Here's a story....Based on real experiance...
I go to a home on Thursday. It's for an annual tune up on a furnace. There is 3 inches of standing water in the basement. I've got Goretex boots, so I'm cool. I put down my tools on a rock big enough to hold it. Put my bucket down and it starts to float away. I check the smokem pipe, chimney base and brush the flue collector, open up the inspection ports and they are packed solid. I fill up my new vacuum bag. So I vacuum them out and look for any obvios cracks. Filter, nozzle, strainer. Check the end cone and coupling. Slap old girl back together, Bachrach and i'm outta there.
I'm sitting in my van at the shop finishing up an estimate and a customer shows up. Tells me who his is and why he's there. The address sounded familiar. So I call Mothership and go check out the furnace. Walk in the door and you're smacked with fumes. Downstairs, you don't smell anything. I open the access doors for the ports and check to make sure that I didn't forget to tighten the cleanouts. They're good. Checked the flu pipe and there was good draft. Removed the humidifier and spotted a crack.
This is where it gets interesting. Now the landlord is yelling at me for cleaning the unit. Telling me that I should have left it alone so he could get a few more seasons out of it. Now he has to put in a new furnace and It's my fault. So I tell him that I am instructed by my companies rules to RED Tag the unit. Then his body language changes to agressive.
I wasn't about to get into a fight over this, It's his problem. This is when this job sucks.