All primary pipework must be free from air locks. Although air locks can often be cleared at time of initial filling, simply by the force of water entering the system, some air will usually be left in the system. Air will also come out of solution when the primary water is heated. Along with any air introduced at a later date, this air can collect in pipework forming air locks, resulting in reduced water circulation. Typical symptoms include a loss of heat to radiator, poor recovery rates for hot water, system noise, and boiler overheating.

These air locks may be very difficult to clear so it is imperative that potential air locks in pipework are fitted with a manual or automatic air vent to allow air to be released. If automatic air vents are used then they must be under a positive head (with the primary pump both off and on) to prevent air from being sucked into the system.

Some installations, for example where pipework is run inside a suspended ceiling, do not provide access to the highest point in the pipework to operate an air vent. Even if venting is possible, the point of venting may be difficult to access. In these situations, a small diameter pipe should be taken off at the highest point and run continually downwards to a manual air vent with easy access. A label should be sited next to the air vent to inform any servicing engineers that when venting air, a small volume of water (equal to the volume of the down-pipe from the highest point in the pipework) will be discharged through the air vent before any air can be released. This label is important since a typical air vent, once opened, will first release any air, with the release of water indicating that all air is bled.

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