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).
| 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.
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.