Best Of
Re: Heat loss calc..
Typical residential boiler is around 3x oversized, residential furnace 2x.
Fuel use is usually the most accurate method, the only time it will be off is if you have a lot of glazing and solar gain, like an MCM house with a wall of glass.

Re: Oil Boiler Issues
But a tiny leak should not make problem with a two line system, should it?

Re: How Does This Look?
Looks good but where the backflow preventor and a electronic low water cut off unless its equipped w a digital aquastat w a lwco feature ,both are code in my area if getting inspected . Other wise welcome to doing it the correct way now dont fall back into old habits . Its a wonderful thing when ya don't have to remove convector covers to bleed rads and convectors . on another note if looking to get better try califee demineralizing filter to bring your fill water within what the manufacture's specs . Keep up the good work .
peace and good luck clammy

Re: Bit different
Thanks, clammy. A fun project. Yeah, I think most of them are gone now. That thing will survive this whole so-called human race.
Re: Indirect HW off steam- shaking circulator
May need to add the bypass that is shown in most diagrams for hot water loops off a steam boiler or a ball or globe valve on the discharge of the circulator so you can throttle it.

Re: Heat loss calc..
Trust your fuel bill. It is correct. That's a design heat loss of 18 BTU/hr/sq ft, which is easily achievable in your zone with average amount of insulation and reasonably tight envelope.

Re: Bit different
The funny thing i used to see them all the time about 25 to 30 years ago in paterson nj in a scrap yard where i would occasionally get salvaged radiators never thought much about it . Since then i don't think i ve seen even one in any scrap yard guess they all are ancient maya history now . Thanks for sharing your find .
Ps Ed your looks sweet
peace and good luck clammy

Re: Drip to nowhere.
No evidence there were ever under the floor wet returns. This is the only drip in an otherwise conventional one pipe system.
I have to open up more wall to see where it ends. Being in line with the sink drain raises suspicions.