Condensation caused by boiler flue?

[electroman68]

Hi There,

I have condensation in my loft in the form of beads of water on the underside of the bitumen roofing felt.

I have a combi boiler located in the upsatirs airing cupboard which has a vertical flue which goes up through the loft space and vents out of the roof.

I have noticed that when the heating is on and I have been up in the loft I can feel warmth against my face when near the flue pipe.

The flue looks to be in good condition and is the type that 'push fits' together. I have put my carbon monoxide alarm up there to see if the flue is leaking and the alarm doesn't sound. Before anyone asks, yes the alarm is working!

My question is, could just the heat radiated from the metal flue be sufficient to be the cause of the condensation problem. The loft feels cold and there are cobwebs blowing around so looks to be sufficient ventilation. There is also the required depth of insulation laid between the joists and across the rafters.

Has anyone heard of lagging the flue pipe? I assume that special high temperature and fire retardent material would need to be used?

If anyone can advise on this then it would be most appreciated.

Thanks.

 

[MJN]

Heat doesn't cause condensation, moisture laden air against a cold surface does. Warm air holds more water vapour than cold but that's a different issue.

It could be that warm, moisture laden, air is rising up from the house through gaps in the loft/house interface whether that be through a badly fitting loft hatch, thru-ceiling light fittings (e.g. unshielded downlighters particularly), perhaps up around the flue itself etc. Do you have any such gaps? Do not underestimate the potential amount of air passge through even the smallest of gaps.

Obviously when the boiler is on the house air is warmer hence more rising air, and likely more populated thus more water vapour being produced (breathing, bathing, cooking) hence why you are seeing the effects during such times.

Mathew

 

[electroman68]

Hi Mathew,

Many thanks for your reply and for clarifying things. By the sound of it I don't need to worry about heat from the boiler flue, provided that it isn't leaking which it doesn't appear to be.

I have looked at where the boiler flue goes up through the ceiling into the loft, and there are some gaps around it. Looks like someone has used expanding foam filler to try and fill the gaps but not done a very good job!

Is it ok to use expanding foam filler to seal the gaps? If so then I will get some and remove the old stuff and put fresh stuff in so it is sealed all the way round the pipe at the point of entry into the ceiling.

Thanks,
Rob.

 

[MJN]

Many thanks for your reply and for clarifying things. By the sound of it I don't need to worry about heat from the boiler flue, provided that it isn't leaking which it doesn't appear to be.

I obviously cannot say for certain it is not leaking, however there's nothing to suggest it is. Note the flue is actually two concentric pipes - an exhaust pipe inside an inlet pipe. As a result leaks will normally under negative pressure, although obviously there can be circumstances where that won't always be the case.

The heat from it should be expected - it is afterall passing gases at 50C+ through it so some radiated heat is entirely normal.

I have looked at where the boiler flue goes up through the ceiling into the loft, and there are some gaps around it. Looks like someone has used expanding foam filler to try and fill the gaps but not done a very good job!

Is it ok to use expanding foam filler to seal the gaps?

I'll let one of the qualified, or at least more knowledgable, posters answer this one as from an engineering perspective expanding foam provides good sealing properties however there may be guidance, if not defined rules, about what can/should be used.

Mathew