Recess: Expansion vessel

The task of an expansion vessel in a heating system is to enable the volume of the water to increase. There are two types of expansion vessel, Open expansion vessel and closed expansion vessel.

Open Expansion vessel

An open expansion vessel differs, as the name suggests, from a closed expansion vessel where the expansion vessel is pressurized. The open expansion vessel is often placed at the highest point of the heating system, which is usually the attic, and then the expansion line must go straight up from the boiler. The expansion line must be larger than DN20 for copper pipes and larger than DN25 for steel pipes, this to meet the requirement that the expansion vessel must be more than 5% of the heating system's total water volume. It is customary to install a safety valve on the expansion line to minimize the risk of freezing damage.

Closed Expansion vessel

The closed expansion vessel does not need a pipe that goes up into the attic, but is a pressurized vessel that contains some form of rubber membrane or balloon. The rubber's job is to keep liquid and air separate and to be able to handle pressure changes. Nowadays, it is mainly the closed expansion vessel that is installed for new heat pumps. It is important that the expansion vessel is properly sized, otherwise the safety valve will release too early because the more liquid you have, the more volume change it can make.

There are two different types of closed expansion vessels, those with a balloon and those with a membrane (see picture at the bottom). There are several differences between these two:

• The membrane expansion vessels are usually a bit cheaper than the balloon expansion vessels.
• With a balloon, heat loss is reduced because the expanded water in the balloon is not in contact with the metal and rubber conducts heat worse than metal.
• There is less risk of contamination with a balloon as the water does not come into contact with the steel.
• There is a higher degree of efficiency on an expansion vessel with a balloon, as this can expand in all directions, unlike the membrane.

How can the expansion vessel break?

The rubber inside the expansion vessel is usually of very good quality and usually lasts many years. But the rubber is worn by frequent pressure changes that cause the rubber to stretch and contract. The rubber also wears more in contact with water and also the quality of the water, i.e. impurities such as magnetite.

The most common manufacturers of closed expansion vessels are:

• Zilmet
• CIMM
• Flamco
• Altech (Dahl)
• Heat pump manufacturers' own
• a-collection (Ahlsell)
• Winkelmann
• Varem
• Imi (Balloon/bellows)

What happens if the expansion vessel breaks?

Expansion vessels are a vital part of a heating system. If the expansion vessel breaks, it is usually that the rubber has lost its ability and the liquid in the heating system has entered or passed through the rubber membrane. This ultimately means that the function of the expansion vessel to regulate pressure does not work, as the rubber bladder does not work, and that the liquid cannot expand. This in turn leads to the pressure in the system increasing and the safety valve opening.

It is a relatively cheap spare part, but due to the fact that they are often built into the heat pump, the work of accessing and replacing the expansion vessel becomes expensive. There are some heating systems that have external expansion vessels, which makes the work less complicated and thus cheaper.