access to the pipe: an exposed pipe is ideal, while a pipe in a chase, behind tiles or tight against a wall has to be uncovered first
• Whether the flow can be stopped: if the pump can be switched off and the system allowed to cool, everything goes smoothly, but with constant flow the job takes longer or cannot be done at all
• Condition of the pipework: rusty or previously repaired pipes need a more careful approach
• Follow-on work: freezing is almost always combined with replacing a radiator, valve or length of pipe, and the length of that work decides how long the plug has to hold

Most problems are caused by poor preparation, not by the freezing itself. In practice the same situations keep coming up:

• Freezing hot heating pipes or freezing with the pump still running, so the plug never forms or gives way during the work
• Choosing the wrong spot, such as a bend, a joint or a damaged section where the pipe is weakest
• Freezing two points close together with a closed section between them: ice expands, and pressure in the trapped section can rise enough to damage the pipe
• Soldering or welding too close to the plug, where the heat melts it, which is why engineers keep a safe distance or use press-fit joints
• Speeding up the thaw with a blowtorch or other open flame, which can distort the pipe or cause a leak later

For thawing there is one simple rule: let the plug melt slowly, and only then restore full pressure and start the pump.

Once the engineer has gone, make sure the system works as well as it did before. At handover, go through these points:

• The new or replaced joint is dry and does not drip, even after a few hours of running
• The pipe where the plug was shows no visible distortion or cracks
• The pressure on the boiler or system gauge is within the range stated by the boiler manufacturer
• The radiators have been bled and heat evenly across their whole surface
• The circulating pump is running and there is no gurgling or hissing in the pipes
• The engineer has told you exactly what was replaced and what to do if any damp appears later