Looking for a water tank?

actually an old non leaking water heater will work like a

charm. Just get Pipe plugs that will fit in where the heating elements

once lived install only the hot water line but install it to the coldwater inlet.

Place the pipe from the hot water outlet into the house hot water supply line.

You can install if you want a 2 valve bypass circuit which can if necessary

bypass the storage tank by isolating it from the circuit

Remove the electrical guts from the old water heater and purchase an

insulation Blanket and mount it to the outside of the storage tank.

This will help maintain the water temp.

Not a bad idea!

Google 'pressure tanks'

(It will be exposed to system pressure.)

A set of isolating valves and a bypass would be a good idea.

You might try one from one of of these suppliers:

http://www.amtrol.com/index.htm

http://www.wellmate.com/