Bit dull this one but could do with some advice. Our old oil fired boiler is giving me a few headaches. Currently where the first part of the flue, that comes directly out of the boiler, meets the second part of flue there is a join that had previously been sealed by the previous owners with some sort of sealant. Over time that sealant has degraded and the the flue now leaks at that join. Its leaking dirty water whcih i’m assuming is just the condensation running down the inside of the flue.

My problem is finding something that will seal it adequeatly. Every high temperature product i have looked at seems to be water soluble so won’t seal it for long before it starts to break down.

Can anyone recommend something – a high temperature resistent sealent that isn’t water soluble is basically what I need…. i think.

There are high temp silicone sealants used in the automotive world
which would very likely do the job.
Some are not cheap but you can by them in std gun cartridges if you
go to trade type suppliers.
Note that the risk of carbon monoxide is when the burner is not working correctly – but if you do not have an alarm get one.

high temp silicone sealant also used in the stove world.

some of which are not water soluable – and wont come off your hands short of sanding it.

Automomtive exhaust sealant?

Edit: Beaten to it. Holts do a putty based one, fireguard I think it’s called.