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Scald Protection Nine times out of ten, using a water heater for heating means cranking up the water temperature to unsafe levels for bathing or laundry, so anti-scald valves and other tempering devices will be necessary for safety. The tempering valve works by mixing the outgoing hot water with cold line water to lower the overall temperature (Figure 6).

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Figure 6. When water heaters are used for both space heating and domestic water, the domestic side must usually be mixed with cold water for safety's sake. A mechanical tempering valve is a simple way to do this.

Think of it as a secondary thermostat on the domestic water side.

Choosing a Water Heater

For a low-budget system without a heat exchanger, any standard water heater will do as long as the Btu output is enough to cover the heating load. I would always choose one with an extra tap on the side (in addition to the tap on top for the pressure relief valve), because it makes the piping a lot easier. The Bradford White TTW2 meets this criteria.

High-efficiency units.

For a heat-exchanged system, I prefer using a high-end sealed combustion water heater with an external heat exchanger. There are a couple of ultra-high efficiency water heaters on the market that are ideal for hydronic applications. The Polaris (American Water Heater Group, Dallas, Texas; 800/937-1037) and SuperStor's Voyager (Heat Transfer Products, East Freetown, Mass.; 800/323-9651; 508/763-8071 in Mass.) both use stainless-steel tanks with sealed condensing burners.

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SuperStor's Voyager water heater works well in dual-purpose applications, for both drinking water and space heating. Voyager offers a built-in heat exchanger, but the author prefers to use an external unit, shown preassembled with expansion tanks and circulators in the photo (above left). The heat exchanger is fully insulated and is located between the tank and the red and blue heating loop manifold (above right). These units draw outside air for combustion and operate at efficiencies that rival even the highest-efficiency (and highest-priced) boilers. The downside is their initial cost, which, starting at around $1,500, is ten times the cost of a cheap hardware store water heater. Still, these are appliances that will probably last for the life of the mortgage, and the lower operating costs pay back quickly. In a northern climate (6,000 degree days or more), the homeowner might realize a savings of $300 to $400 per year on space heating and another $150 for domestic water heating. (Of course, the final savings will depend on the heating climate and fuel costs in your area.) If I'm installing a heat-exchanged system but the budget is limited, I pipe the space heating side exactly the same way as I would with a high-end water heater, but substitute a conventional "hardware store" unit like the Bradford White TTW2. We install them with shutoffs and unions so the homeowner can easily retrofit a higher-efficiency water heater later. When the local building official demands the extra protection of a double-wall heat exchanger, I go for a Combi-Cor, with its built-in coil exchanger. It's easy to pipe, and only requires one pump instead of the two that are needed with an external exchanger. The Combi-Cor's drawbacks are that it has to operate at very high temperatures to get substantial space heating output, and that the components are not separately replaceable — when the heater tank goes, the exchanger goes with it.