Solar Thermal Water Heating: A Full Guide

Solar Thermal: The Basics

Solar thermal systems work in a similar way to the more well-known Solar PV (Photovoltaic) systems. Rather than generating electricity, the devices attached to your roof (known as collectors) capture heat from the sun and use it to heat water. Your immersion heater can then be used to make the water hotter, or to fully heat the water as a backup when there is no solar energy available.

Currently, it is estimated that you could potentially save up to 50% on your annual hot water bill by using a solar thermal setup, if you are currently using gas, however due to the volatility of energy prices, this is only an estimation.

In the past when the government had numerous incentive schemes, it was possible to get £300 towards the cost of installation from the government’s Renewable Heat Premium Payments scheme and also receive payments for the heat you generate through the Renewable Heat Incentive scheme, however most subsidised schemes have now ended.

Despite this, if you are currently loooking into the solar thermal as a means of heating and hot water, it’s certainly worth seeing if there are any schemes available.

How Solar Thermal Systems Work

Solar thermal systems consist of a set of collectors on the roof connected to a solar cylinder (containing your home’s hot water supply) via pipework containing a carrier fluid, which is pumped around the system to transport heat from the collectors to the cylinder.

This carrier fluid flows through a coiled, tube-like ‘element’ in the solar cylinder to heat your water. A second element in the cylinder is connected to your boiler to heat the water further if necessary.

A controller uses sensors in the cylinder to monitor the difference in temperature between the water in the cylinder and the collector, and if the difference is large enough the controller will automatically switch the pump on to heat the water further.

There are two main types of collector used in solar thermal systems:

Evacuated Tubes

This collector is made up of several large glass tubes side by side attached to a ‘manifold box’. The tubes are double layered with a vacuum in between the two layers to insulate the tubes and minimise heat loss.

The inner tube layer is coated in a dark absorber material which captures the Sun’s heat, which is then concentrated onto a central receiver. Evacuated tubes are more efficient than flat plate collectors.

There are two types of evacuated tube collector:

• Heat pipe evacuated tubes: In a heat pipe collector each tube contains a copper heat pipe. This is a pipe containing a partial vacuum and a volatile fluid. When the heat pipe is heated up by the sun’s energy, the volatile fluid vaporises and rises up the tube to the manifold box (the heat exchanger) which transfers the heat to a fluid, either the cold water you wish to heat or a carrier fluid, usually water or a mix of water and antifreeze (glycol) which then transfers heat to the water you’re trying to heat
• Direct flow evacuated tubes: In this collector the evacuated tubes contain a carrier fluid, usually water or a mix of water and antifreeze. The tube coating captures the sun’s heat and transfers it directly to the fluid, which flows around the tubes, out of the manifold and into the piping inside your house to transfer its heat to the water you want to heat.

Flat Plate Collectors

These collectors are more similar to standard solar panels. They are flat plates made up of a layer of pipes covered by absorbent material, which is then covered by transparent glazing. The pipes contain carrier fluid, usually water or a mix of water and antifreeze which then transfers heat to the water you want to heat. Flat plate collectors can be attached on top of your roof tiles or in amongst the tiles.

Some collectors come with a drain-back mechanism. This drains the fluid from the collector whenever the system isn’t running to make sure the fluid doesn’t boil or freeze inside the collector.

The Solar Cylinder

The solar cylinder will typically be 30-50% larger than your current hot water cylinder. There are two types of solar cylinder:

• Vented: This cylinder works with your current cold water feed tank if you have one, which means that it will have to be installed close to it.
• Unvented: This cylinder does not need a cold water feed tank in your system. Simply connect the cylinder to your mains cold water supply. This means that the cylinder can be installed anywhere in the house. The cylinder supplies mains pressure hot water, which means that there is no need for pump to boost the pressure. However, this also means that more safety checks will be required.

Installing Solar Thermal Systems

If you’re considering installing a solar thermal system it’s important to think about the location of your collectors – ideally they should be in a sun exposed location at a 20-50 degree angle of incidence. You should also check with your installer that your roof is strong enough to bear your chosen collectors as they can be heavy.

