Solar Thermal Hot Water: Is It Worth It in London?

How solar thermal heats your hot water

Solar thermal panels, also called solar collectors or solar hot water panels, heat a fluid rather than make electricity. The fluid is a mix of water and antifreeze, usually glycol, that circulates in a sealed loop between the panels on the roof and a coil near the bottom of your hot water cylinder. When the panels are warmer than the water in the cylinder, a small pump runs and carries that heat down to the tank.

A controller does the thinking. It reads a sensor on the panels and another in the cylinder, and it only starts the pump when there is useful heat to collect. The pump, controller, pressure gauge, expansion vessel and safety valve for the solar loop are normally grouped in a pump station mounted beside the cylinder.

Because the solar coil sits low in the tank, the sun preheats the bottom of the cylinder. The boiler coil or the immersion heater higher up then tops the water up to a safe storage temperature when the sun has not done enough. That is why a solar system needs a cylinder with two heat inputs, which is what a twin coil cylinder is.

Solar thermal vs solar PV for hot water

This is the comparison most people are really making. Solar PV panels make electricity. On a sunny day, a solar diverter can send the surplus that would otherwise be exported into your immersion heater, so the same panels that run the fridge and the washing machine also heat the cylinder.

Solar thermal turns sunlight into heat more directly, so a given roof area collects more energy for hot water. The catch is that it can only ever heat water. In summer, when the sun is strongest and the household often needs least hot water, a solar thermal system can collect more heat than the cylinder can use, and the surplus has nowhere to go. PV surplus, by contrast, can go to the house, a battery, an electric car or the grid.

There is also more to look after. A solar thermal loop has fluid that degrades, a pump, a pressure vessel, sensors and a controller, and it can leak on the roof or in the loft. A PV system with a diverter has none of that on the water side, which is why it is now the more common choice for a new installation on a typical London roof. If you already have PV, our guide to a solar diverter for an immersion heater explains how it works and when it pays.

When solar thermal is worth it

It is harder to justify where one or two people use modest amounts of hot water, where the only suitable roof is shaded by taller buildings, which is common in terraced streets and mansion blocks, or where the cylinder is small and already sits in a tight cupboard. In those homes the solar coil spends much of the year doing little.

• The household uses a lot of hot water through the day or early evening, for example a family, or several people who shower after work
• There is a roof facing south, south east or south west with little shading from neighbouring buildings or trees
• The cylinder is due for replacement anyway, so a twin coil model adds little to the work
• There is no space or appetite for PV, or the PV roof area is already used
• Someone will keep an eye on the pressure gauge and the controller, and have it serviced

What goes wrong with older solar thermal systems

We see a lot of solar thermal systems in London houses that were fitted ten or fifteen years ago and have been quietly doing nothing for a while. The owners often do not notice, because the boiler or the immersion simply takes over and the hot water carries on as normal. The first sign is usually a bigger energy bill in summer, a pressure gauge on the pump station reading zero, or a controller screen that is blank or showing an error.

• Loss of pressure in the solar loop, from a slow leak at the roof, in the loft or at the pump station
• Glycol that has broken down after years of heat, leaving the fluid acidic or sludgy so it no longer protects the loop
• Summer overheating, where the panels sit hot with no demand and cook the fluid
• Failed pump, controller or temperature sensor
• Air in the loop after a refill that was not purged properly

The solar loop is a pressurised circuit that can get very hot on a sunny day, even when the pump is off. Do not open valves or drain points on it yourself. If the gauge is at zero or you can see fluid leaking, switch the controller off and get it looked at.

Repair it, keep it or decommission it?

If the panels and the cylinder are sound, the fault is usually in the parts beside the cylinder: the pump, the controller, a sensor, or a loop that needs refilling with fresh fluid and repressurising. Those are repairs, and on a system that still has useful life in it they are often worth doing.

If the system has leaked for years, the panels are damaged or the roof is due for work, it can make more sense to decommission it. That means isolating and draining the solar loop, safely disposing of the old fluid, and capping the pipework at the cylinder. The solar coil is simply left unused. The cylinder carries on heating from the boiler or the immersion, and nothing about your hot water changes except that the sun no longer helps.

If the cylinder itself is failing, you have a choice at replacement. Keep the solar system working and fit a new twin coil cylinder, or decommission the solar and fit a standard single coil cylinder that may suit the space better. Our twin coil cylinder guide covers that choice in more detail, and signs your cylinder needs replacing will help you judge whether the tank has reached that point.

Some systems were installed under the domestic Renewable Heat Incentive, which closed to new applicants in 2022. If yours was, check the terms you signed before you decommission anything.

Solar thermal and Legionella

Solar preheat warms the bottom of the cylinder to whatever temperature the sun allows, which in spring and autumn can be lukewarm for long periods. That is the temperature range where Legionella bacteria can grow. A properly designed system deals with this by making sure the whole cylinder is regularly heated to a safe temperature by the boiler or the immersion, and the controls should be set up to do it.

If you have taken over a solar system with a house purchase and nobody knows how the controls are set, have them checked. Our guide on what temperature a hot water cylinder should be covers safe storage temperatures.

What decides the cost

For a new system, the price depends on the collectors, the roof access and scaffolding, the pipe run to the cylinder, and whether the cylinder has to be replaced with a twin coil model. On an existing system, a repair is labour plus the part or the fluid, and a decommission is mostly labour.

Our hourly rate and call-out are on the pricing page and agreed before anyone attends. The total depends on what the system needs once it has been looked at, so ask any installer to separate the roof work from the work at the cylinder when you compare quotes.

Getting a solar hot water system looked at

If your solar thermal panels have stopped working, the pump station gauge reads zero, or you are replacing the cylinder and need to decide what to do with the solar coil, call 020 4652 5111. We look after the cylinder, the pump station and the pipework indoors, tell you whether the system is worth repairing, and if it is not, shut it down safely and fit a cylinder that suits the way you heat water now. We have 20 engineers across London.