Terminal Valve Installation

In practice, "terminal valve" covers several similar parts. The main difference is what connects to them:

• Angle valve under a basin, sink or by the toilet, where the flexible hose of the tap or cistern connects
• Washing machine or dishwasher valve with a male thread for the hose, often with a built-in non-return valve
• Ball valve at the end of a branch, which closes with a quarter turn of the lever
• Radiator valves: a thermostatic valve usually sits on the flow side and a lockshield valve on the return, which lets you isolate and balance the radiator

Some valves are motorised and connected to a control unit or smart home system. The electrical part of these belongs in the hands of a qualified electrician.

The plumber first closes the main stopcock or isolates the heating branch, then releases the pressure by opening the nearest tap. Next comes removing the old valve. If it has seized, a second spanner holds the pipe steady so the pipework in the wall does not twist or crack.

The thread then gets cleaned of old sealant. The plumber seals it with PTFE tape, sealing thread or hemp with jointing paste. Some valves come with a factory-applied sealant, and then nothing extra goes on. The valve is screwed on and tightened so the outlet points the right way, for example towards the tap hose.

Copper pipe without a thread often gets a compression fitting instead. Here the olive and nut do the sealing, so no tape belongs on that joint.

Finally the water goes back on slowly. The plumber checks the joint with a dry cloth and opens and closes the valve a few times. With new pipework or larger changes, the whole branch gets a pressure test according to the pipe manufacturer's instructions.

The valve has to suit both the pipe and what flows through it. When choosing, check:

• Thread size: ½" is usual in homes, ¾" on main branches
• Medium: drinking water needs a valve approved for drinking water, heating needs one rated for heating water and its temperature
• Pressure rating, marked PN, which should be comfortably above the working pressure of your system
• Material: nickel-plated brass is standard, and in hard water areas a better-quality valve pays off
• Operation: a ball valve closes quickly and reliably, a screw-down valve regulates more finely but tends to scale up faster in hard water

Why do plumbers recommend a valve with a non-return valve for a washing machine? It stops water from the appliance flowing back into the drinking water supply.

The simplest case is swapping a valve on an easily reachable thread under a basin, with a main stopcock that shuts off properly. These things make the job longer and more complicated:

• A seized or corroded old valve that will not come loose
• A damaged thread in the wall, where a section of pipe and sometimes tiling has to be redone
• A main stopcock that lets water through and needs replacing first
• A valve in a tight space behind a cabinet or bath panel
• Radiator valves in a block of flats, where the riser has to be drained

In a block of flats with central heating, agree any radiator valve replacement with the building management or owners' association first. Draining the riser is usually done by a contractor the building appoints, often outside the heating season.

In practice the problem often does not show straight away, but only after a few days or weeks. These are the usual culprits:

• Tape wound against the thread, which bunches up as you screw the valve on and the joint seeps
• Overtightening, which cracks the valve body or damages the thread in the wall
• Backing off after tightening to line up the outlet, which breaks the seal
• Valve fitted against the arrow on valves with a set flow direction, typically thermostatic and non-return valves
• Extra sealant on joints with a rubber washer, which is unnecessary and harms the joint
• A valve unsuitable for the temperature or pressure, such as a cheap water valve fitted on a heating circuit

With a threaded joint, you only ever line up the outlet by tightening further, never by turning back. If the valve goes past the right position, unscrew it and seal it again.

If your main stopcock works and the valve sits on an easily reachable thread, you can do this yourself. For radiators, corroded pipework or a damaged thread in the wall, call a plumber instead.

1. Close the main stopcock and open the tap to release the pressure.
2. Put a bucket and a cloth under the valve.
3. Disconnect the flexible tap hose from the old valve.
4. Unscrew the old valve anticlockwise, holding the pipe in the wall steady if it starts to turn.
5. Clean the thread of old tape, hemp and limescale.
6. Wind PTFE tape or sealing thread in the direction of the thread, which means clockwise when you look at the end of the pipe.
7. Screw the new valve on by hand, then tighten it with a spanner until the outlet points towards the hose.
8. Reconnect the hose; its nut has a rubber washer, so tighten it gently and use no tape.
9. Close the new valve and slowly open the main stopcock.
10. Open the new valve and wipe and check the joint several times with dry paper.

When you take over the job, ask to be shown everything on the spot. Check that:

• The joint at the wall and the hose joint stay dry after several hours of use
• The valve turns easily and really stops the water when closed
• The outlet points straight at the appliance and the hose is neither kinked nor stretched
• On a radiator, the return valve closes and the radiator heats evenly once reopened
• The arrow on valves with a set flow direction matches the direction of the water
• The area is clean, and with new pipework you have a record of the pressure test