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I have an older 275 gallon oil tank in basement that could be 50 years old and no signs of any leaks. It has a bottom spout on tank that connects to line to burner and the tank is slightly pitched towards the spout when I put a level on it

The metal plate on top says: "Underwriters Laboratories, Inc" "Inside Oil Tank" "Metal No Gage no F?" I think

Regards,

RTW

Thank you all for your help!

First off, apologies for the long delay. I have been fighting some personal problems that prevented me from getting back to read/reply.

I did decide to stick with traditional design approach using separate circ pump for the radiator loop. And of course the main loop is not interrupted by the mixing valve — closely spaced Ts off loop. Lots of good information here, and seems not worth experimenting on something that seems unlikely to work well. I really appreciate all of the thoughtful comments!

Comment to a couple of the posts — the mixing valve does properly regulate floor temperature to the set temp, while the furnace itself decides what the loop water temp should be. It's actually not an outdoor reset (though that can be easily added with an optional temperature sensor). Without the reset, the boiler uses some sort of "intelligent" algorithm based on overall system demand. I honestly don't think it does anything from energy saving perspective for the underfloor zones since their temperature is controlled by mixing valves so always at steady set temp. But I can see it being beneficial for the radiator zones.

Sadly, it is all cast iron. And yes, I fight the sludge problem (flushing every year and keeping chemicals in helps). One of the zones was totally plugged when I moved in, and initial flushing to get everything going was a PITA. Didn't really know why I keep getting dirty water until I discovered the PB issue and fact that it is not oxygen barrier. I met the plumber who did the whole thing to begin with, and he admitted it was his "first system" and he made "some mistakes". I've learned since then that basically the whole thing was done wrong. Had I known what I know now, I would have just pulled it all out a long time ago. After replacing the PB, I will have essentially replaced everything except one of the circulator pumps.

The system works properly in some parts of the house, but not in others. Two main is days. The loops were not done correctly, and I've learned that the worst of the rooms has no insulation under the floor (explaining why the basement room below stays sufficiently warm without it's radiator ever running). The tubes do not have heat spreaders, so as you can imagine I don't get nearly enough coming up through the floor. Fixing this would require tearing out ceiling under both rooms, so I'm inclined to abandon the underfloor for those two rooms and use radiators instead.

Second issue, as has been discussed, is I have PB and cast iron everywhere. Installed new boiler a couple years ago because the original was breaking down, and didn't know about the non-oxy-PB issue at the time. I'm starting to replace all the PB with poly to deal with this (as well as the inspection issues I will run into when I sell if I leave PB in place). Fortunately, I have fairly easy access to everything except those two rooms.

I'm now looking more closely at those radiators, and will probably start a new post on it if I can't find my error. I'm having a lot of difficulty believing what I'm coming up with on required length based on BTU calculations…because I've never seen a small room with as much baseboard as the calculators are telling me I need! BTU is calculating around 13,000 for a 138 Sq Foot room. That's 20+ feet per room for a 11 1/2 x 12 ft room! Way more than my original 8' that came from a generic estimator. My neighbor has all baseboard in a similarly built house, with nowhere near that much baseboard in his rooms. And I've never seen baseboard around here that covers two walls of any bedroom. But, maybe I just haven't been sufficiently observent.