Solar Cylinders - Twin coil storage cylinders explained

By far the most common way of incorporating a solar system into your home is with the use of a twin coil hot water store. Such is the speed at which solar thermal has taken off in recent years, that the vast majority of cylinder manufacturers now have different sized solar cylinders in their range to suit all applications.

twin coil solar cylinder

Sizing the cylinder

The solar thermal circuit should always be plumbed into the bottom coil of the twin coil cylinder. Heat rises, and the draw-off point for the water to the taps is at the top. This means your solar system is capable of heating the whole volume of water stored in the cylinder. So on a sunny day, when solar works at it's very best, this entire volume of water will get hot. (Yes, even in Britain!)

On gloomier days, when there is little solar gain, your boiler will need to 'top up' the heat - although you'll be very surprised at just how much heat a solar system will capture even on an overcast day! You may find that the temperature of the stored water may well reach 30-40 degrees, so for tap water being delivered at 50 degrees, your boiler will only have to top up those last 10 degrees, so you can see how having a solar system helps cut your energy bill all year round.

What you do have to remember, is that in Winter on very overcast days when there is little or no solar gain available, only the volume of water from the boiler coil to the top of the cylinder will be heated For this reason, it is recommended that solar cylinders are sized correctly. They should be one and a half to twice the size you need. Your installer should carry out a full system design, to accurately size a suitable cylinder.

solar cylinder

For example:

Average hot water consumption is approximately 40 litres per person per day.

If you currently have a 150 litre cylinder for your hot water requirements (and this is currently adequate for your needs), you may look to a 250 litre solar cylinder. Also as a rule of thumb, there should be around 50 litres of stored water per square metre of flat plate collector area (solar panel) On no account, fall below 35 litres of stored water per square metre of collector area

Based on these calculations, between 4 and 6 square metres of flat plate collector would be adequate. Four if your house was perfectly South-facing with the correct pitch, six if you need that bit extra because of partial shading, South East facing etc. It's not an exact science, but for safety reasons, there are limitations.

Consideration has to be made not only for the extra height this cylinder will need, but also the extra weight involved (250 litres of water weighs 250 kgs, before you add on the weight of the cylinder and connections.

What else is different about a solar cylinder?

Solar cylinders also need pockets for sensors. There are typically 2 sensors used in a solar cylinder, this is so that the solar controller can accurately turn the solar pump on and off. Usually, there will be two sensors. One towards the botttom of the cylinder, and one near the top. The solar controller uses these to detect when useful solar energy is available, and whether it is needed. In all instances, a blending valve must be installed on the hot water outlet from the cylinder to prevent scalding.

Read about solar blending valves

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