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Solar Thermal System Design

Solar Thermal water heating systems capture the sun's energy in the form of heat which is transfered to hot water cylinders. Replacing the need to burn fuels such as gas, coal or wood.

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Solar Thermal Overview

Solar Thermal water heating systems capture the sun's energy in the form of heat which is transfered to hot water cylinders. Replacing the need to burn fuels such as gas, coal or wood.

Depending on how much hot water you need, the hot water system you already have, your budget and where you are located there are a few ways you can put a system together. Below we outline some of the system designs that you could implement

Active "Direct" Open Loop

In an active Direct open Loop System the water you use in your bath and taps would be the same water that at one time would have been heated by the sun in your solar panel (or other collector).

Active "Direct" Open Loop System
Colder water is taken from your hot water cylinder and pumped (the active part) to the solar collector to be heated. Hot water is then returned to the cylinder. In the UK to protect from freezing we install a controller that will shut off the supply to to the collector once the outside temperature drops to a certain level. You can click on the picture to see a bigger picture of how this system fits together.

We often recommend Active "Direct" Open Loop systems when customers are looking for a simple or low cost solution. In many cases an Active "Direct" Open Loop system can be installed using existing hot water stores and cylinders, most of the existing pipework remains the same and these systems integrate nicely with existing boilers.

Active Drain-Back System (with Water)

With an active Drain Back system using water the temperature of the water in the cylinder is monitored, once the temperature drops below a certain level water is pumped up to the Solar Collectors to be warmed. When the water is not being used it drains back into the system to avoid freezing.

Drain Back systems are also popular due to their ease to install and simple design.

Active "Indirect" Closed Loop (With Antifreeze/Glycol)

Active "Indirect" systems use a dedicated loop to transfer a fluid such as Glycol between the Solar Collector and the hot water storage cylinder. The fluid is heated up in the collector, this heat is then transfered to the water in the cylinder via a heat exchanger (often a heating coil similar to a heating element in a kettle).

The second coil in this kind of system is used to transer heat from other heating methods such as boilers and immersions when the Solar Collectors aren't being used.

Active "Indirect" Closed Loop systems are very popular in the UK but slightly more expensive then the direct systems as a twin coil cylinder will need to be installed if you don't already have one and there is more pipework.

Passive Thermal Siphon

This is where a water storage tank is placed higher then the solar collectors to allow the heat captured by the collectors to rise into the tank and warm the water. This is not a very efficient system but as it doesn't expend any energy it may be useful in some situations.

Batch Heaters

This is where the storage tank or cylinder is the collector, the water is warmed by the sun as the cylinder warms up. With this type of system thought needs to be given about what happens to the stored heat at night.

Solar Thermal Equipment

Choosing a Solar Thermal Collector

Solar Thermal Collectors form the head end or energy capturing end of a Solar Thermal water heating system. There are broadly two types of collector, Flat Plate Collectors and Evacuated Tube Collectors.

When evaluating the two different technologies the three main things to consider are:

What are Flat Plate Solar Collectors?

Schuco Flat Plate Collector
Flat Plate Solar Collectors can be descibed most simply as a heat absorbant material mounted in a sunny spot. The collector absorbs heat from the sun and then transfers that heat into fluid that is circulating around the system. Where a traditional boiler would burn gas, oil or coal to heat the water you would use the sun's heat, not needing to burn anything.

What are Evacuated Tube Collectors?

Evacuated Tube Collector
Evacuated tube collectors look a bit like strip flourescent lights, although they work quite differently. An evacuated tube system consists of individual tubes with fluid circulating through them absorbing the sun's heat. From there the system is much the same as the Flat Plate system mentioned above.

Flat Plate Vs Evacuated Tube

A flat plate system is generally slightly cheaper as they usually have less engineering costs associated with their manufacturer. Yet in ideal conditions, measuring peak performance a flat plate system could compete or even beat the performance of an equivelant evacuated tube system.

An evacuated tube system is likely to perform slightly better in lower light environments so you may find in a like for like comparison with an Evacuated Tube system, the tubes would squeeze a bit more heat out of the available sun in Spring/Autumn or on duller days.

Both have their uses and both work effectively in the UK, we don't believe that one technology is better then the other. The right collector to choose will be the one that looks best on your property and is big enough to provide for all of your hot water requirements.

Cost Vs Performance

When evaluating collectors it's always useful to think about cost Vs mounting area Vs performance, for example if you had a large south facing roofspace at the optimum pitch and a big flat plate collector this system is likely to deliver equal performance or more likely better performance when compared to a smaller, more expensive evacuated tube system.


If you had only a small amount of available space on a wall, roof or balcony or less the perfect conditions, you may find the curved nature of the tubes makes up for lack of space by always offering a larger surface area to the sun, you may find you get more energy per sqm from Evacuated Tubes in less then ideal conditions.


Both Flat Plate and Evacuated Tube systems require very little maintenance. There are few moving parts and the inner workings are well protected from the elements. If something happens to the collector it's easy to swap out whole collectors and individual tubes.

Both systems will perform better when clean and free of dirt/dust so if dirty and where feasable its worth washing them both in much the same way as you would clean a window using just water.

