we have a combi boiler and 10Kw electric shower, which is plenty fine for us. When the combi was installed the plumber disconnected the cold water tank ... not sure why, but we have very good mains pressure ....
are there any reasons we should have a cold water tank ?
A standard cylinder would not be able to with stand mains pressure, and would split. A direct cylinder costs around 5 times more then a standard. So unless there is no space for a water tank the standard cylinder is the cheaper option.
The only benefit that I can see from a direct cylinder is mains hot water pressure.
Just a small point. A direct cylinder is one where the water is heated by passing it directly through the boiler rather than having an arrangement with a coil in the cylinder with separate water that is part of the heating system water fed through it. These were more common when hot water was generated from back boilers etc. and CH not necessarily implemented.
A cylinder that is fed directly from the mains is referred to as a pressurised or sealed cylinder. THe copper is thicker, and there are arrangements with an integral or external pressure vessel or other arrangement filled with air or nitrogen to absorb the expansion of the water as it is heated. There are also pressure and temperature safety valves.
An open vented cyinder uses the tank in the loft to absorb the expansion and the safety is through the vent pipe going up and over the top of it.
Where a cylinder has a coil for heating the water, it is referred to as indirect and can be either open vented or sealed.
A sealed cylinder does have the advantage of not needing a roof tank (if space is of concern).
Since the combi is directly fed from the cold mains (and therefore all of the taps/showers in the house) there's no need for a cold water tank (or any other tanks for that matter).
Our previous house had a combi (Vaillant Sine 25W) and was excellent. Our current house has tanks and boiler (Ideal Standard [Stelrad]). Wish we had the Vaillant... sigh!
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