We live along the northern edge of Ohio and have 3 adults in the house. We are looking at the 50 gallon units and will be replacing two 40 gallon units (tied in series) that are 15 years old.
We only have a 1/2" NG connection at the water heaters and tankless heaters need a minimum 3/4". Our basement is finished with a drywall ceiling and running 3/4" through it (from a 1" in an open ceiling utility area 25 feet away) may be difficult or impossible.
Our water is fairly cold in the winter and I do not think a tankless will be able to heat it up adequately, even those rated at 6+ gpm.
Based on cost and ROI I have ruled out the tankless units.
So we are resigned to replace the 2 tanks with 1, but a 50 gallon unit.
The plumber is proposing a basic Rheem 22v50, natural gas fired and venting through the roof using most of the existing venting. I cannot find any information on this unit and would like to compare fuel burn efficiency between it and the direct vent unit.
For a total installation cost (including removing the 2 existing 40 gallon tanks, replumbing with new pipe and valve) I am looking at $750 complete for the basic tank.
It appears the first heater was installed as a pre-heater and as it supplied the second heater in series, this heater did not need much time to recover as hot water is used. The thermostat of the first heater was probally set a bit lower then the second. This is a necessary arrangement if other options are not considered.
One other option is to install a large holding tank to give the incoming water a chance to warm up to room temperature and let this be the feed to your hot water tank.
If you were satisfied with the performance of the two in series, then just replaced them. I don't know if a single 50 gallon hot water tank will do the job of two 40 gallon due to the temperature differance of the incoming water and the time needed to recover heated water. Someone else may have basic information on water heater performance, the average amount of hot water used in three showers in a row. It all depends on how long each individual spends running the water.