Connecting Unvented Cylinders to Solar Systems

In the UK the G3 regulations specify the three tier level of protection required to ensure safety. All unvented installations are controlled and installers must be qualified to be able to certify the installation and sign the commissioning certificate.

A pressurised cylinder should only be filled by a competent person. A pressurised cylinder incorrectly connected has the energy potential to destroy a house. Under Irish law, a plumber is considered an expert in the field, and must inform himself of the dangers of any installation. If a dangerous system is installed and fails in a manner that causes injury or death, then the plumber is liable to be charged for manslaughter.

Safety interlocks on pressurised vessels

As the hot water cylinder is a pressurised vessel, there are important safety considerations.

Pressurised cylinders should never be connected directly to uncontrolled heat sources such as solid fuel.

The danger of unvented cylinders are both temperature and pressure related. An unvented cylinder operates above atmospheric pressure therefore the boiling point of water is raised. Water at a 3 bar gauge pressure will boil at 143C. If water was stored at this temperature and a tap was opened, the water at the tap would violently flash to steam causing serious, perhaps fatal burns.

If no pressure relief is available on a cylinder and the cylinder is heated until it ruptures, this results in a catastrophic explosion. All the water in the cylinder suddenly finds itself de-pressurised and a portion of it (more than 20%) flashes to steam which expands by 1600 times it’s own volume (Gelignite has an expansion rate of 2200 its own volume). The resulting explosion is powerful enough to demolish a house and kill the inhabitants.

For this reason it is very important that several levels of fail-safe safety interlocks are incorporated. These are IN ADDITION to the normal controls which should prevent such an occurrence in the first place.

Mythbusters Water heater Rocket

Do a search for Mythbusters Water heater rocket on www.youtube.com to get a good apprecition of how much damage an exploding unvented cylinder can do to a house and the people inside. Its quite frightening really.

In addition, a spring return (normally closed) two port valve must be fitted on the coil of the cylinder, so that the coil is isolated if power should fail.

As can be seen we have 3 levels of safety interlocks in addition to the normal controls.

Firstly two electrical interlocks wired in series which are designed to “fail-safe”. These are normally incorporated into the same housing and use the same terminals.

The “back-up” T&P valve which is a last resort safety device. This should never be capped or removed.

Correct way to connect a solar panel to an unvented cylinder.

As can be seen the solar pump is wired through a dual thermostat, this will prevent the cylinder from being overheated by the solar if the controller is left in manual or if sensor S2 shows a inaccurate/false reading (e.g. S2 falls out of the sensor pocket). This is acceptable provided that the panel is mounted higher than the coil in the cylinder so as to prevent thermosyphonic circulation.

If the cylinder is above the panel, then a 3 way divert valve is necessary to isolate the coil of the cylinder in over temperature conditions. Needless to say the valve must be fail safe shut and only open on an electrcial signal wired through the dual thermostat.

Solid Fuel Back Boiler

The Flue and the installation of the Solid Fuel Back Boiler is outside the scope of this document, however good guidance is given in the SEI document

“Burner Heating and Domestic Hot Water Systems for dwellings – Achieving compliance with Part L 2008”

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18691,en.pdf

Outlet fittings, Pipe Sizes to taps