Designing your solar system

While we run courses on designing and installing your own solar system, doing this is not beyond the scope of most competent DIY enthusiasts. If you are building a new house, or renovating your house, you will probably already be using the services of a plumber. Either way, you will find useful information here to help you design your system and if you are using a plumber, to specify the equipment you want to use.

Size of your domestic hot water cylinder

The size of solar panel you require will be determined by the size of your cylinder. Many solar companies love to sell a standard system with a 300L cylinder and 6M2 of panels.
However, a smaller cylinder and small panel will provide a quicker return on investment, and many smaller households will get on fine with a low-cost system based on a 135L solar cylinder and a 20 tube ( 2M2) panel.

The standard small cylinder in most houses is about 90L. However, when you fill the bath in the evening, the cylinder quickly gets re-heated by either a boiler or an electric immersion. A solar system will only heat your cylinder once – during the daytime. For this reason, there is some logic in having a slightly larger cylinder on a solar system. If you regularly need two baths a night and use a lot of hot water, get a larger cylinder, and a larger panel to match. But don’t feel you have to!

Matching a panel to your cylinder

A small panel and large cylinder will give you a huge amount of lukewarm water. A small cylinder and large panel will regularly over-heat, wasting valuable energy.

Vacuum tube solar water heaters are typically more efficient than flatplate collectors during cold, windy weather and low light situations like a cloudy day. For this reason, the panel size required for vacuum tubes is somewhat smaller. Our 20 tube panel will provide sufficient hot water for solar hot water cylinders between 135L and 160L if the roof is facing due south. If your roof deviates from south by more then 20 degrees, you should install slightly more panel area. A 30 tube panel would heat a cylinder sixed between 150L and 190L, and above that to 250L you would use 40 tubes (2 x 20 tube panel). A 300 Litre cylinder would require 50 – 60 tubes which comprises a combination of 20 tube and 30 tube panels.
If using flat plate panels and assuming they are installed facing south the rule of thumb is one 2m2 panel to heat every 100L of water in a cylinder. So a 200L cylinder would require 4m2 of flat plate panel installed, a 300 litre solar cylinder would require 6m2 and so on..

Locating your panel

Most houses put their panel on a south-facing roof. If you have a complex roof shape which causes your south-facing roof to be shaded, you should put the panel near the top of the roof and as far away from the shading factors as possible. Otherwise, it may be preferable to put the panel quite low in the roof space. This is so you can fit a header tank in the attic space above the level of the top of the panel. If you do this, the system can be open-vented as a fail-safe in the event of any overheating. However, in many cases the panel is above the level of any attic tank, and in this case a pressure kit will be needed.

Thermo-syphoning system

If you are lucky, it may be possible to position the panel either on the ground, or on a conservatory roof in such a way that the hot water cylinder is almost directly above the tank. Because hot water rises, the hot water may automatically flow from the panel to the cylinder (called thermosyphoning) without the need for any pump. For this to work effectively, the horizontal distance from the cylinder to the panel must not be more than twice the difference in height between the bottom on the cylinder and the top of the panel. Even at this, a larger pipe size (3/4") should be used, and there should be no sharp bends in the pipework. A plumber would be able to advise on the feasibility of this. Because you have no controller on this system, you should use a thermostatic mixing valve on the outlet from your hot water cylinder to prevent scalding as the water in your cylinder could get extremely hot!


Unless you have a thermosyphoning system, you will need a small pump to bring the hot water from the panel to the cylinder. A controller or "differential thermostat" has two temperature probes, one of which is in the panel and one in the cylinder. When the panel is hotter than the cylinder, the pump is switched on.

We have a more sophisticated controller (the Resol BS3) which has a third probe in the top of the cylinder, and has a digital readout which can tell you the temperature of the various parts of your system. This provides useful information on how much hot water you have. The controller also enables you to divert surplus heat from the panel into a radiator if the system is over-heating. The BS3 controller also provides an anti-freeze function to prevent the panel from freezing by pumping water from the cylinder back up to the panel if the temperature goes below a pre-set level.

So what will you need?

Please click here to see an example of the cost of a fully installed 40 tube system you require for a typical, smallish system to be installed in a south facing scenario. If you require a bigger system you will be enlarging the cylinder and the panel accordingly.

You will also need pipework and pipe insulation. Pipework from the panel to the cylinder is normally 1/2" copper or DN16 Stainless Steel pipe in a domestic installation , but must be insulated with High Temperature rated insulation rather than regular pipe insulation which will not withstand the heat generated by solar systems.

We highly recommend installing a heat dump particularly with vacuum tube solar systems, this is to cater for a situation where the cylinder has been heated to it’s maximum but the panels are still producing heat – you need to be able to safely dissipate this heat to avoid putting stress on your system. Solutions such as dump radiators and or dumping into your central heating system are available. Feel free to give us a call to discuss this option.

Because you have certain fixed costs (controller, pump, pressure kit, labour etc.) doubling the size of your solar system does not double the price. However, it is better to build a modest solar system within your budget than to do nothing!

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