CAP AHU User manual

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 1
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
INSTALLATION AND MAINTENANCE INSTRUCTIONS FOR AHU
HORIZONTAL, MODULAR, VERTICAL AND ROOFTOP UNITS
SIZES FROM 600 CFM TO 9,000 CFM
SIZES FROM 600 TO 4,000 CFM SIZES FROM 600 CFM TO 9,000 CFM

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 2
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
TABLE OF CONTENTS
Introduction 3
Unit Inspection 3
Unit Storage 3
CAP Unit Description 4
Safety 6
Locating the Unit 6
Lifting and Handling the Unit 6
Electrical 7
Cutting Electrical Openings 9
Start up List for duct Heaters 10
Minimum Air Flow Required Across the Heaters 11
Electrical Duct Heater Maintenance guide 12
Duct connections 13
Condensate drain Piping 14
Heating Coils, Piping 14
Chilled Water Coils, Piping 15
DX Coils, Piping 16
Prior to Start-Up 17
Operation and Maintenance 21
Limited Warranty 27
Installation Checklist 28
Maintenance Log 29

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 3
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
INTRODUCTION
READ THE ENTIRE INSTALLATION, OPERATION AND MAINTENANCE MANUAL. OTHER IMPORTANT
SAFETY PRECAUTIONS ARE PROVIDED THROUGHOUT THIS MANUAL.
The following information is to be used by the installer as a guide. Since each installation is unique, only
general topics are covered. To order in which topics are presented may not be those required by the
actual installation.
This guide does not supersede or circumvent any applicable national, state or local code.
The installer must read the entire contents of this guide and develop a thorough understanding before
beginning installation.
Note: Due to continued product research and development, Commercial Aire Products reserves the right
to discontinue or change without notice, any or all specifications or designs without incurring
obligations.
INSPECTION
Receiving Unit
When received, the unit should be checked for damage that might have occurred in transit. If damage is
found it should be noted on the carrier’s Freight Bill. A request for inspection by carrier’s agent should
be made in writing at once. All our sales are FOB our warehouse in Fort Worth Texas and any in transit
damage is carrier responsibility.
Nameplate should be checked to ensure the correct model sizes and voltages have been received to
match the job requirements.
If repairs must be made to damaged goods, then the factory should be notified before any repair action
is taken in order to protect the warranty. Certain equipment alteration, repair, and manipulation of
equipment without the manufacturer’s consent may void the product warranty. Contact the MORTEX
MANUFACTURING Warranty Department for assistance with handling damaged goods, repairs, and
freight claims: [phone] ext 225.
Note: Upon receipt check shipment for items that ship loose such as filters and remote sensors. Consult
order and shipment documentation to identify potential loose-shipped items. Loose-shipped items may
have been placed inside unit cabinet for security. Installers and owners should secure all doors with
locks or nuts and bolts to prevent unauthorized access
Thoroughly inspect all packages upon receipt of product. Ensure pallet(s) have not been dropped,
crushed or punctured. Inspect all contents for damage. If damage is found, immediately file a claim with
the delivering freight carrier
Storage
This equipment is not suitable for outdoor storage. If installation will not occur immediately following
delivery, store equipment in a dry protected area away from construction traffic and in the proper
orientation as marked on the packaging with all internal packaging in place. Secure all loose-shipped
items.

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COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]

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COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 6
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
SAFETY
The installation and/or servicing of comfort conditioning equipment can be hazardous due to system
pressures and electrical devices.
ONLY TRAINED/QUALIFIED PERSONNEL SHOULD PERFORM SERVICE AND/OR INSTALLATION
OBSERVE ALL PRECAUTIONS AND WARNINGS IN PRODUCT DATA OR INSTRUCTIONS ON UNIT.
FOLLOW ALL SAFETY CODES.
Wear eye protection and gloves. Have a fire extinguisher readily available.
