Most cost effective used mining rig to use as a space heater?

I see that this has been discussed at various times in the past, but I don't see anything recent in the last year so thought I might throw this out again.

I did a lot of mining back in 2014 and still have like 30 280X cards, but at this point I think those are pretty much useless.

I see some used S9s available in my area for $90 pickup, plus ebay for like $40 + $50 shipping (about the same). Would those be the most "cost effective" used gear to use as a space heater?

Long story short, I have a property that currently uses baseboard heat and has a crawlspace. I figured that instead of just getting absolutely nothing for using electricity to heat the place, I could set up a few rigs in the crawlspace and pipe the heat into the rooms instead.

I know that I won't cover the cost of electricity, but right now I'm getting absolutely nothing for the electricity spent. I also don't want to spend thousands on a more modern rig that will still lose money.

So just wondering what the best bang-vs-buck would be for used mining gear to use as a space heater.

TIA!