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Vaillant Gas Boiler and Windhager Pellet Boiler

Solar Thermal

Solar Thermal Illustration

This section is titled "Solar Thermal" but what exactly does Solar Thermal provide?

A Solar Thermal System generally consists of one or more Solar Panels, pipework, circulation pump, expansion vessel, fill/flush point, controller and storage vessel.

All equipment used for a Solar Thermal system must be rated for solar thermal use as the temperatures regularly encountered are hotter than normally experienced.

The equipment can be used for heating Domestic Hot Water, Space Heating, Swimming Pool Heating or Commercial Processes. Domestic Hot Water is probably the most common use, although it is particularly suitable for low temperature applications, such as swimming pools.

Solar Panels are commonly mounted on roofs, although any location without shading from the sun that is not subject to vadilism threat or health and safety concerns (the panels become very hot), is suitable for consideration.

The solar panel/s contain a water which is heated by radiation emitted from the sun. The temperature of the solar panel is constantly monitored and typically, when 6 deg c warmer than the storage being heated, turns on the solar circulation pump.

The solar circulation pump continues to operate if the solar panel temperature remains greater than 3 Deg C warmer than the storage vessel.

There are various types of Solar Thermal System, we concentrate on two variations, Conventional and Drainback. The applications for both are numerous.

If you wish for your Solar Thermal to be repaired, replaced or serviced, simply call us on 01206 432 999.

However, are you Interested in Heat Lab Ltd improving the efficiency of your Solar Thermal?

We are glad you are still reading and are interested in energy efficiency, our passion. Solar Thermal systems are potentially incorrectly specified, installed or commissioned, reducing the user satisfaction and system confidence.

The efficiency can be improved by ensuring correct operation, general maintenance or improving system design;

All of these products are available from ourselves and have a thorough description in their relevant section of this website or just click on the description above.

There are a number of common problems encountered with Solar Thermal and the most damaging of those problems is stagnation, a condition where the solution within the panel boils or overheats.

The Technical Side of Solar Thermal

Types of Solar Thermal Systems

There are predominently two types, Conventional and Drainback. Each is available from a multitude of manufacturers, costs vary, various efficiency ratings are available and personal preference or previous experience often dictates buying of the appliance.

This section will explain the key differences between each type of System and will also show schematics for their conventional installations.

Regular Gas Boiler and Solar Thermal Schematic

Conventional Solar Thermal

This is probably the most common configuration, a twin coiler cylinder is used and a primary heat source provides Domestic Hot Water when insufficient solar gain is available. The remaining system can be a "S", "Y" or "W" plan, with a Domestic Hot Water Cylinder.

As can be seen by the plumbing schematic to the left, there are a greater number of external components and as such, the space required for the installation is slightly increased. The installation is not particularly flexible in design, with inclusion of additional boilers and wood burning stoves, whilst possible is complex.

The Domestic Hot Water governed by the performance of the solar panels, the selection, sizing, orientation and storge volume.

Solar Thermal is often used with open vented type hot water cylinders, the system relys on the height difference between the cold water storage tank and the hot water outlet. This type of installation is simple and effective, although pressure and/or water flow can be limited.

Also available are Unvented Cylinders or Thermal, which can provide much better hot water performance if correctly installed and again, we would recommend referring to the links above for further information.

Conventionally, the configuration of the Solar Panels are selected based upon expected hot water usage of the property and not occupancy or use. Hence, if the property is unoccupied or partly occupied, the heat generated is not used and there is a potential for the solar panel to stagnate or overheat.

Simple Solar Drainback

Drainback Solar Thermal

These systems are very similar to conventional systems and feature only one additional component, the Drainback Vessel.

The Drainback Vessel must be installed atleast 1 metre above the circulation pump on the flow pipework and the pump is mounted on the return. This configuration will provide a positive inlet pressure for the circulation pump.

The circulation pump however must be capable of providing sufficient pressure at the required flowrate to overcome the total system height and pipework resistance, not just the pipework resistance as in a conventional system.

The required flowrate is typically very low, around 0.5 to 1.5 litres per minute for a single flat plate panel under 2m2.

The volume of the drain back vessel sufficient to accept the complete volume contained within the Solar Thermal Panel and when the circulation pump turns off, the water drains out of the panel/s into the vessel.

The inclusion of the Drainback Vessel ensures the Solar Panel/s can not overheat or stagnate - enabling a much more flexible design configuration.

The drainback vessel is commonly used in europe, although it is less common in the UK due to reduced solar gain and therefore, is not as easily sourced.

Drainback Vessel

Below is a Schematic of a Simple Installation with a Direct Domestic Hot Water Cylinder and Drainback Solar Thermal System, hover over the image to view the difference in fluid levels with the system activated.

Example of How a Solar Thermal Drainback Works


With no Solar Gain available, such as at night, the solar panel cools and the circulation pump turns off.

When the solar circulation pump turns off, the water "Drains Back" into the drainback vessel.

When Solar Gain becomes available, the solar panel heats and as this temperature is being monitored, the circulation pump turns on.

Water is then pumped in to the Solar Panel and the level within the drainback vessel drops but critically, the water level must never drop to below the level of the circulation pump.

