About Solar Thermal


Solar Thermal systems can generate up to 70% of a households annual hot water requirement and are an excellent source of renewable energy on their own or as a complement to solar PV and ASHP.

A solar thermal collector consists of a dark flat-plate absorber, a transparent cover that allows solar energy to pass through but reduces heat losses, and an insulated backing. Glycol anti-freeze fluid is pumped around the circuit - to the solar collector to heat up and then through the lowest coil in the cylinder to transfer that heat to the surrounding water in the cylinder, then back up to the collector and so on. This process is controlled by a display connected to a number of temperature sensors fitted on the cylinder and collector so that the pump is only operating when the conditions are bright enough to make it worthwhile.

Sizing the system:

The collector(s) are sized according to the volume of the hot water cylinder(s), which in turn should be calculated according to the number of occupants using the hot water. If the collectors are too small for the size of the cylinder and the amount of consumption, the system will be ineffective and the boiler (or other primary heat source) will need to work harder. If the collectors are too large, they will heat the cylinder quickly in the morning and then have nowhere to transfer the heat to in the afternoon. This will cause stagnation, where the glycol becomes thick and tar-like and will soon damage the collector and other components.

The Two Most Common Installation Methods

Fully Filled / Pressurized Systems Drain back System
  • Replacement of glycol necessary every 5-7 years
  • Low power pumps adequate
  • Systems may stagnate if collectors are oversized
  • Stagnation and fluid failure can occur due to pump failure or loss of electrical power
  • Simple to install.
  • The glycol drains from the collector to the drain-back tank when the system is not operating to avoid stagnation.
  • Replacement of glycol necessary every 7 - 10 years
  • Requires a powerful pump
  • Can be complex to install.
  • Collectors and piping must be carefully sloped to ensure drain-back occurs.

Water Cylinders

Twin Coil Hot Water Cylinders (vented or unvented)

In a twin coil hot water cylinder the upper coil is normally heated by a boiler, and the lower coil is used for the solar thermal circuit. This is the preferred system in most cases.

Solar thermal on a vented cylinder Solar thermal on an un-vented cylinder
  • Inexpensive  to buy and maintain.
  • More commonly Installed as a retro fit or replacement system
  • Requires a cold water storage tank, often in the loft
  • Requires maintenance & annual servicing.
  • Provides constant hot water at mains pressure
  • Commonly installed during a refurbishment or new build
  • Must comply with building regulation G3

Thermal Stores

Thermal stores are a reversal of a twin coil cylinder in that the hot tap water is passed through a coil to be heated by the surrounding water in the cylinder. There would be an additional coil for the solar thermal to transfer it's heat to the cylinder.

As well as not needing to comply with Building Regulation G3, one of the advantages of a thermal store is the ability to combine many heat sources in one system, such as:

  • Wood burners
  • Gas or oil boilers (vented or pressurised system boilers)
  • Solar thermal
  • Heat pumps
  • Electric elements (powered from mains supply, or from wind turbines)
  • Central boiler plants on a communal system

The Controller

While the control systems for solar are sophisticated, they primarily rely on sensing temperatures in the hot water cylinder and comparing the temperature within the solar array on the roof. Once the control system senses a solar array temperature 7 deg greater than the temperature within the lower part of the cylinder, the system pump will begin to operate at the best speed to gather the maximum possible solar energy. When these two temperatures come within 4deg of each other the pump will stop.

Maximising Performance

You should set the boiler to not heat the cylinder during the day when there is no hot water demand and there is the opportunity to gather solar energy. For an average household we would suggest heating the hot water by the boiler in the late afternoon (once the sun has had an opportunity to make its contribution) ready for the evenings demand and then again briefly in the early morning, ready for the morning demand.

In the Summer months when solar production is high, and there is no central heating demand, it is often possible to leave the boiler switched off and let the solar thermal do all the work and provide the hot tap water alone.

Holiday function

Stagnation is a serious problem at times of high production / very low consumption such as Summer holidays, and to mitigate this, the controller should have a 'holiday function' which runs the pump for a short period every night so that some of the heat in the cylinder is lost to the cooler night air. This means the system doesn't begin another sunny day with a cylinder already up to temperature and no-one there to use it.

