Solar water heating systems = Solar panels + solar energy. Solar thermal: solar power & solar hot water.
Solar water heating systems with conventional twin coil solar cylinders.
We are sometimes asked how to reduce Legionella risk if you already have twin coil solar cylinders fitted.
Here are 4 approaches to consider with reasons, costs, pros and cons for each. We hope this Legionella safety guidance is useful. The first is the cheapest to do…
1 – Zero cost and immediate:
The simplest is to stay clear of hot water spray. Take baths, not showers. Contemplate while you are in the bath what you plan to do next: of 2-4 below.
Why? This is because Legionella infects by being inhaled into your lungs. Taking baths instead of showers will stop you inhaling water droplets which might be contaminated. If you actually disconnect all hot or warm sprays, then the home may well comply with L8.
2 – Cheap and cheerful (under £100)
Buy and fit a long immersion heater which is the full length of the cylinder and which will reach to the bottom.
Time this to come on at the end of the day to heat the whole cylinder to 60C.
Why? This will heat the cylinder daily to the bottom and complies with L8.
- Pros: Good if you can buy cheap rate electricity.
- Cons: Not all cylinders will have a suitable top entry hole in the cylinder for the immersion heater. I am not sure if long immersion heaters are available for solar cylinders since these tend to be taller. Heaters may not fit – they may touch or get stuck on the heat exchangers inside. If they touch they may “spot-overheat” and fail too soon. Peak rate electricity is costly and is 2-3 times as carbon intensive per unit of energy than mains gas. So your net carbon savings from having solar may fall, possibly even to zero.
3 – Medium cost. Requiring a plumber and electrician and keeping the cylinder: (£200-500)
Have the twin coil cylinder fitted with a mains powered destratification pump which comes on:
- Every day
- For one hour
- At the same time as the water heating is turned on whether immersion or other water heater.
Why? This will heat the cylinder daily to the bottom and complies with L8.
- Pros: medium cost.
- Cons: cost money to run. Uses electricity. Reduces carbon savings.
4 – Higher cost. Requires a plumber only: replacing the cylinder with a safer one: (£500-£1000)
Replace the twin coil cylinder with another twin coil cylinder which has BOTH heat exchangers (solar and backup) at the bottom. Also time the backup heating to come on in the evening as we do.
Why? Again this will heat the cylinder daily to the bottom and complies with L8.
- Pros: greener in that you don’t need to run an extra mains pump.
- Cons: arguably less green in that you are throwing stuff away. More costly than a shunt pump.
Prevention by good design is much better than a cure after the event…
At Solartwin you can be sure that today’s installations all comply with HSE’s code of practice L8′s guidance to heat the stored hot water in the cylinder to the bottom. We do this by: either having the backup heating positioned at the botom of the cylinder and ideally it coming on at sunset (dedicated solar volume in space) or by using thermal stores which have a low volume of stored water in the heat-out heat exchanger.
Solartwin have been advising Which? magazine (The Consumers’ Association) on how to make solar water heating installations safer. To date Which? have not taken our advice.
