Although I don't really have a satisfactory answer for the cause of the explosion, and that worries me. My entire system was dry, so the plumber put water in it and bled all the radiators and now it's working fine. But a dry system doesn't really explain the BOOM that happened, AND since I've never seen water in the basement there's no explanation for why the system was totally dry in the first place. AND AND it really is only a matter of time before this furnace just dies.
But still, HEAT! And only $125 (ouch, but much less ouch than I feared). And my wonderful friend's wonderful husband is going to stop over sometime in the next week or so and just give it the once over too because a thorough cleaning would have cost another $200 so I didn't get that done. So while it's gonna be tight for a few weeks financially I'll manage.
Thank you again, Groupthink friends, for thinking of me yesterday and offering the extra collected money to help. I honestly was and am so incredibly moved by how generous and supportive this community is and I'm really lucky I found you all and that you let me hang out here with the cool kids.