Scavenging heat from return hot water baseboard to preheat DHW

jinbtown

1900's era house in northern NH, 99% design temp is -4F here. Unconditioned uninsulated basement that currently stays *very* warm with 2 Peerless gas boilers, uninsulated copper piping running everywhere, twin 40 gallon LP water heaters, etc. Manual J comes to about 60k per hour via Coolcalc.
Lochinvar Noble NKC150N (newly purchased, not yet installed)
We are changing this from a duplex to a triplex with 3 zones and 3 thermostats. One zone will be very small, so likely installing a buffer tank to prevent short cycling if just that zone is calling for heat. Other two zones are plenty large and well radiated (60' total baseboard per zone). Single ECM pump and valves. Noble on LP will supply heat for all 3 zones as well as DHW

3 kitchens, 3 dishwashers, 3 washing machines, 3 bathrooms. I already know I'm going to run into problems with GPM on the DHW side. 2 Bosch ES4 POU water heaters on the upstairs kitchen/laundry and bathroom.
Option 1: 80 gallon Aerotherm heat pump water heater. Set the Aerotherm at 100 degrees in efficiency mode and let it preheat the incoming water
Option 2: Indirect water heater set to ~100 degrees, plumb output to the input side of the Noble and output of the Noble to DHW distribution
Option 3: Radiant return lines through an indirect water heater and scavenge whatever heat is left in the lines to preheat incoming water. Send that water to the Noble for DHW heating. Would this also have the advantage of keeping the Lochinvar in condensing range? Is that scavenged heat not going to be enough to overcome possible usage? I wouldn't anticipate 3 bathtubs running at once, but 2 is certainly possible.

Any thoughts, comments, suggestions?

Zman

The Noble combi is not the right product for the job. It is not even going to be even close to satisfying a triplex, even with little booster heaters.
A condensing boiler with indirect water heater is what you need. I would return the combi and purchase the correct appliance rather than try to coble something together.

jinbtown

thanks for the insight @Zman , despite it being a triplex, there will only be 3-4 people living there at any one time, so I'm going off number of people (as suggested here) rather than number of fixtures. I agree that the Combi is undersized in that scenario, even with high efficiency fixtures, energy star dishwashers and washers, and low flow aerators, but I also don't want to go with a 199 and oversize the heating side of things. The suggestion of a boiler with an indirect is a good one. Let's say for the sake of argument that our first home needing $125k+ in renovations means that the financial feasibility of updating a currently working heating system is dependent on the fact that we got a brand new Noble 150 for 1/5 of MSRP from a friend. In that case, it's what we have and we are determined to make it work at this point in time. I wouldn't call any of the proposed options "cobbling something together", at least the relatively simplicity of the entire system seems to be a lot less than some of the installations I've seen in my research, where people go 1000x overboard in complexity.

I guess my main question is, is option 3 viable in terms of preheating my DHW, or is that not going to yield dividends on the efficiency side (keeping the boiler in the condensing zone) or in the preheating side (keeping the DHW ~100 degrees). If it isn't, then the 25 degree rise/8 GPM spec of the Noble will have no issues keeping up with 3-4 people, if fed from an indirect, or from the Aerotherm.

thanks again for your help and thoughts.

Jamie Hall

You have what you have, and it sounds as though you are determined to make the best of it. That's OK, so long as you recognize that it's only adequate -- marginally -- in your current situation, and wouldn't be with changes in use of the apartments.

That said, keep it as simple as possible. You will, in my view, be better off arranging your heating piping and pumping and controls to keep the Noble in the condensing range on its own. If I were to go to the expense of Option 3 -- having an indirect -- I would run it off its own zone from the Noble and stop right there.

jinbtown

@Jamie Hall , thanks, that's what I'm looking for. I believe that with a small buffer we can keep it in the condensing zone.

mikestooneat

How about using uninsulated tank as preheat tank for DHW.

jinbtown

I had considered that @mikestooneat , we did that in a house we built 15 years back. Worked very well in a house my parents built about 15 years ago where they had geothermal but no desuperheater

Branch

May I suggest a variation on option 3?
A reverse indirect tank, like an ErgoMax or TurboMax, would effectively work as both a buffer tank and to preheat your DHW.

jinbtown

well, I thought I'd give an update. After considering all options and talking it over with our friend, we got the Bradford White 80 gallon hybrid heat pump water heater. Set to efficiency mode, preheating 80 gallons to 100 degrees, should cost us <100 a year in electricity and let the Noble supply 8-10 GPM at a 25 degree rise