Radiant, heat pump, DHW, repipe — oh my!

JeremyG

I'm probably a rare homeowner who daydreams about upgrading my hydronic heating I'm scheming a multi-phase upgrade plan and could use some input on the high-level design. I'll engage with a local pro when it's time to do the stuff that's out of my league.

I live in a 1930s brick colonial in the DC area (CZ 4a). The current system is a 1984 W-M HE6 cast iron boiler (167 MBTU/H) feeding the original cast iron radiators via converted gravity piping. The house has the original footprint and has not been substantially modified. Insulation has been added, and a design day heat load is about 40 MBTU/H, according to the load calc I had done post-insulation. I have about 100 MBTU/H worth of radiaton at 180º, meaning a) with the hilariously oversized boiler, flue gas condensation is a potential problem a lot of the heating season and 2) radiator temps rarely exceed 130º. DHW is supplied by a 50-gal gas water heater (circa 2003), which shares the flue with the boiler.

The basement is unfinished, and the boiler and water heater are in the portion that we want to finish. But that project is probably 3+ years out. In the meantime, I'd like to add radiant floor heat for the kitchen, add primary-secondary piping for the existing boiler and be ready to replace the water heater with an indirect tank. Two radiators were removed from the kitchen during the remodel in the late 1990s (long before we moved here) and put in the basement. Those total about 11 MBTU/H @180º. A kickspace heater (4.2 MBTU/H @180º) is the sole heat source now, and it's undersized before accounting for the low supply temps that rarely start the fan even with a low-temp aquastat. The house is comfortable and reasonably well balanced until average temps fall below freezing. Then the lack of heat in the kitchen and brick-on-block (with plaster) construction rear their heads. I added TRVs to a couple bedrooms this summer that tend to overheat.

To try to manage the flue gas condensation issue, I've set a minimum runtime of 12 min on the Ecobee thermostat and installed a Taco delta-T circulator. (The pump allows a 20-25º delta-T, whereas the old oversized B&G pump ran at more like 5º. Heating the emitters rather than the return pipes helps with comfort.) But I think a P-S piping setup with a provision for DHW (perhaps via zone valve) would be much better. I'm comfortable sweating copper pipe (though only 1/2 and 3/4" thus far), so I'd probably do that myself.

Another goal is to get rid of fossil fuels. We don't use a ton of hot water (now), though I'd love to get rid of the gas water heater in favor of an indirect. Seems like those tanks (HTP, Lochinvar) are easily $1000+ in 30-40 gal capacities. Can I expect a stainless tank to last a lot longer than a bog standard gas water heater? A heat pump WH isn't appealing in terms of noise or heat removal from the basement. When my girls hit teen years I'm guessing DHW needs will increase a good bit. We have 2 showers now and will add a third in the basement.

When we get to the basement finishing project, I'd like to replace all the iron pipe below the ceiling with PEX, relocate the WH tank (maybe under the stairs? Smaller would be better) and put in an air-to-water heat pump. I can meet my design day load with 130º water, so I think that puts me in the ballpark. I'd need a larger indirect DHW tank than with the gas boiler, so trying to consider tank sizing assuming a 4-ton HP. I'd need to consider the mass of the iron and water in the distribution system, less basement pipes, in terms of avoiding short-cycling. I'd like to avoid an additional buffer tank due to floor space constraints.

If you're still reading, what have I missed? Or could improve?

Thanks!