Unvented Cylinders

Unvented cylinders are a replacement for the traditional open vent hot water cylinder.

Heat Only Boiler diagram

The traditional open vent cylinder requires a cold water storage cistern above it to provide a supply of cold water to replace the hot taken from the cylinder. In addition a vent pipe from the hot cylinder/ pipe work terminates over the cold water storage cistern to provide a safety outlet in the case of excess temperature expansion.

The hot water pressure and flow generated in a traditional cylinder depend upon the height of the cold water storage above it that is used to replenish the cylinder. The height difference is known as “Head”. 10metres in height generates 1 bar of pressure. In a normal home the height difference is usually only 2 meter’s or so resulting in 0.2 bar pressure hot water with resultant flow and pressure issues.

The unvented cylinder provides a solution to the low pressure and flow rate issues.

The unvented cylinder is fed directly from the cold main passing through a pressure reducing valve to restrict pressure usually to 3 bar. A balanced cold supply to the household can be taken off at this point.

Heat Only Boiler

Instead of a vent pipe the unvented cylinder has a temperature and pressure relief valve to provide an external safety outlet against excess temperatures and pressures. It also has an expansion relief safety valve to allow for excess expansion.

Unvented cylinders tend to be of two main types, either “Bubble Top” or with an external expansion vessel. The bubble top types have either an air bubble in the top or a moving diaphragm with air above it to allow for expansion as the water in the cylinder gets hot. The external types have a separate expansion vessel to provide for this expansion.

It is normally recommended that a minimum inlet water flow rate of 20 liters per minute is available to run an unvented cylinder.

The distinct advantage of the unvented cylinder is the high hot water flow rates achievable and their ability to cope with shock demand.

They are more expensive than the combination boiler route as they are usually fitted with a system boiler and associated external controls. However the cost of unvented cylinder has fallen dramatically over the last few years.

Unvented cylinders can only be worked upon or installed by engineers with G3 Building regulations qualification.

Frequently asked questions

Unvented Cylinders

Yes. Most modern unvented cylinders are made from stainless steel and therefore are less affected by calcium deposits. They do not need a sacrificial anode.
However, ancillary pipe work and valves will need protection against hard water.

Typically we are looking for an incoming water supply flow rate of at least 20 litres per minute. The incoming water pressure is restricted to 3 bar by the pressure reducing valve on the cold water inlet.

  • High pressure and high flow hot water.
  • Ability to cope with shock/high demand such as multiple showers and bathrooms.
  • Highly efficient insulation and reduced energy cost.
  • Flexibility for direct or indirect cylinders.
  • Hot water can be generated from gas boilers, heat pumps,solar panels and solar PV.
  • The cylinder has an immersion heater to provide an emergency back up.
  • The heating coil in an indirect cylinder will typically operate at 17/18 kilowatt heat input. The heat recovery time is therefore very quick at around 20 minutes or so.

In normal operation when water gets hot it expands up to 10%. It therefore follows if it is I a sealed cylinder system such as an unvented cylinder this expansion could be dangerous if not provided for. A bubble top cylinder has an air bubble in the top of the cylinder so that when the water expands it can compress the air bubble.
The air bubble needs to be replenished regularly when the cylinder is serviced.
If the air bubble is depleted the expansion relief valve will begin to discharge.
An alternative to the bubble type, used on many unvented cylinders ,is a separate expansion vessel connected to the incoming cold water supply to allow expansion back down the cold supply pipe into the vessel which gas a rubber membrane to cope with the expansion.

A 200/210 litre cylinder will be in the region of 1650 to 1700 mm high and approximately 550 mm in diameter.
Slimline versions are available for tight spaces as are horizontal cylinders for use in lofts etc.
It must be remembered that water weighs 1 kilo per litre. A 200 litre cylinder is therefore nearly a quarter of a ton. Siting of the cylinder therefore needs to be correctly supported.

This depends upon a number of factors. The number of hot water outlets and the demand at any given time are the key issues. The unvented cylinder is a better solution for multiple outlet usage and can cope with shock demand where multiple outlets operate together. Combi boilers have a restricted hot water flow rate and are not always suitable for high demand.

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