Wondering why you need an expansion vessel? The reasoning behind it is simple. Your central heating system contains a closed pipework filled with water. The boiler heats the water and pumps it through your radiators.
All substances, including water, expand when they’re heated. So as the water in your system heats up, it tries to expand. If the system were completely sealed, the hot water would burst the pipes and spray all over your house.
We don’t want that to happen, so we use an expansion vessel – it’s basically a rubber balloon inside a metal tin, which is joined into the heating pipes, usually near the boiler.
As the water heats up and expands, the balloon stretches to accommodate the extra space. When the system cools down again, the balloon contracts, keeping the water at a constant pressure the whole time.
Another (more old-school) way of achieving this is to use a header tank, sometimes called a feed & expansion tank. This is a small water tank that’s open to air, and located in a ceiling space (above the highest point in the system).
The header tank is connected to the heating system so that when the water in the heating system expands, the water level in the tank simply rises a bit. When the water cools down, the tank level subsides.
Because it’s open to air, some water will evaporate from the tank, so it’s fitted with a ballcock valve to top it up when the water level gets too low.
So which is better?
A header tank requires more space and cost to install, but has fewer ways it can fail. The expansion vessel stops any evaporation, so it’s more efficient. It’s also cheaper and easier to install.
But if the rubber membrane fails and isn’t repaired then it can cause serious damage to the system because there’s no way to maintain pressure.
Each option has its proponents. I favour the expansion vessel as it saves money and space. They’re generally reliable if you get a reputable brand. The system should be serviced annually anyway, so that’s a god time to test the expansion vessels.
However there’s no right or wrong answer. Both solutions work and it’s just a weighing the pros and cons and choosing what’s more important to you.