Will an inline tankless heater help my 50 gallon water heater fill my 70 gallon tub?

Question

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.

Thoughts?

Answer 1

I believe you are over estimating how much hot water is needed to fill the tub. It will be filled below the overflow drain that should be built into the tub. You can reduce the total by the size of the person getting into the tub. And most importantly you can reduce the amount of hot water by any cold water you include to regulate the temperature, perhaps you are only using 2/3 hot water to 1/3 cold water.

The important question you'll want to determine is how quickly the water heater can regenerate a full 50 gallons of hot water because you may want to heat the bath back up after a while, or use hot water for some other purpose after taking a bath. If you find yourself near the threshold of available hot water, you may find it easier and cheaper to increase the temperature of the hot water in the existing tank, rather than adding a tankless water heater.

Answer 2

Running the tank at a higher temp, and using a mixing valve to temper it down to a safe level would be the way to go. The mixing valve can be at the tank, and would serve the whole house.

At present 50 gal of 140 averaged with 20 gal of 50 degree water averages 114 degrees. Still way too hot. Even half and have gives you 90 degree water -- Warm swimming pools are usually 88.

Other ways around this:

• Temper your cold water supply. Have an auxiliary tank in your utility room where water can sit and come up to room temperature. Downside: Unless you change your plumbing, you are warming the water you use to flush. Making this available to the showers, but not the baths is a lot of pipe to re-arrange.
• Put a second tank in that is just a tank. Insulate it. Connect it with large pipes to both inlet and outlet of the hot water tank. Move the make up water line to the bottom of this tank. Under normal operation, water in this tank will circulate with the main tank by convection. This doubles your hot water supply at the cost of having slightly higher losses. When you draw hot water it takes water from the top of both tanks. Cold water enters the bottom of 2nd tank, and flows through the link to the primary tank.

If you use all of the hot water then the primary tank turns on, and heats up. Heat doesn't circulate to the secondary until the primary is significantly warmer than the secondary.

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Downsides: If you have teenagers, you may never get them out of the shower. A teen is capable of standing in the shower with unlimited hot water for time intervals large enough to be concerned with erosion processes.