A.O. Smith AWH-35, Instruction Manual, Page: 20 / 48

20
Negative Pressure
Ducting supply air only to an alternate location, such as the
outdoors, may cause excessive negative air pressure inside the
building envelope.
Excessive negative pressure inside the building structure may
result in cold or hot air from outdoors being drawn inside the
building and place additional load on space heating and cooling
equipment. Negative air pressure in buildings can also cause
reverse flow in chimneys and gas vents.
Positive Pressure
Ducting return air only from an alternate location, such as the
outdoors, may cause excessive positive air pressure inside the
building envelope.
Excessive positive pressure inside the building structure may
place additional load on space heating and cooling equipment by
interfering with the delivery of conditioned air.
WHEN TO INSTALL DUCTING
HPWH units are often installed in unoccupied spaces or
equipment rooms where there is an ample source of ambient
heat and no need to redirect the supply air to another location.
Ductwork is not necessary in these circumstances. See Heat
Source and Conditioned Space on page 10 and Unconditioned
Space on page 11.
Typical applications when ducting is installed.
• Duct supply (outlet) air to alternate location for spot cooling
or discard supply air not wanted in the installed space.
• Duct return (inlet) air from an alternate location (outdoors,
warm equipment room) to optimize efficiency.
SUPPLY AIR DUCTING
Observe and follow these guidelines and instructions when
installing supply (outlet) air duct:
Sharp Spinning Blades!
The blower operates at a high RPM that can cause injury. To
prevent inadvertent access to the blower opening, supply
ducting or other suitable means to prevent access must
be provided.
DANGER
1. When duct work is not field connected to the supply air
outlet on the HPWH unit a field supplied safety guard
must be installed over the supply air connection. Failure
to comply could result in severe injury.
2. All ductwork and plenums shall be field supplied or fabricated.
3. The total equivalent length of all supply and return air ducting
must not exceed the maximum equivalent lengths shown in
Table 6 or Table 7 on page 21.
4. Cooling output (supply air) from the HPWH is supplemental
only and must not be factored into sizing calculations for
space cooling equipment. Cooling output is only produced
when the HPWH is operating to satisfy a water heating
demand. Once the storage tank temperature is satisfied,
the HPWH will stop until the next water heating demand is
initiated regardless of space cooling needs.
5. Depending on the temperature of the air entering the HPWH
unit the supply air may not be suitable for supplemental
space cooling purposes.
The air temperature drops approximately 12°F to 20°F (7°C
to 11°C) as it flows through the HPWH unit. If the return air
to the HPWH unit is derived from a location that is above
80°F (27°C) the supply air from the HPWH unit will not be
suitable for offsetting the building cooling load. This will
often be the case when the return air is ducted from a warm
equipment room or from the outdoor atmosphere. In these
circumstances do not duct supply air to another location for
spot cooling.
6. Ducting supply air only to an alternate location, such as the
outdoors, may cause excessive negative air pressure inside
the building envelope.
Provision must be made to prevent a negative pressure in
the installed space or building envelope. Return air must be
supplied to the HPWH from the alternate location through
ducting or louvers that communicate with the alternate
location where the supply ducting terminates. See the Return
Air Ducting section that follows.
RETURN AIR DUCTING
Observe and follow these guidelines and instructions when
installing return (inlet) air duct:
1. All ductwork and plenums shall be field supplied or fabricated.
2. The total equivalent length of all supply and return air ducting
must not exceed the maximum equivalent lengths shown in
Table 6 or Table 7 on page 21.
3. When installing return air duct to the HPWH unit a field
supplied/fabricated filter access door must be installed.
4. Ducting return air only from an alternate location, such as the
outdoors, may cause excessive positive air pressure inside
the building envelope.
Provision must be made to prevent a positive pressure in
the installed space or building envelope. Supply air must be
supplied to the HPWH from the alternate location through
ducting that communicates with the alternate location from
where the return air is derived. See the preceding Supply Air
Ducting section.
5. If the alternate location where return air is derived from
has temperatures above 95°F (35°C) for extended periods
the accessory High Ambient Air Kit should be installed for
optimum efficiency. See page 39 .
BLOWER ASSEMBLY ADJUSTMENTS
Sharp Spinning Blades!
The blower operates at a high RPM that can cause injury. To
prevent inadvertent access to the blower opening, supply
ducting or other suitable means to prevent access must
be provided.
DANGER
The HPWH blower is a belt driven assembly. The blower belt
and sheaves are factory pre-set and should not require any field
adjustment. The blower assembly should be checked to ensure
the blower wheel moves freely and that sufficient tension is on
the blower belt. If belt looseness is suspected, check the blower
maintenance section of the manual for re-set recommendations.