Abstract: A versatile hot water supply system incorporating a feedback control network and vapor generator of the kind in which a fuel air mixture is combusted in a chamber through which water is flowed. The vapor generator produces a low pressure steam which is permitted to mix with a low pressure water supply at a controlled rate dependent upon the desired temperature and rate of flow of the resultant mixture. The steam formed in the vapor generator is a product of fuel combustion and evaporated feed-water accompanied by the noncondensibles remaining after combustion in the vapor generator. The vapor generator may be run on transportable fuels and therefore affords portability to the system. Control systems are coupled to temperature sensors and related feedback devices and permit the efficient and advantageous use of low pressure steam and condensibles to produce high temperature water at low or high pressures. An upstream, cold water reserve provides high volume, variable temperature capacity to the system.