A float valve is a valve on a tank’s inlet that a floating ball closes when the water reaches the set level and reopens when users draw the level down, so the tank refills itself without attention. The float has to push the valve shut against the pressure of the incoming water: a 100 mm float on a 5:1 lever closes a 12 mm seat at 2 bar, with 25.7 N available against 22.6 N needed, but not at 4 bar, so put a break pressure tank ahead of a high-pressure inlet.
Part of Water Distribution
A float valve is a level-controlled inlet valve, like the mechanism in a toilet cistern but larger. It keeps a storage tank topped up and stops the inlet when full, which makes the whole supply system run itself.
The Float Has To Beat The Water Pressure On The Seat
The incoming water pushes on the valve’s seat with a force equal to pressure times seat area. The float must exert more than that, through its lever, to close the valve.
For an illustrative valve (assumed values, not a standard):
- Seat diameter 12 mm: area = π × 0.006² = 0.000113 m² = 113 mm²
- Inlet pressure 2 bar = 200,000 Pa: force on the seat = 200,000 × 0.000113 = 22.6 N
- Float ball 100 mm across, fully submerged: volume = 4/3 × π × 0.05³ = 0.000524 m³; buoyant force = 1,000 × 9.81 × 0.000524 = 5.14 N
- Lever with the float arm 5 times as long as the arm to the seat: force at the seat = 5 × 5.14 = 25.7 N
25.7 N exceeds 22.6 N, so the valve closes at 2 bar. At 4 bar the seat force doubles to 45 N and the float cannot close it: the tank overflows. The remedies are a smaller seat, a longer arm, a bigger float, or lower inlet pressure from a break pressure tank. A ball of this size is never fully submerged in normal use, which lowers the real force, so treat the figures as an upper limit.
What You Need
- A float valve rated for your inlet pressure, or a lever valve and float you build to the arithmetic above
- A float of suitable size — a sealed plastic or copper ball
- A firm mount at the top of the tank, with room for the arm to swing
- An overflow pipe anyway — the float can stick
- An isolation valve ahead of it, so you can service it (Isolation Valves)
Method: Mount, Set, Test
Step 1: Mount the valve on the inlet, above the overflow level.
Step 2: Set the arm so the valve closes at the chosen full level, a little below the overflow.
Step 3: Fill the tank and watch it shut. It should close firmly with no drip.
Step 4: Draw water down and check it reopens and refills.
Step 5: Check it every season, for a stuck arm, a leaking seat or a float full of water.
Common Mistakes
- No overflow. If the float sticks the tank runs over.
- A high-pressure inlet on a small valve. It will not close.
- Setting the level at the overflow. A little ripple spills over.
- Closing too fast. A valve that shuts hard sends a pressure surge up the pipe (Isolation Valves).
- An inlet that dips below the water. Keep an air gap between the inlet and the water surface, so the tank can never draw back into the supply.
Safety
Close the inlet before servicing
Shut the isolation valve ahead of the float valve and let the line pressure off before opening the valve body: at 2 bar the 12 mm seat in the example is pushed with 22.6 N, and at higher pressure the parts can spring out. Never climb into a lidded tank to adjust the valve; adjust it from the hatch.
What’s Next
- Learn next: Pressure Zones — handling a site with large height differences
- Combine with: Storage Tanks — the tank the valve controls