Solar Thermal Systems “Are they a good investment?”

One of the most common questions we are asked; “Are solar thermal systems a good investment?”



The simple answer in most cases is NO. While the principle of using FREE energy from the sun to heat water makes sense, how does it work in a real-life application in the UK. The average family will spend around £1000 per year on heating and hot water. Hot water accounts for around 20% of the annual domestic fuel bill (£200). A correctly sized solar thermal system will cover on average 70% of your annual hot water consumption or save £140 per year, or does it? A circulation pump is required to move the fluid from the roof array to the cylinder as well as the control system. An ErP compliant A rated pump will cost around £20 per year to run and controls (£2). This gives a real life saving of £118 per year. An MCS supplied and installed system with new cylinder costs between £4000 & £5000.

Under the governments Renewable Heat Incentive scheme, a home owner would receive between £1500 & £3000 (dependant on occupancy) paid quarterly over 7 years. Servicing and maintenance must be factored in with an average service charge of £250, as it can only be carried out by a qualified specialist. We would recommend the system is serviced every 5 years to check integrity of the pipe work, panels, expansion vessels and most importantly the antifreeze. As with your car, part of the major service is to replace the antifreeze as it deteriorates over time creating sludge which inevitably leads to terminal problems!

Using an average installation cost of £4500, RHI return of £2200 and 3 services (£750) over the systems lifespan, there is a deficit of £2950. This is reduced by the offset in fuel (£118pa). In this scenario the Return on Investment (ROI) would be 25 Years (excluding fuel inflation).


Design considerations

We are one of the few companies in the South West that offer servicing & repair for any manufacture of solar thermal system and the biggest cause by far of system failure is over sizing of the array. Once the hot cylinder has reached its target temperature (65C-70C) the solar thermal system becomes static as it can’t reject the energy from the sun. This is called stagnation, where the fluid (antifreeze) boils and reaches temperatures of 200C – 300C. Stagnation is normal but can be reduced with a correctly sized array and storage cylinder. Excessive stagnation will cause system components to fail; expansion vessels, pumps, sensors and the panels or tubes.

The number of panels or tubes must be calculated based on occupation and actual usage. Not the number of bedrooms and bathrooms. It is a lot easier to increase the size of a system than to reduce it.



With modern high efficiency buildings, solar thermal can be integrated in to a low temperature heating system and cover around 20% of the annual heating load. Unfortunately, the RHI does not provide payments to solar thermal systems for space heating.

Solar thermal systems for indoor and outdoor swimming pools and can provide huge fuel savings over conventional heating methods and even heat pumps.

Commercial applications attract the highest ROI of any technology with a 20-year RHI tariff; swimming pools, district\communal hot water, energy harvesting for ground source heat pumps, process plant, etc.



Solar thermal systems are good addition as a supplementary or primary heat source but only in the right application. The effect of stagnation can not be under estimated.

You can reduce fuel costs and your carbon foot print with simple measures; loft insulation, draft proofing, LEDs and most importantly, bad habits. The next step would be installing Smart thermostats to control the heating or upgrading to an A rated efficient boiler or heat pump. A Solar PV system still offers a ROI of 7-10 years even though the tariff has recently been reduced.

If you want to know more about the options available speak to our Technical team at

Join the Heating Revolution – Boiler Plus Regulations

From April 2018 new standards for domestic boiler installations (Boiler Plus) will apply, improving the way people use energy in their homes by giving them more choice, greater control and tangible savings on their energy bills. The new minimum performance standard for gas boilers in England will be set at 92% ErP. The standard will also make timers and room thermostats a minimum requirement.

When a gas combination boiler is installed, an additional energy efficiency measure will be required. This requirement is flexible to allow a suitable choice to be made that reflects the diverse nature of housing stock, and the needs of the household. The energy saving technologies that can be selected are;

  • Flue gas heat recovery system
  • Weather compensation
  • Load compensation
  • Smart controls featuring automation and optimisation functions

What do I need?

The range of controls and options from manufacturers and 3rd parties (Nest, Tado, Honeywell, etc) can be confusing and with the new Boiler Plus regulations it couldnt be more important to select the right package. This is where you rely on your installer to give you the right advice and not what is easiest to fit or most profitable.

At South West Energy Services we have built part of reputation on applying advanced integrated controls to domestic and commercial heating\cooling systems for the past 10 years. This has culminated in to developing our own automated building control software for commercial applications.

