As part of the graduate scheme at FairHeat, I rotate between the new build and operations departments. I spent a year in the operations department during a time where we helped our clients receive £2.8 million of HNES funding across 33 projects. For me this meant I was going to be performing a lot of optimisation studies, where performance issues are identified and then work packages are created to resolve them.

Unfortunately, many of the issues found with these networks are difficult and expensive to rectify now that the system is in operation. This means that, even with the improvements made, there could have been further savings if the issues were prevented prior to installation.

Annual savings of c. £330 per dwelling could be found during a typical optimisation study by ironing out some of the issues with the design and installation of the original system, with the work packages to achieve this costing c. £1,800 per dwelling. For a 100 dwelling site in operation for 10 years that would be £330,000 of excess spending on operation and £180,000 of works that could have been avoided with good design practice.

Additionally, 80% of the buildings which will be around in 2050 already exist which means the buildings we design now need to be compatible with the net zero future. Many of the existing buildings analysed in the HNES optimisation studies would require investment in the heating system beyond the £1,800 per dwelling level to make them compatible with heat pumps or connection to district heating networks.

In October I moved over to the new build department and started to work on design reviews, where we prevent issues prior to installation! In the short time I’ve spent in the department I’ve worked on several design reviews on projects from RIBA stage 2 to 4, and many of the issues I found while working on operations are still present in new build designs today. This is unsurprising as several of the operating systems I investigated previously were less than a decade old and performing well below their potential.

Here are some of the plant room and network issues which you should be looking for when reviewing a design:

Hydraulic Arrangement

The most common issue found in design reviews is incorrect hydraulic arrangement of the system. Many designs we review still include a number of bypasses which will increase system flow rates and temperatures whilst providing little benefit to the system. Large bypasses can even prevent delivery of heat at peak load and cause overheating in the summer. Some systems have a hydraulic arrangement which prevents some heat sources from directly serving the network, reducing the heat fraction which can be supplied by these sources.

Peak load sizing

Accurately sizing peak loads is important as the values cascade throughout the rest of the design. The peak load not only affects the sizing of the heat source but also the distribution pipework and equipment. Oversizing the heat source unnecessarily increases the upfront cost of the system, whilst undersizing the heat source will lead to a system that cannot meet the peak demand. Oversizing the heat source is now more impactful as new build systems move away from relatively cheap gas boiler systems and towards more capitally intensive electrical systems.

Pipe sizing

Pipe work is often found to be oversized due to an implementation of a pressure drop based approach at peak load which does not account for the fact that peak load operation is a rare event for a heat network. Oversizing the pipework not only increases upfront costs but increases the heat losses during operation. Undersizing the pipework will increase pump energy consumption and may even prevent delivery of heat at peak demand.

Water quality

Many of the networks I performed optimisation studies on had issues with poor water quality that had affected the performance and lifespan of equipment and even the delivery of heat. Networks should be designed to prevent corrosion and remove solids and air from the system. Many designs include provision for bulk solids removal to protect equipment in the short term but lack provision for removing finer solids which will degrade the system water quality over time. Additionally, water quality equipment is often found arranged in a bypass configuration which unnecessarily causes poor return temperatures and increases system flow rates.


Ensuring that the thickness of insulation is in line with the guidance of CP1 is effective in reducing overheating and minimizing the lifetime cost of supplying heat, however it is often not cost effective to increase the insulation thickness after it has been installed. CP1 insulation thicknesses are much higher than required by British standards and have greatly reduced heat losses, so it is important to make sure that this is carried through in the design. Care should also be taken to ensure that the insulation is made of the correct material. Using a material with a low thermal conductivity such as phenolic insulation will reduce the thicknesses needed to achieve the same heat loss reduction which will make installation much more practical.

Case Study

I recently reviewed the stage 3 design of a site which is due to be constructed in London and supplied with heat via a district heating connection.

The system had bypasses which needed to be removed from the primary side of the district heating connection as these would have prevented delivery of heat to residents and drastically increased primary network return temperatures.

The peak load was found to be oversized by c. 25%, this was due to the allowance of an excessive space heating load in the calculations. However, the plate heat exchangers were only sized to 2×50% instead of the agreed 2×60%. This meant that the plate heat exchangers were sized just 5% larger than necessary.

Despite the oversized peak load, the distribution pipework was found to be generally undersized. This would have increased the pump energy consumption and potentially prevented the delivery of heat to residents.

The side stream filtration unit was shown upside down in the schematic. Had the equipment been installed like this, it’s effectiveness would have been reduced and system water quality may have been compromised. The unit specified was also incompatible with the hydraulic arrangement in the schematic, ordering this equipment would have set back the project timeline and wasted money.

On this project, no insulation specification was provided for review. However, we still recommended to our client that phenolic insulation is specified and that it should be installed to the recommended thicknesses in CP1.

The issues we raised to our client through the design review process will reduce the operating costs of the network, improve reliability, and simplify the design. By dealing with these issues now, the project is more likely to be delivered on time, within budget, and the need for future work is reduced.

If you have a heat network project in the design phase, then you can get in touch with the new build team at FairHeat to get the design reviewed and prevent yourself from having to call our operations team later!