Roof Suitability

As with a PV array, the thermal collector will ideally be mounted at a 30deg pitch facing South. However where planning is restricted we can design systems to be laid flat or even fitted to a gable end wall. Evacuated tube collectors allow even more design flexibility as you can angle each tube one way or the other.

The "solar sundial" below shows that a fair bit of deviation in the positioning of the array will still yield a large percentage of the energy expected from an optimum orientation (a pitch of 30/40 degrees facing due South).

solar sundial


For an optimum system it is important to avoid shading as much as possible. However, unlike PV, solar thermal systems are quite resilient to partial shading, and by slightly over-sizing the collector we can compensate for this. Our engineer would assess the site and can provide a simulation report showing predicted performance outputs.

Roof Area Required

Flat plate collectors - Allow approximately 1m2 of pitched roof area roof  for every 1 person occupant or 50L of stored water capacity.

Evacuated tube collectors - Allow approximately 0.5m2 of pitched roof area roof  for every 1 person occupant or 50L of stored water capacity.

E.g. 4 person household with a 200L twin coil cylinder = 4sqm of flat plate collector or 2 sqm of evacuated tubes

Flat Plate Collectors OR Evacuated Tubes?

Most evacuated tube systems lose less heat to the surrounding air (because of the vacuum), and are more efficient per square meter than equivalent flat plate systems. This makes them ideal where roof space is limited.

More expensive flat-plate systems are often considered most attractive however, and lend themselves well to high-spec integrated (in roof) systems.

Planning Permission

Planning permission is not normally required unless it is a listed building or in a conservation area - please see the Town and Country Planning Amendment of April 2008. If in doubt, check with your local council, and if you do need to make a planning application we will advise you and supply all the drawings, photographs etc that you need for a successful outcome.


To claim the Renewable Heat Incentive, you need to ensure your panels are installed by an MCS accredited installer, and comply with Building Regulations Parts A and C.

G3 Building Regulations and the supply of hot water

These regulations aim to ensure that wholesome water is supplied, that the water is not wasted and that the use of the water does not result in the supply becoming contaminated.

G3 used to relate only to unvented hot water storage, but now relates to all hot water systems and includes provisions to prevent scalding, as well as requirements governing the installation and use of hot water systems, expansion vessels and storage tanks.

Renewable Heat Incentive

The RHI is an incentive similar to the FiT (Feed in Tariff) to encourage consumers to generate heat energy from renewable sources. The most up-to date information can be seen here.

The scheme will support air source heat pumps (ASHP), biomass systems, ground source heat pumps (GSHP) and solar thermal technologies. The support rates vary depending on the technology installed:

  Air source heat pump Biomass Ground source heat pump Solar thermal
(p/kWh renewable heat)
7.3 12.2 18.8 19.2

However the generation of heat energy is generally less easy to measure than electrical generation from PV's, and the operation of the scheme is slightly different from the PV Feed-in tariff. The installer (MCS accredited) will declare an estimate of how much heat your renewable energy system will produce and you then get paid at the applicable rate over the seven year period of the RHI payments.

For larger systems, the heat output will be metered rather than estimated.

Existing systems

Solar Thermal installed before the launch of the domestic RHI in summer 2013, but after 15th July 2009, will still be able to join and benefit from the Renewable Heat Incentive scheme.

Renewable Heat Premium Payment

There is an additional incentive payment of £600 available for domestic solar thermal installations to encourage uptake yet further. Full information is on the EST site here.

Commercial systems

For information on applying for commercial RHI see www.ofgem.gov.uk/e-serve/RHI/howtoapply/Pages/howtoapply.aspx

What next?

If you are interested in installing a solar thermal system please get in touch. We will need to know:

  • Is your existing boiler a conventional boiler or combi?
  • If conventional, is your existing hot water system gravity fed or pressurised?
  • Can you tell us the capacity in litres of the existing / proposed cylinder?
  • How many people use the hot water and what is your pattern of use (all in the morning or spread out)?
  • Do you have a preference for evacuated tubes or flat plate collectors?

Don't worry if you don't know all the answers, a photo or two of the existing heating system will often be enough, and we can arrange a site visit after sending you an estimate.

If you are enquiring about a commercial system please get in touch. We will need mechanical schematics and information on any target requirements and predicted consumption.