A tank bung is a bung used for blocking a cistern outlet, such as the outlets on a cold water storage cistern. Typically available in 15 mm, 22 mm, and 28 mm sizes, a tank bung consists of a plug made out of rubber, or made out of plastic with rubber edges, which is inserted into a cistern outlet from within the cistern itself.
Tank bungs may also be referred to as radiator bungs – they are usually the same piece of equipment. Bunging a radiator will allow for a radiator to be changed without having to drain down the entire central heating circuit and refill it with inhibitor.
Tank bungs are used when it is necessary to shut off the supply of water from the cold cistern to another part of the plumbing system (e.g. to the hot water cylinder), and a gate valve on the supply pipe is faulty or absent.
The main advantage of using a tank bung is that a tank/cistern outlet can be isolated without having to drain down the cistern in the absence of a working gate valve, potentially wasting up to 50 gallons of potable water.
Why might I need a tank bung?
As mentioned, bunging a tank will allow you to carry out work on the plumbing system without having to pour 50 gallons of water down the drain. An example of this scenario would be if you needed to replace a washer on a hot tap, but there is no gate valve on the supply pipe to the hot water cylinder, or the gate valve is faulty.
Once a tank bung is in place, it is the ideal opportunity to install a gate valve where one is missing, or to repair one which is faulty. Valves which isolate the supply of water from a cistern are required by water bye-laws.
While it may be possible to improvise a tank bung with a cork or piece of rubber, it is possible that a device which has not been specifically made for the purpose may not cause a water-tight seal. Where maintenance is being carried out on open pipework in connection to a cistern full of water, the end result could be catastrophic if the seal should fail.
Unfortunately it is not always possible to fit tank bungs due to the shape or design of the tank connector on a cistern, e.g., the presence of brass lugs on the outlet prevent the tank bung from forming a water-tight seal. In such cases, it will be necessary to isolate and drain the cistern down.
What’s the difference between a tank bung and a radiator bung?
As mentioned, there is little to no difference – both will allow you to carry out work on a plumbing system without having to isolate and drain down the relevant cistern.