Hot water heater

Hey, this is a great site I've just found and I'll be with it for a long time.
Would my 20 gallon HWH be undersized. new 4 years ago.
I use it to shampoo hair in a small salon and after one wash the water runs cold.
If I use the 70% method what water supply does that give me ?
The basin has a tempering valve (divert faucet) for temperature control.
I also need to purge the tank - any ideas what to use to dissolve any calc thats buildup.
The thermo is on highest setting.

Thanx in advance for any suggestions.

Here is a water heating sizing link

http://rightspec.bradfordwhite.com/Sizing/BeautyBarber.aspx

heater

There could be any of several things that would cause your symptoms, but we would have to be there and test the water heater to see if it is something other than an undersized water heater.

If you are needing HW for a client and don't have it after doing 1 hair wash then I would have it looked at and if all is well with it then you would need to upgrade. Your sprayer may have a large GMP rating, you may also have high water pressure and be using more water than needed because of it. Your whole system needs to be looked at.

How many chairs do you have and how many other hairdressers / barbers are working the floor at the same time.

You need a much bigger water heater. Even if you only have one chair, I would look at a 40 gallon, minumum. You will probably need a new electric circuit.