Hot Water Air Lock - Vented System

With vented hot water systems the hot water may be either direct or indirect, but both have a hot water cylinder where the water is heated and stored. This cylinder is supplied with cold water from the cold water storage cistern in the attic to replace water being drawn off at the taps. It is most likely therefore that if you have a hot water air lock in your supply, this is due to the cold water storage cistern running dry.

Cold Water Storage Cistern Running Dry

There are a number of reasons why this might happen. The size of the cistern may be insufficient to meet the amount of water being drawn, the ball valve controlling the filling of the cistern may be faulty or stuck, or the water supply to the cistern may have been shut off or blocked.

If the cistern runs dry regularly and cannot cope with the amount of water being drawn, you will get frequent hot water air locks and it would be advisable to fit a larger capacity cistern. If the ball valve is stuck or faulty, repair or replace as necessary. A blockage may be caused by a frozen supply pipe. Another reason for there being no supply might be if the mains in the street have been turned off to allow repairs to be carried out.

Spluttering Hot Water

If the water being drawn at a hot water tap splutters regularly and is not being caused by an air lock, it may be because air is being pulled in via the vent pipe. If the water level in the vent pipe drops below the point where hot water is being drawn off between it and the cylinder, air can get drawn in here, resulting in air and water coming from the tap. Fitting a larger diameter vent pipe may resolve this.

Clearing Air Locked Pipes

This is a two person job. The aim is to use the mains water pressure to push the air lock back up through the hot water system and one person will need to watch what’s happening at the vent pipe and cold water storage cistern in the attic. Before starting, switch off the boiler and immersion heater, and allow the water to cool.

Hot Water Air Lock Between Cylinder and Taps

Attach a length of garden hose with a suitable backflow prevention device to the mains cold water tap at the kitchen sink using a jubilee clip to secure it. Attach the other end to the hot water tap at the sink, again using a jubilee clip to hold it in place. The mains water pressure can push the hose off if it’s not properly secured making quite a mess.

Open the cold tap gradually and this will force water back up the hot tap, forcing any air trapped to be pushed up to the hot water cylinder where it will escape via the vent pipe. When water begins to discharge at the vent pipe, close off the cold water tap and then the hot.

Hot Water Air Lock Between Cylinder and Cold Water Storage Cistern

This will require a longer hose. Identify the drain point at the bottom of the hot water cylinder. Attach a length of garden hose with a suitable backflow prevention device to the mains cold water tap at the kitchen sink using a jubilee clip to secure it. Attach the other end to the drain valve on the cylinder and once secured with a jubilee clip, open the valve. Open the cold water tap at the sink gradually and this will push any air locked water back up the feed pipe to the cold water storage cylinder. As soon as the water here begins to rise, or water comes from the vent pipe, switch off the cold tap at the sink. Close the drain valve on the cylinder and remove the hose.

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