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CNT-SVX11A-EN

Unit Operation

EWT sampling will be invoked when
the following conditions have been
met:

• EWT is not communicated via a BAS

and:

• Unit is equipped with a WSE

The the following must occur:

• There is a new control request for

cooling and isolation valve 1 is not
open and more than 1-hour has
elapsed since the last EWT sample

When these conditions are met, circuit
1 isolation valve is opened for 3-min-
utes and the EWT reading is taken and
WSE operation feasability is deter-
mined. The isolation valve will remain
open regardless.

Sampling for Electric Heat
Units

For units equipped with electric Heat
and configured for boilerless control,
EWT is used to determine whether DX
heating should be disabled and electric
heat enabled. EWT sampling is invoked
when the folowing conditions are met:

• EWT is not communication via a

BAS and:

• Unit is equipped with electric heat

configured for boilerless control and
is not a cooling only unit

Then the following must occur:

• There is a new control request for

heating and isolation valve 1 is not
open and more than 1-hour has
elapsed since the last EWT sample
or:

• Boilerless electric heat is running

and more than 1-hour has elapsed
since the last EWT sample

When these conditions are met, circuit
1 isolation valve is opened for 3-min-
utes, the EWT reading is taken and
electric heat operation feasability is
determined. If boilerless electric heat is
enabled, the isolation valve will be
closed, halting water flow to the unit.

The following chart provides a quick
reference to unit configurations in
which EWT is enabled:

Water-side Economizer
Operation

The ZN524 supports the use of a 2-posi-
tion Water Side Economizer (WSE). The
WSE is only active in cooling mode and
the EWT setpoint has been met. If the
zone requires cooling and the WSE is
enabled, the WSE valve will open and
begin controlling the zone. If the WSE
capacity can not cool the zone, then 1-
stage of compressor cooling will be
allowed to operate with the WSE. The
second stage of compressor cooling
will not be allowed to run, under any
circumstances, when the WSE is in
operation. Note: If the zone tempera-
ture continues to rise, the WSE enable
setpoint may need to be redueced in
order to increase unit capacity.

When the WSE and the compressor are
running simultaneously, coil frosting
may occur. The ZN524 controls two
devices to prevent this from occuring.

Coil Icing Protection

A fixed temperature device is mounted
to the evaporator coil and wired to a
binary input on the ZN524 controller.
When the binary input on the ZN524

trips ( in alarm situations) the following
actions will occur:

Note: The fan will remain on during
this operation

This low-level diagnostic will automati-
cally rest when the binary input
changes to the normal state. The com-
pressor minimum on and off times (3
minutes) will be enforced during this
mode.

Low Leaving Air Protection

This mode is activated during WSE
operation and controlled by the dis-
charge air sensor. If the discharge air
temperature drops below 45 F for 1-
minute while the unit is economizing,
the following actions will occur:

Note: The fan will remain on during
this operation

This low-level diagnostic will reset
when the discharge air temperature has
risen above 50 F. The compressor mini-
mum on and off times (3 minutes) will
be enforced during this mode.

EWT

Sampling

Enabled

Unit Build Configuration

Heat pump (HP)

Yes

HP with WSE

Yes

HP + electric heat (boilerless)

Yes

HP + electric heat + WSE

HP + electric heat (concurrent)