Common cold-water pipe materials, isolation, and why and where cold pipework must be protected from freezing.
Cold water pipework has to deliver wholesome water reliably, so the material, the joints, the supports and the frost protection all have to suit where the pipe runs.
Inside a dwelling, copper and plastic pipes such as polybutylene and cross-linked polyethylene are common. Underground, blue MDPE is the usual choice for the supply pipe. Each material has its own jointing methods, and pipe clips or supports must be spaced to suit the material so the pipe does not sag.
Fitting service (isolation) valves at appliances and key branches lets you shut off one fitting for maintenance without draining the whole system, which makes repairs quicker and cleaner.
Water expands as it freezes and can split a pipe or fitting, which floods when it thaws. Protect exposed pipework by routing it well, covering it and insulating it. Insulation only slows heat loss, so an unheated space in a long cold spell may need more protection. Water Regs UK advises turning off the supply and draining the plumbing if a home will be empty and unheated through cold weather: lagging cannot stop still water freezing for ever. If the heating is left on instead, WaterSafe's winter advice is to keep it at no less than 14 °C overnight and while the home is empty. Cold pipework also needs protecting from heat: keep it away from sunlight and heat sources, with insulation that keeps the water below 20 °C. Warmed water can taste unpleasant, and people waste water running the tap until it cools.
Underground, the supply pipe is laid deep enough to stay below the frost line and clear of other services, with protection where it rises into the building. Water Regs UK's building-entry guidance calls for a smooth-bore duct where the pipe passes through a wall or is shallower than 750 mm, including inside. Seal the entry against gases and vermin using a method the water undertaker accepts. Under a suspended floor with air vents, carry both the duct and insulation up to finished floor level, protecting the pipe throughout the cold void.
In short: match the material to where it runs, support it properly, fit isolation points, and protect it from frost. Check the material and the depth and insulation details against current guidance.
Choosing and protecting the pipe material
A pipe material carrying wholesome water must resist corrosion where it is used, and must not contaminate or taint the water. Copper and approved plastics can both do this. A run does not have to be one material throughout, and the pipe does not need to conduct electricity.
Blue MDPE is made for burial: prolonged ultraviolet light in sunlight can degrade it, so it is not left exposed above ground. Where a run has to come up into daylight, use a pipe made for that exposure instead.
The normal buried service-pipe cover is 750–1350 mm. Agree an arrangement with the water undertaker before laying it outside that range: insulating the pipe does not remove the need to notify and get consent.
Never fit galvanised steel pipe downstream of copper in the same run. Tiny traces of copper carried in the water settle on the zinc coating further along and set up galvanic corrosion, attacking the galvanised pipe.
Direct and indirect cold water systems
A direct system feeds every cold outlet, including the WC, straight from the incoming main at mains pressure, with little or no water stored on site.
In a common indirect arrangement, the kitchen drinking tap remains on the main and other cold outlets are supplied from a cistern. Confirm the actual branches: mixed arrangements are possible.
The cistern's stored water is a finite reserve. It keeps the outlets it serves running for a while if the main is shut off or its flow dips, which a fully direct system cannot do, since it has nothing stored to fall back on.
Reading common symptoms on cold pipework
Cold water that looks milky or cloudy for a moment and clears from the bottom of the glass upward is just tiny air bubbles releasing; it disperses harmlessly and is not a sign of contamination. Discolouration that settles out from the top down is more likely sediment or limescale disturbed by recent work.
Droplets spread evenly over a cold pipe in a warm, humid cupboard point to condensation. Dry the pipe and inspect it and its joints first, as a joint could still be leaking. If the pipe is sound, continuous insulation will limit condensation on its cold surface.
Support spacing for pipe clips
Plastic pipe is more flexible than copper and generally needs closer support. Wavin’s Hep2O guide gives 0.3 m horizontal and 0.5 m vertical clipping distances for its 15 mm polybutylene pipe. A 15 mm copper horizontal run commonly uses 1.2 m centres; do not transfer that spacing to flexible plastic.
A horizontal pipe spans between supports and can sag under its own weight and the water in it. A vertical run is loaded differently and can often have its clips further apart, but its weight still needs supporting. The fixings carry that load: a vertical pipe does not hold itself up just because it stands upright.
Always use the support-spacing table for the pipe system you are installing; never carry spacings over from another material.
Dezincification-resistant fittings
Dezincification draws zinc out of ordinary brass and weakens it. The guidance to the Water Fittings Regulations says two kinds of fitting must be made of gunmetal or another dezincification-resistant material: every concealed fitting other than a terminal fitting, including buried ones, and every backflow prevention device. Fittings laid underground should be dezincification-resistant too. Underground fittings should also be installed to allow for ground movement, and should not be jointed or connected with adhesives. Such fittings are marked with the symbol CR or DRA and tested to ISO 6509.
Concealed fittings
Schedule 2 counts a water fitting as concealed when it:
- is installed below ground;
- passes through or under a wall, footing or foundation;
- is enclosed in a chase or duct; or
- sits anywhere else that is inaccessible or awkward to reach.
No water fitting may be embedded in a wall or solid floor, and paragraph 7 says a fitting designed to be operated or maintained, or a joint, shall not be concealed. The guidance also keeps water fittings out of the cavity of a cavity wall and from below a suspended or solid ground floor. The exceptions are an internal wall that is not solid, a chase or duct that can be readily exposed, and a suspended floor that can be readily taken up or reached. Water Regs UK's guidance adds that a fitting below ground but not buried, such as one in a chamber or basement room, is not treated as concealed, and nor is one under a floor or in a wall that can be readily reached. Keep valves, backflow devices and joints where they can be reached, and avoid joints on concealed pipework.
Solder and flux on water pipework
Lead-based solder has been banned on domestic hot and cold water systems since 1987, and anywhere the water has to be wholesome for drinking, cooking or food production. A water supplier that finds it can make the installer cut out every joint containing lead solder at their own cost, and the installer can also be prosecuted. Leaded solder is still allowed only where the water never has to be wholesome, such as the pipework of a sealed central heating system.
On water pipework, make capillary joints with fittings that have integral lead-free solder rings, or end-feed lead-free solder from a pack marked as lead-free. End-feed only enough to fill the capillary gap: on small tubes, a length of solder wire roughly equal to the tube's diameter is plenty, so about 15 mm on 15 mm tube.
Flux residue has to come out afterwards, because excess flux corrodes copper and can turn the water blue. Wipe the outside of each joint with a wet cloth and warm water, then flush the pipework through as soon as possible. A cold flush usually clears water-based fluxes, but a grease-based flux, such as one mixed with petroleum jelly, is not readily shifted by cold water and needs a thorough hot water flush. Water-soluble fluxes are preferred with lead-free solder because they wash out more easily.