We have a 3 storied building where the water tank is located at the top of the building.We have fixed a float valve for the water tank on the municipal line so as to avoid the water from over flowing.But the float valve often does not work properly and the water overflows.Since the tank is at the 3rd floor roof the overflowing of the water comes to our notice after a long time and by then lot of water is wasted.Any suggesstion?

Hi cool plumber the problem looks like is from the ball of the float valve.You have not mentioned what type of ball you have installed is it Pvc or some metal.What ever it is it looks like the ball may have some tiny holes and so it is not being lifted properly and so the valve in the tank is not working.

i too think that your ball valve is damaged and by changing that you can have the float valve working.But you cannot use the float valve for pump lifted water.As if the water inlet closes and the pump still running may cause the pump to break down.So you can have a cat and mouse level indicator to check the level of water in the tank and switch off the pump by which you will not waste water

Leaving the float valve problem as it is clear that you ball in the float valve is damaged. If you have a problem with the pumped main over flowing and george has advised to have a level indicator in the tank which is a good idea, but i have a solution which is float switch operated pumpset, by this you dont want to bother about the tank water level.The pump operates on the level of the water level in the tank which is sensed by the float switch.
You can set your on and off level so that you dont have to bother at all.

That is good point that if you have a pumped main then you can have the float switched opereated one.Also I have to make a point that you can have a alarm when the water is getting full so that you can immidieatly notice and take the necessary action as soon as you hear the alarm.Even if the filling is not by pump then also you can use this