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Do I need to change my hot water cylinder?

Probably, because your existing cylinder may be too small, and you must have a suitable means of introducing heat to the cylinder.

The hot water cylinder is used to store hot water ready for use.  Because solar heating is less controllable than a boiler or immersion heater, it is generally advisable to fit a larger cylinder than otherwise would be the case.  If the cylinder is too small, solar heat will be wasted during sunny periods once the cylinder water has reached its maximum temperature, and conversely, during dull periods solar heated water will be used up more quickly, requiring more use of a boiler.

A typical standard cylinder has one internal heat exchange coil which is connected to the boiler.  Hot water from the boiler is passed through this coil, and the heat is transferred to the water in the cylinder.  This is called an indirect system and keeps the water in the cylinder (which will become the water in your bath) separate from the water which runs through the boiler.  When a solar system is installed, a cylinder with an additional heat exchanger is used, and the hot water can be heated by either the solar or the conventional one as required.

Generally solar cylinders should be relatively tall and thin, with a heat exchanger at the top for a conventional boiler, and one at the bottom for the solar loop.  This is because of the following.  A usual basic requirement of a hot water system is that hot water is available at all times.  This means that there must be hot water in the cylinder.  However this presents a problem for a solar system in that the solar heating is unpredictable, and if the cylinder is full of hot water the solar energy would be wasted.  This is resolved by the arrangement mentioned above.  A fairly large cylinder, typically at least 200 litres, is used.  The boiler, when necessary, ensures that the water at the top of the cylinder is hot at all times.  Because cold water is denser than hot, this hot water will float on top of the colder water.  It is possible for water at the top of the cylinder to be at 60C and that at the bottom 10C, with a remarkably sharp transition between them.  This is called stratification, and works best with tall thin cylinders.  Thus, when solar heating is available during the day there is cold water to heat.  When water is drawn off at the top, cold water enters at the bottom and remaining hot water moves up.  This is why it is possible to draw off almost a full cylinder worth of hot water with little decrease in temperature.

Vented or Unvented?

A vented cylinder is fed by a cold water tank, often in a loft.  This is sometimes called a feed and expansion, or F & E tank, because as the water in the cylinder is heated and expands, it can escape back up the pipe into the tank.  The outlet pressure is provided by the head of water between the cold tank and the tap.  This pressure is generally less than cold mains pressure, sometimes greatly so, and can make showers seem lacklustre.  In a very unusual situation where the water in the cylinder boils, for instance if an immersion heater was to become stuck in heating mode, steam would pass back up into the cold water tank without too much drama.  An unvented cylinder is filled directly by the mains cold water.  Expansion is taken care of by an expansion vessel similar to the type installed on solar loops.  Various safety devices are employed to prevent the build up of excessive pressure.  Advantages of the vented system are relative low cost, ease of installation, and lack of interest from building control who administer the building regulations (ie you can do it yourself).  The main disadvantage is lower pressure hot water, plus space is required for a cold water tank somewhere higher up.  Unvented systems cost more because of the safety equipment required, and because they must be stronger.  Also, they must be installed by a registered installer, and require (although don't always get!) an annual safety inspection.   However, they provide mains pressure hot water.

It is possible to some extent get the best of both worlds using a thermal store.  Here, water is heated and stored in a large cylinder but not used directly.  Mains cold water is heated via a heat exchanger which is heated by the water in the store.  This gives mains pressure hot water without worrying about the regulations surrounding storing a volume of water at pressure.  A thermal store may also be the best way to utilise multiple heat sources, eg solar, biomass and heat pump.

For more information about thermal stores contact:

Dedicated Pressure Systems Ltd.
0845 241 1441

Inventors and manufacturers of the Heat Bank range of thermal stores, specialising in the design of multifuel hot water and central heating systems.




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Last modified: 30-06-10