Hot Water Airlocks in Indirect Systems


Air trapped in the water supply pipework can cause homeowners considerable inconvenience, particularly when it becomes a recurrent problem. Central heating airlocks are generally easy to resolve and may simply require the bleeding of radiators.

When the air becomes trapped in the domestic hot water supply, dealing with it can become a little more complex.

The presence of low pressure at a hot water outlet may be caused by a number of problems. If the problem seems to be restricted to one outlet, it may be worth checking to see whether a filter is incorporated in the tap or device. Filters in tap fittings occasionally become blocked with limescale and other debris or mineral deposits. The filters can be removed and cleaned.

In old hot taps, damaged washers or excessive wear and tear can lead to flow problems. Older taps are better replaced with newer ones.

Internal filters in shower devices, particularly electric ones, are a specialist task and should be serviced by a competent person.

Where low hot water pressure occurs frequently, with or without air locking problems, it is important to check whether the mains supply to the water storage tank in the loft is sufficient. It might also be necessary to check the tank’s capacity to ensure it is sufficient to meet demand. Where water flows out of the tank faster than it can be replaced, the potential for air to enter the system is greatly increased.

Blockages can also occur in ball valves that restrict the flow of mains water into the tank. To check mains pressure at the ball valve, place a finger over the valve outlet and press down the ball cock. It will be impossible to prevent good mains pressure water from escaping around a finger. A blocked valve should be cleaned.

Airlocks may also occur if a storage tank runs dry when the mains water supply is interrupted, for instance due to a provider carrying out maintenance work on their system.

Where the mains supply is sufficient and yet all the hot taps are experiencing low or non-existent flow rates, an airlock in the system is the likely cause.

There are a number of ways to try to remove an airlock. If the mains water supply is delivered by a single cold-water tap at the kitchen sink, it is possible to firmly attach a length of hose from the spout of the cold tap to the spout of the hot tap. With the hot tap turned on, the cold tap should be gradually opened. Cold water at mains pressure will flow into the hot water system forcing any air through the system and back into the cold-water storage tank in the loft. The cold water should only need to flow for a few seconds to clear the pipes.

If the tap at the sink is a mixer type, it may be possible to cover the outlet spout with one hand. Then, with the other hand turn on the hot tap followed by the cold tap. Water will be forced from the cold into the hot tap completing the process outlined in the above method. A short length of hosepipe firmly attached to the spout and with both taps open can offer an alternative to using a hand. With the cold water flowing, tightly nip the hosepipe. This will force cold water into the hot water system.

On some mixer taps, non-return valves are installed internally to prevent backflow occurring. If this is the case, the above method will not work. However, an alternative method can be used. If a washing machine is installed and connected by hot and cold separate pipes, it is possible to turn off the supply valves to the hoses, disconnect them from the machine, join them together and turn on the hot, and then cold supply valves. Provided that the cold supply is at mains pressure, the cold water will force itself through the hot water system, hopefully taking the airlock with it. The supply pipes can then be reattached to the washing machine.

If these methods fail to remove the airlock, a final method is to turn off a suitable hot tap at its isolation point. This is usually a small valve located on the pipework supplying it. It is then necessary to dismantle the tap to a point where it is possible to attach a length of hosepipe. The other end must be attached securely to a cold tap supplying mains pressure. With the hot tap isolation valve open, turn on the cold supply and allow the cold water to enter the hot water system and flush out the airlock into the main storage tank. Once this is accomplished, turn off the main water tap. Close the hot tap isolation valve and reassemble the tap.

It is a good idea to have an observer stationed at the water tank in the loft with any of these methods to confirm successful operation and to warn of any danger of the tank overflowing during the process.

Repeated airlocks can sometimes be caused by air being pulled back into the system through the water expansion pipe. It is worth getting someone to go into the loft and cover the expansion vent pipe exit with their hand. At the same time have someone turn on the lowest hot tap on the circuit. This can force trapped air out of the system.

If these methods fail, it may be necessary to drain down the entire domestic hot water system.

First, ensure that the boiler and all pumps etc are turned off. Turn off the water feed to the storage tank. Then open all the hot taps to drain down the system. If any sludge is visible in the bottom of the tank, avoid allowing this material to be drawn down into the pipe-work. Sludge should be removed with a suitable scoop and bucket and the tank disconnected and cleaned correctly.

Once the system has drained, go round and close each hot tap until they are about three-quarters closed. Now turn the water supply to the storage tank back on. The tank will start to fill and eventually all the taps should have a gentle flow of water. Adjust each tap slightly so that the flow rate through each is similar. Starting with the lowest in the system, work upwards with each hot tap, turning them on further about half a turn. Return to the first tap and repeat the process again. Keep doing this until all the hot taps are fully turned on and all the air has been removed. The next step is to slowly turn them off until each tap has just a trickle. Let them run for a minute or so and then turn them all off. Do not forget to restart the boiler and any associated pumps when the task is completed. If the water pressure is particularly low, it may be necessary to fill the storage tank prior to operating the taps.

If this does not solve the problem, it is time to call in a professional to ensure that the problem does not lie elsewhere within the system.