New used boiler internal cleanup

Well, I finally did it, after lots of research and contemplation, bought a Central 5036 on Friday, been fired only 2 weeks, good condition, saved mucho $$, dealer will honor full warranty, but will not be getting it on line until this coming fall. He showed me a water sample from it, and it looked a little yellow (sulfur?), and recommended I fill it and hang a circ. on it and keep the water flowing thru it until I get my piping and install done. Has anybody tried TSP, or any other boiler water cleaner, other than the PH and corrosion inhibitor that CB sells?

I don't see what's wrong with draining it, but I guess I'd be inclined to do what the guy honoring the warranty suggests.

Do you know this boiler's history? I'm wondering why it was only used for two weeks. Sized wrong, maybe?

I'm trying to think of a way to verify that based on the condition of the boiler, but nothing comes to mind offhand. I would inspect the back plate to look for dings caused by wood being carelessly tossed into the firebox, and also the condition of the firebrick.