Hot Water Cylinders

vented cylinder

Solar hot water cylinders are differentiated by the addition of a coil, usually low down in the cylinder, which is meant for a solar panel to be plumbed into. They will also have the standard boiler coil about half way up the cylinder. A cylinder with a coil designed for solar will be more efficient than a standard coil or a retro-fit coil.

People who are buying a solar panel often choose to go the extra mile and replace their cylinder as well to get the best out of the panel. It is not always easy, however, to decide what type of cylinder you want.

cyiinder cross section

A standard vented hot water cylinder for domestic use will be layed out as per the above diagram. Your cold water from the header tank enters at the bottom, is heated by the solar and boiler coil and is drawn off from the top. There is usually an immersion socket at the top for topping up with heat if necessary and temperature sensor ports at the top and bottom. Vented cylinders require a header tank as they are not capable of directly accepting mains pressure water. Typically a vented cylinder will be made out of copper. Although an expensive resource, copper is better at transfering heat (than stainless steel for example) and has very good anti-bacterial properties. A copper cylinder needs to be well insulated. Most copper cylinder companies insulate their cylinders as standard with at least one layer of polyurethane foam or similar, non-toxic, coating. Cylinder covers with high insulating properties are also available on the domestic water heating market.

Most good companies offer complete customisation on cylinders. Additional coils, tappings, header tanks, shower coils, layers of insulation and the ability to make your cylinder horizontal should all be readily available at an affordable price. If you have special requirements for a cylinder make sure you are specific about what it is you need. Send the manufacturers a sketch showing exactly where you want each addition. A cylinder made incorrectly is an expensive task to rectify.

When choosing a twin coil copper cylinder there are a few manufacturing standards which should be looked out for in order to get the most heat transferred from the solar panel to the water within. These points are:

Solar Coil Material: Should also be copper and finned for better heat transfer.

Area of Solar Coil: Should be at least 1.5m2, the larger the cylinder the larger this should get. The higher the surface area the better the heat transfer to the water.

The Cylinder has Clearskies, British Standard and Water Manufacturers Association approvals.

The Grade of copper used: Should be at least Grade 3 for domestic use.

Our vented copper twin coil range of cylinders is specifically designed to work in conjunction with standard system boiler heating systems and solar thermal systems. These cylinders are manufactured as standard with two coils but bespoke cylinders with additional coils are available. The boiler coil is manufactured to BS 1566 and comes with 22mm compression fittings as standard although different fittings can be specified. The solar coil is manufactured from high efficiency finned copper tube, the finned copper has a comparatively large surface area when compared with standard tube and this allows for an efficient transfer of heat. The solar coils comes with 15mm compression fittings but these can be changed to suit individuals requirements.

All our cylinders can be customised to suit any heating and hot water requirements with the following options.

Optional Extras

- Insulation: All Cylinders come with 50mm as standard, extra layers can be added if required

- Coil Upgrades: Boiler or Wood Burner

- Additional Coils: Up to 2 additional 22mm or 28mm Boiler or Woodburner Coils Heat Pump, Underfloor Heating & Thermal Store Coils also available

- Combined Header Tank with ballcock

- Direct Tappings: 15mm, 22mm, 28mm at any position on the cylinder

- Essex or Wessex Flanges

- Horizontal Cylinders: all cylinders (except thermal stores) can be made to sit horizontally and include cradles for it to rest on.

We can supply vented and unvented copper cylinders. Unvented cylinders are used to supply mains pressure hot water - although it is a more expensive option, and should be inspected annually. A more cost effective method (which does not require annual inspections) is to use a 'thermal store coil' - this is a high efficiency coil that runs the length of the tank, which heats mains pressure water as it is required.

Thermal Stores.

thermal store coiled cylinder

In a vented system, heat is imparted to the incoming mains water by means of a secondary heat exchanger either within the vessel or externally mounted.  Water in the vessel is used rather like a battery for heat - a ‘Thermal Store.’ The incoming mains water passes through it's heat exchanger drawing heat from the store. The Thermal Store temperature is maintained in the usual way - form whichever sources you choose - solar, oil or gas boiler, heat pump, biomass boiler, wood burner, immersion heater - you can utilise all heat sources that are available. Because the water within the Thermal Store does not need to be under pressure (as with an unvented system) it is vented to atmosphere. The body of water within the Thermal Store is used purely as a medium for storing and imparting heat to the secondary exchanger.

We supply thermal stores from Akvaterm and a range of bespoke thermal stores in copper or stainless steel.

Unvented Cylinders

In an unvented system, incoming cold mains water typically enters a pressure vessel where it is heated either Directly by means of electrical heater/s, or Indirectly by means of a primary heat exchanger within the vessel being supplied by your central heating boiler. The hot water stored within the vessel is forced out by the incoming cold water when you open a tap hence - Mains Pressure Hot Water. 

With an Unvented Cylinder you will be storing a large volume of hot water under pressure. For reasons of safety such systems must be installed by qualified technicians with relevant experience, CITB training and G3 certification. As a precaution, pressure-relief pipework and valves must be installed to protect against unsafe pressure build-up within the vessel which could result in explosion. 
Your Local Authority (Building Control Dept) will need to be advised of your intention to install an unvented system. For reasons of safety, your system will require annual maintenance to ensure safety equipment is functioning correctly (BS2870). An unvented system must be commissioned and certified by the installer. You should not fit uncontrolled heat sources such as stoves to a mains pressure tank. Copper Unvented Cylinders tend to be considerable more expensive than vented cylinders. Stainless steel cylinders are cheaper but lack the benefits gained from copper.


