In an indirect-fired water heating system, the domestic water is heated by hot water from the boiler. In many oil-heated homes, you may find an indirect-fired system that is also tankless. A tankless, indirect-fired water heating system can work in conjunction with either a hot water boiler or a steam boiler.
There are three variations: internal tankless coil, external tankless coil and tankless coil with storage tank.
Types of Indirect-Fired Water Heaters
- Internal Tankless Coil: In this type of indirect tankless water heating system, the coil containing the domestic water supply is located in the water jacket of the boiler. One drawback to this design is the absence of hot water storage; another is the need to maintain the temperature of the boiler water at a high level, high enough to heat the domestic water as it passes through the coil.
- External Tankless Coil: This indirect-fired water heater has a separate storage tank that contains an internal coil. The unit connects to the sides of the boiler. A newer version of the external tankless coil water heater is the plate heat exchanger. It contains a series of wafers or plates with internal porting; plates alternate between boiler water and domestic water.
- Tankless Coil with Storage Tank (or Aqua Booster): This indirect water heating system was introduced to boost water heating capacity. After water has been heated by going through the indirect system’s coil, it is stored in a vertical storage tank. The temperature of the hot water in the tank is usually maintained by means of a recirculating loop; it allows water to be reheated by going back to the coil, either by gravity or forced circulation.
Note: Some local codes require an anti-scald control, also called a tempering valve, for tankless coil indirect water heating units.