What Causes Boiler Flues to Leak?

The majority of installed boilers are condensing. This means the heat from the exhaust is made use of to preheat the water coming back into the system. The advantage of this is it greatly increases the efficiency of the boiler as it makes use of energy that would otherwise be wasted. However, this means the waste products exiting the flue are much cooler than in non-condensing boilers. This cooler plume causes acidic condensation to form in the flue. This acidic water is the key problem when it comes to leaking boiler flues.

It is mandatory that when a boiler flue is installed there is a consistent slope all the way back to the boiler to allow the condensate to run back down the flue to the boiler where it runs into a drain. We advise a minimum of 3-5°. On longer flue runs there should be suitably located drain off points for the condensate waste in the flue. This avoids it running back to the boiler where it could corrode the heat exchanger.

If a flue is not installed correctly this causes problems as the condensate waste is very acidic. It is critical that the correct flue material is selected otherwise it will eat away at the flue causing the flue to leak. Also, if the flue is not sloped properly or drains are in the wrong location then the condensate could end up sitting in the flue pipe and will eventually find its way out. The acidic water will then damage whatever building material that its dripping onto.

When you have a new boiler installed, the installer must ensure that there are no joints in the flue that are hidden or difficult to get to. This is because if one of them works loose or is not connected properly chances are it will not be picked up till it is too late. Any flue that is concealed for aesthetic purposes must have an inspection hatch so it can be checked when it is serviced, or a suitable liner must be used rather than pipe so that there are no joints.

Where you have two or more boilers in a row connecting into the same flue header, our recommendation is that the flue coming off each boiler goes into the side of the header rather than underneath. The reason for this is that the condense waste runs to the end of the header and down the drain at the end of the header. This is because when they enter the underside of the flue header the tendency is that all the condense runs back to the first boiler which can result in damaging the boiler that is first in the line.

If you do find that your flue is leaking the best thing to do is to catch the leak in a plastic bucket and contact a gas safe registered engineer to sort the problem. The engineer will need to analyse the flue readings once it has been fixed and check the boiler/s to ensure there is no other damage that may not have yet made itself manifest.

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