Sidearm water heaters were developed decades ago so that boilers could provide both space heating and domestic hot water. Generically, they are water-to-water heat exchangers that transfer heat from the boiler water to domestic water whenever the latter is flowing through. Figure 1 illustrates how they were originally used along with boilers that maintained a constant minimum temperature.

Sidearm water heaters rely on buoyancy-driven flow through a piping path that connects the upper and lower portions of the boiler. This phenomenon goes by many names, including “gravity flow,” “thermosiphoning,” “ghost flow,” and “heat migration.” Before circulators, it was the only propulsion effect for moving heated water from a boiler in the basement to the heat emitters in the building above.

In Figure 1, hot water from the boiler rises upward and then over and down through one chamber of the sidearm water heater. As water from the boiler gives up heat to the cooler domestic water in the other chamber, it cools and its density increases. This causes it to drop through the remaining piping and eventually flow back to the boiler.

This flow occurs regardless of whether the boiler’s space-heating circulator is operating. The sidearm circuit in Figure 1 includes a balancing valve that limits bypass flow through the sidearm circuit when the space-heating circulator is operating.

Given that older boilers operated at relatively high temperatures (180-230ºF), a single pass through the sidearm heater was adequate to bring cold domestic water up to (and at times above) a reasonable domestic hot water delivery temperature. Eventually, thermostatic mixing valves were developed and installed to reduce scalding risks with this type of installation.

Natural vs. Forced Convection

The heat transfer rate allowed by a classic sidearm water heater is limited by the bouyancy-driven flow through the chamber connected to the boiler. Heat transfer associated with bouyancy-driven flow is called natural convection and is relatively weak compared to forced convection (e.g., when water is forced through a heat exchanger by a circulator).

This difference also explains why a 14-inch-wide, fan-forced, kick-space heater can match the heat output of several linear feet of fin-tube baseboard. The kick-space heater uses forced convection on both its water and air side. The baseboard uses forced convection on its water side, but natural convection on its air side. The latter is what limits its output.

Higher flow velocities always improve the thermal performance of any heat exchanger. This happens because higher flow velocities create turbulence that reduces the thickness of the “boundary layer,” a thin layer of very slow-moving fluid that clings to the inner surfaces of the heat exchanger. The thinner the boundary layer, the less resistance to heat transfer between the fluid and the surface.

If you’re not convinced that higher flow velocities produce higher rates of heat transfer, have a look at the heat output ratings of fin-tube baseboard at flow rates of 4 gpm vs. 1 gpm. All other things being equal, the higher flow rate always increases the rate of heat transfer. You also can look at the heat output ratings of fan-equipped convectors and find that increasing the water-side flow rate or the air-side flow rate always increases the rate of heat transfer.

The Makeover Begins

The original configuration of sidearm water heaters was good for the days of high-temperature boilers and low fuel costs. Today, we have significantly better hardware that can transform the original concept into a state-of-the-art subassembly, one that can be used in a wide variety of modern hydronic systems.

The first change is to use a small, low-power circulator to create higher flow rates on the boiler side of a sidearm water heater. This will significantly decrease the surface area required for a given rate of heat transfer.

The small circulator will have to operate whenever domestic water is passing through the heat exchanger. This can be managed by a flow switch that detects a demand for domestic hot water. An example of such a switch is shown in Figure 2.

A modern brazed-plate, stainless-steel heat exchanger will be used to transfer heat from the storage tank to the domestic water. An example of such a heat exchanger is shown in Figure 3.

Years ago, I designed systems around bulky shell-and-tube heat exchangers. Today I continue to be amazed and delighted by the thermal and hydraulic performance of brazed-plate heat exchangers. The rate of heat transfer per unit of volume and weight is incredible compared to other options. They have relatively low head loss, which conserves circulator wattage. And they are available in a wide range of sizes that allow the sidearm concept to be scaled up or down as required by the load.

Figure 4 shows how the flow switch, heat exchanger, circulator and valves are configured into an “instantaneous” domestic water heating subassembly.

