What is an expansion vessel in a combi bolier?
An expansion vessel or expansion tank is an important part of a pressurized central heating system. It can be found on combination boilers, system boilers, pressurized central heating system or pressurized hot water cylinders.
What is the function?
Water is heated through central heating boiler. When water heated to certain temperature its expands. Water can not be compressed. An expansion vessel takes the expanded water volume by compressing the air in the chamber.
Why water is dripping from pressure relief valve?
Its is necessary to check the air pressure in the vessel regularly. Highly trained boiler engineers can tell by checking the sound of the vessel got full of water or air by gently tapping the vessel.It should be done during annual boiler service. If the vessel loses air or the expansion vessel connection pipe become blocked due to sludge or the membrane got worn out, expansion is lost in the central heating system. The expanded water has to go some where safely. The heating system is protected by the pressure relief valve. So the safety system is activated.
I always ask my customers how often you top up you car tyre pressure? Same principle. Vessel need to be checked and charged if necessary.
Why my heating is taking too long to warm up?
A typical 3 bed room house central heating capacity is 100 litres of water. A normal combi is fitted with 8 litres expansion vessel. Large boilers can be 10 litres. If the vessel is under sized or become redundant. Expansion can’t take place. So the boiler will cut in and our quickly. This is one of the reason heating takes too long to warm up.
Do I need an additional expansion vessel?
If you live in a large house or have got many double radiators, may need an additional expansion vessel. Water expand 4% when heated. A 150 litre central heating system need minimum 12 Litre expansion vessel. This would be a 4 bed room house.
Why I need to fill my boiler regularly?
If you are regularly topping up your boiler it may be few things to check before call an expert heating engineer Check for any water leaks. If you can’t find any visible leak it may be a flat or redundant expansion vessel or leaking main heat exchanger.
How do I check and recharge expansion vessel?
It’s a job for the heating professional. During the annual boiler service engineer will drain the system(partially) and using a cycle pump to re charge the vessel according to manufacturer instruction. When we found out the vessel is faulty we will replace the Shrewder valve first to see vessel hold air charge if not we will replace the vessel. If necessary we will replace the pressure relief valve. Normally a good clean will do.
Some of our recent work
A large house with 17 radiators. We installed a Worcester Bosch 30 CDi system boiler (10 litre internal expansion vessel). We also put an external 12 litre additional vessel to take care of the heating system. It was sited in the airing cupboard.
We were called out for a boiler repair in Hinckley. There was no power to the boiler. When our customer turn the boiler on it trips the fuse board. On arrival we found out boiler is leaking on to external diverter valve.(Honeywell 2 port valve). Auto air vent on the top of the boiler was leaking. The water was travelling through condense pipe and dripping onto diverter valve. We replaced diverter valve motor head, expansion vessel and auto air vent. Also installed an external 12 litre expansion vessel, because a large detached house with 21 radiators.