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Commercial Boiler Replacement Case Study

  • Site Location : Ascot, Berkshire
  • Application : School
  • Date of Installation : Nov 2010

Client Brief

  • To provide new replacement gas fired boilers for a school block, due to the inefficiency of the existing plant compounded by ongoing reliability problems
  • To retain the existing hot water system
  • To provide a new plant room arrangement that was simple to control, reliable and brought value for money in terms of capital expenditure

Survey of Existing Installation

The existing heating system at this school consisted of two 50Kw floor standing conventional flue gas fired boilers, which were approximately 20 years old. The boilers had an efficiency rating of 75% but were in a poor order – resulting in efficiencies far less than this original badge rating. They were controlled by a complex optimising and compensating energy management system.

The boilers served thirty large low-surface temperature (LST) radiators, each with thermostatic radiator valves which provided local temperature control. The pipework arrangement was a sealed system with a very old pressurisation unit to maintain water levels.


In order to ensure we met the value for money criteria outlined by our client, we initially carried out heat loss calculations for the building. This study enabled us to determine that the original boiler plant was oversized by 20%, which meant we could select newer and smaller boilers that had ample capacity to meet the building load.

The boilers we selected were two Worcester Bosch 40Cdi Greenstar wall mounted high efficiency condensing boilers. Each boiler met two thirds of the building capacity, which is the correct criteria for installing a twin boiler plant installation.

The boilers were installed in a reverse return two pipe arrangement with a common header. The pipework was extended to connect onto the existing unvented hot water plant as stipulated by the client – and the heating pipework had two zone control valves fitted in a parallel arrangement. In order to provide effective but simple controls we installed a Drayton optimising programmable room thermostat and a Danfoss domestic type twin channel programmer.


On completion of this installation we had provided a cost effective, reliable system with low running costs. Gas consumption savings were delivered by installing boilers with a 92% efficiency rating. Improved controls brought about further savings, particularly the optimising programmable room thermostat, which is able to control the building at different temperatures at different times of the day.

In addition to the optimisation feature is a self learning control that is able to determine how long it takes to heat the building, which means the boilers only fire when they really have to. Capital expenditure was significantly reduced by simply calculating the correct boiler plant size and applying simple but effective controls.

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