Bryant 50YEW Design Manual page 27

When generating DHW with a heat pump or boiler, potable water
must never come in contact with heating water. Therefore, an
indirect water heater or secondary heat exchanger is required. As
shown in fi gure 2-7, an indirect water heater has a coil inside the
tank to isolate the two liquids (potable water and heating water).
Figure 2-8 shows a brazed plate heat exchanger that can be used
in between the heat pump and direct water heater (electric, oil,
natural gas, propane). Only one pump is needed for an indirect
water heater (the water-to-water unit's load pump circulates water
between the heat pump heat exchanger and the water heater
heat exchanger), but two pumps are required when a secondary
or brazed plate heat exchanger is used (one pump between the
water-to-water unit and the brazed plate and one pump between
the brazed plate and the water heater).
Figure 2-7: Indirect Water Heater
Potable Water
H
Load Connections
to Boiler/Heat Pump
Figure 2-8: Brazed Plate Heat Exchanger
IN
TOP VIEW
OUT
SIDE VIEW
Bryant Geothermal Heat Pump Systems
Part II: Load Side Design / Distribution Design
Some indirect water heaters have electric elements for use as
backup. The 50YEW series equipment has an emergency DHW
function that will send a 24VAC signal to a fi eld-installed contactor
to energize the backup electric elements if the unit is locked out.
A direct electric water heater could also be used for backup when
a brazed plate heat exchanger is installed.
IMPORTANT DESIGN NOTE: Most indirect water
heaters are designed for 180°F [82°C] water circulating through
the heat exchanger. At lower water temperatures capacities are
signifi cantly reduced. Make sure that the heat exchange capacity is
adequate at the lower water temperatures used by water-to-water
heat pumps. Some indirect solar water heater manufacturers
publish data at lower water temperatures, and some European
manufacturers of indirect water heaters have signifi cantly more
heat exchange surface (i.e. more coils), which will allow the use
of cooler water. Brazed plate heat exchanger sizing is also critical
for the same reason. Larger heat exchangers will be required for
lower DHW temperatures.
Figure 2-9: Indirect Water Heaters
Indirect
C
(water-to-water)
Heat Exchanger
OUT
IN
Water-to-Water System Design Guide
Indirect water heater with more surface
"Typical" indirect water
area (photo courtesy of TURBOMAX).
heater rated for 180°F
Consult manufacturer's data for operating
(82°C) or hotter water.
at lower water temperatures.
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