The secret life of boilers: Dynamic performance of residential gas boiler heating systems - a modelling and empirical study

Residential space and water heating accounts for 23% of UK final energy demand and combination gas boilers are the dominant technology. Performance gap issues in gas boiler systems have been reported, with previous studies unable to isolate or quantify root causes for performance issues. Dynamic behaviour of boiler heating systems is important to their overall performance, issues of plant size, supply temperature regulation and system control effect the performance of the system. The UK Standard Assessment Procedure (SAP), contains simplifications to aid ease of use and comparability of resulting Energy Performance Certificates (EPC) but they partly overlook these key dynamic issues. Three complementary methods were used to analyse heating system dynamics; a cross-model comparison, real world data from four case studies and a statistical analysis of a 217 boilers sample. Comparing SAP with a dynamically modelled system, showed that SAP results correspond closely to a model using an idealistic heating system, with perfect control and instant responsiveness. Introduction of a physically realistic gas fired boiler and water-based heating system to the model results in a consistent increase in internal temperature (0.5°C) and energy demand (ca. 1000 kWh/a). Variation of controls and plant size show an efficiency penalty for oversized boilers with limited modulation and poor thermostat controls. The key issue of boiler cycling was highlighted as a dynamic symptom of poor efficiency performance. From in situ analysis of boilers, cycling behaviour was observed indicating widespread performance reduction, as was seen in simulation. Most observed combi-boilers appear oversized for space heating and despite available modulation are unable to prevent rapid on-off cycling. Per day, half of combi boilers studied average more than 50 starts and 70% of cycles average less than 10 minutes during space heating operation. Boiler dynamic performance is not reflected in UK and EU efficiency testing standards, which assume steady state operation. The characterisation of the dynamics of gas boilers highlights issues of oversizing and excessive cycling, and reveals opportunities to improve the current building stock energy demand/emissions through better installations, EPCs, and energy labelling.