Installing a Sidearm Heat Exchanger on a Hot Water Tank

This is another episode of Alternative Heating & Supplies' tips and in this blog post, we are going to talk about the sidearm heating exchangers that are mounted on the side of domestic hot water tanks. We get a lot of phone calls from people who have purchased sidearms from other companies and even our own company and they don't have it working properly because of simple mistakes.

So the most common way to install them is to mount them off the side of the domestic hot water tank. Let's take a further look at how to install.

Sidearm and Tube and Shell Heating Exchangers

Tube & Shell heat exchangers are suited for higher-pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it.

One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. Ideal for many residential and industrial applications. Their complete stainless steel welded structure ensures product strength and high-quality performance.

Applications: Pools, spas, hot tubs, transmission and engine coolers, oil coolers, boiler sample cooler, waste water heat recovery.

 

Sidearm Heat Exchanger Installation Guide

How Do Heat Exchangers Work

The heat exchanger works by heating the water inside the domestic hot water tank. As the water cools, the coldest water settles at the bottom of the tank and flows into the lower portion of the sidearm heat exchanger. Hot water from the external boiler circulates through the tube-and-shell exchanger, transferring heat to the domestic water. As the domestic water warms, it naturally rises.

The heated domestic water then returns to the tank, while cooler water sinks to the bottom to be reheated. This natural convection cycle continues, maintaining the tank at a consistent and comfortable temperature.

Because the system relies entirely on natural convection, no pump is required. The continuous movement of hot water from the boiler and the domestic tank provides a steady and reliable supply of hot water.

Causes of Heat Exchanger Failures

The problems and issues arise because many people who experience problems do not install a sidearm heat exchanger correctly. As shown below, water exits the bottom of the tank and flows downward, which is expected since colder water sinks. As the water heats up, it rises. The mistake is typically made at this point in the system.

Thermal Blocks and Convection Systems

Many people route the piping up, across, and then back down into the tank at this point. The problem is that hot water naturally rises rather than falls, so this configuration creates a thermal block. When a thermal block occurs, hot water can only move downward after it cools, which prevents proper convection. Without continuous convection, the system cannot circulate water effectively.

For the system to function correctly, the water must return directly without being forced to drop. This is why the installation must be done in this manner.

Many people run into issues because their hot water tanks do not have side ports. However, most hot water tanks sold today are equipped with both a pressure relief valve and a drain located at the bottom of the tank, which can be seen beneath this unit. This particular tank also includes dedicated hot and cold side connections.

If a tank does not have side ports like the ones used here, the installation can still be done correctly. The bottom drain can be removed and used as the lower connection, as shown. The pressure relief valve can be teed off by installing a tee fitting, with the pressure relief valve reinstalled on one side and the top of the sidearm heat exchanger connected on the other. This configuration keeps the piping level, allowing heated water to rise naturally into the tank.

This is the proper way to install a tube-and-shell sidearm heat exchanger. When installed this way, the convection system will function efficiently and reliably without issues. Even if side ports are not available, the drain connection is always present, and while pressure relief valves are sometimes located on the back or top of the tank, they can still be used to complete a proper installation.

Material Selection for Shell and Tube Heat Exchanger

When installing the tube and shell heat exchanger, you can use a couple of different materials.

• PEX tubing is one option and is shown here, but it is generally not recommended for this application. As PEX heats up, it becomes more flexible and can lose rigidity, which may cause the installation to sag or appear poorly supported over time.

• In this installation, lead-free brass fittings were used. These fittings are readily available and provide a clean, durable connection. As shown, the finished installation is solid and well-supported.

 

• Another suitable option is high-quality copper piping. Copper can be soldered, threaded, and sealed with pipe dope, offering flexibility depending on installer preference.

It’s important to remember that the tank operates under significant pressure, particularly on the domestic hot water side. For this reason, unions are strongly recommended. Unions make it easier to disconnect components if repairs, adjustments, or maintenance are ever required.

Be sure to use Teflon tape and pipe dope that are specifically rated for domestic hot water and potable water systems. Using non-rated sealants can introduce contaminants into the drinking and bathing water supply. Even small amounts matter, so all materials should be approved for potable water use.

The Plate Exchanger

Okay, if you have this, then we're going to recommend you go to the plate exchanger, which is another easy install, actually a little bit more simplified, and that works on a thermal pressure syphon, which we’ll go on to explain deeper in the next video that I have, and it's called "Plates Exchangers For Domestic Hot Water Tanks."

 

If a sidearm is not suitable because you lack the necessary ports or your pressure relief valve is not ideally located, we suggest a plate exchanger as an alternative.

Why You Might Choose a Plate Exchanger

A plate heat exchanger can be a smart option for many systems because it delivers very efficient heat transfer in a compact design. Unlike larger shell-and-tube-style sidearms, plate exchangers use a series of thin metal plates that expose a greater surface area between the fluids, which speeds up the transfer of heat and allows you to get more hot water from the same amount of boiler output. They are also easier to maintain, since you can disassemble them for cleaning or add plates to increase capacity if your needs grow. In tight mechanical spaces or for systems where maximizing efficiency and flexibility matters, a plate heat exchanger can offer long-term performance and lower operating costs.

Ensure Proper Installation

A properly installed sidearm heat exchanger can significantly improve the efficiency and performance of your domestic hot water system. When following the correct piping configuration and using suitable materials, you allow natural convection to do the work without relying on pumps or additional equipment. Whether your tank has side ports or you adapt the pressure relief and drain connections, the key is to avoid creating thermal blocks so the system can continuously circulate water as intended. 

With thoughtful installation and attention to detail, you’ll enjoy reliable hot water delivery and a convection system that works smoothly and efficiently. Let the experts at ALT Heat help get you the right parts for your heat exchangers.