Lets see if I can add anything useful here, It is really difficult, they are so cool you just don't want to put them down... First I agree with Jesse... exhaust leaks are likely to be significant, those old coal doors don't exactly seal well. I really like your boiler plate (pun intended) also Bill, and Kurt is right also, efficiency is likely around 50% at best, and with todays fuel prices... (how often can you offer a client a near 45% increase in efficiency with a new boiler install these days...) Now to add a few things, maybe. The flu, having been originally sized for coal is likely not drafting very well and probably unlined unless it has been since modified. Was there any kind of draft diverter/damper installed? Any condensation damage near the top of the chimney? Those old gas guns I typically find in a pretty dangerous state, if your standing there when it ignites you may be able to show the client some nice flame blowout. This one is interesting, doesn't look too bad... is that actually a thermocoupler I see? I still find the really old ones with the independent feed to the pilot and no thermocoupler, which usually turns out to be the best reason you can offer to put er to bed. Lastly, depending on the client being one of the new breed of men who probably doesn't know which end of a hammer to hold on to, it's getting more and more difficult to get service people to work on these things, No company wants to put their neck on the block and say "ok, it's good for another year, nice and safe"... Personnally it makes my skin crawl a little seeing the Hydrostat & PRV mounted up there on piping branches. That said, I don't see a pump anywhere... is it just not in the pic or is this still gravity. Was the rest of the house as interesting & free from modern intervention?