HPAC Magazine

Only As Needed

A new approach to on-demand domestic water heating.

June 1, 2013   By John Siegenthaler

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.

The original sidearm water heaters rely on buoyancy-driven flow through a piping path that connects the upper and lower portions of the boiler. This phenomena goes by many names including “gravity flow” and “thermosiphoning.”  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 or not. 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 (typically 180-230F), 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 in such installations.

The heat transfer rate allowed by a sidearm water heater is limited by the thermosiphon flow through the chamber connected to the boiler. Heat transfer associated with thermosiphon 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 explains why a 14″-wide fan-forced kick-space heater can match the heat output of several linear feet of fin-tube baseboard. The kick-space heater used forced convection on both its water and air-side. The baseboard used forced convection on its water side, but natural convection on its air-side. The latter is what limits its output.


The original configuration of sidearm water heaters was good for the days of high temperature boilers and low fuel cost. 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 heat input side of a 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 was passing through the heat exchanger. This can be managed by a flow switch, which 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 a storage tank to domestic water. An example of such a heat exchanger is shown in Figure 3.

Brazed plate heat exchangers offer excellent thermal and hydraulic performance. The rate of heat transfer per unit of volume and weight is incredible compared to other options. They also have relatively low head loss, which conserves circulator wattage.

Figure 4 shows how the flow switch, heat exchanger, circulator and valves are configured into an “instantaneous” domestic water heating subassembly that draws heat from a thermal storage tank.

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 assumes that water near the top of the tank is maintained at a temperature hot enough to produce domestic hot water whenever it is required. If you want to use this subassembly in a system where this is not the case, you will need to include an auxiliary heat source. We will 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 is 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 0.6 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.

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. Heated domestic water will emerge from 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.


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

For most residential applications, I have found that heat exchangers using a 5″ x 12″ plate size works well. The greater the number of plates in the heat exchanger, the smaller the approach temperature difference will be for a given rate of heat transfer.

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

Formula 1:

ftotal@115F=(105-50/115-50) x fshower + flav

ftotal@115F=(0.846) x 2.5 + 1.5 = 3.62gpm

The total load on the heat ex
changer is:

 Load=500 x (3.62) x (115-50)=117,650 Btu/hr

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.

Using the above assumptions, an approach temperature difference of 5F, and a maximum pressure drop of two psi on the tank side, the online software suggests the required heat exchanger as a 5″ x 12″ x 30 (i.e. 5″-wide, 12″-long, and 30 plates deep). Consider this the minimum size. A larger heat exchanger will also work, its primary benefits being slightly lower approach temperature difference, and less pressure drop, albeit at a higher installation cost.


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 an auxiliary heat source will be required.

My choice for boosting preheated domestic hot water to final delivery temperature is a thermostatically controlled electric tankless water heater, such as the unit shown in Figure 6.

The electric tankless water heater would be installed as shown in Figure 7.

The tankless electric water heater should be sized to bring the water from its minimum preheat temperature of about 65F, up to the desired delivery temperature – typically not higher than 115F. The assumed starting temperature of 65F is based on the tank cooling to approximately room temperature after several days of little if any heat input from a heat source such as solar collectors or a wood-fired boiler. If you plan to keep the minimum storage tank temperature higher, the size of the tankless heater can be reduced accordingly.

Most thermostatically controlled electric tankless water heaters can accept preheated water over a wide range of temperature. 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.


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

1. It is 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 are also available.

3. The external stainless 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 elevated temperatures by an electric element or other heat source.

Figure 8 shows how this subassembly could be used in combination with a drainback-protected solar thermal system.

The solar thermal collectors heat the storage tank to whatever extent possible, and the electrical tankless water heater provides the necessary boost. On hot summer days the  tank will likely be hot enough that the domestic water will not need any temperature boost. It could even be substantially hotter than required at the fixtures. Therefore, the ASSE 1017
thermostatic mixing valve is installed to protect against scalding.


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 well-insulated (R-4 ºF•hr•ft2/Btu minimum). 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 fluid as necessary over the life of the system. Several companies offer specialty valves for this purpose.

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 domestic water from being delivered to the fixtures at temperatures that could cause burns.

Also, be sure the circulator that moves hot water from the storage tank to the heat exchanger has an internal check valve. If not, install an external check valve down stream of the circulator. This prevents water returning from the space heating portion of the system from short circuiting through the heat exchanger, rather than passing through the storage tank.

Ensure the building’s circuit breaker panel can accommodate the amperage required by the electric tankless water heater before committing to this approach. For electric tankless water heaters operating on single phase 240 VAC circuits, the maximum amperage draw can be determined using Formula 2.

Most electric tankless water heaters have an internal temperature limit switch that automatically stops the elements from further heating if the water temperature passing by the switch exceeds a preset limit. On some heaters this limit is preset at 140F. On others it can be as high as 180F. If you are going to use this system with either solar thermal collectors, or a wood-fired boiler as the heat source to the storage tank, the 180F limit is preferred since it would prevent what otherwise might be nuisance tripping of the limit switch at times when the storage tank is heated to relatively high temperatures.

Formula 2:



A = required amperage (amp)

P = Rated power (kilowatts)

Finally, I want to stress that this is not a new concept. Very similar methods have been used in European domestic heating substations (associated with heat metering), as well as many combi-boilers. It is widely applicable to almost any hydronic system that has thermal storage, and could even be built into a compact pre-manufactured module. Perhaps you can find a place to apply it in an upcoming project. <> 

John Siegenthaler, P.E., is a mechanical engineering graduate of Renssellaer Polytechnic Institute and a licensed professional engineer. He has over 34 years experience in designing modern hydronic heating systems. He is also an associate professor emeritus of engineering technology at Mohawk Valley Community College in Utica, NY. This fall, Siegenthaler will be teaching an online course entitled “Mastering Hydronic System Design.”  Details are available at http://bnp.cammpus.com/courses/hydronic-system-design-training–online