Our expertise in this field means we can give you the right advice every time!

To help understand the options we have put together a simple guide for you.


Flue gas heat recovery system

A flue gas heat recovery unit is an option provided by manufacturers and 3rd parties to recover the rejected energy from the flue gas and transfer it back to the heating or hot water systems.


Provides good efficiency gains for high temperature heating systems and large hot water demands


This is an external unit which fits on top of the boiler so available space must be considered


Weather Compensation

Weather compensation adjusts the water flow temperature from the boiler  based on ambient air temperature. The colder it gets outside, the higher the heating flow temperature. The reverse for cooling. At a fixed external temperature (summer disconnect) the heating system will shut down automatically.

Weather compensation will save 10% with preset set points based on (A0˚C\W60˚C) and a summer disconnect of 18˚C.


Simply to install

Low cost

Highest return on energy savings


There aren’t any!


Load Compensation

Load compensation monitors internal and external temperatures to enable the boiler. Dead bands are created where the boiler  is in standby even though the external temperature is lower than the internal temperature.

Load compensation will delay the start up of the heating\cooling system to the latest possible moment to avoid unnecessary energy use. The sensors calculate the rate of change and amount of time required to raise the temperature by 1C.

To improve efficiency weather compensation needs to be integrated.


Simply to install

Low cost


Can cause issues with comfort levels when applied to buildings with high solar gain (lots of glass)

Requires weather compensation to be fully optimised


Smart Controls

Smart controls are all around us and we have more options than ever before. Smart controls are basically an App which utilises Load compensation and geo location with the ability to adjust the internal room temperature and hot water functions remotely.

For existing boiler installations with one or two heating zones we would recommend the Nest or Honeywell Lyric. They are good value for money, easy to install & setup and provide annual savings of 2-5%.

New boiler installations will be specified with the Vaillant vSMART and VRC700 which incorporates; Weather compensation, Load compensation, Smart controls, all in one package.


Simple to install

Low cost


Having the ability to control your heating remotely is a useful tool but now we have the control do we understand how best to use it?

More guides to follow on how to use your Smart Control

Domestic RHI (Renewable Heat Incentive) Update March 2016

At South West Energy Services, we provide constant updates of changes to industry funding and regulation. There have been some changes to the RHI for 2016. Below is a summary of the changes from Ofgem.

1. Green Deal Assessments

Currently we require applicants to the domestic RHI to have a Green Deal Assessment (GDA). This is to provide them with information about the efficiency measures that are suitable for their homes. This initially applied to all applicants but Registered Social Landlords (RSLs) were made exempt from this requirement in February 2015.

It was announced in July 2015 that there would be no further funding to the Green Deal Financing Company. This, alongside consistent feedback that a GDA is an unnecessary burden to scheme participants, has resulted in us changing the Regulations by removing the need for a GDA as an eligibility requirement for the Domestic RHI. Applicants will still need to meet minimum insulation requirements of loft and cavity wall insulation and provide a valid Energy Performance Certificate (EPC) that is no more than 24 months old.

2. New Build Properties – 183 Day Occupancy

The domestic RHI requires that if a property is occupied for less than 183 days, in a 12 month period, a heat meter must be installed to measure renewable heat use and determine payments, instead of payments being based on the annual heat demand specified on the property’s EPC.

This occupancy requirement ensures that the Domestic RHI delivers value for money for the taxpayer by not overpaying for renewable heat systems installed in properties that are not continuously occupied (for example, second homes).

This has resulted in the unintended consequence of eligible new self-builder properties having to wait 183 days prior to scheme application or installing heat meters as they are unable to provide evidence that they have lived in their homes for at least 183 days in the previous 12 months.

To overcome this, we are amending the regulations to make eligible new build properties exempt from the 183 day occupancy requirement in the 12 months prior to application to the scheme. Thereafter, these participants will be subject to the standard ongoing obligations which include the annual declaration that the property has been occupied for 183 days or more in the last 12 months.

3. Tariff Indexation

Tariffs in the both the Domestic and the Non-Domestic RHI are subject to an annual adjustment in line with inflation to take into account changing prices in the economy. The indexation rate used to date has been the Retail Prices Index (RPI).

