The present invention provides an improved frac water heating system to fracture a subterranean formation at a remote work site to produce oil and gas. The present invention includes a single-pass tubular coil heat exchanger contained within a closed-bottom firebox having a forced-air combustion and cooling system to heat the treatment fluid. In another embodiment, the invention includes multiple, single-pass heat exchanger units arranged in a vertically stacked configuration to heat the treatment fluid. In a preferred embodiment, the improved frac water heating system is used to heat water on-the-fly (i.e., directly from the supply source to the well head) to complete hydraulic fracturing operations. The present invention also includes systems for regulating and adjusting the fuel/air mixture within the firebox to maximize the combustion efficiency. The system may also include a novel hood opening mechanism attached to the exhaust stack of the firebox.