Is the boiler a Combi? Green stain could be from condensate outlet. Insulated pipe. Gas inlet, or hot water to downstairs loo?
Yeah - combi. It's not just a stain, right by the pipe, it's about 15mm thick.
If it is the condensate outlet, as the condensed liquid is acidic, it is meant to be plumbed into your foul/effluent drain. The stain/growth is probably some acidophillic algae/moss/lichen amalgam.
@Soul started
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Is the boiler a Combi?
Green stain could be from condensate outlet.
Insulated pipe. Gas inlet, or hot water to downstairs loo?