How bung kits work?

Can somebody explain how a bung kit works (to replace a TRV without draining down)?

Have tried searching but struggling to find a clear description...

IF no air or water can enter the system then none can leave it!

Is that simple enough?

A bit like a pipette!

explain how a bung kit works

Click to expand...

Unreliably!

In standard house with normal domestic systems installed i have a 95% success rate using bungs out off the remaining % half these although not completly bunged stop the flow enough to do what i want without getting soaked..

So one bung in the vent pipe... is that it?

Goldspoon said:

So one bung in the vent pipe... is that it?

Click to expand...

Only if you want an unexpected shower!!

One in cold feed as well usually helps!!

The water level in the vent pipe is at the same level as the water in the F/E tank. Inserting a bung in the vent pipe creates a potential vacuum above the water level in the vent pipe. If the water tried to escape from the break in the circuit, the vacuum will increase but, as this is not 'cool' (or, as I learnt years ago, "nature abhors a vacuum"), the water is prevented from escaping.

If there was no bung in the feed pipe, the water would just run away until the F/E tank was empty - assuming the ball valve had been tied up

The only problem is that, if the vertical height between the water level in the tank and the disconnection point is more than about 10 metres, the vacuum will be broken. So bungs are OK for two floors and can be used for three with care, but are no use if you are in a 10 storey building with the F/E tank in the roof and you want to remove a rad on the ground floor.

There is also a slight water loss to start with while the vacuum is established, but it is usually only a towel's worth and is minimised if the bungs are securely inserted.