What is an Open Vented Central Heating System?

A central heating system is called open vented if there is an open vent pipe
which is bent over the top of a feed and expansion tank (F & E tank).
The F & E tank is often called a header tank. It allows water to be added to the
heating system to keep it topped up and it accommodates the expansion of the hot water.
Click here to see how the F & E tank works.
If there is no F & E tank, the central heating system is called sealed.

The water in an open vented heating system is at a low pressure (typically 0.1 to 0.5 bar).
The water contains additives and inhibitors which slow down the rate
of corrosion of the boiler and the radiators.

The advantages of an open vented central heating system are
1. The heating system is less likely to leak because the water is at a low pressure
(but see number 4 below).
2. Water needed to top up the system is added automatically.
3. The system is quite simple and reliable.

The disadvantages of an open vented central heating system are
1. The heating system is open to the air which allows oxygen to enter
and speed up the rate of corrosion.
2. The float valve in the F & E tank can go wrong and cause the overflow to drip.
3. Black sludge can form in the heating pipes and block the F & E tank.
4. If there is a leak, water will not stop leaking out of the system because it is being
continuously replaced by the F & E tank (compare this with a sealed system).