Today, more and more home heating systems are sealed systems. Heating engineers who design and work on such systems need to understand the importance of sufficient system expansion so, in this blog, Phil Stanley, Grant UK’s Training Manager, takes a closer look at this topic and explains the importance of the expansion vessel.
Understanding the expansion vessel
Since their introduction into the UK in the early 1980’s, sealed systems have become the norm for domestic heating systems. A sealed heating system must include all of the following components: expansion vessel, pressure relief valve (PRV) (sometimes referred to as the safety valve), pressure gauge and filling loop. Of these items, perhaps the expansion vessel is the one that is least understood.
As we all know, when water is heated it expands and wants to take up more volume. As the system is basically a closed loop, consisting of the boiler, pipework and radiators, it is necessary to provide some space for the water to expand into. This is why the expansion vessel is required.
An expansion vessel basically consists of a container divided by a watertight rubber membrane – with the system water on one side and usually nitrogen or air on the other side.
To check and either increase or decrease the pressure on the air or nitrogen charge, a Schrader valve is fitted on the air side of the expansion vessel. This is exactly the same type of valve used on a motor car tyre. In fact, if you have a leaking Schrader valve, causing the air pressure in the vessel to continually fall, it may just require the valve insert (available from any car or bike accessory store) to be replaced!
Selecting the right size
Using the correct size of vessel is vitally important for the system to operate correctly and also to avoid potential damage to the boiler.
The correct vessel size is dependent on several factors:
- The pressure relief valve setting
- The vessel charge/ initial system fill pressure (when cold)
- The total water content of the system (including all pipework, radiators and the boiler itself).
With this information the required expansion vessel size can be easily found using the selection table given in the ‘Sealed Heating Systems section’ of the CIBSE Domestic Heating Design Guide (or in BS7074-1:1989). The CIBSE Domestic Heating Design Guide also gives a method for estimating the system water content.
Some boilers come with a factory fitted expansion vessel. Common examples of these are most Combi boilers or System boilers, but don’t forget to check the vessel size required for the total system water content (as above). It may be that the vessel fitted in the boiler is not big enough for the system and an extra vessel is required to make up the difference.
Checking the air charge pressure
It must be remembered that the air charge pressure for an expansion vessel can only be correctly checked when the system is cold and the system pressure is zero. The initial air charge pressure should always be greater than the static pressure of the heating system at the vessel.
For example, if the height to the top of the highest radiator from the centre of the expansion vessel is, say, 4 metres, this would be a static head of 0.4bar at the vessel. So in this case the air charge pressure would have to be a little higher than 0.4bar when measured at the Schrader valve on the expansion vessel.
Typical air charge pressures are 0.5bar for bungalows and 1.0bar for two storey houses. When the system is filled, the system pressure (visible on the system pressure gauge) should ideally be 0.2 bar greater that the air charge pressure. So, if the system is in a two-storey house with an air charge pressure of 1.0bar, the system pressure (cold) would be 1.2bar.
Remember…it is important to check the system and expansion vessel air charge pressure on every service to ensure that the vessel is still operating correctly and is correctly set for the system in which it is fitted.