Hot Water Cylinder

General Information

A solar compatible cylinder includes a second, longer coil, suitable for the lower water temperatures typically produced by solar. Coils are mounted one above the other, the solar coil at the bottom, resulting in a taller thinner cylinder. One or two electric immersion heaters can be included. It is also possible to have three coils and use hot water from a stove or range as well.

If you are replacing your cylinder for any other reason, but are not ready to install solar panels, you would be well advised to fit a dual-coil, solar compatible cylinder. The incremental cost over a normal cylinder is not great, but the subsequent cost of fitting a solar panel system would be much reduced. A similar argument applies when building a new house.

Copper or Stainless Steel?

Hot water cylinders are available in copper or stainless steel; while copper is cheaper, especially for smaller cylinders, stainless steel is the better product, and must be used if you require a pressurised hot water supply. A stainless steel cylinder usually includes an outer skin and is typically better insulated than a copper cylinder, for which the insulation is probably sprayed on. When a copper cylinder is replaced with a stainless steel cylinder it can noticeably reduce the temperature of the hot press, reflecting the reduced heat losses; this is a significant secondary benefit of installing solar.

What size of hot water cylinder is advised?

A cylinder size is chosen to meet the expected hot water demand; for solar it is usual to use a bigger cylinder than would be used for a conventional heating system, to allow for the slower but more continuous heating. 50 litres per person per day is the standard recommendation; so a normal size would be from 150l for a “small” household, up to 300l for a “large” household. See also Facts & Figures.

What type of water circulation system is advised?

A conventional indirect or vented heating circuit is fed from a header tank, which must be at the highest point of the system; because the solar panel is usually on the roof, which is already near the highest point, this is rarely possible for a solar water circuit. Instead a sealed pressurised system is usually required.

What about Legionnaires Disease?

Legionnaires’ disease is a type of pneumonia that is caused by Legionella, a bacterium associated with warm water environments. However, where backup heating is used to top up the water temperature in winter or on dull days, and where the hot water system is flushed regularly, there is usually no significant risk in a domestic environment. Legionella is killed by raising water to 60°.

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