Decommissioning and recommissioning water and heating systems

What decommissioning means

Decommissioning takes a system, or part of one, out of use. It can be temporary, such as a holiday home closed for the winter or a rented home between tenants, or permanent, such as a bathroom being stripped out or a cylinder and its cisterns being removed when a combination boiler goes in. For cold water, hot water and heating the order is the same: isolate, drain, then disconnect and either cap or remove.

Plan the isolation

• The Water Fittings Regulations require every water system to be capable of being drained down, with enough stopvalves, servicing valves and drain taps to isolate and empty parts of it without wasting water.
• A stopvalve isolates a zone of the premises; a servicing valve isolates one fitting or appliance.
• Valves should be labelled with the part of the system they control, so the right section can be isolated and drained, including when a building is left empty in cold weather.
• On premises other than a house, altering the water system is on the list of work that must be notified to the water undertaker unless an approved contractor does it. If in doubt, ask the undertaker.
• The gas side of taking a boiler or other gas appliance out of use is covered in the gas notes.

Isolate and drain

1. Isolate the electrical supply to anything that heats the water, such as the immersion heater and the boiler, and make it safe.
2. Close the stopvalve or servicing valve that feeds the part being taken out of use.
3. Open a tap on that part to let air in; on a mains-pressure system this also releases the pressure.
4. Connect a hose to the drain tap at the lowest point, lead it to a suitable discharge point below, and open the drain tap.

For an unvented cylinder, one maker's decommissioning order is: isolate the electrical supplies, isolate the water supply, drain the cylinder, drain the primary circuit if the cylinder is indirect, remove the cylinder and cap the pipework. To drain it, turn off the mains supply, isolate the electrics and switch off the boiler, run a hose from the drain cock to a point below the cylinder, open a hot tap near the cylinder and then open the drain cock.

For a sealed heating system, fit a hose to the drain valve at the lowest point and open the radiator air vents so that the whole circuit empties. One boiler maker's permanent decommissioning is: switch the boiler off, disconnect it from the electricity supply, close the gas and cold water isolation valves, then drain the boiler through its own drain valves.

Disposing of heating water

Heating water is not drinking water. It usually contains inhibitor, and it is classed as fluid category 3 in a house and category 4 in other premises. Drain it only to a foul drain or a septic tank. Never let it go down a road drain or rainwater gully, or onto the garden: surface water drains discharge straight into local streams and rivers.

Permanent decommissioning: remove, do not just cap

• When an outlet is permanently removed, disconnect its branch at its source, where it leaves the pipe that stays in use.
• Pipework that no longer has water flowing through it, such as the feed to a removed appliance, is a dead leg. The water in it stagnates, so it is a contamination source and a Legionella risk for the rest of the system.
• Remove dead legs. HSE guidance is to cut redundant pipework back to the common supply, preferably by taking out the tee that fed it. Where a dead leg really cannot be removed, keep the blind end as short as possible.
• Capping a branch behind a wall and leaving it full of water creates exactly the dead leg these rules exist to prevent.
• Turn the supply back on and check every cap and joint for leaks.
• If a building will need no water supply at all, for example before demolition, the customer gives the water company notice and the company can then disconnect the service pipe from its main.

Temporary decommissioning: empty and seasonal properties

Two risks pull in opposite directions. For a let home the landlord has a legal duty to assess the Legionella risk, so agree the approach with them.

• Frost. If a property will be left empty and unheated in cold weather, turn off the supply and drain the system before freezing starts; insulation only delays freezing. Isolate and drain seasonal systems such as garden irrigation, and outside taps, and disconnect the hoses. The alternative is to keep the heating on: WaterSafe suggests at least 14 °C while a home is left. One maker's built-in boiler frost protection only works with the gas and electricity left on, and it is there to protect the boiler itself.
• Stagnation and Legionella. Standing water lets bacteria grow. For empty homes, HSE advises running every hot and cold outlet at least once a week, or another flushing regime, and considering draining the system if the home will stand empty for a long time. HSE's guidance for buildings and systems mothballed for a period is that they are normally left full rather than drained, because damp pockets left after draining let biofilm grow.

Whichever route is chosen, label the valves that have been closed. Where a landlord or employer has Legionella duties, their records should show whether the system is in use or drained down.

Recommissioning

1. Close all drain taps. Where there are storage cisterns, empty them and refill them from the incoming main before flushing the taps.
2. Fill a cylinder with a hot tap open, let it run until the water is clean, then open the other hot taps to clear the air. Never switch on an immersion heater or boiler before the cylinder is full, or if its water might be frozen.
3. Flush every outlet, starting with the one nearest where the supply enters and working out to the furthest, until the water runs clear and cool. Keep spray down, for example by taking shower heads off first.
4. Disinfect as well as flush where the system has not been in regular use and has not been regularly flushed, and this applies to houses too. A mothballed system is recommissioned as if new: flushed, cleaned and disinfected. BS EN 806-4 gives the method.
5. Check any water softener or filter before use; units left standing need servicing.
6. Refill a sealed heating system and vent it back to its working pressure. If it is filled through a temporary filling loop, disconnect the loop afterwards. Check the inhibitor level.