wwhitney
In the Trades
The way I'd put it is that a hot water tank hold "gallons * delta T".
Say your cold water is 50F. You have a 40 gallon water heater set to 120F. You have nominally 40 * (120-50) = 2800 gallons * degF stored. [You may not be able to get it all out at once, as you can't perfectly remove all the hot water and replace it with cold water, the incoming cold mixes with the remaining hot.]
You want to draw 60 gallons of water at 100F. You need 60 * (100 - 50) = 3000 gallons * degF. Your hot water tank is not going to cut it, as 2800 < 3000 (and the full 2800 may not be available).
So you raise your tank temperature to 150F. Now you have nominally 40 * (150-50) = 4000 gallons * degF stored. You should be able to get the desired 3000 gallons * degF out of the tank.
But 150F water is hazardous, so you don't want to push that to all your fixtures. You use a mixing valve to temper the water down to 120F. That will work fine for any fixture that doesn't need water hotter than 120F. It just means the hot water is mixed down with cold right at the tank, rather than at the fixture.
Cheers, Wayne
