Low Pressure Hot Water Float Valve

Gravity feed low pressure hot water

A customer in Ashburton this week had water overflowing from their roof!

As it turned out it was coming from an overflow pipe which was discharging from their electric low pressure hot water service in their ceiling space.

There was a faulty float valve that was leaking and causing the flood. We replaced the valve and purged the lines to remove airlocks within the hotwater lines. The header tank was flushed and cleaned and the plumbing reconnected. After adjusting the valve to suit the water levels, the customer was curious of the hot water service that they had inherited as they were new owners to the property and were not familiar to low pressure hot water units.

Tom Plumb explained that the low pressure hot water unit that is found in the ceiling of older style homes has a smaller header tank that supplies the cold water feed to the hot water service. Also known as a gravity feed hot water unit, the smaller cold water feed tank is sometimes attached to the side of the hot water unit or it is installed at a higher level above the hot water unit with plumbing to suit.

On some occasions the hot water unit can be on the ground floor and the cold feed tank installed high up in the roof space to gain water pressure for the bathrooms. As it is the height difference between the cold feed tank and the taps that makes a difference with gravity feed hot water supply. Which is completely different to a mains pressure hot water unit where it is the pressure of the cold water that pushes the hot water through the unit.

In a gravity feed hot water unit there is a cold feed from the base of the header tank to connect to the hot water cylinder and a expansion tube that feeds back up into the top of the cold feed tank. This expansion pipe allows for the expansion of water as it is heated within the hot water service unit to rise in the expansion pipe and discharge to the cold feed tank on occasion.

On the side of the cold feed tank is the mains pressure cold water inlet. There should be an isolating stop nearby that can close the water supply to the feed tank for future maintenance. The cold feed from this isolating stop is connected to a “float valve” that is securely fastened to the side of the cold feed tank.

The horizontal float valve operates by opening and closing a plunger against a seat through the action of a lever arm attached to a float. The lever arm is interconnected to the plunger via a cam. As the water level drops, the float and lever arm move in a downward direction allowing the plunger to move away from the seat, which opens the valve. When the water level rises, the float and lever arm move in an upward direction and the plunger moves towards the seat until it seats fimly against the seat and shuts the valve off. 

There are a few maintenance issues that do occur with these type of float valves.