A fluid heater for heating fluid moving along a fluid path between a fluid inlet and a fluid outlet. The fluid heater comprises a fluid preheating assembly configured to directly heat fluid moving along the fluid path, a fluid holding tank for receiving preheated fluid moving along the fluid path from the fluid preheating assembly, a primary fluid pump configured to pump fluid along the fluid path, and a primary fluid heating assembly configured to indirectly heat fluid moving along the fluid path from the fluid holding tank. Fuel is combusted in the primary fluid assembly and an exhaust gas movement path is defined to guide the exhaust gas from the primary fluid heating assembly through the fluid holding tank to the fluid preheating assembly.