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Direct Storage Cylinders
Some System Considerations
Calorifiers can be supplied for open vented or unvented (sealed) systems. In open vented systems the vent pipe allows escape of air from the calorifier, ingress of air during drain-down, thermal expansion of water and (in the event of control failure) escape of steam from the calorifier. The vent pipe should never be blocked. No valves should be fitted to it except, where more than one calorifier share a common vent, special 3-way vent/bypass valves. These ensure that the calorifier is always open to atmosphere.
Most large systems circulate DHW around a building and back to the cylinder. This ensures that all draw-off points have hot water available quickly. The pipe-work should be lagged and the re-circulation rate minimised to reduce heat loss. The heat loss should be taken into account when selecting a cylinder.
When it is not practical to fit a vent, an unvented system will be used. Certain additional precautions and equipment are necessary to ensure that an unvented system will be safe:
- The calorifier must be designed for the maximum working pressure - after thermal expansion of the water
- A temperature (or combined pressure/temperature) relief valve must be fitted in case of control failure
- An automatic air vent
- An anti-vacuum valve
- An expansion vessel & expansion vessel isolating valve (lockable)
- An expansion relief valve (to protect the expansion vessel)
A water booster set may be required to provide water at the required pressure and flowrates.