Should I replace tank with Tankless
We are remodeling a bath for our son who is disabled. As a part of the remodel, we are adding a 6' whirlpool/air spa tub, so that he can "soak" in the tub at times.
We currently have a 50 gallon condensing hot water heater (about 10 years old - put in during construction of the house).
I am thinking about replacing the water heater with a tankless. We have a Takagi Jr. on the other side of the house (guest bath, laundry, kitchen) and it works great, with the typical problems of cold water sandwich and long start up times to get hot water. Not a big deal on that side of the house.
The 50 Gallon supplies 3 bathrooms, which is what the tankless would replace. My concern is that to fill the tub, 50 gallons would not be enough, and kind of defeats the purpose of having the tub !
I am looking at the Eternal (GU145) but have not found a lot of good reviews lately on these. It looks like it eliminates the "cold water sandwich" with it's onboard mini storage, which is what draws me to it. I CAN'T have the sandwich, or my wife will KILL ME !!
I would love to hear suggestions/recommendations. Efficiency is important, but I want something that will work and last!
you're ignoring the mixing valve and cold volume
So let's suppose a 70 degree rise is needed to go from 50 degree water to 120 out of the heater. One is not going to fill the tub with 120 degree water, so some of the flow will come from the cold side of the mixing valve. Besides won't the spout be plumbed in 1/2" pipe? I know in my new bathroom I feed the 1/2" thermostatic valve with 3/4" copper, then 1/2" to the tub filler. I can get 6 - 7 GPM out of it, which seems quite high and fills my (admittedly standard-sized) tub pretty quickly.
7 GPM would fill a 100 gallon tub in 14 minutes. Which might seem a while, but that is a big tub!. Unless you're willing to install a 100 gallon high-BTU tank with 3/4" valve and tub filler, the fill time does not seem totally unreasonable. And that assumes you have the upstream plumbing (1") to support the flow rate. And even then you'll get the gradually cooling effect on the hot water side.
So I do think this is a reasonable application for a tankless. But do check about the gas meter capacity. I had the gas company upgrade mine for free when I repiped.
Here is the spec sheet for the big Noritz.