I frequently run out of hot water in the shower. I have a furnace that heats the water directly. My question is; Can I use a hot water heater as a holding tank, so I don't run out of hot water? If so what do I need to complete this task?

I'm sure most codes won't alow you to mix domestic water and water for a boiler. You can hook up a second storage tank to the water heater and use a circulating pump to keep them both hot. But first I would do an on demand water heater. Endless hot water at almost half the operating cost. I like Noritz and Rinnai. I get allot of service calls on two of the other brands.

I have a car wash we have a 60gallon hot water tank with another 40 g tank as a holding tank to maintain hot water. My question is what temperature do you set them at ,the second tank is hooked to a makeup pump to circulate water.