RPI is no longer classified as a National Statistic, therefore, for all applications accredited on or after 1 April 2016 tariff adjustments will be linked to the Consumer Price Index (CPI). CPI is already widely used across Government, for example for the uprating of pensions, wages and benefits. Current scheme participants will continue to have their tariff linked to RPI.

4. Degression Triggers

Spending in the RHI scheme is controlled through the degression mechanism. This operates by applying an automatic reduction to a technology tariff if pre-determined expenditure trigger points are exceeded. Expenditure is assessed every 3 months.

Individual technology trigger points for the scheme are set out in the Regulations. Currently the Regulations only specify triggers up until the 31 January 2016 assessment date; the next assessment against expenditure is due 30 April 2016, which will inform any tariff reductions that take place on 1 July 2016. In these Regulations we updated the expenditure trigger points for the scheme until the 31 January 2017 assessment date.

Are air source heat pumps noisy?

The simple answer is NO but what is noisy? Modern air source heat pumps are significantly quieter than units produced say 5 years ago. This is primarily due to advances in the development of fan technology and improved chassis design which reduces vibration. 99% of sound generated by an air source heat pump is the movement of air. The sound levels will vary depending on the ambient temperature. The colder it is outside the faster the fan spins and vice versa. In the middle of summer the fan may not even spin.

For the purposes of the MCS Planning Standards for air source heat pumps the back ground noise is assumed to be 40 dBa or lower. For compliance with planning regulations and permitted development rights the air source heat pump must be 42 dBa or lower from the assessment position.  The assessment position is the distance between the unit and a neighbouring property.

The positioning of the unit is critical to meeting the maximum sound levels. Obstacles such as walls and fences can increase the noise levels over the design conditions. A well designed air source heat pump system must allow for the displacement of the air. This not only reduces noise but also increases the efficiency of the heat pump.

When the noise exceeds the maximum permitted levels the unit can be installed in an acoustic enclosure or sitted far enough from the assessment position to meet the regulations. Heat pumps can be located up to 30m from the property before other design considerations are imposed.

To reduce sound levels  manufactures are encouraged to meet the standards set out by the Noise Abatement Society and  awarded the Quiet Note if the heat pump is within the standards. The Mitsubishi Ecodan was the first heat pump to be awarded the Quiet Note in the UK.

DeLonghi Air source heat pump - high temperature - acoustic louver enclosure

Specification of Commercial Ground & Air source heat pumps

The unique thermal characteristics and demands of a commercial building often means a traditional approach of separate boiler and chiller is taken to meet the peak heating and cooling loads. Due to increasing operational and energy costs, renewable technologies are becoming more common in modern developments but very rarely used to their full capability. There is a growing demand to increase sustainable value to a development and utilise natural resources more efficiently. Ground and air source heat pumps can replace or integrate with new and existing applications to provide; heating, hot water and cooling, efficiently as possible while reducing carbon emissions.

Although heat pumps in general offer the highest return on investment and lower energy bills, the initial capital cost can discourage developers and end users from investing in the technology. This is not to say heat pumps are the only solution. A majority of commercial heating and cooling applications operate at part load conditions throughout the year. The integration of a heat pump with gas\lpg or oil boilers can significantly reduce investment and operational costs while still meeting the design conditions.

Research carried out by construction consultants Sweett Group and BRE found that office developers typically invest up to 2% more when targeting higher BREEAM ratings, and recover that additional investment in two to five years through savings in their energy and water bills. The same research found that achieving lower BREEAM ratings can incur little or no additional cost.

There is a greater opportunity to specify recovery heat pumps which recover rejected energy from the cooling cycle to provide simultaneous heating and cooling with huge energy savings. This can be improved even further with a recovery ground source heat pump as rejected energy can be stored in the borehole array and harvested during the heating operation increasing the COP.

We offer a turnkey solution for all types of commercial heat pump systems from 5kW to 1MW.

We supply equipment as approved installation and service partners for; Dimplex, Thermia, Danfoss, Carrier, Clivet, CIAT and Climaveneta.

South West Energy Services – Thermia 42kW Ground Source Heat Pump

New Build - Thermia - Ground Source Heat Pump - Vertical bore holes - heating - hotwater - swimming poolThis exclusive new build development in Cornwall has been built with no expense spared. To meet the standards required for an A rated energy efficient house a Thermia ECO 42kW ground source heat pump with 8 x 100m vertical boreholes distributed from a sealed manifold chamber. The heat pump supplies a 500L heating buffer tank and 2 x 250L heat pump optimised hot water cylinders. The buffer tank supplies the wet under floor heating system and indoor swimming pool.

