A simple formula for matching your bathtub needs with your water heater capacity.
Don’t bother taking a bath after my daughter takes a shower. She starts singing Adele songs and before she’s done, all the hot water is gone.
Picture this; the house is quiet, the water heater is full of hot water and you finally get a chance to take a bath in your brand new tub.
But even though the heater started fully charged, the tub is too big for the heater. You don’t have enough hot water to fill the tub.
What to do?
Get a bigger water heater.
How big? Let’s look.
How big is the tub?
First, let’s figure out how big the tub is. Measure your tub; take all the measurements in inches. Measure the inside width and the inside length. Measure the depth as from the inside bottom of the tub to the bottom of the overflow drain hole. This is the typical fill level. (Note that you are not in the tub, displacing the water. This is for calculating the capacity of the tub, not determining the actual water needs).
Then do a little math. Multiply the width by the length. Multiply the result by the depth. Divide the result by 1,728 for the number of cubic feet of water the tub will hold. Multiply that result by 7.5 for the number of gallons of water held in the tub.
A little more math
Just a bit more math and we’re done. A water heater should be at least 3/4 the size of the tub. So if your tub has a capacity of 100 gallons, you will need a 75 gallon water heater.
But I don’t want to buy a new water heater!
If your water heater is just a little too small, there are other options. You could install a second heater to increase capacity without taking out the old one. I know, I don’t like that option either.
How about increasing the water temperature?
That actually is an attractive option, but is horribly unsafe without a tempering valve. That kind of valve is designed to blend super hot water with enough cold water to avoid scalding at the taps.
A very good article about temperature control can be found by clicking: here.