Replace the float valve in a loft cold water tank

Isolate the water, drain the tank, fit a WRAS-approved float valve, then reset the water level and check for leaks.

FV-01Get set up in the loft safelyTake a torch, kneeling board, gloves and a dust mask, since old loft insulation sheds fibres. Lay the board across the joists to reach the tank, never step on the ceiling itself between joists, or you risk your foot going through the plasterboard below. Clear a path to the tank before you start so you are not reaching over pipework once your hands are wet.

1 each · Kneeling board or loft boards

Buy:Tooled UpB&QAmazon

1 each · Dust mask

Buy:Tooled UpB&QAmazon

1 each · Torch or head torch

Buy:Tooled UpB&QAmazon

FV-02Check what the tank is made ofModern tanks are grey or blue plastic and safe to work on. Older tanks may be galvanised steel, or a grey, rough-textured asbestos cement (AC) tank, common in houses built or fitted out before the 1980s. If you suspect an asbestos cement tank, stop, do not drill, cut, or disturb it, and get a specialist assessment before going any further.

RULES

KNOW THE RULES

Disturbing asbestos cement products is controlled under the Control of Asbestos Regulations 2012. If in doubt, get it checked before touching the tank.

FV-03Turn off the water supplyTurn off the stopcock, usually under the kitchen sink, and run a cold tap to confirm the supply has stopped. If the tank has its own isolating valve on the feed pipe, close that instead so the rest of the house still has water. Lay a batten across the tank and tie the float arm up to it, so no more water can enter while you work.

FV-04Drain the tank downOpen a cold tap fed from this tank, such as a bathroom basin tap, and let the water run until the level in the tank drops well below the float valve. If nothing drains it, siphon the last of it out with a length of hose into a bucket. You only need it low enough to work on the valve without water pouring out as you remove it.

1 each · Bucket

Buy:Tooled UpB&QAmazon

1 each · Length of hose (for siphoning)

Buy:B&QAmazon

FV-05Remove the old float valveSupport the valve body from outside the tank and undo the backnut on the inside with a spanner, using a second spanner outside to stop the valve turning. Lift the float arm and float out, then withdraw the valve tail through the hole in the tank wall. Check the old valve's type and the thread size, so the new one matches.

2 each · Adjustable spanner (pair)

Buy:Tooled UpB&QAmazon

FV-06Fit the new float valveBuy a WRAS-approved equilibrium or diaphragm-type valve (such as a Torbeck) to match the old thread size. Wrap the tail thread with PTFE tape, pass it through the tank wall, and fit the sealing washer and backnut from inside, tightening by hand plus a light turn with a spanner. Do not overtighten on a plastic tank, as this can crack the wall. Fit the float arm and ball onto the new valve.

RULES

1 each · WRAS-approved float valve (equilibrium or diaphragm type)

Buy:Tooled UpB&QAmazon

1 roll · PTFE tape

Buy:B&QAmazon

1 set · Tank connector washer set

Buy:Travis PerkinsB&QAmazon

KNOW THE RULES

Float valves fitted to a mains-fed cold water storage cistern must comply with the Water Supply (Water Fittings) Regulations 1999. Buy one marked WRAS-approved.

FV-07Reconnect and set the water levelUntie the float arm, then turn the water back on slowly at the stopcock or isolating valve. Watch the backnut for drips as the tank fills. Bend the float arm so the valve shuts off with the water level around 25mm below the overflow (warning) pipe outlet.

FV-08Test it properlyRun a tap fed from the tank to drop the level, then let it refill and recheck the shut-off point. Leave it for 20 to 30 minutes and check for any drips at the backnut or tail, and that no water is running out through the overflow pipe. Adjust the float arm again if the level is wrong.

FV-09Tidy upWipe up any spilled water and remove your tools, board and bucket from the loft. Dispose of the old valve, and note the make and model of the new one somewhere handy in case you need parts again. Give the tank a final visual check before you close up the loft hatch.

Travis Perkins and Tooled Up links are affiliate links: if you buy after clicking, we may earn a small commission at no extra cost to you. B&Q and Amazon links are plain searches and earn us nothing.

Save this as a project

Back to all guides

Related guides