What a Drain-Off Valve Is For

Where you will find drain valves in a house

Water only drains downhill, so a drain valve is only useful at the lowest point of whatever it serves. The Water Supply (Water Fittings) Regulations 1999 require water systems to be capable of being drained down, with enough servicing valves and drain taps to keep the water wasted during maintenance to a minimum, so a repair does not mean emptying the whole house. In practice that puts them in a handful of predictable places.

• Just above the internal stopcock, usually under the kitchen sink. Sometimes it is built into the stopcock body as a small side outlet. With the stopcock shut, this one empties the pipework above it
• At the bottom of the cold feed into a vented hot water cylinder, in the airing cupboard. It empties the cylinder
• On the lowest run of central heating pipe, often on a downstairs radiator tail or beside the boiler
• On the pipe to an outside tap, just inside the wall, so the tap can be drained before a frost
• On a combi or system boiler itself, but inside or under the casing. That one is for the engineer, not for you

How to recognise one and which way it turns

Most drain valves in UK homes follow BS 2879, the standard for draining taps, and look much the same: a small brass or chrome body, a ribbed nozzle about the width of a garden hose, and on top a square or slotted spindle instead of a handle. The missing handle is deliberate. It stops anyone knocking it open by accident. You turn it with a drain valve key, a cheap square socket tool from any merchant, or with a small adjustable spanner.

Anticlockwise opens, clockwise closes. A turn or so is usually enough for a good flow; there is no need to wind it right out. There is often a small plastic cap over the nozzle. If the cap is missing, fit a new one: it is the only thing stopping a slightly weeping valve dripping onto the floor.

The spindle on older valves can be stiff after years untouched. Apply steady pressure. If it will not move, leave it, because snapping the spindle on a valve that is still under pressure turns a small job into an emergency.

Isolate the supply before you open a drain valve, and never open one on a hot boiler circuit or cylinder. Switch the heat off and let it cool first, because that water can scald.

How to drain a section without flooding the room

Most flooding during a drain-down happens because the hose slipped off or ran uphill. Take a few minutes to set up properly and the job is clean.

• Isolate the water. For cold pipes, close the stopcock or the valve that feeds that section. For a hot cylinder, switch off the boiler and immersion heater and close the gate valve on the cold feed into the cylinder
• Push a hose firmly over the nozzle and secure it with a hose clip, or a cable tie at a push
• Run the hose to an outside drain or gully that is lower than the valve. Water will not flow uphill without a pump
• Open a tap or a radiator bleed valve at the highest point of the section so air can get in. Without it the water barely trickles
• Open the drain valve with the key a quarter-turn, then more once water is flowing
• When it stops, close the valve fully and refit the cap before refilling

Sealed heating systems carry inhibitor and sludge. Do not drain them onto a lawn, flower beds or into a surface water drain. Use a foul drain such as the gully under a kitchen waste pipe.

A drain valve that drips: what is wrong and what you can do

Drips come from one of two places. Water from the nozzle means the valve is not sealing on its seat, usually because grit or scale got trapped when it was last used. Water from around the spindle means the small O-ring or gland packing has worn.

For a nozzle drip, try opening the valve a quarter-turn and closing it again with the hose on. The flow can flush the debris off the seat. If it still weeps, a tight-fitting cap over the nozzle will usually stop the drip as a temporary measure until the valve is replaced.

For a spindle drip, the gland nut under the spindle can sometimes be nipped up an eighth of a turn. Go gently; overtightening makes the valve impossible to turn later. If neither works, the valve needs replacing. A leaking drain valve right beside the stopcock is also a sign that the stopcock is getting old, and our stopcock repair visit often deals with both at once. For the similar little valves on appliance and tap supplies, see an isolation valve leaking.

Replacing one, and what it typically costs

The part is cheap, typically a few pounds for a standard 15mm compression drain valve. The cost is in isolating the section. On a cold mains pipe that means shutting the stopcock, or the outside stop tap if the valve sits below the internal one. On a heating circuit it means draining or partly draining the system, then refilling, bleeding and adding inhibitor afterwards.

An engineer will often avoid a full drain-down by freezing the pipe. A pipe freezing kit forms an ice plug on each side of the valve, so the old one can be cut out and the new one fitted while the rest of the system stays full. It is a standard tool for exactly this job, and it matters in a flat where draining means other people lose water too.

If the valve you need is the one above the stopcock and the stopcock itself will not shut, the job has to start at the boundary. Replacing a stopcock explains that sequence. The £60 call-out covers the visit at any hour, and the repair price is agreed with you on site before any work starts.

When you will actually need to use yours

Most people never touch a drain valve until something breaks. The times it earns its place are a burst pipe you cannot isolate at a local valve, a stopcock replacement, a leaking cylinder, a heating system being drained for a new radiator, and an empty house being winterised.

For a burst, the drain valve above the stopcock is useful once the stopcock is closed: opening it empties the water still standing in the rising main, and with the hot and cold taps open upstairs that takes the pressure off the leak much faster. Our guide to what to do when a pipe bursts puts that in order. For a heating drain-down the full sequence is in how to drain central heating, and for the cylinder see how to drain a hot water tank.