Gas tankless water heaters are designed for high temperature deltas, and are just fine with an 83F delta.
The biggest residential versions have 199,000 BTU/hr burners. A condensing 199K burner can deliver about 190,000 BTU/hr out, which at an 83F delta is 190,000/83F= 2290 lbs/hr, or (/60=) 38 lbs/ minute, which at 8.34lbs/gallon is 4.6 gpm. At a lower delta-T it can deliver more.
Tub-filling temps are about 110F, so with 35F incoming water that's a 75F delta, and you'd get about 5.1 gpm out of it, filling a typical tub in well under 10 minutes, but more than 5.
Showering temps are about 105F at the shower head, which would be about a 70F delta, and you'd have enough for ~5.5 gpm, which is one MAJOR gusher head or 2-3 low-flow heads.
With a drainwater heat recovery unit big enough to test at 50% heat recovery @ 2.5 gpm (under the Natural Resources Canada test protocol) there is a LOT of margin available for other hot water draws, or multiple simultaneous showers, or even a smaller tankless if slow tub-filling isn't a problem.
If you're heating the place with a hydronic boiler you'll get faster tub filling times and bigger simultaneous flow capacity out of an indirect fired tank sized for the biggest tub you have to fill. (Most families do just fine with a 40-50 gallon indirect.) Don't upsize the boiler for the hot water heating- just make the indirect the "priority" zone on the zone controller. |