Solar thermal Installation Checklist

You will need to think about the following:

• Space on your roof: Usually you will need a little over 1m2 of collectors for each person living in your house. The Energy Saving Trust also suggests that 5m2 is ideal for the average home. Ideally the location should be south facing and exposed to sunlight for most of the day. You can also use a wall or a frame attached to a flat roof



• Compatibility of your current system: Most systems are compatible, however if you currently have a combi boiler without a cylinder it may be problematic to install this system. Please check with your installer for more information
• Space available for a solar cylinder: You may well need to replace your current cylinder with a solar cylinder as it is usually larger, at 30 to 60 litres. You can alternatively add another cylinder with a solar coil or add a solar coil to your current cylinder if it is large enough
• Planning permission: Usually you will not need planning permission for installing one of these systems. There are permitted development rights for this installation unless the collectors are more than 20cm tall when installed
• Current heating fuel: A solar thermal system will deliver significant energy savings if your existing system is solid fuel, electric or oil based. Traditionally, if you were using gas savings would be minimal, however due to the massive hike in energy costs over recent years, savings could now be significant.
• Will you be carrying out other building work? You may be able to save money on the cost of your pump installation by combining it with any other building work you have planned.

Solar Thermal Costs

At present, the Energy Saving Trust estimate that the cost of installing a solar thermal system to be in the region of £3000 – £5000. Generally, the more capable the system is of generating heat, the more expensive it is but the more it could save you on your heating bills.

When it was available, the government-backed Green Deal scheme offers a loan service which can pay for all or part of your installation costs.

You then pay back this amount on your energy bill, but because the payment amount shouldn’t be more than the saving you make by using the system the installation should be free to you. However, the payments do include interest and so your overall savings won’t be as large as if you had paid for the installation yourself. Unfortunately, this scheme was pulled some years ago now and as we have said, schemes and grants are now pretty much non-existant, but it’s worth looking about if you are considering.

Solar thermal systems require little maintenance and are generally cheap to maintain and run, as your fuel – the sun’s energy – is free. The system usually comes with a five to ten year guarantee.

You should check the system at least yearly to make sure everything is in working order and be aware of any carrier fluid leaking from the collectors. Some companies do offer an annual check. You should aim to get the system professionally checked every 3-7 years. The carrier fluid will need replacing every few years at a cost of around £500 as it does break down.

Installation Time

This does vary and depends on the size of the system, though 2 days is suggested as a guide.

Solar Thermal Installers

To ensure that your installation is of sound quality and complies with the government’s Renewable Heat subsidy schemes, you should always use an MCS accredited installer to install your solar thermal system.

The Benefits of Solar Thermal Systems

The obvious major benefit of a solar thermal system is that it uses free energy to heat your water, so once you have covered the initial outlay, then any heat it generates from that point on will be free to you.

Sadly, the many grants that were historically available to those wanting a solar thermal system installed have all dried up, unless you joined when it was up and running, so can still enjoy payments etc.

Despite this, if you do have the funds available to install such a system it will certainly save you in the long run, especially considering todays energy costs.

• Reduce your carbon emissions: Solar thermal systems lower your home’s carbon emissions by up to 510kg a year because they reduce the amount of fuel that you need to consume to provide hot water to your home
• Almost no maintenance costs: After you have paid for the installation you hardly have to spend any money to keep the system running, giving you hot water at a reduced cost
• Year round hot water: Though you will need to use your boiler more in the winter the solar thermal system will continue to heat your water through the winter months

Solar Thermal – Maximising Your Savings

You can maximise the benefit of the system by adjusting your usage:

• Make sure you use as much solar heated water as possible, rather than water heated in an appliance: Try to install a mixer shower instead of an electric shower, and locate any ‘hot fill’ appliances as close to the solar cylinder as possible
• Set your boiler to come on at the right time: You will need to adjust your boiler’s timer settings so that it only comes on when the solar thermal system needs topping up