Solar Cylinders

The efficient storage of the solar energy gained from the collectors is crucial to the successful operation of the whole system. The storage volume must at a minimum provide for the normal daily consumption of the household, but ideally, given the variability of our weather should also provide for the next day as well. This raises issues with the level of heat loss from the cylinder, the integration of the auxiliary heating system, the size of the area available for the hot water cylinder (the strength of the supporting floor and the size of doorways for installation access) and the prevention of overheating of the solar collectors in spells of prolonged hot weather. The type of hot water cylinder chose will depend on the type of system design that has been adopted. The range of cylinders employed varies from the modified use of the existing cylinder if it is still compliant with insulation regulations (BS ***) to the use of system specific polymer stores. While much attention is provided to the main cylinder insulation, the heat loss through the connections is often neglected. These can be substantial particularly for connection near the top of the cylinder, where it is warmer. Check valves, s-bending (180 o loops) or specilaist connections such as Convectrol can be used to minimise these losses. Desirable Solar Cylinder Properties High level of insulation of the cylinder. Good stratification of heated water in the cylinder. Prevention of heatlosses from cylinder connections. Sufficient storage volume for the daily demands of the household. Ability to control the auxiliary heating system. Provision of a reserved solar space that may be defined as a permanent physical volume or a volume reserved in time by interlock of the auxiliary heating Means of minimising legionella risk

Draft Only.

The immersion heater is normally pre-fitted to the cylinder. All heaters are factory pre-wired with a 1 meter long heat resistant 4 core cable, ensure that all connections are tight before switching on.

The thermostat is wired through the neutral, the brown wire goes to the bath element, and the black wire goes to the sink element. It is very important to ensure that the heater is earthed correctly.

With the cylinder full of water, heat the cylinder to 60C via the immersion heater or central heating system. Adjust the thermostat so that it just turns off at 60C.

Anti-Legionella. Normally an immersion heater timer is wired in front of the Sink/Bath Switch, so that this can be used to provided a timed source of heat. This is a SEI requirement. It is common necessary to replace the cylinder when installing a solar system.

A pressurised cylinder should only be filled by a competent person. A pressurised cylinder incorrectly connected has the energy potential to destroy a house.

Anti-Scald Valve

Best practice calls for the fitting of a thermal mixing (anti-scald) valve. This applies to all hot water systems and not just solar heated water systems. Many solar systems in Ireland are installed with a large collector area and will heat the cylinder to high temperatures during good weather. It is imperative to reduce the risk of sclading at taps that a good quality anti scald valve is installed. In addition storing water above 60°C will legionella bacteria so is becoming more of a consideration to install thermal mixing valves. The ideal maximum, safe temperature for hot tap water is 50 C.

The maximum bathing temperature recommended for young children is 38 C.

Minimising flow problems when installing Anti scald valves.

A 3/4" Mixing valve will typically have a KVS of 1.5. Using a 2 meter head (20 kPa) the valve will allow about 13 litres/minute flow. A typically 1" mixing valve has a KVs of 3.0, using the chart above, a 2 meter head drop, the valve will allow approx 22 litres/flow. Experience has shown that a vented cylinder fed from a low attic tank can experience flow problems in showers unless a 1" valve is used.

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.

Unvented Cylinders and Irish Law

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.

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