A Takagi or Rinnai 190000+ btu unit will do the job itself without a
tank if the Gpm of the shower head matches flow capacity and temp rise
of your propane tankless. You need to know exact gpm output of showers,
incomming water temp lows, and measure temp drop from present tank to
shower head. Proper gas flow is the critical part 190000 btu is im sure
more than your present furnace, you need to have gas flow measured with
competing apliances on with a manometer. You might need all new propane
pipe and regulator to get the flow needed.
I use a small 117000 bosch with 35f incomming water I get a hot 105f
shower without unit set on high, without a real restrictive shower head.
You can get 1.2? gpm heads that a small 117000 Bosch will handle 2
showers, with 190000 btu you wont have an issue or need a tank , unless
altitude issues cannot be rejetted for.
I would hook up the tank but have a bypass valve set up since you may
not need it. Gas flow measured, incomming water temp low measured [ but
figure 35f] shower gpm output, and temp drop in pipe are what you need
I have my tank I replaced infront of the tankless , it is there only to
temper water to house temp. The most efficient, best unit made is a
Takagi TK1 a 94% propane fuel unit. I dought you will need the tank but
keep it inline with a bypass anyway.
Contact Takagi and Rinnai to go over the specs you need and for real
guidance, there might be a high altitude larger model avalaible that is
jetted and sized to get full Btu. It is all in temp rise, gpm, gas
supply+competing users, line temp drop, and knowing incomming temp low.
People unhappy with tankless are the ones that did not calculate
everything first, or installers cutting corners and guessing on gas
supply and pipe sizing.