Drain-Off Valve: What It Is, How to Use It and Why It Drips

Updated July 2026 · Emergency Plumber Chelsea

What a drain-off valve looks like. Most are about the length of a thumb, made of brass, with a ribbed nozzle for a hose on one side and a small square head or a slotted spindle on top. Some have a little lever or a wheel instead. They are usually left closed for years at a time, so they are often painted over, hidden behind furniture or tucked under floorboards, and many people only discover theirs when they need to drain the system.

Where to find a radiator drain valve. On a central heating system the drain-off is fitted at the lowest point, so gravity can empty the pipes. In a flat that is usually on the pipework below a radiator in the lowest room, or on the pipe tail beneath one of the radiator valves. Some radiator valves have a small drain point built into the valve body. In the taller Chelsea townhouses, the lowest point is often in the basement or lower-ground floor, sometimes in a boiler cupboard or under the stairs. If you cannot find one, it may be under floorboards next to a radiator, or the system may not have one, in which case an engineer can drain it another way or fit a drain-off.

Other drain-offs around the house. There is often one just above the main stopcock, so the cold water pipework can be drained after the stopcock is shut. Many stopcocks have one built in, which shows as a small outlet on the side of the stopcock body. A hot water cylinder usually has one at the bottom on the cold feed, which is used when the cylinder is drained. Our guides on how to drain a hot water cylinder and how to turn off the water at the stopcock cover those two.

How to open a drain-off valve safely. First make sure the system it serves is off and cool: turn the boiler off and let it cool if it is the heating, or shut the stopcock if it is the cold supply. Push a hose firmly over the ribbed nozzle and secure it with a hose clip, because a hose that slips off under flow will soak the floor. Run the other end outside to a drain that is lower than the valve, so the water keeps flowing. Then open the valve slowly with a spanner or drain-cock key, a quarter to half turn at first. Water will run once air can get into the top of the system, which on heating means opening the bleed screws on the upstairs radiators. Our guide to draining a central heating system walks through the full sequence.

Hold the valve body while you turn the head. This is the most common way people cause a leak. On many drain-offs the valve is screwed into a fitting, and if the head is stiff, turning it can unscrew the whole valve from the pipe instead. Use a second spanner on the body of the valve, not the nozzle, to hold it steady while you turn the head. If the head will not move with moderate force, stop. Forcing a seized valve can snap it or crack the joint, and on a sealed heating system under pressure that can mean a sudden spray of dirty water.

Do not open it too far. On some drain-offs, if you keep turning the head anticlockwise after the water starts running, the spindle can come right out of the body, and water will pour from the valve rather than through the hose. A half turn to one full turn is usually plenty. If water starts weeping from around the spindle as you open it, that is the seal on the spindle and it will usually stop once the valve is closed again.

Closing it again. When the water has finished running and your work is done, turn the head clockwise until it stops, firmly but without heaving on it. Overtightening crushes the washer inside and makes the next drip more likely. Take the hose off, dry the nozzle, and watch it for a few minutes. Then refill or repressurise the system as normal, bleed the radiators, and check the valve again once everything is back under pressure and hot.

Why a drain-off valve drips after use. A slow drip from the nozzle after closing is very common. The usual causes are a small piece of sludge or scale caught on the washer, a washer that has hardened after years of not moving, or a valve that was not quite closed. Try opening it a quarter turn and closing it again once or twice with a hose attached, which often flushes the debris off the seat. If it still drips, many drain-offs can take a cap over the nozzle as a stop-gap. Water weeping from around the spindle rather than out of the nozzle usually means the gland needs a gentle tightening. If the valve body or the joint behind it is leaking, the valve needs replacing.

Replacing a drain-off valve. On a heating system this usually means draining the system down below the valve, which is why it is often done at the same time as other radiator or pipework jobs. A plumber can sometimes avoid a full drain-down by freezing the pipe either side with a pipe-freezing kit, which lets a single fitting be changed without emptying the system. On a sealed system, the pressure has to be restored and the radiators bled afterwards, and inhibitor topped up if much water was lost. Our guide to why a radiator is leaking covers the other places heating systems commonly weep.

When to use a drain-off, and when not to. You need one to drain the whole heating system for work such as a power flush, replacing several radiators or moving pipework. You often do not need it to remove a single radiator, because most radiators can be isolated at their two valves and drained on their own, which is quicker and loses less inhibitor. Our guide on how to drain a radiator explains that method, and our guide to power flush cost covers when a full clean-out is worth doing.

Autumn is when drain-off valves show up. Many are last used when work is done in spring or summer, and the first time the heating runs hot again in October, a valve that was left slightly open or has a tired washer starts to drip. Check your drain-offs and radiator valves when you first switch the heating on, especially any that sit under floorboards or above a ceiling in a flat, where a slow drip can go unnoticed until a stain appears below.

Call a plumber if the valve is seized, snaps, or is leaking from the body or the joint, if you cannot find a drain point at all, if the water coming out is black and sludgy, or if you are not confident repressurising a sealed system afterwards. Emergency Plumber Chelsea fixes and replaces drain-off valves and drains down heating and water systems across Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham. We explain what we have found and the options before any work starts, and there is no call-out fee on jobs over 3 hours. See our central heating repairs, plumbing repairs and radiator power flush pages, or call 020 3002 6667.

Quick tips

• Secure the hose to the drain-off with a clip and run it to a drain lower than the valve before you open it.
• Hold the valve body with a second spanner while turning the head, so the whole valve does not unscrew.
• Open it no more than one full turn. On some valves the spindle can come right out if you keep going.
• Close it until snug, not as tight as it will go. Overtightening crushes the washer and causes drips.
• If it drips after closing, open and close it a quarter turn with the hose on to flush debris off the seat.

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