Ok I have a daka indoor wood furnace , works awesome, 2500 sq feet ,72 degrees all the time , went through 11 face cords last year , Now I want to set it up to preheat my water supply to my hot water tank , if i was to put coils in the heat chamber ( not the fire box ) it will heat up the water but will it build up preasure ? city water so not a closed system , I am a fabricator by trade so making it not a problem , come on put your 2 cents in , THANKS FOR YOUR EXPERTISE