The storage tank holds heated “system” water. This water could be heated by a boiler, solar collectors, wood-fired heater, heat pump or a combination of heat sources. The schematic in Figure 4 on page 30 assumes that water near the top of the tank is maintained at a temperature hot enough to produce domestic hot water whenever it’s required. If you want to use this subassembly in a system where this is not the case, you’ll need to include an auxiliary heat source. We’ll get to that shortly.

In theory, the flow switch could be located in either the hot or cold domestic water piping. However, mounting it in the cold water piping reduces thermal stress and prolongs its life expectancy. Also notice that it’s mounted upstream of the mixing valve (on the cold pipe). This allows it to detect the total flow rate of domestic water, rather than what might be a portion of that total flow passing through the heat exchanger. The latter occurs when some cold water passes into the cold port of the thermostatic mixing valve.

The flow switch contacts close whenever a hot water flow of 0.5 gpm or higher is detected. Most small flow switches use sealed magnetic reed contacts that are not rated for line voltage switching. Because of this, the flow switch contacts are wired to activate the coil of a relay. The contacts of that relay connect line voltage to the circulator.

Ideally, the return pipe should be located so that the expected temperature drop across the heat exchanger corresponds to the temperature difference between the upper and lower piping connections due to stratification within the tank. This helps preserve stratification and is especially important if water from the bottom of the tank supplies a solar collector array.

This arrangement also allows the storage tank to provide hydraulic separation between the sidearm heat exchanger circuit and piping that serves other loads in the building. Because of this separation, there is no need to install a check valve in the sidearm circuit. This provides an additional benefit by allowing a very slight thermosyphon flow through the insulated sidearm heat exchanger circuit, even when the circulator is off. It keeps the sidearm circuit “primed” with warm water. This further reduces the response time of the subassembly when a de-
mand for hot water exists.

When the flow switch turns on the circulator, heated water from storage immediately flows through the primary side of the heat exchanger, as domestic water passes through the other side. Brazed-plate, stainless-steel heat exchangers have a very high ratio of internal surface area to volume. They also have very low thermal mass. These characteristics allow them to transfer heat almost instantly when fluids at different temperatures flow through. My estimate is that heated domestic water will emerge from the domestic water outlet of the heat exchanger two to three seconds after the flow switch is activated. This is significantly faster than the response of a gas-fired tankless water heater or a combi-boiler starting from room temperature.

Exchanging Btu

For this subassembly to function well, the heat exchanger should be generously sized. My suggestion is to select a heat exchanger that will provide the design heat transfer while operating with an approach temperature difference of 5º. This means the heat exchanger should provide the design heat transfer rate when water supplied to its primary side is only 5º higher than the desired domestic hot water supply temperature. Thus, to produce the design flow rate of domestic hot water at 115º, the water supplied from storage should not have to be above 120º. Keeping the minimum usable storage tank temperature low improves the performance of any renewable energy heat source supplying heat to the tank.

The design domestic water flow rate is determined by estimating the number of hot water fixtures that are likely to be operating simultaneously and then adding up their total flow rate.

In some cases, the design flow rate needs to be adjusted based on the fixture delivery temperature. For example, assume that a shower requires 2.5 gpm at a delivery temperature of 105º and that it operates simultaneously with a lavatory requiring 1.5 gpm at 115º. The design outlet temperature from the heat exchanger must be at least 115º, but the total flow rate of this 115º water can be adjusted downward because the shower only needs 105º water. Here’s the calculation, assuming cold water enters the heat exchanger at 50º:

The total load on the heat exchanger is thus:

Once the temperatures and flows are determined, I suggest using one of several readily available software tools provided by manufacturers of brazed-plate heat exchangers to select an appropriate model. An excellent online heat exchanger sizing tool is available at

Using the above assumptions, an approach temperature difference of 5º and a maximum pressure drop of 2 psi on the primary side, the online software suggests the required heat exchanger as a 5 x 12 x 30 (5 inches wide, 12 inches long, and 30 plates deep). Consider this the minimum size. A larger heat exchanger also will work (its primary benefits being a slightly lower approach temperature difference and less pressure drop).

Topping It Off

One variant of this approach is for systems with intermittent heat input to the storage tank. This would include systems supplied by solar collectors, wood-fired boilers, or off-peak electrical elements. These systems typically produce a wide range of storage tank temperatures. At times, the storage temperature may be much higher than the required domestic hot water delivery temperature. At other times, the tank will only be providing a preheating function and thus an auxiliary heat source will be required.

