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Central Heating

Boiler replacement

Which Worcester combi boiler would be best for 4 bed semi with ten rads? The Greenstar 42CDi or 25si

1 Answer from a MyBuilder Heating Engineer

Best Answer

Hi Terez,
When selecting a boiler, you could benefit the choice by applying a little bit of calculation. Boilers are sized by the kw heat output. The two boilers output are broadly referenced as 25 & 42 kw respectively. These outputs are nominal. Take a modern double panel 1200mm radiator, this could have an output of 2kw. Based purely on these two facts, one could assume that 10 radiators @ 2kw each, could be fully delivered by a 20kw output boiler. Personally I over-rate boilers to allow for the boiler reduction in efficient over its life cycle (SEDBUK avg 90%). This being said, if you moved your central heating system and radiators into an extremely large house, 20kw heating would probably not offer sufficient heat overall. One man's four-bed house is another man's castle. Please factor in your hotwater usage, the flow rate of the CDI is greater than the SI and this is impacted further by Bath / Shower requirements. Sorry that I can't suggest Boiler A over Boiler B, but the selection invariably will require you to determine heating demand, which can't be determined on just property type and radiator quantity. Best of luck.


Answered 12th Jun 2015

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