DEEP Report 5.02 Salford Energy House Heating Systems Testing

Farmer, David; Henshaw, Grant; Roberts, Benjamin; Fitton, Richard; Swan, Will

Abstract

The primary objective of the Energy House DEEP heating system tests was to investigate the unintended consequences of performing fabric retrofit without any modification to a dwelling’s gas central heating (GCH) system. The research was designed to measure the impact of post-retrofit oversizing and test low-cost strategies to mitigate it. A secondary objective was to identify the point during the piecemeal retrofit process when it becomes viable to replace GCH with an air source heat pump (ASHP). The heating system tests also provided the opportunity to assess the impact of fabric retrofit on occupant thermal comfort.

A whole house approach to retrofit must include consideration of space heating provision. Post-retrofit oversizing resulting from fabric retrofit can reduce the efficiency of GCH systems that use traditional Class I (on/off) boiler controls. This can result in retrofits failing to deliver the anticipated savings in space heating energy use. The extent to which efficiency is reduced is directly related to the size of the dwelling heat load reduction. Any measure or set of measures (for example solid wall insulation and room-in-roof improvements) that result in large heat load reductions are likely to have a significant impact on GCH efficiency and should trigger some evaluation of mitigation measures or an adjustment in energy saving estimates.
GCH systems that use Class V modulating boiler controls are less susceptible to post-retrofit oversizing. Class V controls could provide a low-cost measure for mitigating post-retrofit oversizing. They also have the potential to optimise existing heating systems that are incorrectly sized, so their adoption should be considered as part of any insulation improvement package. However, Class V controls require a boiler to be capable of modulation, so they may not be suitable for older boilers.

An alternative and complementary low-cost post-retrofit oversizing mitigation measure to Class V controls is to modify the boiler flow temperature setpoint to provide better agreement between the heat output of the existing radiators and the heat load of the dwelling. It is a strategy that can be adopted for boilers that are incapable of modulating output and can also be used to improve the performance of incorrectly sized heating systems without significant detriment to their responsiveness and ability to provide space heating in cold weather.

Many solid wall dwellings will require their external walls to be insulated to make the transition from GCH to ASHP viable. However, ASHPs that are incorrectly installed could result in increased energy bills. ASHPs should provide clear diagnostic feedback to installers and householders about installation issues. ASHPs require the appropriate heating curve (weather compensation) to be selected to achieve good control of space heating and optimal efficiencies. Selecting the correct heating curve is difficult due to lack of industry knowledge and tools, leaving installers and consumers reliant on trial and error. User-friendly tools are required to assist with heating curve selection.

The performance of a dwelling’s heating system is linked to its fabric performance. Ideally, boiler flow setpoint modification, heating system sizing, and ASHP heating curve selection should be based upon the measured fabric performance, however this is not yet feasible. Current practice is to use two different methodologies to calculate the fabric heat loss of an existing dwelling and size its heating system. Improving the accuracy and alignment of these methodologies would allow heating systems to be optimised for the dwelling’s fabric heat loss.

Citation

Farmer, D., Henshaw, G., Roberts, B., Fitton, R., & Swan, W. (2024). DEEP Report 5.02 Salford Energy House Heating Systems Testing. Department for Energy Security and Net Zero Website: Department for Energy Security and Net Zero

• Report Type: Technical Report
• Acceptance Date: May 15, 2024
• Online Publication Date: Oct 23, 2024
• Publication Date: Oct 23, 2024
• Deposit Date: Nov 21, 2024
• Publicly Available Date: Dec 3, 2024
• Pages: 59
• Keywords: Building Thermal Performance, Retrofit, Central Heating, Gas Boiler, Air Source Heat Pump, Oversizing, Insulation, Solid Wall House
• Publisher URL: https://assets.publishing.service.gov.uk/media/6717bf3938149ce9d09e3840/5.02_DEEP_Energy_House_Services_and_Comfort.pdf