More About Indirect Water Heaters
Unlike traditional water heaters, indirect WH’s do not heat water using gas or electricity on their own, but instead use the hot water produced by the boiler, solar, or similar heating system. A heat exchanger inside the indirect water heater transfers heat from the heating system to the domestic water stored in the tank and effectively isolates one from the other, preventing cross-contamination.
Indirect water heaters are best used in combination with oil and wood-fired boilers or solar heating systems. Water can be stored at high temperatures and used in combination with a mixing (anti-scald) valve to achieve more water output at desired temperature.
Buderus, Triangle Tube and Bradford White indirect series comparison
- S & SU series (S32, SU54/80/100) - vertical, single coil, glass-coated tank & coil
- LT series (LT160/200/300) - horizontal, single coil, glass-coated tank & coil
- SM series (SM80/100) - vertical, dual coil, glass-coated tank & coil
- SST series (SST150/250/300/450) - vertical, single coil, all-stainless steel tank & coil
- SMART series (SMART 30/40/50/60/80/120) - vertical, tank-in-tank, all-stainless steel tank
- PowerStor series (SW-2-30/40/50/65/80/120) - vertical, single-coil (single-wall heat exchanger coil), glass-coated tank & coil
- PowerStor2 series (DW-2-40/50/65/80) - vertical, single-coil (double-wall heat exchanger coil), glass-coated tank & coil
Difference between Single-coil and Dual-coil design
Dual coil indirect water heaters are intended for dual heating systems, such as oil-fired boiler and solar, where the domestic hot water can be heated intermittently by one system and then by the other (i.e. solar in summer and oil in winter).
This design allows both heating systems to operate without mixing their contents and without considering the differences in temperatures, operating pressures and other factors.
Coil vs. Tank-in-Tank Design
Standard indirect water heater design incorporates one or two heat exchanger coils.
With a tank-in-tank design, domestic hot water tank is located inside a slightly larger tank, which, in turn, contains the heating system water. Steel wall of the DHW tank act as a heat exchanger and accomplishes the thermal energy transfer from one type of water (heating system) to the other (domestic).
The main advantage of tank-in-tank design is larger heat transfer area, which allows for higher recovery rate and consequentially, higher continuous hot water flow rate. The downside is somewhat larger standby heat loss due to heating system’s water being the outermost in the tank.
Glass-lined vs. all-stainless tank
In glass-lined tanks, manufacturers coat regular steel components (tank and coil) inside the indirect water heater with epoxy lining to prevent corrosion. This is the most common and cost-effective practice in the indirect WH industry. The downside of this design is the brittleness of the glass lining, which can crack and chip with improper handling or installation.
All-stainless units have stainless steel (i.e. 316L) components which are inherently corrosion-resistant and do not require protective coating. They are more robust, but also tend to cost more.
Difference between Vertical and Horizontal indirect WH models
Horizontal indirect water heaters are generally intended for installation directly under a boiler (such as Buderus G215 boiler + LT160/200/300 indirect WH). This design saves space, simplifies and reduces the amount of piping. Be sure to check compatibility between your boiler and horizontal indirect tank of choice.