DISCONNECT ALL POWER SUPPLIES BEFORE REMOVING ANY PANELS.
DISCONNECTING MORE THAN ONE POWER SUPPLY MAY BE REQUIRED FOR SOME EQUIPMENT.
Locating the Unit
Placement of the unit relative to ductwork, electrical and plumbing must be carefully considered. Return
air plenum or duct can be mounted directly to the return air flanges. Use flexible gasket material to seal
the duct to the unit.
Verify floor, foundation or suspension support can support the total unit weight, including accessory
weights. Unit must be level in both horizontal axes to support the unit and reduce noise and vibration
from the unit.
Allow adequate service clearances as shown on the unit nameplate and unit drawing. Consult your local
building codes for additional service clearance requirements.
Allow adequate space for piping access and panel removal. Condenser water piping and condensate
drain connections are located on either side of the unit.
Suspended Units
Horizontal air handling units are equipped for suspended installations. The unit should be lifted into
position by supporting the unit with the skid used for shipping. The air handling unit must be installed
level and care should be taken to prevent damage to the cabinet. Other installation provisions may be
necessary according to job specifications.
LIFTING AND HANDLIND THE UNIT
Horizontal, Vertical and Modular Units have channels underneath the base which provide lifting access
to the underside of the unit and allow moving without physical damage.
Before lifting the unit, be sure that all the shipping materials has been removed.
Incorrect lifting can cause damage to the unit, injury or death. Lifting equipment capacity should exceed
unit weight by an adequate safety factor. Always test lift unit not more than 24 inches to verify the
proper center of gravity lift point.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 7
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
Floor Mounted Units
Make sure the unit is level and mounted on a field supplied platform with a minimum height to allow for
proper depth of the condensate line p-trap. Other installation provisions may be necessary according to
job specifications. Vertical air handling units are designed for upflow applications only.
ELECTRICAL
Verify the unit nameplate agrees with the power supply. The wiring diagram is located inside the unit.
Route the power and control wiring separately, through the utility entry in the unit. DO NOT RUN
POWER AND CONTROL WIRES IN THE SAME CONDUIT.
ELECTRIC SHOCK HAZARD. Before attempting to perform any installation, service, or maintenance, shut
of all electrical power to the unit at the disconnect switches. Unit may have multiple power supplies.
Failure to disconnect power could result in dangerous operation, serious injury, and death or property
damage.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 8
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 9
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]

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COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 11
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 12
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
All units require field supply electrical over-current and short circuit protection. Device must be sized
larger than Maximum over-current Protection (MOP).
Codes may require a disconnect switch be within sight of the unit. It is recommended that over-current
protection and disconnect switch not be installed in the unit.
Locations for field cut electrical entries are marked on the unit. Field openings must be at least 6” away
from any component to prevent damage during the cutting procedure.
To pass wires through the wall or roof of the unit, a hole should be cut and a conduit should be passed
through it using the following procedure.
CUTTING ELECTRICAL OPENINGS
1- Locate the placement of the hole. Be sure that the hole will not interfere with any component
operation or prevent access to any part of the unit.
2- Drill a pilot hole all the way through the panel, insulation and interior panel.
3- Using a hole saw cut the hole through the outside metal, insulation and inside metal
4- Install the conduit through the panel and caulk the entire perimeter of the conduit in both sides
with and industrial grade caulking or duct seal compound.
Size supply conductors according to the Minimum Circuit Ampacity (MCA) rating. Supply conductors
must be rated a minimum of 75ºC. Protect each branch circuit according with the codes and
requirements. All the units must be electrically grounded in accordance with local codes or, in absence
of local codes, as per the current National Electric Code.
Supply voltage must be within the minimum/maximum range shown on the unit nameplate, available
short circuit current should not exceed the short circuit current rating (SCCR) shown in the unit
nameplate.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 13
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
Installing contractor must check for proper motor rotation and verify the bower motor amps are not
exceeding the name plate.
