Air locks can occur in radiators, towel rails and in the circulating pump of a heating system. They must be cleared by means of “bleeding” the air.
Radiators. If any one radiator or a heated towel rail fails to reach normal temperature when others are operating correctly, suspect an air lock. First check that the radiator control valve is fully turned on. To clear an air lock you open the air vent valve which is usually located at the top of one end of the radiator. To open it you need a square-ended hollow key.
Place a bowl under the valve and turn the key to release the air. When the water flows, turn off the valve. Note: Where a radiator has a thermostatic valve in place of a hand-operated control valve, failure to heat up is probably due to a faulty thermostat. But first check that the thermostat is set at the required temperature. Never adjust the lock-shield valve. This is set when balancing the heating system. Circulating pump. If water does not circulate when the pump is running, the probable reason is an air lock in the pump. To “bleed” the air, open the air lock valve located on top of the pump, using a special key or a screwdriver. Place a bowl or drip tray under the pump and close the valve when the water flows.
Note: Every six months check that the pump is operating correctly. The temperature of the water flowing out from the boiler should be 35 deg F (7 deg C) higher than the water returning in the lower pipe. You can check temperatures by means of strap-on thermostats. If the temperature difference is much greater or smaller than 35 deg F the water flow through the pump must be adjusted — but that is a job for an expert.
What to do about scaling
In hard water districts, scale can form on the interior of pipes, the hot water cylinder and the boiler. It is usually indicated by hissing and knocking noises. Scale restricts the flow of water and, if very extensive, could cause an explosion. You remove scale from the system by introducing a chemical remover, which should be bought locally to suit the local water. Most “wet” systems are completely independent and the same water is re-circulated, but a chemical called an inhibitor should be added to water which is fed into the system after it has been drained and cleaned. A central heating system equipped with an indirect hot water cylinder containing a heat exchanger should not produce scale, for the water is continually recycled.
What to do about over-heating
If the water in the system is over-heating, indicated (again) by hissing and knocking and by the escape of steam in the expansion tank, the system is obviously faulty. Causes include: a damaged thermostat; a faulty circulating pump; a jammed ball valve in the feed water tank. As soon as you notice over-heating, switch off the central heating boiler or power unit. If it is a gas or oil boiler, turn off the gas or oil supply. If it is a solid fuel boiler, empty the hot fuel from the fire bed into the ash-tray and remove it. Allow the system to cool. This is assisted by keeping the flue fully open, opening the boiler door and, where possible, running the circulating pump to distribute the heat throughout the system. Call in an expert to diagnose the trouble precisely.