There are three particular type of boiler installations available in today’s market and these are combination, system and open vented. if you have central heating in your home then you will certainly have one of these types of installation, this will vary from home to home.
What is an Open Vented Central Heating System?
A central heating system is called open vented if there is an open vent pipe which is bent over the top of a feed and expansion tank (F & E tank). The F & E tank is often called a header tank. It allows water to be added to the heating system to keep it topped up and it accommodates the expansion of the hot water.
In an open vented type of central heating system there is a small water tank which is usually in the loft. It is always higher than the highest radiator. The F & E tank has a pipe carrying the mains water supply which enters through a float valve at the side of the tank near the top. There is another pipe called a vent which is bent over the top of the F & E tank. The vent pipe has an open end. This forms part of an open vented central heating system. There is a third pipe near to the bottom of the tank which is connected to the central heating system. The F & E tank allows water through the float valve to flow down this third pipe to add to the central heating system to keep it topped up. This is how the feed and expansion tank feeds the system.
When the central heating is switched on, the water in the heating system gets hot and expands and takes up more space (it has a bigger volume). As the hot water expands it runs back up the third pipe (see above) into the F & E tank. This is how the feed and expansion tank allows for expansion (of water) in the system. The water in an open vented heating system is at a low pressure (typically 0.1 to 0.5 bar). The water contains additives and inhibitors which slow down the rate of corrosion of the boiler and the radiators. If there is no F & E tank, the central heating system is called sealed.
What is a sealed system?
A central heating system is called sealed if there is no vent pipe which is open to the air. If you do not have a feed and expansion tank in your loft, then you more than likely have a sealed central heating system.
Water can be added to the heating system to keep it topped up by using a filling loop. The pressure of the water in a sealed heating system is usually around 1 to 1.5 bar. The water in the heating system contains inhibitors which slow down the rate of corrosion of the boiler and the radiators.
The expansion of the hot water is accommodated by an expansion vessel. The expansion vessel can be anywhere in the heating system or can be inside the boiler. The expansion vessel allows the hot water in the heating system to expand while maintaining the water pressure. If the water pressure in the heating system gets too high (above 3 bar) then a pressure relief valve will open and discharge water from an overflow pipe. If this happens it shows that there is something wrong with the system and professional advice should be sought immediately.
What is a combination boiler system?
A combination boiler (also called a combi) provides hot water for central heating radiators and also heats water as it is required for hot water taps and showers. There is no need for a hot water cylinder (compare this with a heat only boiler) or for water storage tanks in the loft (which frees up loft space for other uses).
A combi boiler is part of a sealed central heating system. The boiler usually contains the pressure gauge and expansion vessel. It may have a temperature control on the front panel. In a property with a combi, cold water at mains pressure (high pressure) runs to all of the cold taps, showers, toilet cistern and other services. Cold water also runs through a pipe to the combination boiler which heats it as it flows through the boiler. The heated water runs through pipes to all of the hot outlets, taps and showers.
The water system with a combi boiler is called a high pressure system because all of the pipes contain water at a high mains pressure. The hot and cold water is at the same high pressure but the flow rate of hot water through the taps and showers may not be constant if more than one outlet is used simultaneously. This this can be a problem for showers. There are two types of combi boiler, the instantaneous combi and the storage combi.