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there is also a table in the code that will say something similar to the table in the boiler instructions.

@2x_Tom

Don't know your location but in many areas "chimney sweeps" have to be licensed or certified or something.

I wouldn't get involved and it is good that you didn't recommend them.

It's the homeowner's issue and they should contact their building dept. I would go as far as sending the building dept the information, boiler size and input, chimney height etc and let them do their thing.

Maybe your state or location has a "consumer protection agency" that can help.

That chimney guy missed that day in school.

Even a 225K boiler needs a 7" diameter x 20' high vent.  So, reducing from 8" to 7" might be OK but you have the right idea.  That 5" is not going to work.  

I would craft a letter with the statistics of the customer’s boiler and indicate that you do not believe that the 5" liner is adequate for the 225k input boiler (enclose the rating of the boiler) and send the letter by certified mail so you know that your customer received the letter.  Give your recommendations in the letter and inform them not to operate the boiler until the problem is taken care of. 

It should include statements like “I have made an inspection of this Make… Model number…   Serial number…  Boiler at this location and have determined that the carbon monoxide problem is a direct result of insufficient combustion air and/or inadequate venting.  I recommend the customer have a competent chimney professional look at the venting system including the chimney and the combustion air requirements.  After the work was completed by XYZ chimney company I was requested to inspect the problem again and discovered the work performed by that company has not solved the venting and combustion air problem .  The 5“ chimney liner installed by XYZ Chimney company, may have actually worsened the problem."   

You can also add that you said you are not a chimney man, you are a plumber and the venting system is not up to the specification that the plumber recommends, the manufacturer recommends, or the local authority having jurisdiction would approve. 

"If you have any other problems with your heating system regarding carbon monoxide, please refer them to the XYZ chimney company, the local fire department, or the local authority having jurisdiction. As you can not create the needed venting and combustion air requirements within the job description of a plumber. You can only tell the other trades what the specifications and recommendations are for the equipment.” …Or something like that.

And in the meantime, lock out the boiler before someone dies.