Hello, all. I've been reading through the archives for the tank and tankless water heater forums trying to decide how to spec a new water heater. I have a similar post on the Tank forum, but I thought I’d explore the question on this side as well.

We’re in Rochester, NY where the winter groundwater temp hovers between 40 and 45 degrees. It’s currently just me and my wife, but kids are a definite possibility in the near-term. We’re in the process of a master bath remodel and are installing a 100-gallon soaking tub, which replaces an existing 85-gallon jetted whirlpool tub. Our current heater is a 13 yr old 50-gallon natural gas tank (State, I think). It still works but is probably nearing the end of its useful life. The current water heater just barely filled the old tub, so I don’t expect it to do the job on the new tub. The new tub would be likely used a few times per month on average. So the question is tank or tankless. We heat with GFA, so indirect / boiler options are out. I’ve long been drawn to the tankless concept, but much of what I’ve read that’s relevant to our location and usage raises a lot of red flags. I’m contemplating a bid for the Rinnai R75LSi tankless with an installed cost (after NYSERTA rebate) of roughly $2,200. We’re also replacing our HVAC system which is using up the entire $1,500 federal credit so none of those dollars can offset the water heater.

My question for this forum is whether the Rinnai R75LSi tankless would be advisable for us? From a water flow standpoint I’m assuming my worst-case scenario supply (dead-of-winter, 80 degree temp rise from 40 to 120) would be 3.8 gpm. That’s sufficient for a single shower and perhaps some simultaneous hand-washing, given a 2.5 gpm shower head. I calculate 105-degree water at the shower to require 81% hot water (120 degrees) and 19% cold (40 degrees), so the hot water flow with a 2.5 gpm showerhead is 2.03 gpm (did I do that right?) If we put a low-flow shower head in the 2nd bath we could eek out 2 simultaneous showers on the coldest day of winter. In the summer when ground water temps are 50 degrees or so we’re looking at 4.3 gpm maximum flow. That’s enough for 2 showers or a shower and perhaps a load of laundry (not sure about the water flow for the washing machine – what’s a good assumption for a 10-yr old Maytag top-loader?). The tub filler flows at about 4gpm on hot only, so it looks like nothing else is happening when we fill the tub.

A few other factors – our water (Monroe County Water Authority) is “moderately hard” at between 5.7 and 7.7 particles per gallon. I understand that annual flushing / descaling is required – would we need to do so more frequently given our water hardness? Would some type of pre-heater filter help? We don’t have and won’t add a water softener.
One final question – what happens when you exceed the flow rate? Say we’re filling the tub and someone jumps in the shower – does the temperature drop off or does the pressure drop? A less powerful 105-degree shower may be tolerable. A colder one is not.

I’ll stop with my rambling and let those who know better than I weigh in. Does anyone else in a similar climate and household demand situation use this or a similary sized unit? To what effect?

I appreciate any suggestions or feedback you can offer.