Defects in hot water systems
Generally speaking, most problems in domestic hot water systems are the result of lack o knowledge
and appreciation of basic plumbing principles.
Air Locks
Air locks are a very common cause of trouble with both hot water and heating systems.
Most air locks are caused by unventilated arches formed in badly fitted pipe work. An air lock is a small
quantity of air trapped in a pipe which due to the very low circulating pressure available prevents water
passing through the pipe. Even if an air lock does not completely stop the flow of water it can reduce
the flow considerably. Air locks are very often the results of pipes sagging, or not being laid truly
horizontally or to appropriate falls.
Fig 1 shows how an air lock occurs in a pipe run, although they are not always as obvious as this.

Another common cause of air locks  where the cold feed, instead of falling away from the cistern, rises causing an unventilated arch in the bend.

Air may also be locked in a boiler due to defective tapping

Boiler manufacturers are aware of this and usually the flow connection is the position

Probably the worst effect of defective tapping would be a rather noisy boiler. Care must therefore be

exercised when installing boilers and primary circulating pipes to avoid collections of pockets of air.

Yet another common cause of air locking is where the hot draw off has been badly fitted

causing it to sag and create an air trap at the cylinder connection.

Scale Deposits

The formation of scale deposits in temporary hard water areas can obstruct the primary circulating

pipes, but this is unlikely to happen to both pipes simultaneously as the flow pipe carrying the hotter

water tends to scale up first and the resultant noises in the boiler would prompt and early investigation.

Other common faults in hot water systems.

1. Undersized distribution pipes.

These will result in poor supply of systems with low head pressure. In the main’s fed systems they can result in noisy operation. If the circulation pipes are too small in diameter, the time required to heat the storage vessel can be excessive because of sluggish circulation.

2. Over sized distribution pipe.

These will result in a waste of water and fuel due to dead legs run offs before hot water reaches the taps.

3. Air locks

These are usually caused by badly installed pipe work. If the pipe is installed so that it forms an unventilated arch shape, it will then trap air. The very small circulation pressure will not be able to overcome the problem and the cylinder will not get hot. Similarly if hot horizontal runs of pipe sag because of inadequate support or are installed with a back fall an air lock can result. Air locks can cause noises in the system in the pipe work or from the boiler due to lack of water circulation.

4. Boiler explosions.

Fortunately these are rare: however an explosion can occur due to blockages in the cold feed and open vent pipe due to scale formation.

In the larger installations, it is often the case that boilers are fitted with isolation valves for maintenance purposes. These must be operated with care by experienced personnel who understand the operation of the system where boilers are fitted with pressure relief valves; it is good practice to check their operation periodically.

5. Cylinder collapse

This can result if the open vent and cold feed pipes become blocked. If a hot tap at a lower level in the system is opened siphonage can result in atmospheric pressure inside the system.

If the storage vessel is made from copper, it will have quite a thin wall, air pressure acting on the outside of the vessel can then crush it.

6. Boiler noises.

Where scale forms inside the boiler heat exchanger, it can have an effect of an insulator, this can make the boiler less efficient, it can also reduce the water flow and this can lead to the boiler becoming noisy in operation.

7. Corrosion.

Because the system operates hot, the rate of corrosion will be increased compared to cold water system in the same building.

Therefore it is necessary to select materials for pipes and fittings that are able to stand up to the service conditions required if a long trouble free life is to be provided to your system.

 other causes can be found by your experience at site.

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