In reply to message posted by Harold Kestenholz:
You need more information abnout your plan. A tankless heater would be subject to freezing in an unheated barn. They also have difficulty keeping temperature at low flow rates. Heating a room to contain one would be more of a waste of energy than using a standard tank to heat the water in an unheated area. In either case, you would use a pressure-balanced temperature-limited valve to prevent surprises and possible high-velocity groomers.
Harold, I am appalled at you for not figuring out how many gallons will be needed to was amount of small, big, medium sized horses on a 24-7 bases like a car wash.
Lets think for a minute Harold figure are we are washing Morgan’s and Budwiser horses herd of say 1,000 horses and 22 miniature ponies .
Just take the measurements of the largest horse and the smallest AVERAGE out the square food of hide your washing THEN figure 1/4 gallon per cubic foot at 90 degrees with a one GPM flow rate of 40-90 degree rise.