How to choose a cylinder and cylinders working with solar systems

Solar panels are critical to an excellent solar system performance, but just as critical is the place where the energy is stored. In Britain these are called cylinders, in other places “tanks” and in other places “geysers” but they all look the same and serve the same purpose.


I will explain cylinders insofar as they affect the United Kingdom; other countries have different plumbing laws and regulations as well as different ways of getting potable water into homes, so I shall concentrate on the United Kingdom position.


Most homes have a small cylinder – typically 80 litres or so, hidden in an airing cupboard. Before homes had central heating (like my home in Poplar when I was a child) the cylinder had an immersion heater, just like an electric kettle. When you needed hot water in the bathroom you turned on the immersion heater and the cylinder warmed up.


Most homes have central heating these days and although the cylinder usually has an immersion heater this is there for back up. The central heating boiler usually has a pipe connecting to a heating coil inside the cylinder and then back to the boiler (a flow and return). When the cylinder needs hot water the boiler pumps hot water through the coil. This exchanges heat from the boiler into the water inside the cylinder. That way the potable water is kept apart from the boiler and central heating water, making the heating of water hygienic.


When water is heated in a cylinder it will expand, and you need somewhere for the water to expand into. Traditionally this has been a tank in the loft (or higher than the cylinder), open to the air. A pipe runs from the cylinder to the tank and terminates over the tank leaving the cylinder open to the air. This is called a vented cylinder.


Many quality systems prefer to use a sealed boiler which is connected to a pressure vessel close to the cylinder. That covers the safe expansion of water, and the resultant hot water pressure provides an excellent shower pressure without pumps and can usually be arranged so that there is no pressure loss or hot water imbalance when several hot water taps are turned on at the same time. This is called an unvented cylinder.


Unvented cylinders have to be made well, and out of strong durable materials such as stainless steel, and they have a long life span, subject to the water hardness where you live.


When you have a solar system installed in most cases it is better to change the cylinder. A solar cylinder is much larger than a traditional cylinder – usually between 150 litres and 200 litres. As a rule of thumb a two square metre Genersys panel is sufficient to heat 100 litres of stored water. As you can store your solar heat energy in the cylinder it makes sense to store what you need for toady and a little to tide you over for the next day. An average home uses around 113 litres of hot water a day.


There is another important difference with a solar cylinder; it has two coils. The coil connection to the boiler should be in the upper part of the cylinder and a much larger coil will be in the lower part of the cylinder connected to the heat transfer pipes that connect to the panels. If you reverse the connection the performance will be poor because the solar will ”fight” with the fossil fuelled coil.


An average lifespan of a vented cylinder is less than ten years but people tend to replace them at longer intervals (usually when they break) because out of sight is out of mind. They still get hot water from their system with an old cylinder but do not appreciate that it operates more inefficiently and more expensively in fuel terms with age.


If you do need to change your cylinder my advice is


  • Also think about a solar system at the same time
  • If you can go for a stainless steel unvented cylinder – you will lose a lot of tanks and pipes in the loft and will have drinking water from bathroom taps, but you need your plumber to check the local water pressure first
  • Go for quality; the more expensive cylinders have better insulation and you will save the price differential in the long run
  • Get a competent plumber who has the qualifications to work with unvented cylinders – not every plumber has these.


I am not an expert in plumbing so I ran this post past Chris Flaherty of Vietec Heating who helped me out with it and I thank him for it. You can find him at