Most Solar Collectors have been designed to expell water and dirt effectively on their own so if you can't get to them and don't notice a drop in performance they are likely to be ok.

Choosing a Solar Hot Water Cylinder

The Solar Hot Water Cylinder is arguably the most important and influencial part of a Solar Thermal hot water system design it determines:

If you get the size and type of cylinder for the installation right then usually the other elements of the system will fit nicely into place.

Single Coil Cylinders:

Single Coil Cylinders can be used with Solar Thermal installations, if you already have a water heating system with a single coil cylinder then you do not neccessarily need to replace it. When using a single coil cylinder a 'direct' heating method is used, this is where the hot water from the collector warms the water in the cylinder directly whilst returning colder water to the collector for re-heating.

Twin Coil Cylinders:

Having a twin coil cylinder installed is like having a combined dedicated solar cylinder and a normal cylinder at the same time. Fluid in a closed system circulates from the Solar Collector to the Cylinder through a coil running through the cylinder. Colder water is returned to the collector to be heated. The second coil is connected to a traditional boiler or other heating source. A Twin Coil cylinder is essentially getting hot water from 2 independent sources. When looking at twin coil cylinders, long thin ones are generally better.

Investing in a twin coil cylinder is great for those who:

Solar Thermal Benefits

Will we need planning permission to install a Solar Water System?

Typically planning permission doesn't need to be granted before installing a domestic Solar Water Heating system under 9m2. Exceptions include listed buildings or homes within protected areas or national parks. In our experience when presented wih sympathetic system designs local authority planners look favourably on the installation of these systems.

What will a system look like on my roof?

An average family home's domestic hot water needs can be supplied by 1 or 2 neat panel, similar in appearance to a large flat black velux window. Collectors can also be made from evacuated tubes which will look like flourescent tubes mounted together on a roof or wall.

How long will it take to get a system installed?

The first step is to carry out a survey of your property. The survey will cover off physical aspects such as where to place equipment, what style of panel(s) best suits the property and an evaluation of your existing energy infrastructure such as boilers and pipework. An average domestic installation takes 2 engineers between 1-2 working days to complete.

Can a Solar Hot Water System heat radiators?

As a rule a Solar Hot Water System wouldn't be able to heat the radiators found with typical Gas Central Heating Systems. This is due to not being able to heat the same volume of water neccessary due to space constraints on the roof and the size of the panels that would be needed. However Solar Hot Water systems can be succesfully integrated into underfloor heating systems. Also intelligent placement of storage tanks can negate the need for or replace some radiators. A lot of the options available are dependent on the existing layout of the house.

Do Solar Water Heating systems and Solar Electricity systems need different equipment?

The simple answer is yes, Solar Hot Water systems work on the principle of converting light into heat wheras a Solar Electricity installation will convert light into DC power i.e. electricity.

Do we still need a boiler?

A Solar Water Heating system is capable of producing 100% of a households hot water in the summer months. In winter when the days are shorter this can drop down to as low as 10%. To make up the difference in the winter moths you will either need to retain your boiler or find another back up water heating source. A good CO2 neutral complement to Solar Water Heating is a Biomass or wood burning boiler.

Is my property suitable for a Solar Hot Water system?

A quick look at your location will give you a good idea if a Solar Water Heating system can be installed. You are looking for physical free space of around 4m x 2m (front or back) with a solid structure that can hold the weight of the panels. The area of roof should also be unshaded by chimneys, trees or other properties and face anywhere from East through South (Ideal) to West.

Does it need to be sunny in order to work?

No, Solar Water heating systems utilise both direct and indirect sunlight. What's important is the amount of hours of daylight that is available. This is why 100% of your hot water needs can be easily achived in the summer months with quite a bit less (can be as low as 10-15% in some parts of the UK) in winter.

What’s my options if I don’t have a South-facing roof?

You can install an 'East-West' system which has a panel facing each direction, each panel working at different times of the day AM / PM.

What types of solar collectors are available?

There are two types of solar collector

  1. The most common type consists of flat plates with water pipes inside an insulated box with a glazed front. These are called Flat Plate Collectors.
  2. The second type, know as evacuated tubes, looks like an array of oversized fluorescent tubes. They produce more energy per unit area but are generally more expensive.

Do solar panels have to be fitted to a roof?

No, they can be mounted on a free-standing frame in the garden or on a flat roof or they can be mounted vertically on walls and balconies.

What Types of Solar Thermal Collectors are available?

Roof Integrated Flat Plate Solar Thernmal System
Roof Integrated Flat Plate Solar Thermal Systems: Offer a consistent finish and clean roof lines. Flat plate collectors are installed in much the same way as a velux window, being fitted into the roof using a colour matched frame. Flat Plate Solar Collectors are supplied with 25 year guarantees and can be connected in parallel. Full systems can be supplied with and without hot water cylinders and can also be supplied for self installation.
Slimline Evacuated Tube Solar Thermal Systems
Slimline Evacuated Tubes for Solar Thermal Hot Water: Have the dual benifit of high performance and flexible mounting. They are designed to operate at the low light levels found in the UK maximising solar gain even at freezing temperatures. For a system that performs well throughout the year in challenging conditions or off perfect mounting these slimeline evacuated tube systems are hard to beat.

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