Thanks in advance for any thoughts/input...

- Complete two-story reno.  1800 sq.ft., utilizing spray foam on all walls and roof deck. 

- Design heat loss comes in around 37,000 btu/hr.  Radiant heating via tubing laid in basement pad, RauPanel on main floor, and Stelrads on upper floors.  Going with rads due to some upper-floor room-specific heat losses which exceed output of in-floor installation.

- Boiler/IWH suggested by installer is Triangle Tube Prestige Solo 110 and Smart 60.  Boiler, IWH and all tubing in heated/conditioned space.

- Two adults, three younger children


Although the installer is adamant these sizes are necessary, he won't provide any analysis and keeps saying that I'm paying for his professional experience.  But I just can't see it...  When I do my own back-of-the-envelope, it strikes me that the Prestige Solo 60 is ideal for this situation, and that a Smart 50 would provide more than enough on the DHW side.  

Specifically, the Solo 60 will provide 54,000 btu/hr, which seems to be perfect for the design heat loss of the building.  As well, the Smart 50 has DHW priority.  This should provide DHW capability of about 1.8gpm (assuming a 60 F deg 'Domestic T').  When combined with the initial storage of the Smart 50 (46 gal x 80% useable / 0.67 mix ratio = 55 gal), it seems to me that the combination allows for two simultaneous showers of 10min each (@ 2.5gpm per shower head) AND a follow-on 10min shower.  I can't see where we'd ever even use this scenario... 

No issue with having to schedule dishwasher/washing machine/bath at non-peak times.  Dishwasher is used late at night/overnight.  Washing done late at night or during the day (when no other DHW demands).  Bath (rare) would be late at night. 

My understanding (basic) was that one should try to achive all the efficiencies of the mod-con boilers by properly matching heat losses with outputs.  Oversizing would seem to defeat the purpose.

Help? 

if your heat load is 37kBTUs, you want the solo 60.

If you plan to have 2 back to back sets of 2 showers someday, 5 GPM for 16 minutes straight, you would need a 100 gallon storage tank, assuming 50 in the ground, 110 at the fixture and 140 starting tank temp. That's a robust demand of course, but I would say possible in a family of 5 with two showers.. though realistically, there is a pause between showers and it could even be a several minute pause, so downsizing the tank to, say, 80 gallons is probably fine.

Or you can use low flow showerheads (1.5 gpm each) and a 50 gallon tank would be plenty for the two sets of two showers.

if you match a 60 to a 100 gallon tank, you'll be told "the tank is too big for the boiler". I would typically say I don't care about recharge rates.. I just sized the max demand, so if the hot water is not fully recharged for 20 minutes, who cares.

Right as usual Rob. In addition, many ModCons boilers have a DHW priority with controlled "limited" operating time. So the boiler will not operate "too long" and let the living space drop below set point. It is an option I seldom use but is yet another reason ModCons RULE!

hehehee

Those are great (and very helpful) responses.  Thanks so much guys...