Hi there, looking for some advice like many others, Im an RGI, mostly do installs and was asked by a friend to have a look at a boiler he was stuck on, existing ideal logic, with a vertical flue. At first by the rumbling sound after firing I thought it to be the condensate choked, tried that which it did need cleaned anyway, fired the boiler up and worked perfect until I put cover on. thought then maybe the flue was blocked/seals broken. Didn't have my Fga in my car, but seals of flue did look fine with no signs of leakage. On a older boiler I'd certainly think this to be the Venturi blocked? Was just wondering if this could be fault? Or if anyone has had this fault before?