The term float valves refers to the valve arrangement either with toilet cisterns or water tanks. Its function is to start the top up of water when the water level drops due to a release of water or conversely to stop the flow of water when the level reaches the required level. Float valves come in many different designs but most follow the same principal, a plastic (occasionally brass) float attached to an arm on a pivot which in turn attaches to a shut off valve. The end of the arm nearest the valve usually has one or more threaded stops which allow set up of the levels at which the valve operates.
When setting up or adjusting the float valve it is essential to ensure the movement of the plastic float and arm is unhindered, otherwise it can become stuck in any given position, possibly allowing the cistern or tank to overflow. All cisterns and tanks should also incorporate an overflow in the event of float valve failure. This overflow can either be an outlet pipe which allows water to run away safely to the outside or the soil system or alternatively it can be an internal overflow which directs any overflow back into the toilet pan. Either way this should be monitored periodically as water consumption can be significant, even for small overflows, over long periods of time.
Float valves come in cheaper plastic models or more expensive brass models, dependent on the budget and usage, both working on the basic same principal. There are a few variations to the design where the “float” can ride up a guide to operate the shutoff valve, or the “float” is a simply plastic cup, usually these are more compact but not necessarily more reliable. Failure of the traditional float valve can be in several ways, the valve seat or seal within the valve assembly itself can be faulty, these seals are easily available, not always easy to identify the correct one. Sometimes isolating the water supply, dismantling the valve assembly and cleaning out the valve seat can help free movement, enabling a better seal, although the build up of limescale can seriously inhibit free movement and is difficult to remove. Secondly the brass valve arm can become weak or bend allowing the operation to be mis-adjusted.
Gently bending the arm (if it is brass) is possible, but care should be taken not to further weaken the brass so the adjustment on the threaded stop should be the preferred option. Replacement brass valve arms are available from on-line plumbing suppliers in different lengths.
A leaking plastic ball float will also cause mis-adjustment of the float valve. Lastly the most common cause or incorrect operation is a toilet cistern plastic float is the ball float riding up the edge of the toilet cistern preventing smooth operation, sometimes intermittently. Simply loosening and twisting the main body of the entire float valve assembly to move it away from the edge of the cistern is all that is required. Depending on the amount of movement required this can require emptying the cistern by isolating the water supply, flushing the toilet and removing the rest of the water manually. Be sure not to over-tighten the main securing thread particularly if it is of the plastic design. If the water supply pipe is solid copper that is not perfectly aligned with the valve this can make it difficult to maintain a seal where the valve enters the cistern body or can easily strip the connecting thread, consider replacing with a flexible pipe incorporating an isolating valve. After any adjustment of a faulty float valve periodic re-visiting of its correct operation is strongly advised particularly when the overflow is not readily visible.
Finally some European float valves, often supplied with toilet cisterns from DIY shops are supplied with float valves with 3/8" thread, a little smaller than the most common 1/2" thread, 3/8" flexible connectors are available to connect directly.
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