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Heat Exchanger Systems Heat exchangers keep the potentially stagnant heating water completely separate from the domestic water system. In a heat-exchanged system, the space heating side is set up almost exactly as it would be if you were using a conventional boiler as the heat source (Figure 4).

Dual-Purpose System With Heat Exchanger

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Figure 4. The author's favorite system uses an external heat exchanger to isolate the heating loop from the home's drinking water. The tank temperature is set high enough to meet the heating demand, while the domestic water is tempered for safety.

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Figure 5. This flat plate heat exchanger (left) is adequate for heating a 4,000-square-foot house in the author's climate (75,000 Btu/hr.). Where code officials insist, the author uses a Combi-Cor unit (right), which has an internal coil exchanger. The coil's double wall offers secondary protection against contamination of the drinking water by the heating fluid. Given the choice, I would always opt for a generously sized flat-plate stainless-steel heat exchanger mounted on the outside of the water heater. The flat-plate exchangers transfer heat more efficiently than the double-wall coil types, so they allow for lower water temperatures on the domestic side. Efficiency aside, I prefer external-mount exchangers for practical reasons. Since the exchanger is likely to outlive the water heater, or vice versa, it makes sense to me to keep them separate. It's like having separate stereo components — if one thing breaks, that's all you have to fix. I always install all the components with isolation valves and unions for easy servicing.