Sealed System Central Heating

Expansion vessel

Though no feed-and-expansion cistern is required in a sealed system, it is necessary to provide for the molecular expansion of heated water, since this cannot be compressed. This requirement is met by a component called a diaphragm expansion vessel or tank, filled with air or nitrogen and directly connected to the system water, against which the expanded volume can ‘flex’ and be absorbed. The diaphragm is usually of a rubber composition.

While the expansion vessel can be placed anywhere in the system, it is usually located near the boiler. This vessel must, for systems of up to 24kW rating, be connected by 15mm pipe and above this by 22mm pipe.

Pressurisation of the system is governed by the size and capacity of the vessel, it’s given pressure, the static head available, and the water content of the system. These are taken into consideration when sizing the expansion vessel. These vessels are factory pre-pressurised from between 0.5 bar to 2 bar. Under expansion, pressure increases to between 2 bar and 3 bar. Two bar is equivalent to about 21.3m water ‘head’.

Safety valve

A safety valve should always be incorporated into the circuit. The pressure vessel should be sized to be at 0.5 bar (9.15m head) below this. The valve is usually set at 2-½ – 3 bar, representing, at 3 bar, a 25.9m head.

The vessel takes up the thermal expansion of about l/20th of the water content in an average circuit. For normal installation, the water content of a system using radiators is about 30 litres for every kW and around 15 litres per 3kW for convectors.

The pressure vessel should contain no water at all when the circuit is filled. Once the water becomes hot, this gradually overcomes the pressure behind the membrane. Once the system cools, the membrane will force out the contracting water.

Manufacturers’ data enables the size of vessels to be easily established. A good working formula is: for homes with a heat requirement of 14.5kW (50,000 Btu/h) a 4-litre vessel is needed; between 14.5kW and 29.3kW (100,000 Btu/h), a 7.5 litre vessel is adequate, and over 34kW (150,000 Btu/h), a 12-litre vessel is necessary.

The three basic domestic temperatures are:

• low-up to 82°C;

• elevated: 82°C-99°C;

• medium: 99°C-110°C.

Automatic air vent

Regardless of how well a system is vented during filling and after initial heating, a certain amount of air will be released in subsequent heating and cooling cycles. In conventional systems, this air is released by the vent pipe.

An automatic air vent is used in sealed systems. Most of these operate on a float principle which relies on the tendency of air to rise above the body of water. The lowering of a float within a collection chamber triggers off a trip mechanism which releases collected air.

High-limit thermostat

The other basic safety device, needed on elevated and medium-temperature equipment, is the high-limit thermostat. This is designed to shut down the boiler in the event of failure of the main boiler thermostat and the pressure-release valve. Control on boilers are designed on a fail-safe principle, for added security.

Venting and make-up unit

An automatic venting and make-up unit consists of a non-return valve, to prevent backflow of water to its supply reservoir. It is fitted at the highest point in the system; the automatic air vent is attached to the body of the non-return valve.

Air separator

This enables speedy removal of entrapped air from the system. The inlet and outlet of the unit are set at a tangent, creating a vortex which starts a centrifugal action and forces water to the head of the chamber and air to the centre. Centrifugal action is effective even at high-water velocities. This unit should be mounted on the suction (S) side of a pump, since air in water is more finely dispersed after passing through the pump.


It is sometimes suggested that sealed systems are, in some way, dangerous, largely as a result of the pressure element. The pressure element is a source of misconception. In fact, this is a good deal lower than in many larger open central-heating systems.

An important aspect of diminishing pressure is the effect of leakage from badly made joints. Leaks will mean a loss of pressure, as well as water, and if the loss is allowed to continue, a condition, in some installations, could occur where the pressure becomes less than that of the atmosphere.

A steam ‘flashpoint’ could be reached at temperatures below boiling, and temperature-actuated high-limit controls are designed to deal with this should it occur.

Pressure gauge

It is desirable that a sealed system is fitted with a temperature and a pressure gauge which may be a combined type.

Always ensure that the pressure within a sealed system, when cold, is not allowed to fall below 0.5 bar and regularly check the pressure-gauge reading.

Regular checking of readings is no more involved than checking the setting on a room thermostat. If leaks occur, they will usually show up during the first few weeks that the system is in operation.

Radiators and convectors While many advantages are gained by using higher water temperatures, these eliminate the use of the conventional steel-panel radiator. Water flowing at these temperature levels would cause severe burning if the surface were touched. This also applies to exposed pipework which must either be covered with a protective coating or boxed in.

Care should be taken when choosing fan convectors that the components, particularly the fan and blades, are able to withstand higher temperatures.

System choice

The size of the system helps to decide whether to use small-bore or microbore. If this is to consist of a small back boiler serving a hot-water cylinder by gravity circulation, with only two or three radiators, using only the minimum of controls, then a small-bore installation may be cheaper to install.

A single-pipe loop could be used as opposed to a two-pipe design needed on microbore systems. The rule is that the greater the number of radiators, the cheaper a microbore system tends to become, compared with a small-bore system.

10. November 2011 by admin
Categories: Featured, Handyman Tips | Tags: , , , , , , , , , | Comments Off on Sealed System Central Heating


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