What I've found after getting into working on unvented cylinders about 10 years ago, most people don't know what they do and that they need yearly maintenance and inspection.
The water comes into it at the bottom, that water is then heated up inside the cylinder by the boiler or the immersion heater.
So when no taps are running the water is just sat at mains pressure, when water is heated it expands, the expansion vessel (fitted on most models) absorbs this extra pressure so its not pushing on the walls of the cylinder.
There are various safety valves/devices fitted on unvented cylinders, these let water out when the pressure reaches a certain level should the expansion vessel fail, which they do, a lot. So an annual inspection is required to check the expansion vessel and these safety devices, and manufacturers require an annual service to maintain the warranty should the cylinder fail and leak.
Here is a list of checks that should be carried out during a service (as a minimum);
- Checking water flow rate & temperature
- Cleaning of line strainer where possible
- Testing of pressure relief valve.
- Testing of temperature/pressure relief valve.
- Testing of safety cut off devices.
- Testing of immersion condition/resistance.
- Check drain off works.
- Checking expansion vessel pre charge/reinstate air gap.
- Visual inspection of the unit.
Any engineer carrying out work on any unvented cylinder must have their unvented certification (referred to as G3), here at HiPro Heating we only have G3 qualified engineers and cover the Yorkshire area.
Cylinders that haven't been maintained or inspected can and do split. If this happens it just like opening a tap onto the floor and can wreck properties, especially apartments where the water can travel down 5 or more floors.
I have known home insurance policies that have been void when a cylinder has split, as they can class the lack of maintenance as neglect. They then won't cover the cost of replacement and repair to the property.
With lots of new build properties, particularly in Wakefield at the moment, using unvented cylinders to meet the hot water demand of 2+ bathrooms, I think people need to be made more aware of the importance of servicing and maintenance of their units.