How should a boiler flue be tested

So much for so called competent RGI's!

First RGI couldn't find the leak and said it must be rain blowing into the flue. Funnily enough, it would do it when it wasn't raining.

So i called out a different RGI. He couldn't find a fault either but wanted to replace the two 45 degree elbows. I agreed for him to do that and he went off to get the parts.

In the mean time i thought i'd have a look down the end of the flue with a torch.

The flue was full of rusty water and there was condensate dripping very slowly from the joint in the telescopic straight section.

I took the flue out myself and found that the straight section of flue was so rusty where condensate had been sitting i could push my finger through it. I stripped the joint and found that the so called competent RGI who'd installed the boiler in the first place hadn't even made a token effort at deburring the sawn end of the inner flue and this had damaged the seal.

Quick call to second RGI as i already had the straight section from the flue kit the 1st RGI had supplied when he replaced the elbow and he came back and put the new flue back in, tested it and told me it was all fine.

However, i shone my torch down the flue and you could see water dripping from the same seal that it turned out he hadn't replaced despite me showing him the damage.

In the end i made him strip the lot, replace all the seals and reassemble it and there was finally no leaks.

What a bloody shambles and what a mockery of the RGI scheme.

It is indeed embarrassing to read stuff like that.
The RGI scheme is only a test (not a very good one) to see if people are capable of being competent.
Whether or not they can be arzed to do a competent job (even if capable) is down to the individual.
We've all seen jobs like yours down to incompetence or sheer lazyness, the problem we face is that people who don't care what corners they cut can always beat people who do on price.
The only answer is to go on recommendation from someone who's had a good job done, and obviously don't recommend gobsh*tes like the ones you seem to have had.