Types of central heating systems

There are two main types of systems: vented and sealed. They vary in the way they are set up and have different components, yet each satisfy the requirements of the central heating.

Two fundamental requirements of central heating
For your central heating to operate both safety and effectively there are two main requirements that must be addressed. The first is expansion, think about what happens when water is heated – it expands, this expansion needs to go somewhere. The second is the ability to top up the system with water. Over time the central heating system will lose water and needs a way to refill in order to run correctly.

Open vented central heating

Vented systems have an open vent pipe which is terminated above a feed and expansion cistern. This allows for expansion when the central heating water is heated. The feed and expansion cistern as the name suggests also satisfies the second requirement– to refill. Water is fed directly into the system and requires no intervention. Water pressure in this type of system is much lower at around 0.1 to 0.5 bar.

Advantages and disadvantages of central heating with an open vent
+ No intervention is needed to refill the system.
+ The likelihood of leaks on the system is reduced as it operates at low pressure.
– Feed and expansion cisterns need to be maintained.
– Additional pipework is required.
– As the system is open to the atmosphere oxygen enters the system and speeds up the rate of corrosion. This increases sludge (black iron oxide) related issues.

General information about vented systems
– The feed and expansion cistern is referred to by a few different names: header tank, feed and expansion tank, F&E tank.

– The cistern will be located at the highest point of the heating circuit.

– Most cisterns will have four pipes: a cold feed that relies on a float valve, a pipe that feeds the central heating, an overflow pipe and an open vent pipe. The cold feed will hopefully have an isolation valve to stop the flow of water. If not you will need to switch off the mains stopcock.

– Modern cisterns are made out of plastic, they must have a solid base such as marine ply. Resting over joists is NOT a solid base. The cistern should be covered and insulated to prevent freezing, so too should the pipework.

– A common problem is float valves failing, causing the cistern to fill and drain via the overflow pipe. This should be addressed immediately, switch off the cold-water supply and have the defective float valve replaced by a competent plumber such as Boilers Direct Scarborough.


Sealed systems

Sealed systems are more modern, and take up the expansion of heated water using an expansion vessel. The expansion vessel is precharged with air pressure equal to the normal pressure of the system (1 bar), when the pressure rises it is taken up by the expansion vessel. In order to fill the sealed central heating system a filling loop is required. The filling loop must be connected to a mains cold water supply and be protected by a non-return valve, to prevent contamination. A pressure relief valve (PRV) is also required should the expansion vessel fail and the pressure rises above 3 bar. The PRV will discharge water from the system to outside and in doing lowering the pressure.

Advantages and disadvantages of sealed central heating
+ Cleaner systems with less sludge related problems, as the water is sealed from the atmosphere. So corrosion is slower.
+ Higher pressure helps with heat up times of radiators.
+ No requirement for feed and expansion cisterns or open vent pipes.
+ You know if you have a leak on the system as the pressure will drop.
– Every so often you will be required to manually top up the system.
– Expansion vessels can perforate requiring replacement, which can be costly.
– PRV can let-by which will let most the water out of the system, a faulty PRV needs to be replaced.

General information about sealed systems
– The majority of new boilers are sealed systems.
– Easier to install than open vented systems.
– Sealed systems have a filling loop and a pressure gauge.
– The ideal pressure the sealed central heating system should be is between 1 and 2 bar.
– Pressure in the heating system will drop if there is a leak.
– You should only have to top up the boiler every now and again about once or twice a year. If you are topping it up more than this you have a problem, it might be the expansion vessel, PRV or leak on the system.

Sealed systems are superior to open vented
Sealed central heating systems are better than open vented systems. This is due to faster warm up times, the ability to know if you have a leak on the central heating circuit, cleaner systems with less sludge and no requirements for cold water storage cisterns. When you require a boiler replacement choose a sealed system.