Control wiring must be wired in separate conduit from the power to avoid inductance problems and
must follow the local codes for low voltage wiring. The following table is a guide for control wiring size
and allowable distance:
20 AWG 200 FT
18 AWG 350 FT
16 AWG 500 FT
14 AWG 750 FT
12 AWG 1250 FT
Duct Heaters Installation
Our duct heaters may be used with heat pumps, cooling units or force air systems. They are suitable for
zero clearance installations in vertical or horizontal duct systems.
Duct Heaters Installation Guidelines:
1- Install it at least 4 Ft. Downstream from air source or elbow. If closer than 4 Ft. may require
turning vanes, baffles or other devices to assure an even distribution of air over the heater.
2- Install at least 2 Ft. before an elbow or may require devices as #1
3- Transitions to and from a duct heater should be limited to 20% of the duct area per linear foot.
4- Always mount in the side of a vertical or horizontal duct. NEVER MOUNT FROM TOP OR
BOTTOM OF A HORIZONTAL DUCT.
5- All duct materials must be suitable for 250 º F operation.
6- The air duct should be installed in accordance with the standards of the NFPA for installation of
air conditioning and ventilating systems pamphlet No. 90A and 90B.
7- Locate the heater so that it is completely accessible and normal ventilation is assured.
8- The amount of air curtain between the heater elements and limit switches must not be reduced
by internal duct liner within 1 ft before or after the heater. In fiber glass duct systems, a metal
sleeve inside the duct to support the heater must be used. If the base of the heater is not flush
with the air stream, nuisance cycling may result.
9- Field connected wires entering the heater controls compartment must be copper suitable for 75
º C ( 167 º F ) Field wire the supply and control circuits in accord with the National and Local
Codes and use the wiring diagram supply with each heater as a guide. The heater must not
operate unless the fan is on. Never use a fan delay with these heaters.
START UP LIST FOR DUCT HEATERS
1.) Electric duct heater must be installed according to manufacturer’s installation instruction manual and
must be in compliance with all NEC, local and state codes.
Failure to observe all of the installation guide lines will void the warranty and listing of the product.
2.) Review heater data information label found on outside of heater panel cover.
3.) Voltage, phase and frequency of heater must match heater data label. If field voltage and/or phase
does not match heater data label, do not operate heater.
4.) The proper field wire size to heater must be used, copper conductors only. See wire size data label
adjacent to heater terminal block or disconnect switch in heater. Heater must be properly ground to
accommodate NEC guide lines. Failure to do so may result in shock or death.
5.) Ensure all electrical connections are tight before energizing heater circuit. #10-14

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 14
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
AWG should be 35 inch/pound and 8 AWG should be 40 inch / pound wiring in heater.
6.) Review the proper size disconnect means and / or fusing has been applied to heater power supply
circuit Only qualified individuals experienced in proper installation of heating and cooling should
perform any start-up operations of HVAC systems.
Turn disconnect switch (s) to the on position. Energize heater circuit with thermostat to 100% rating of
heater KW rating. First start-up of heating element will burn off any element manufacturing oil that may
cause some smoke in the air system.
7.) Using proper measuring devices to measure voltage to heater, line voltage should be plus or minus
5% of heater rated voltage. Never operate 208V rated heater on 240V line voltage. Measure line 1 to
line 2 on single phase heaters and line 1 to neutral on 120V and 277V heaters.
On three phase heaters measure line 1 to line 3, line 2 to line 3 and line 1 to line 2 for proper line
voltage. Measure amp draw on supply conductors to heater disconnect switch or terminal block, for
proper amp draw with proper instruments. Amp draw must match heater data label with 100% heater is
energized; reading should be plus or minus 3%.
8.) Air flow over heating elements must be even across the face of the heater. Duct system must have
.08 minimum static pressure duct system 2
9.) Automatic reset will de-energize heater if heater is not installed to manufacturers duct installation
manual or low air flow/uneven air across face of element.