The ground collector is flushed and filled with a diluted Ethylene Glycol mixture (20%). THERMOX DTX has been tested and classified as Non-Toxic 1 by an EPA 2 certified Laboratory.

The heat pump is capable of storing domestic hot water at 60˚C with a secondary return that pumps the hot water around the building meaning you have instant hot water at tap outlets.

To increase system efficiency and performance, the Thermia Solution Controller has been specified. The web based user interface allows remote monitoring of the system with;

  • Access to control strategies and interface parameters
  • System overview
  • Datalogging with presentation in graphical charts
  • Alarm notification through e-mail or SMS

Due to the varying load conditions during the `bedding in` period (12 months), South West Energy Services Ltd, provide a FREE service where we will monitor and optimise the system remotely.

As this is a self build the client will be entitled to the Domestic RHI which will generate a revenue of £40,000 over the next 7 years.

South West Energy Services - Cornwall - Thermia ECO 42 Ground Source Heat Pump - 500L Buffer Tank - 2 x 250L DHW Cylinders - 8 x 100m boreholes - - Domestic RHI

The importance of borehole design for ground source heat pumps

Here is some guidance on the importance of a professionally designed and installed borehole for a ground source heat pump. I have included some technical guidance from the Ground Source Heat Pump Association and MCS GSHP Design Document, regarding the sitting and installation of bore holes. This will hopefully provide you with a better understanding of the correct process for drilling and grouting. Unfortunately a majority of companies do not use a thermal grout, partly due to keeping costs down but also a complete lack of understanding to what is happening in the ground. The water table alone will not suffice.

Borehole spacing and sitting is critical to optimising the extracted energy from the ground and sun. Where boreholes are positioned in close proximity to fixed structures (>7m) the impact of reduced ground surface area on the recovery rate from solar irradiation must be considered.(there is very little geothermal energy extracted, 99% is from the sun). The efficiency and composition of a borehole and probe is calculated using the following criteria;

Abstraction rate for the type of ground\geological make-up.

  • Borehole depth
  • Borehole spacing (distance between boreholes or structures)
  • Solar irradiation
  • Borehole probe construction (32mm or 40mm HPDE)
  • Type of thermal grout

Yes there is the option of energy piles but would you really use this on a domestic project?

We have been designing and installing ground source heat pump systems for over 15 years and built a reputation nationally as a market leader and pioneer in the industry for both domestic and commercial projects. We provide consultancy, design services and training, not only to our competition but also to heat pump manufactures\suppliers. We are one of only a handful of contracting companies in the UK, who provide CPD training to Architects on ground & air source heat pump design.

We are happy to loose a project to a well designed and installed system, unfortunately there are very few out there which is why we try and provide as much guidance and knowledge as possible even if not our job. Anyone can install a heat pump its no different to a boiler. It comes down to design, best practice and experience. The most important part of a GSHP is the ground array as it provides 75% of your energy demands and it’s the one part you can not see once installed.

If you have any questions regarding the drilling process please speak to our drilling partner ADP to see how one of the largest drilling companies in the UK operates.  Tom Page is the MD and can provide you with impartial advice on how a bore hole should be designed and installed. Dont mention you are speaking with South West Energy Services, and see what he has to say.

2.8 Party Wall Issues 

All systems shall be designed assuming that adjacent systems will be installed and will therefore have a right to the heat under their property.


8.1 General 

Grout thermal conductivity is an important aspect of ground heat exchanger design. The designer shall be responsible for selection of the grout to support the foundation of the ground heat exchanger design. The following standards relate to the materials and methods of testing and verification of properties, as supplied by the manufacturers.

8.2 Grout Thermal Conductivity

Grout material thermal conductivity shall be determined by the following tests or comparable to EU standards:

  • Pliable materials – American Society for Testing and Materials (ASTM) D-5334 “Standard test method for determination of thermal conductivity of soils and soft rocks by thermal needle probe procedure”
  • Rigid materials – ASTM C-177 “Standard test method for steady heat flux measurements and thermal transmission properties by means of the guarded hot plate apparatus”

Materials that are bentonite-based are classified as pliable materials and cement based products or grout mixtures containing cement that cause the product to become rigid once cured are classified as rigid materials.

