This article was originally published in the November/December 1999 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online November/December 1999

in energy

Building a Boiler Side-Arm

Figure 2. The single-wall heat exchanger should be made as long as is practically possible, for better heat transfer.
Figure 3. The double-wall heat exchanger is required in some jurisdictions to separate potable water from non-potable.
A boiler is generally more fuel efficient than a standard gas water heater, especially when the boiler is fully loaded. During the winter, the fuel savings from combining a space heater with a water heater compared to operating two distinct systems can amount to about 20%. For this reason, indirect-fired water heaters--basically a hot-water storage system coupled with a boiler or other water-heating source--have been used for many years.

However, for the owner of a single-family dwelling, the cost of an indirect-fired water heater--anywhere from $500 to $1,000 and up--can be an insurmountable obstacle. For owners or contractors looking to cut the energy costs associated with heating potable water, the indirect-fired water heater does have a cheaper cousin: the boiler side-arm. It works in much the same way--during the heating season, the tank heats up from the boiler's burner and not from the more costly tank heater--but a side-arm costs a lot less to assemble; about $200 in parts if a functional water heater tank already exists. Be forewarned, though, that boiler side-arms tend to be less efficient than indirect-fired water heaters and are much slower at recovering from large uses.

The side-arm strategy will lead to a 5% to almost 20% savings in energy costs over maintaining two distinct heating systems--and that's only during the heating season. During the summer, when the boiler is cold, the domestic water heater uses its own heater, or preferably a solar water heater, to supply the energy needed for water heating.

The big difference between an indirect-fired water heater tank and a side-arm is in recovery time. The side-arm is much slower. If water use in the house is fairly low, this isn't a big concern. However, with larger users the water heater thermostat should be set on a base temperature of 110°F-120°F so that the users will always have at least some hot water available.

A boiler side-arm may be especially beneficial in homes with oil-fired boilers, since many of these homes have an electric water heater tank or a boiler with a tankless coil. Side-arm installations can be especially cost-effective if done during major repairs or when replacing a boiler. If the boiler needs replacing, get one with a tankless coil and hook it up as shown in Figure 1. Although tankless coils have earned a tarnished reputation thanks to their requirement for boiler firing during the summer and to customer complaints about summer shortages of hot water, don't automatically dismiss them. In this case, the tankless coil is a good idea. The tankless coil is a more efficient heat exchanger than a homemade side-arm is. With a tankless coil, the recovery rate for the storage tank is faster.

If a new boiler is not needed, buy or make a boiler side-arm. A side-arm is similar to a shell-and-tube heat exchanger and gets installed on the boiler's supply pipe (see Figures 2 and 3). The other necessary components are: a small circulator pump with a 1/2 to 2 gpm flow capacity, an open-on-rise aquastat, a few feet of copper pipe, some pipe insulation, and an ordinary water heater.

The boiler side-arm's circulator is wired through the aquastat and in parallel with the main circulator for the house loops. When the boiler fires, both circulators come on. For maximum efficiency, the circulator should run immediately after the burner fires. If more than one circulator exists, hook up the side-arm circulator in parallel with the boiler circulator that runs most often. If you later find that the side-arm is hooked up to a circulator that is not the most active, it's a simple matter to move the electrical connection to one of the other circulators. The boiler's supply water flows through the inner tube of the side-arm and heats the domestic water in the outer tube. The collected heat is then transferred to the storage tank by the small circulator.

Install the aquastat near the top of the water heater tank with the sensing bulb under the insulation, so that the side-arm circulator turns off when the tank reaches about 150°F. (As the heat exchanger's capacity is small compared to the water heater's element or burner, this relatively high temperature is a way to increase the storage.) If this isn't done, the tank will eventually get as hot as the boiler--a surefire method of producing scalding burns on unwary hot-water users. As an additional safety precaution, install a tempering valve set at 120°F on the output of the water heater. You'll also need to relocate any heat traps upstream of the cold water connection.

Although some jurisdictions require a double-wall heat exchanger between potable and nonpotable water, in areas where this is not a code requirement, a homemade side-arm can be assembled for about $10; it's basically a pipe inside a bigger pipe. Make it as long as is practical for better heat transfer--lengths of 2-6 ft are most common. However, if the section of pipe being replaced measures 10 ft, then that's how long the side-arm should be. For 3/4-inch boiler supply pipes I use two 1 inch x 1/2 inch x 3/4 inch copper tee fittings, one for each end of the assembly. File out the pipe stops for the 3/4-inch thru part of the fittings and push the 3/4-inch pipe from the 1-inch end through until a couple of inches extends past one of the tees. Install the 1-inch diameter outer pipe over this. Slide the second tee on the other end and solder it all up. I also like to tightly wrap a piece of #10 AWG bare copper wire around the 3/4-inch inner pipe to increase the turbulence and thus the heat transfer. You don't need to solder the wire in place. I've found that friction with the pipe does the job very well.

Be sure to set the water heater's thermostat below that of the aquastat, at about 110°F. This way, the boiler will do most of the water heating in the temperature range between 110°F and 150°F.

--Michael Lamb
Michael Lamb is a Certified Energy Manager on the staff of DOE's Energy Efficiency and Renewable Energy Clearinghouse.
Figure 1. Shown above are two alternate means of adding storage to a boiler, depending on whether or not the existing boiler has a tankless coil.



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