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CRR 406/200 - Assessment of methods to detect leaks in the casing of room sealed appliances

A series of experiments was performed to determine suitable procedures for detecting leaks from the case and seals of fan pressurised central heating boilers. The work was done using a type of boiler which has been involved in several incidents associated with the escape of carbon monoxide into a property. Procedures involving the following equipment were found to have the potential to detect leaks. A visual and tactile inspection of the case and seal. Smoke tubes to produce smoke for flow visualisation. Ordinary matches and wax tapers to produce a flame for flow visualisation. A flue gas analyser detects the drop in oxygen concentrated or the presence of carbon monoxide due to leak of combustion products. A boiler burning well but leaking slightly often produces only a small change which may be missed using a flue gas analyser. The method is not recommenced.

As a result of this report new guidance to gas engineers on the procedures to be followed before and after the outer case is replaced to ensure that no significant products of combustion are leaking into the room. Copies of this guidance can be found on the Gas Safe Register web site at

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Updated 2020-08-05