9.4 Ground Heat Exchanger Installation

The borehole shall be grouted from bottom to top with a suitable recognised geothermal grouting material and tremmie pipe as outlined in section 8.


3) MIS 3005 requires that the design of a GSHP system will lead to a minimum entering water temperature (EWT) into the heat pump from the ground of 0°C over the lifetime of the installation. Assuming a possible temperature difference of 3°C, an average minimum ground loop fluid temperature of -1.5°C has been adopted here.

This temperature defines some of the properties of the antifreeze solutions.

A) Heat pump thermal output. This is can be taken as the manufacturer’s MCS approved quoted output at B0/W35.

I know this is a bit War and Peace but I am a big believer in pass on the knowledge, it’s the only way the industry will improve and grow.

Please look at our case studies page for the types of projects we are involved in.


Are Solar Thermal Systems a good idea?

One of the most common questions we are asked; Is solar thermal a good addition to a ground or air source heat pump system?

The simple answer in most cases is NO. Hot water accounts for 20% of the annual domestic fuel bill. Solar thermal will cover on average 70% of your annual hot water consumption, so really doesn’t provide very much unless you have a very high hot water demand. The RHI pays back around 80% of the installation cost paid back over 7 years. Financially it doesn’t stack up. Plus you have the annual service cost which would be higher than what you would save. With modern high efficiency buildings, solar thermal can be integrated in to a low temperature heating system and cover around 20% of the annual heating load. Unfortunately the RHI does not provide payments to Solar thermal systems & space heating.

In our opinion you are better off in investing in a Solar PV system even though the tariff has been recently been cut. A 4kW PV system can reduce the heat pumps predicted running costs by around 20% plus generate a revenue for 20 years with the FIT. PV still makes sense if you have a heat pump. It would also provide more free hot water than a solar thermal system. Why pay for 2 installations when 1 does it all!

At South West Energy Services we highly recommend Solar thermal systems for indoor and outdoor swimming pools and where there is a higher than average hot water demand (changing rooms or communal showers).

Solar thermal system - evacuated tubes or panels with air and ground source heat pumps

Under Floor heating design and installation with a heat pump or boiler

One of the most important factors when designing a heating system is to understand the integration of the heat source be it a ground\air source heat pump or gas\oil boiler and the effects flow temperatures have on performance, efficiency and comfort. This is even more important when using under floor heating due to the slow reaction time, meaning any action you take today (raise room temperature) takes effect tomorrow. We provide a complete design and installation service for all types of under floor applications.

Our standard solid floor design is based on a screeded floor construction with the underfloor heating pipes clipped to floor grade insulation and embedded within the screed layer. With a standard sand/cement based screed the thickness of screed should comply with the requirements of BS 8204, which specify a minimum screed thickness of 75mm except for domestic and light loading applications where a thickness of 65mm is acceptable. Alternatively a liquid flow screed can be used with a minimum thickness of 50mm.

Where under floor heating is required in suspended timber floors there a number are number of different methods to consider; metal diffuser plates, over lay systems and pipe clipped to insulation between the joists.

Due to the different construction methods we offer a free technical design service for all types of under floor heating to make sure the right system is applied every time.

Uponor under floor heating design, 16mm 20mm PEX pipe, manifolds, diffuser plates, supsended and soild floor

What type of ground collector do I need for a ground source heat pump?

Solutions ground source slide 9

Ground source heat pumps can extract energy from the ground or water using a closed loop or open loop collector. The size of the array is dependent on a number of factors including but not limited to the peak heating and cooling loads and usage of the building.

The most common design is to use a horizontal collector buried in a trench 1.2m deep x 1m wide. This method does require a large amount of ground and can not be used to build on. A typical 3 bedroom house would require 300-400m of pipe.

If space is an issue then a vertical borehole can be drilled. The bore hole is drilled to depths of up to 150m for residential properties. One of the most important factors with drilling is to ensure a thermal grout is pumped in to remove air from around the bore hole pipe. A lot of companies do not do grout and rely on the water table which will vary throughout the year. This will cause inefficiencies in the heat pump and potential system failure.

Ground source heat pump design and installation open trench slip trench

The slip trench design uses a 200mm wide bucket to install a single collector pipe. Trenches must be a minimum of 1m apart.

 The open trench design uses a 1m wide bucket to install the collector pipe either side of the trench. Trenches must be a minimum of 1m apart.