Open-vented or sealed-system

This page is an explanation of the two different methods of filling, and keeping filled, a central heating system:

Open-vented central heating is where you have a 'Feed & expansion cistern or header tank' in the loft which fills the boiler and radiator system initially and keeps it topped up with water on a long-term basis to replace losses from evaporation and/or minor leaks. A float valve in the cistern (more correctly called a 'feed and expansion cistern', F&E tank) and connected to mains water keeps the water level in the tank and the heating system correct. The primary advantage of an open-vented system is that no user intervention is required to keep the system topped up with water.

Common problems that occur with 'open vented' systems:

1) Float valve seizes up and sticks closed. Then as the months and years pass by, the water level in the F&E tank drops to nothing, and then the water level in the heating system proper falls. Eventually there isn't enough water for the pump to pump and the system stops working. Sometimes boiler banging noises also happen, and tripping of the boiler over-temperature thermostat. Easily fixed by freeing off the float valve and re-filling the system.

2) Cold feed pipe blocks. This is more common that you might first think. The tee piece where the cold fill pipe joins the system proper accumulates corrosion deposits and they form a rock-like blockage which results in similar symptoms to 1), except the F&E tank remains full of water! The fix is to cut out the tee and solder in a new one.




Sealed-system (sometimes known as 'pressurised') central heating is where the F&E tank is dispensed with and replaced with FAR more complicated and prone-to-failure apparatus. Because the F&E tank is no longer available to accommodate expansion in the water contained in the system when it heats up, an expansion vessel is fitted instead. This is a sealed container fitted with a compressible air bladder connected to the heating circuit, and is often fitted inside the boiler.

The second device needed is a method of initially filling the system, and topping it up if/when the system pressure degrades to zero. This is called a 'filling loop', and is usually a silver-braided length of flexible pipe about 12" long which bridges from a mains cold water pipe to a heating circuit pipe, and has valves on both ends. The installer or user turns on the valves to manually fill the system, and turns it off again when filled and the pressure set correctly, and strictly speaking should disconnect the loop (but nobody ever does). There will be a pressure gauge on the system somewhere so the user can see to turn off the filling loop when the system is filled to the right degree, but it will not necessarily be next to the filling loop!

The third device required is a pressure relief valve (PRV). This opens should the system pressure ever rise dangerously high to prevent busting of pipes, radiators etc. It discharges excess pressure through a pipe to outside.



Combination Boilers

Combination boilers are by far the most popular boiler type, accounting for over half of all new boiler sales within the UK. Most wet central heating systems use a boiler to heat a copper cylinder in the airing cupboard to provide hot water. A combination boiler (usually referred to as a 'combi') heats water for the taps from within the unit and combines this with central heating. Combi boilers don't need a water cylinder on a cold water tank to feed it. Hot water running costs are therefore slightly lower. The water is heated as you use it and doesn't sit in a cylinder which is gradually going cold. However, the rate at which hot water comes out of the taps is not likely to be as good as with a traditional storage cylinder, particularly in Winter.

In homes where a number of taps and showers are likely to be used at the same time, you should seek manufacturers' advice on whether or not the combi is suitable. Showers fitted to this boiler must be a 'mains pressure' type. Because combi boilers are fed directly with mains cold water, there must be enough water pressure for them to work properly.