What is a Sealed Central Heating System?

A central heating system is called sealed if there is no vent pipe
which is open to the air. If you do not have a feed and expansion tank
in your loft, then you probably have a sealed central heating system.

Water can be added to the heating system to keep it topped up by using a filling loop.
You can do this yourself if you know how a filling loop works. The pressure of the water
in a sealed heating system is usually around 1 bar. If you are not familiar with a filling loop
then it is best to get advice from a qualified plumber. The water in the heating system
contains inhibitors which slow down the rate of corrosion of the boiler and the radiators.

The expansion of the hot water is accommodated by an expansion vessel.
The expansion vessel can be anywhere in the heating system. It is often inside the boiler.
The expansion vessel allows the hot water to expand while maintaining the water pressure.
If the water pressure in the heating system gets too high (above 3 bar)
then a pressure relief valve will open and discharge water from an overflow pipe.
If this happens it shows that there is something wrong with the system (see disadvantages below).

The advantages of a sealed central heating system are
1. There is no need for a F & E tank in the loft which makes the system good for flats.
2. The heating system is not open to the air which makes it less likely for oxygen to enter.
This slows down the rate of corrosion compared to an open system
(but mains water can enter, see disadvantages below).
3. If there is a leak, water will stop flowing out of the system after a while because
the system is sealed and no more water can enter. Compare this to an open system.

The disadvantages of a sealed central heating system are
1. You may need a plumber to top up water losses (see the filling loop above).
2. The expansion vessel can go wrong and cause the pressure relief valve
to discharge water from an overflow pipe.
3. Mains water can continually enter the heating system through the filling loop
(if it does not turn off properly) or through a faulty combi boiler. This will cause the
pressure relief valve to discharge water from an overflow pipe. Mains water
contains oxygen which speeds up the rate of corrosion of the boiler and radiators.