Correction must be made to operate heater. Manual reset will also de-energized heater if there is low or
uneven air flow over the heating elements. Air flow volume and / or reinstallation of heater must be
done before energizing heater.
10.) If heater has a control transformer it may have a breaker in the circuit. If transformer is shorted,
breaker will open the circuit. You must reset and find the cause of the short. If transformer has no
breaker, it may have an inline fuse. A short circuit will open the transformer. Transformer must be
replaced with an identical transformer.
11.) Air flow switch only proves that air flow exists, not that the minimum proper air flow does exist;
standard air pressure switch set-up is for positive air pressure system.
Heaters in negative air system must have air tubing to air pressure switch relocated to the open port of
the switch.
12.) After heater installation and operation has meet standards and heater data label, closed heater
panel door and energized heater circuit.
Minimum Air Flow Requirements Across the heaters
Each KW produces 3413 BTUH. Divide the total BTUH needed by 3413 to find the KW needed. Use the
chart below to find the minimum air flow required. You will need to know the maximum inlet
temperature of the heater. The outlet air from a Heat pump (sometimes 110 º F) would be the inlet air
to an auxiliary heater. You must also know the Kw per Sq. Ft. of the Minimum Duct Area (Kw/Sq. Ft.
MDA) for the heater. An example is worked out below:
EXAMPLE: Using 12 KW Heater designed for minimum duct size 8” x 16” and being the only heat in
an area. You expect inlet air to the heater will be 77 º F Maximum.
8” x 16” divided by the .888 Sq. Ft. = 13.5 which is the Kw/Sq. Ft. MDA
Locate the 13.5 along the top side of the chart and from there draw a vertical line
downward until the 77 º F inlet air line is intersected. From point of intersection draw a
horizontal line to the left side of the chart to the answer of 800 FPM (Velocity) of air
minimum required over the heater.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 15
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
To convert to CFM multiply the answer by the Sq. Ft. of the heater minimum duct area
design. In the example 800 multiply by .888 would be 710 CFM.
These minimum air flow requirements should be met at any point over the face of the heater. If heaters
are used in ducts larger than the heater minimum duct design, reliable means should be used to assure
the proper air flow through the heater.
ELECTRIC DUCT HEATER MAINTENANCE GUIDE LINES
In most cases electric duct heaters require very little maintenance or service during the life of the
product. The duct heater must be installed according to our installation instruction manual and NEC /
SMACNA guide lines.
1) Disconnect ALL power circuit(s) to heater before any service is to be performed, there may be more
than one disconnect switch that needs to be in the off position.
2) All electrical connections to the heater power & control circuits should be tight. Inspect all electrical
connections in heater, as they may become loose during operation. All wiring must be done in
accordance with the National Electrical Codes and any applicable state & local codes.
3) All air filters in duct or unit system should be checked to ensure clean air is present with no restriction
of air flow to heating elements. Always replace the filter with the same type as originally furnished.
4) Check the field voltage and phase rating is the same as the heater information data. Check full line
amp draw of heater against rated heater data plate/label. Correct field wire size and grade must be used
to heater power circuit(s). Heater must have proper grounded wire to heater.
5) Heater should be free and clean of dust /dirt and moisture in heater control panel.
6) Heater construction or wiring must not be modified or any alterations are preformed to change the
heater construction. This would void the listing and warranty of the heater.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 16
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
7) Replacing unauthorized components must be approved from the factory first, if not of the same part
number and style. Safety automatic and manual resets MUST be of the same temperature, part number
and vendor for proper heater operation. When higher rated temperature automatic/ manual resets are
replaced in the heater circuit, this will void the listing and warranty and could cause damage or result in
a fire.
8) Heater panel cover should not be removed or open during heater operation. Optimal operation of
heater requires cleanliness.
