Draining and Cleaning a Central Heating System

How to give your central heating system a spring clean

With so many components in a central heating system, something must go wrong somewhere in its lifetime. The two main types are the open-vented and the sealed system.

Common Heating System Problems

The open-vented system makes use of a feeder tank in the loft, which fills the boiler and radiator system through gravity-feed and keeps it topped up with water. It works on the same principle as a toilet cistern and is called the ‘feed and expansion tank". The sealed system does not have a feeder tank and is more complicated and if something goes wrong it's best to call in a boiler engineer or qualified plumber. Make sure that the person working on your system is fully qualified.

The most common problems with the open-vented system include the float valve that eventually may get stuck. As time goes by the feeder tank runs empty while the inlet valve is stuck in the closed position, followed by the water level in the system itself. Eventually the pump will run dry and the system will stop. You may be warned by banging noises or by the over-temperature thermostat that trips. Freeing the float valve and refilling the heating system will fix this.

The cold water pipe that feeds the system proper may also become blocked by a build-up of corrosion deposits and similar symptoms will occur as with the first problem, but the feeder tank will remain full. The tee piece that connects to the system would normally be the affected part and must be replaced by soldering in a new one.

Sometimes water discharges from the open vent pipe into the feeder tank when the pump is running. This can be caused by, again, corrosion deposits in the cold fill tee piece or by poor pipework design. This must receive immediate attention as the circulating water gets re-oxygenated, which leads to accelerated internal corrosion and it also constantly fills the loft with steam. It may also heat up the feeder tank to the point of collapse.

The central heating system should preferably be serviced once a year to keep the system in prime working condition. This might save the owner a lot of trouble in the long run and prevent unpleasant surprises. There are some things the owner can do to maintain or service the system. In the case of removing sludge and other deposits the heating system can be flushed quite easily. It must be drained first, especially if a cleaning agent is to be used.

How to Drain Your Central Heating System

To begin with, the boiler must be turned off by turning off the gas or electricity supply to the boiler or any other means of heating. Then the water feed must be stopped. As with a toilet cistern, water from the mains is fed into the feeder tank, which in turn lifts the ball cock. When the ball cock has risen high enough an arm attached to it will close a valve and stop any more water flowing in from the mains. As the water level in the tank drops, the ball is lowered and the valve is opened again, letting in more water to correct the water level.

Before draining the system the valve must be kept close by keeping the ball cock in the highest position. It can be tied to a piece of timber across the tank to ensure that the valve stay closed. Now a hosepipe can be connected to the outlet at the bottom of the radiator and run outside to a suitable point. Open the drain valve to drain the water from the system. By opening bleed valves on radiators, starting with the top ones, the water will drain faster.

Cleaning the Central Heating System

When drained, test if it's really empty by temporarily opening the inlet valve of the feeder tank in the loft. If the water flows through and out at the end of the hosepipe, you can start the servicing. If the water does not run through, there is an air lock. To remove it the process must be briefly reversed by connecting the other end of the hose to the cold tap. Briefly open the tap to send a burst of water back into the radiator being drained. Repeat the test and keep flushing until no more sludge is coming out.

Radiators that are to be replaced can now be replaced. Close the radiator outlet and all bleed valves that are still open. Release the ball cock in the feeder tank to open the valve and fill the heating system again. Bleed the system by starting from the lowest radiators, working upwards until the whole system is completely filled.

When you are sure that all nuts and valves that must be closed are properly closed, you can turn on the power supply and light the boiler.

It may be necessary to rebleed the whole system as it heats up and trapped air expands. Check for any leaks.