Sealed system components

Level 3 6035 Diploma in Plumbing Studies
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Sealed system components
Unit 306: Central heating

Sealed system components
Sealed systems do not contain a feed and expansion cistern, nor open-
vent pipe. Instead, these systems incorporate the following components:
• an external expansion vessel fitted to the system return
• a pressure-relief valve
• a temporary filling loop or a CA disconnection device
• a pressure gauge.
The expansion vessel is a key component of the system. It
replaces the feed and expansion cistern on the vented
system and allows the expansion of water to take place
safely. It comprises of a steel cylinder, which is divided in two
by a neoprene rubber diaphragm. The vessel is installed on
to the return because the return water is generally 20°C
cooler than the flow water and this does not place as much
temperature stress on the expansion vessels internal
diaphragm as the hotter flow water.

On one end of the expansion vessel, is a Schrader air-pressure valve
where air is pumped into the vessel to 1 bar pressure and this forces the
neoprene diaphragm to virtually fill the whole of the vessel. On the other
end, is a half-inch male BSP thread and this is the connection point to the
system.
When mains pressure cold water enters the heating system via the filling
loop and the system, it is filled to a pressure of around 1 bar, the water
forces the diaphragm backwards away from the vessel walls compressing
the air slightly as the water enters the vessel. At this point, the pressure
on both sides of the diaphragm is 1 bar pressure.

As the water is heated, expansion takes place. The expanded water
forces the diaphragm backwards compressing the air behind it still further
and, since water cannot be compressed, the system pressure increases.
When cooling, the water contracts, the air in the expansion vessel forces
the water back into the system and the pressure reduces to its original
pressure of 1 bar.
Periodically, the pressure in the vessel may require topping up. This can
be done by removing the cap on the Schrader valve and pumping the
vessel up to its original pressure with a foot pump.

Pressure-relief valve
The pressure-relief valve (also known as the expansion
valve) is installed on to the system to protect against over-
pressurisation of the water. Pressure-relief valves are
usually set to 3 bar pressure. If the water pressure rises
above the maximum pressure that the valve is set to, the
valve opens and discharges the excess water pressure
safely to the outside of the property through the discharge
pipework. Pressure relief valves are most likely to open
because of lack of room in the system for expansion due to
a malfunction with the expansion vessel. This can be caused
by:
•the diaphragm in the expansion vessel has ruptured
allowing water both sides of the diaphragm, or;
•the vessel has lost its charge of air.

The filling loop
The filling loop is an essential part of any sealed system and should
contain an isolation valve at either end of the filling loop and a double-
check valve on the mains cold water supply side of the loop.
•The filling loop has a type EC-verifiable double-check valve included in
the filling loop arrangement, or
•the filling loop must be disconnected after filling creating a type AUK3
air gap for protection against back flow.
The filling loop is generally fitted to the return pipe close to the
expansion vessel and may even be supplied as part of the expansion
vessel assembly.

System boilers
A system boiler is an appliance where all necessary safety and
operational controls are included and fitted directly to the boiler. There is
no need for a separate expansion vessel, pressure-relief
valve or filling loop, and this makes the installation much simpler. The
system boiler has all the components for a sealed system contained
within the boiler unit.

System boilers
It is filled directly from the mains cold water via a filling loop which is
often fitted by the boiler manufacturer and has all the components for a
sealed system contained within the boiler. It is filled directly from the
mains cold water via a filling loop which is often fitted by the boiler
manufacturer and has all the components for a sealed system contained
within the boiler. It is filled directly from the mains cold water via a filling
loop which is often fitted by the boiler manufacturer.

Combination boilers
Modern combination boilers are very efficient and they contain all the
safety controls, ie expansion vessel or the pressure-relief valve, used
on a sealed system. Most‘combis’ also have an integral filling loop.

Water, when it is heated, expands. The amount of expansion will
depend on the temperature of the water. At atmospheric pressure,
water is at its greatest density at 4°C. At this temperature, 1m3
of water
has a mass of 1000kg. From this point forwards as the water
temperature increases, 1m3
will lose density. At 100°C, 1m3
of water
has a density of 958kg or an expansion rate of 4%. The expansion of
the water in a central heating system, if not accommodated, will lead to
an increase in system pressure and, possibly, a component or
appliance failure as a result. In a sealed heating system the expansion
of water is accommodated for by the use of an expansion vessel. The
expansion vessel is a vital part of the installation will need to be sized
correctly.

Sizing an expansion vessel:
There are several methods for sizing expansion vessels. All methods
must take into account the volume of cold water in the system and the
amount by which it will expand in order to reach its design temperature.
The CIBSE method is shown below. If the system volume is known,
expansion vessels can be sized with the following formula:
V = the total volume of the expansion vessel
C = the total volume of water in the system in litres
P1 = the fill pressure in bars absolute (gauge pressure + 1 bar)
P2 = the setting of the pressure relief valve + 1 bar
e = the expansion factor that relates to the maximum system
requirements.

Where:
d1 = density of water at filling temperature kg/m3
d2 = density of water at operating temperature kg/m3

Example
A sealed central heating system has a total water volume of 600 litres.
The pressure of the water main is 1.5 bar and the pressure-relief
maximum pressure is 6 bar. The system is designed to operate at a
maximum temperature of 80°C, which means the expansion factor will
have to be calculated. The fill temperature of the water is 10°C.
Calculate the expansion factor using:
e = d1 – d2
d2
Calculate the expansion vessel volume using:
V = eC
1 − P1
P2

Example (contd.)
The temperature of the fill water is 10°C with a density of 999.8kg/m3
.
The maximum operating temperature is 80°C with a density of
972kg/m3
. Therefore, the‘e’factor is:
999.8 – 972 = 0.0286
972

Calculate the expansion vessel volume
V = The total volume of the expansion vessel
C = 600L
P1 = 1.5 + 1
P2 = 6 + 1
e = 0.0286
V = eC
1 − P1
P2
Therefore, the expansion vessel volume is:
0.0286 x 600 = 17.16 = 17.16 = 26.68l or
4.44%
1 – 2.5 1 – 0.357 0.643
7
So, the expansion vessel volume is: 26.68l or 4.44% of total
system volume.

Any questions?

Sealed system components

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