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Gas safe ?
Your letter box is supplying the fires ventilation...
FFS stop using the fire , close the window and use the boiler and get a hetas guy round to...
Open fire pal
Its obviously been there years , the way I read your post was it was a newish installation so please ignore the flue gaurd terminal opinion .
Erm again , I was a bit bemused ..
Manufacturers instructions say a min of 300 mm from the end of the flue to the window opening which as the crow flies maybe ok , however...
Have you drained the system down ?
Header tank 1/3 full of water ?
I have serviced nine today and another six tomorrow , they do get serviced !!!
Put pressure up to about 2 bar and try it
Theres always talk about this on vessels but I have never heard that...
Looks like all your pipes are on separate rad drops , they pipe you cant ID will be the gas pipe
Never heard of Lowtons , maybe fluxes are a bit regional lol
Separate names with a comma.