I've been working on an issue with my remolding plan. I want to get a 70 gallon tub, but I currently only have a 50 gallon water heater. The water heater is only 2 years old and is fairly nice, but I'm worried that 50 Gallons @ 140 degrees and 20 gallons at ground water temp (~50 degrees) is not a very hot bath any more.
My first thought was to add an inline point of use heater to the hot water line of the bathroom. It would ideally provide instant hot water to the bathroom, and then scale back when the hot water from the tank in the basement started to arrive. If I was drawing my large bath, and began to run out of water, the heater could take back over. After thinking this plan through some more however I can see that it might not be practical. First, how smooth would the transitions from the inline heater to the basement hot water be? I don't want scalding hot water coming out. Further more, after doing my research, I'm worried about an inline heater limiting my throughput. If I have to fill up my 70 gallon bathtub @ 2 gal/min, it's going to take a long time! Ideally I'd like to keep the water flowing at 4-5+ gal/min.
The only other solutions I could think of: A. Get a 70 gallon tank water heater and replace the 50 (Would hate to do this) B. Get a 30 gallon tank water heater and plumb it in as a supplement when I need it in the basement. Turn it on 30 minutes before I want a bath. (Seems like a PITA). C. Get a large Inline water heater for the bathroom and attach it to the cold line, completely divorcing it from the hot water of the rest of the house.