9) Check operation of heater circuits with thermostat/ controller to ensure proper cycling of heater
relays(s) and to the controller or thermostat set point.
10) If dust or other material is collecting on the heating elements, check filter(s) for replacement. The
heater may have to be removed from duct section for cleaning. Use low pressure air to remove dust
from heating element surfaces. Always wear eye protection and nose protection during this operation.
11) Do not try to repair any of the heater components, as they need to be replaced with original part
number from vendor for safe operation of the heater. Do not file down the relay(s) contacts to stop
relay chatter or to extend the life of the relay. Element male /female chassis insulators that are cracked
or broken should be replaced with the same style and size.
Insulators in the element support frame that are missing or broken must be replaced or replacement
frame assembly should be ordered. Broken heating elements must replace with the same element
gauge, ohms and outside diameter (O.D.) size.
12) Inspect all internal wiring in heater control panel for burned or broken wires. Replacement wiring
must be rated for 105 C grades and of the same gauge for proper operation of heater.
13) Preventative maintenance to achieve maximum performance and service life of heater, a formal
schedule of regular maintenance should be established and maintained.
DUCT CONNECTIONS
Check existing duct insulation and vapor barriers. Previously installed heating supply ductwork may
already have adequate insulation against excessive heat loss. This insulation may be satisfactory for
protection against heat gain from summer cooling. Depending on application, it may require additional
insulation.
External insulated duct systems must have adequate vapor seal for summer operation, particularly
where duct is exposed to high humidity conditions such as in attic, vented crawl space, unconditioned
basement or utility room.
Remove any shipping material from the unit before installing the duct. Be sure that there are no
material inside the unit that can damage the blower wheel when start running. Attach the duct to the
flanges provided on the unit. The installer is responsible for sealing the ducts to the flanges to prevent
any leak or contamination.
Ductwork should be sized according with the existing standards and installed following all the local and
national codes. When attaching the duct to the unit, use a flexible connection to avoid vibrations on the
duct. A three inches flexible connection is recommended.
CONDENSATE DRAIN PIPING
Our units are equipped with more than one drain connection. A p-trap and drain line must be installed
one the drain connections of the unit with a p-trap not to exceed 6” from the drain connection. The lines
should be the same size or larger than the drain connection, include a p-trap, and pitch downward
toward drain. An air break should be used with long runs of condensate lines. Units include an overflow
drain pan to catch any overflowing condensation from the primary. This pan requires a separate ¾” or
1” condensate drains installation. Draw-through units will have a negative static pressure in the drain
pan area; this will cause an un-trapped drain to back-up due to air being pulled up through the

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 17
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
condensate drain piping. Condensate drain trapping piping must conform to all applicable governing
codes
HEATING COILS
One or two row hot water or steam preheating, heating or reheating coils can be factory installing in our
units, all valves and controls are field supplied and field installed. All precautions to prevent freezing
should be taken by consulting engineers or contractors during the installation.
CHILLED WATER COILS
Water piping
All piping must be supported independent of coils to prevent vibration and stress on coil headers. Swing
joints or flexible fittings must be provided to absorb expansion and contraction strains. Rigid piping
reduces the effectiveness of vibration isolators.
Water piping should always be connected so that the entering water is on the leaving air side of the coil.
Coils must be adequately vented in order to prevent air binding. Units are provided with manual air
vents mounted through manifold panel.
Our units can have from two to 8 rows chilled water coils or any combination with pre heat and or
reheat coils up to 10 rows factory mounted. All control valves flow measuring devices and freeze
controls are field supply and field installed. All piping shall be in accordance with national and local
codes. Pressure limiting devices, backflow preventers, flow meters and all safety requirements are the
sole responsibility of consulting engineers and installing contractors.