The potential drawbacks of a mains pressure system are: Under water bye-laws you may not add a pump directly to your mains fed system to increase pressure or flow rate. If the supply in your house is poor there will be no simple way to improve it. Additional performance usually entails bringing in a new, larger supply pipe to the property. Even then, you should ensure the new improved supply will be adequate, as some areas suffer from frustratingly low pressure. Always remember - your system can never be better than the supply to it. Mains pressure systems are at the mercy of fluctuations in supply pressure. This may not be an issue in the majority of cases. Mains pressure systems can give 'power shower' performance, providing supply pressure is good. If you like a vigorous showering experience and your supply pressure is poor, remember - you cannot install a shower booster pump. If you simply must have a power shower then consider a gravity fed system. You can do pretty much anything you want with a gravity feed.

Stainless Steel Cylinders

The Navitron Solar cylinder is a high quality stainless steel unvented cylinder designed for use with all solar systems to provide hot water from a cold mains water supply of between 3bar and 12bar. Reduced performance is available at lower pressures but the units are not suitable for pressures lower than 1.5bar and flow rate of less than 20 litres per minute. The indirect heat exchange surfaces are designed to provide a rapid heat up time. The unit comes complete with all the necessary safety equipment to comply with legislation governing the installation of the cylinder.

The cylinder must be installed by a competent person with appropriate qualifications and registration for the installation of unvented water storage heaters.

All Cylinders include:

Immersion Heater - rated @ 3kw, 240volt, single phase, incoloy 825, with thermostat with independent cut-out

Thermal insulation – CFC/HCFC free, (ODP zero), 60mm fire retar- dant insulation

Outer Casing – grey plastisol

Inner Cylinder – Duplex 2304L stainless steel

Corrugated high performance heating coils - Indirect models with two coils designed for oil, gas or electric boiler auxiliary heat input

Indirect thermostat – factory fitted twin thermostat with independent reset, 30-70degC

Safety Device – factory fitted 7bar/90degC pressure & tempera-ture relief valve

All cylinders are pressure tested to 10.5bar, no anode required, Beab, KIWA, WRAA, WRC and Benchmark approved – 5 year guarantee available on all components except immersion heater and cylinder thermostat (which carry a 2 year guarantee)

Also Included: Safety Control Valves, Pressure reducing valve 3 bar c/w check valve and strainer, Expansion relief valve 6 bar, Pressure & Temperature relief valve 7 bar/90degC factory fitted, 15 x 22mm Tundish with compression fittings, Indirect model – twin thermostat 30-70degC, Expansion Vessel

What size cylinder should I get?

Unfortunately this is a difficult question to answer as one household's daily water usage may be significantly different to another's. If you use baths regularly then you'll want a lot more water than if you shower. The best way to size a replacement cylinder is to use the one you have already and if that either too large or too small for your needs then size your new one accordingly, remembering of course that additional coils will take up some space inside and need to be catered for.
If your buying a new cylinder and don't know what size to get then ask around. If someone you know uses around the same amount of water as you per day then find out their cylinder size. Some houses have water meters fitted which give a good indication.
If you have an area free to house a new water cylinder, this handy cylinder volume calculator may help show you how much water in litres you could potentially have.


retor-fit coilsolarplug


Retro-fitting a system means that you do not have to buy a new twin coil cylinder and is often a cheaper alternative. A retro-fit device fits to an existing hot water cylinder and allows a solar panel to be linked to it.

The left and middle images above are retro-fit devices that fit into the immersion socket on a vented hot water cylinder. The left hand is a simple retro-fit coil which the solar is plumbed into in exactly the same way as it would be in a twin coil cylinder. This, however, means it is heating from the top of tank, not the bottom, and the absense of an immersion heater means the water cannot be topped up with heat if it is too cold for use. As with the solar coil on a solar cylinder, the surface area of the retro-fit coil is important, also the material it is made from. Finned copper is best. Some retro-fit coils are not a closed loop. They have long pipes that run down into the cylinder and directly draw the water from it up to the solar panel. This system can work just as well as closed circuit design, but means chemicals, such as anti-freeze should not be added as it will contaminate the tap water.

The middle image is a retro-fit coil with an immersion heater built in. These are a relatively new design and are becoming increasingly more popular as it allows for both solar and an immersion heater. Some designs also extend when fitted to reach the bottom of the cylinder and can heat the whole volume of water. Although more expensive than the standard retro-fit coil, it should be considerably cheaper than a brand new cylinder. 

The right hand image is an external heat exchanger. This particular one is the willis solaspyhon, which is a highly efficient model. The science behind them is slightly different to the immersion mounted retro-fit coils, as they sit outside the cylinder. One advantage is that you don't have to lose your electric immersion heater to install them. They tie into the hot and cold feed on the cylinder and transfer heat directly from the solar panel.

Whichever you choose, a retro-fit option should not cost more than around £300.

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