Bryant 50YEW Design Manual page 22

Water-to-Water System Design Guide
Part II: Load Side Design / System Design & Selection
Drawing 2-8 – Piping for Backup Boiler (2nd Stage Heating):
Drawing 2-8 may be used for two different types of applications.
A boiler backup may be required because the water-to-water
unit lacks suffi cient capacity at design conditions, or because the
hydronic heating distribution system requires hotter water than the
water-to-water unit can produce.
• Water-to-Water Unit Lacks Capacity: This type of system would
be used when the water-to-water unit has been sized to handle
less than 100% of the heating load. It is common practice to size
geothermal heat pump systems to handle 80-90% of the load
in order to lower equipment and ground loop requirements,
especially when the cooling load is less than the heating
load. In this case, the boiler control should be set at the same
temperature as the buffer tank (or the boiler can be controlled
by outdoor temperature). When the buffer tank begins to drop
in temperature (i.e. the heat pump can no longer maintain tank
temperature), the boiler comes on to make up the difference.
This type of system is excellent for retrofi t installations, where an
existing boiler is in good operating condition.
Drawing 2-8: Piping for Backup Condensing Boiler
(2nd Stage Heating)
Note 1
Radiant
Floor
System
Condensing
20
H
Note 3
Heating
Buffer Tank
OUT
Note 6
Boiler
IN
Bryant: Whatever It Takes.
• Distribution System Requires Hotter Water: This type of system
would be used when baseboard convectors, cast iron radiators or
fan coil units are already installed in a retrofi t application. Since the
50PSW/GSW water-to-water units are only capable of producing
up to 130°F [54°C] leaving water temperature (50YEW water-
to-water units can produce up to 145°F [63°C] leaving water
temperature), and the existing distribution system may require up
to 180°F [82°C] at design conditions, the water-to-water system
should be sized to handle the heating load up to the point where
hotter water is required (i.e. at the outdoor temperature balance
point). Typically, a properly sized water-to-water unit can handle
the load until the outdoor temperature drops to 20 to 30°F [-7
to -1°C]. At that point, the water-to-water unit compressor must
be disengaged (through the use of an outdoor thermostat or
other control means), and the boiler should be started. The water
delivered to the hydronic system now increases in temperature to
help satisfy the increased load.
Notes 1,2
From Water-to-Water Unit
To Water-to-Water Unit
C
Note 4
Thermistor or Aqua-stat
NOTES:
1. Not all components shown (expansion tank, air vent, etc.).
Drawing is for buffer tank connections only.
2. Pump not needed for 50YEW unit with internal load pump option.
3. Mixing valve and appropriate piping required on non-condensing
boilers (consult boiler manufacturer literature).
4. Thermistor or aqua-stat should be installed in an immersion
well. If water heater does not have well, one of the heating
elements should be removed, and a well adapter should be
installed. Locate thermistor/aqua-stat in the bottom half
of the tank.
5. Other components (additional ball valves, unions, etc.)
may be required for ease of service. This drawing
shows only minimum requirements. Your specific
installation will dictate final component selections.
6. Buffer tank must be approved as a heating vessel.
7. Local code supercedes any piping arrangements or
components shown on this drawing.
03Oct07