The installation of an ASSE 1017-rated thermostatic mixing valve (or ASSE 1070-rated point-of-use protection valves at each fixture) is essential to prevent overheated water from being delivered from the fixtures. Never install a domestic water heating system without such protection.

My choice for boosting preheated domestic hot water to final delivery temperature is an electric tankless water heater, installed as shown in Figure 5 on page 32.

The tankless electric water heater should be sized to bring the water from its minimum preheat temperature of about 65º up to the desired delivery temperature — typically not higher than 115º. The assumed starting temperature of 65º is based on the tank cooling to approximately room temperature after several days of little, if any, heat input. If you plan to keep the minimum storage tank temperature higher, the size of the tankless heater can be reduced accordingly.

Most electric tankless water heaters can accept preheated water over a wide range of temperatures. They measure the incoming water temperature and adjust the wattage supplied to their elements to achieve a stable set outlet temperature. If the incoming water is hotter than the heater’s setpoint, it simply passes through without any further heating.

Gas-fired tankless water heaters are more limited in their ability to accept preheated water.

Typically, preheated water supplied to such heaters can be no hotter than the heater’s setpoint minus 20º. Supplying preheated water at temperatures closer to the setpoint causes cycling due to the limited modulating ability of the unit’s combustion system.

Some gas-fired tankless water heaters use the work-around provided by a second thermostatic mixing valve to reduce the temperature of the incoming preheated water low enough to stabilize heater operation. It works, but I view this as a Band-Aid solution, especially when very low-cost renewable energy is providing the preheating.

Instant Benefits

Here’s a summary of the benefits offered by this method of instantaneous domestic water heating.

1. It’s readily adaptable to thermal storage tanks of various sizes and heated by a variety of heat sources.

2. Suitable brazed-plate, stainless-steel heat exchangers are available from several manufacturers. If local codes insist upon double-wall heat exchangers, they also are available.

3. The external stain-
less-steel heat exchanger can be easily inspected, cleaned, and replaced if necessary.

4. The thermal mass of the storage tank stabilizes domestic hot water delivery temperature during long demand periods. This helps eliminate fluctuations in delivery temperature.

5. In the case of a solar drainback system, this approach eliminates the need for any internal heat exchangers in the storage tank. This allows a wider choice of potential tank suppliers.

6. The standby heat loss associated with a separate DHW storage tank is eliminated.

7. The warm-up time of this assembly is very short, significantly shorter than that of a gas-fired tankless water heater because there is no need to initiate combustion.

8. The potential for Legionella growth is reduced since very little domestic hot water is “stored” in this assembly.

9. In the case of a solar thermal system, the storage tank is not heated by the auxiliary domestic water heat source. This improves collector efficiency relative to systems where the upper portion of the storage tank is maintained at an elevated temperature by an electric element or other heat source.

Minding the Details

For best performance, the heat exchanger, circulator, and piping should be kept as close to the tank as possible. This minimizes the water content in the primary side piping and reduces response time.

All piping and components on the primary side of the heat exchanger should be insulated. This helps preserve residual heat in the heat exchanger from one demand period to another that might follow a short time later.

Filling and flushing valves should be installed on the inlet and outlet of both the heat exchanger and tankless water heater. They allow either component to be isolated and flushed with a suitable cleaning compound as necessary over the life of the system. Several companies offer specialty valves for this purpose.

If the DHW subassembly is piped across a set of headers that supply other load circulators, be sure to install a check valve in the piping leading to the heat exchanger. A circulator with an integral check valve also could be used, as shown in Figure 6 on page 32. Without the check valve, there is likely to be reverse flow through the heat exchanger when the other load circulators are active and there is no demand for domestic water heating.

Finally, I want to stress that this is not a new concept. It has been used in tens of thousands of European domestic heating substations (associated with heat metering), as well as many combi-boilers. It’s widely applicable to almost any hydronic system that has thermal storage and could even be built into a compact, premanufactured module.

Maybe you can find a place to apply it in an upcoming project.

The third edition of his book, Modern Hydronic Heating, is now available. For details, visit

Publication date: 6/18/2012