This type of system uses a diaphragm or membrane tank together with a safety valve and pressure gauge, instead of the conventional open system using an F & E tank and open vent.
The sealed system can be used on both smallbore and microbore systems.
It is sometimes called a pressurised sys-
tem, which means that it is not open to the atmosphere and can operate at higher temperatures which in turn can reduce the size of the piping.
Standard radiators cannot be used at these higher pressures as the surface of the radiator would be extremely uncomfortable to touch. Heat emitters that can be used are fan convectors, convector radiators and skirting heaters.
Control of the hot water cylinder water will be by the thermostatic valve which would keep the domestic hot water at 60°C to 71°C.
The position of the diaphragm within the system is important as it has to be in the right place to work with the pump.
Calculating the size of pressure vessel
This has to be calculated using four factors:
1. Static head on vessel
2. Cold fill pressure
3. Final temperature
4. Quantity of water in the system.
Manufacturers of sealed vessels provide information of the correct size to suit the system requirements. Set out right is an approximate guide to the size of expansion vessels required as produced by Wednesbury Tube Co., for a traditional boiler and radiator system.
The diaphragm is usually made of rubber and the expansion chamber is filled with air or nitrogen. Its function is to take up the expansion of water.
You can fill the system directly from a domestic tap via a hose into the stop-cock with a non-return valve.
Another method which is used is that with a break between the mains supply and the system; this is to prevent backflow and contamination.
A leak in the system shows by a drop in pressure and refilling of water.
The safety valve is probably the most important fail-safe device. The correct valve must be used as the pressure in the system is different to that of an open system with an open vent.