Side Arm Heat Exchangers

A side arm heat exchanger is a device used to transfer heat from one fluid to another, typically between a boiler or hydronic system and a domestic hot water tank. It operates passively, utilizing the natural process of thermosiphon to move heated water without the need for a pump. The design features an internally finned stainless steel tube, which increases the surface area and enhances heat transfer efficiency. A larger surface area results in faster and more efficient water heating.

Side arm models are often installed on the side of a water heater tank. The cold water from the tank flows through the exchanger, absorbing heat from the boiler circuit. This results in a steady supply of hot water for household use, without directly mixing the hot and cold fluids. Fin-enhanced units, like those available on our site, offer five to eight times the surface area of traditional designs, making them an ideal choice for faster heating in a more compact form.

If you’re using a wood boiler, pellet boiler, or any other hydronic source, adding a side-arm heat exchanger helps optimize the system’s energy efficiency. It supports long-term savings, provides a consistent hot water supply, and operates quietly. Some models are even available with a double-wall construction for extra separation, typically required in specific code-compliant installations.

The installation of a side-arm heat exchanger depends on your existing system and the selected model, but the concept remains consistent. The exchanger is mounted vertically beside your water heater. The system-side ports connect to the boiler or hydronic loop, and the domestic-side ports connect to the tank using copper or PEX piping.

The unit relies on thermosiphon—hot water rises and cold water sinks—to circulate fluid passively on the domestic side. That means no extra circulator is needed in many setups. Still, adding one can improve flow rates when faster recovery is desired. Before starting, verify the port sizes (typically ¾” and 1” NPT) to ensure they match your plumbing connections. Always install the exchanger with proper support and insulation to avoid heat loss.

For optimal performance:

• Mount it as close as possible to the water tank
• Keep pipe runs short and straight
• Use quality fittings and ensure tight seals

If your system includes glycol or operates under high pressure, a professional installer may need to verify compatibility. Take note: stainless steel components are durable but still need regular inspection for scale or buildup.

Yes, a side arm heat exchanger is specifically designed for heating domestic hot water (DHW). It’s a passive solution that works in conjunction with your hydronic heating source to warm the water in your storage tank. Once installed, it transfers heat from the boiler side to the potable water side, efficiently and safely. No mixing of fluids occurs—only the heat is exchanged.

Fin-enhanced designs make a big difference here. More turbulence inside the system-side tube means better thermal transfer and quicker heat-up times. Faster water heating = less waiting and more consistent supply. Whether you’re heating water for showers, sinks, or appliances, a side arm model helps you do it using the energy you already generate for space heating.

Double-wall variants offer an added layer of protection and are often used where plumbing codes require strict separation between potable and system water. These models still maintain solid heat transfer rates while ensuring compliance with regulations.

Once integrated properly, the result is a nearly continuous supply of hot water, even during high-demand periods—without the need for electric or gas water heating elements to kick in frequently.