One way to get closer towards the approval is to get the process shifted over to the plumbing code and the plumbing officials’ jurisdiction and away from the mechanical code. The plumbing code accepts the use of approved storage type water heaters for heating domestic water, and most jurisdictions approve the use of water heaters providing hot water to a space heating system if it also provides domestic hot water. This includes supplying water directly to the heating system (Open Direct System). These types of systems were originally approved for small fan coils that were placed in the ductwork and provided some heated forced air heat to the structure.