If the water level drops below the circulation pump, the solar pump should be moved to a lower position, the volume of the drainback vessel increased or both.

With the Solar Circulation Pump off, there is no water within the panel and overheating can not occur.

As we now have the option to not to use the panels in the periods of highest gain, we have a much more flexible system and is actually capable of modulating, like a modern gas boiler.

The system is now potentially more useful as we can "Oversize" the Solar Thermal System Collector Area, making greater gains when Solar Gain is not traditionally available and not overheat in periods of high solar gain, or little demand.

Advanced Solar Thermal

Shown below are several more options, with a much greater scope than traditional solar thermal packages and as such are much more viable.

Solar Thermal - Domestic Hot Water Only
Modulating Solar Panel Array

The schematic to the left has a direct domestic hot water cylinder and solar thermal panels, assuming that the installation space is available, an additional panel has been installed.

Traditionally, this would have overheated in the summer when high solar gain in available.

Due to the dual drainback system, this allows only one solar panel to become active in a period of high gain and the other panel remains dormant, with no water inside to overheat.

In periods of lower solar gain, both panels become active and have much better solar yeald, heating the cylinder when otherwise it would have been heated by the primary heat source.

This configuration can also be used for an "East West" configuration, although not being used to the full potential. If space was available, several panels could be used in each orientation and is generally much more tolerant to installation difficulties.

Solar Thermal - Domestic Hot Water AND Heating
Modulating Solar Panel Array with Thermal Store

Designed to heat both the Domestic Hot Water and space Heating, this is making the most out of Solar Thermal.

The solar panels above would normally be grossly oversized if all panels had be designated for Domestic Hot Water Use only. Due to our ability to "modulate" the heat input by only enabling the required quantity of solar panels, we are capable of reheating the Domestic Hot Water very quickly in the summer when solar gain is high and critically, we also have sufficient capacity to also provide some space heating in periods of lower solar gain.

Heat Input from the Solar Panels can be modulated as required without overheating.

Solar Thermal - Heating Only
Modulating Solar Panel Array with Thermal Store

This option does not provide Domestic Hot Water is not produced and as a result, is more limited in application. The system however is capable of providing very effective weather compensated space heating and with the limit being the solar panel area, is worthy for consideration.

With this installation option, the strengths of Solar Thermal and the potential high temperature are not being well utilised.

The important aspect to consider is the temperature of the heating water that is surrounding the solar coil and our need for it to be as cool as possible for ultimate solar gain. For this reason, it is also worthwhile referring to our information on low temperature heat emitters available below;

Solar Thermal - Swimming Pool
Modulating Solar Panel Array with Thermal Store

Swimming Pools are probably one of the most difficult heating loads, the evaporation, conducted and convected losses are substantial, therefore being unable to dissipate the heat collected by the Solar Panels is less likely.

The drainback configuration enables the solar thermal system to be turned off, even in the high solar gain periods in the summer and routine maintenance of the pool, power failures or cooling the pool for competitions, will not cause any problems with the installation.

Intermediate Vessel

Intermediate Vessels are used if a solar expansion vessel is prone to premature failure as Solar Thermal systems experience alot of heating and cooling, with the associated temperature changes. If the system is particularly large or the solar expansion vessel small, the problem will be exagerated.

Below is a Schematic of an Installation with a "Intermediate Vessel" installed on a Direct Domestic Hot Water Cylinder and Drainback Solar Thermal System, hover over the image to view the difference in fluid movement with the system activated.

Intermediate Buffer on Solar Thermal


With no Solar Gain available, such as at night, the solar panel cools, the circulation pump turns off and the system fluid contracts.

This results in the fluid exiting the solar expansion vessel and enters the "intermediate Vessel".

When Solar Gain becomes available, the circulation pumps starts and the system fluid expands as it is heated. This causes the water to leave the intermediate buffer and expand into the solar expansion vessel.

As the water entering the solar expansion vessel is only being stored in the intermediate vessel, it is not being circulated and as a result will always be cool.

As the water entering the solar expansion vessel is relatively cool (20-40 Deg C) and not 80 - 90 Deg C, the solar expansion vessel will last considerably longer.

If a drainback system is utilised, the system effectively has much more expansion allowance (the air in the drainback vessels can compress) and an intermediate buffer is unlikely to be needed.

This type of installation may cause some difficulties in commissioning as the water in the intermediate vessel is not being circulated and therefore receiving the relevant water treatment.

It is general practice to install a drain point before the solar expansion vessel and once the treatment has been added, drain through the appropriate volume of untreated water.


The commissioning of a Solar Thermal System is important if it is to perform efficiently, effectively and most importantly, safely.

Listed below are some key factors which Heat Lab Ltd verify before the Boiler is commissioned;

A little about Heat Emitters

As mentioned previously, there are various strategies available to optimise low grade heating and primarily, they all rely on reducing the temperature of the water flowing through the system.

If these methods are adopted for our Solar Thermal and Thermal Store Installation, the solar thermal system will have greater oportunity to contribute towards the space heating.

Tel. 01206 432 999