You're all set to try out your new whirlpool. You've lit your candles, you're turned on some Enya, you've poured yourself a glass of wine, you cleaned the gunk out of the jets last week, and you've filled your tub with hot water... oh wait. Your tub is only halfway full, and you're out of hot water. Bad times. This will happen if the water heater isn't large enough for the bathtub.
Determining if a water heater is large enough for a bathtub is actually pretty simple; the water heater tank should be about 2/3 the size of the bathtub. For example, if you have a 40 gallon water heater, it would be just large enough for a 60 gallon bathtub. A very small bathtub might hold 40 gallons, while a larger single person bathtub could easily hold 100 gallons or more.
My bath tub holds 110 gallons (pictured above), and my 50 gallon water heater couldn't even fill it with enough hot water to use the whirlpool jets. That's how my house was built, back in 1998. Can you believe it? For a bathtub this size, a 75 gallon water heater would have been about right.
In order to use the whirlpool at my bathtub, I cranked up the temperature on my water heater from a safe 120 degrees to a dangerous 140 degrees (or so). With 140 degree water coming out of the hot water spout at my bathtub, I could mix in a lot of cold water to get the water to a comfortable temperature, and I was able to fill up my bathtub.
That's nice for me, but what about my three-year-old son? This wasn't safe at all - at 140 degrees, it only takes a few seconds to get 2nd or 3rd degree burns.
To make up for this, I installed a tempering valve at the hot and cold water pipes right above my water heater. Now, cold water gets mixed in with all of the hot water coming out of the water heater, making it seem as though my water heater is much larger than it is. The temperature of the hot water that comes out of the faucets can be adjusted at the tempering valve, and I have it set at 120 degrees. This was a much cheaper alternative to replacing my water heater or buying a second water heater, and it was fairly easy to install.
Done and done.