most efficient DHW

keyote

converting a four 1/2 story 5000sf to radiant, the top floor is renovated now, so its time to install a new high efficiency modcon thought TT.
Now the top floor is 2 -1 bed apts which each have their own tankless DHW in bath/kitch ceiling. I set them up with all their own risers so i could switch them to separate gas and elec services if i wanted later. I also installed another tankless in the farthest bed/bath of my own 3 floors. I was thinking of using the nat gas TT 125 challenger for the tadiant heat boiler and also for the DHW for a master bath { and a couple powder rooms and guest bath}, thinking only the master would be used regularly. You guys made me think a bit harder and i decided it wouldn't be sufficient for peak situations or another owner. You guys also made me vaguely understand the buffering advantage of indirect DHW with radiant. And since i was hoping to get some solar added in the distant future this appealed. However some of the arguments cited the inability of these boilers to mod down low enough when need be. which made me wonder what the hell happens during the summer when only DHW is needed.So I guess Im wondering is there any definitive answer to the most efficient DHW in my situation or is everything pretty much an even trade off. The heat load is 110- 125 kbtu and the location is brooklyn ny.
Ill add this follow up in case indirect DHW is still the consensus.
My local supplier has dropped the TT challenger 125 to $1995, while the prestige solo 110 is $3000, and the prestige excellence 110 is $4000. Even if i was doing indirect what am i getting for the extra $1000-$2000 that I wouldnt just use the cheaper challenger and ignore the DHW loop, piping an indirect zone off the heat side as i would with the solo Im not clear whats different between the models besides an extra DHW HX on the challenger and what seems to me to amount to the same thing for twice the price on the excellence [they both boast about 3gpm dhw] Thanks.

Rich_49

First of all , no pricing discussions are allowed .
Why are you married to the idea of one manufacturer as opposed to others that may have the RIGHT solution for your install ?
There are a few other discussions going on here that you may not be aware of . I would point out that at your stated loads you better be damn sure that the boiler and system are working perfectly or you will find yourself very short on BTUs when you need them most .

I might suggest that you find a qualified , very sharp designer to assist you , 72*F consulting comes to mind and if he can't or is unwilling to assist you could contact others that perform this kind of work remotely .

There are many manufacturers that have offerings that fit your bill , while we have already witnessed what a tragedy the unit you specifically mentioned can end up as . See Josie Reynolds .

Another thing that strikes me is that most apartments / rentals in NYC have heat & hot water included . Would it be more first cost efficient to pay for consultation and design support , save tons on equipment and their necessary accompaniments and use one central boiler and water maker and charge accordingly on each rent ? Just a thought . Seems like you will spend heavy to save little . I am designing 2 just such systems at present in Brooklyn . The math really does make sense .

keyote

@ Rich
Thanks for your thoughts. The apts are on the same heating system, or eventually will be. The entire building is being renovated from top down and they are the first. In order to keep heat on the lower floor where i camp out I will soon be running two systems for a while.While its true apts often have hot water included I wanted to keep my options open regarding controlling usage and costs and in case i decided to condo the apts. Since the building is fairly large and stating from scratch it was simpler to run all new risers to the front and back apts. The decision to use tankless DHW units for each apt was partly to keep the units independent gas wise but also because running hot water riser up five stories seemed inefficient.The units sit above the dropped ceilings in the back to back bath /kitch and the longest hot water run is about ten feet.
I had a designer for the radiant who approved my loop design but he got sick and so ill finish it myself im in the hvac business. I chose triangle tube in the end because it was favored by a very wide majority of professionals on here. And yet its reasonably priced, its also very high efficiency as they all are.are. Im not sold on their kind of pricey indirect tanks. Im unaware of the tragedy you speak of please tell.Sorry to mention prices but this is my third post trying to find what the differences are among their models besides the two DHW options.
Not sure what you find particularly alarming about my stated loads I guess youre suggesting that i oversize, im more worried about it being able to mod down to spring and fall loads. I used spray foam and tripple pane and have mini split heat pumps as back up as well it is annoyoying they size their units so far apart though.