REFRIGERANT PIPING
Always use the condensing unit manufacturer’s recommended line sizes. The suction line must be
insulated for satisfactory operation. Observe all condensing unit manufacturer’s installation
requirements. Use refrigerant grade copper only. If unit is to be used when installed as the indoor coil of
a heat pump, a bypass check valve must be used unless unit is equipped with a TXV which includes
bypass and check valve operations.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 18
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
NOTE:
An oil trap must be installed in all vertical suction risers. An additional oil trap must be installed every
15’ for risers over 15’ high. If the condensing unit is located above the evaporator, an inverted oil trap
must be installed at the top of the vertical riser.
Piping from the condensing unit to the air handling unit is responsibility of the installing contractor. The
piping sizes must be selected to meet the codes and the actual installation conditions and not based on
connection sizes of the evaporator and/or condensing unit. Improper installation, adjustment,
alteration, service or maintenance, can cause property damage, personal injury or loss of life.
Installation and service must be performed by a trained and qualified technician.
Only clean ACR tubing should be used. Piping should conform to all the generally accepted practices and
codes. The AHU coils are shipped pressurized with nitrogen the copper caps must be punctured to allow
the gradual relief of the pressure prior to un-sweating the caps. Immediately couple the tubing to the
indoor unit to avoid exposing the coils to moisture. When making solder connections, make sure to have
dry nitrogen flowing through the lines to prevent copper oxidation inside the pipes.
Minimizing the refrigerant line size is favorable from an economic perspective, reducing installation
costs, and reducing the potential for leakage. However, as pipe diameters narrow, pressure-reducing
frictional forces increase.
Excessive suction line pressure drop causes loss of compressor capacity and increased power usage
resulting in reduced system efficiency. Excessive pressure drops in the liquid line can cause the liquid
refrigerant to flash, resulting in faulty TXV operation and improper system performance. In order to
operate efficiently and cost effectively, while avoiding malfunction, refrigeration systems must be
designed to minimize both cost and pressure loss.
Equivalent Line Length
All line lengths discussed in this manual, unless specifically stated otherwise, are Equivalent Line
Lengths. The frictional pressure drop through valves, fittings, and accessories is determined by
establishing the equivalent length of straight pipe of the same diameter. Always use equivalent line
lengths when calculating pressure drop. Special piping provisions must be taken when lines are run
underground, up vertical risers, or in excessively long line runs.

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 19
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
Liquid Line Sizing
When sizing the liquid line, it is important to minimize the refrigerant charge to reduce installation costs
and improve system reliability. This can be achieved by minimizing the liquid line diameter. However,
reducing the pipe diameter will increase the velocity of the liquid refrigerant which increases the
frictional pressure drop in the liquid line, and causes other undesirable effects such as noise.
Maintaining the pressure in the liquid line is critical to ensuring sufficient saturation temperature,
avoiding flashing upstream of the TXV, and maintaining system efficiency. Pressure losses through the
liquid line due to frictional contact, installed accessories, and vertical risers are inevitable. Maintaining
adequate sub-cooling at the condenser to overcome these losses is the only method to ensure that
liquid refrigerant reaches the TXV.
Liquid refrigerant traveling upwards in a riser loses head pressure. If the evaporator is below the
condenser, and the liquid line does not include risers, the gravitational force will increase the pressure
of the liquid refrigerant. This will allow the refrigerant to withstand greater frictional losses without the
occurrence of flashing prior to the TXV.
A moisture-indicating sight glass may be field installed in the liquid line to indicate the occurrence of
premature flashing or moisture in the line. The sight glass should not be used to determine if the system
is properly charged. Use temperature and pressure measurements to determine liquid sub-cooling,
not the sight glass.
Liquid Line Routing
Care should be taken with vertical risers. When the system is shut down, gravity will pull liquid down the
vertical column, and back to the condenser when it is below the evaporator. This could potentially result
in compressor flooding. A check valve can be installed in the liquid line where the liquid column rises
above the condenser to prevent this. The liquid line is typically pitched along with the suction line, or
hot gas line, to minimize the complexity of the configuration.
Liquid Line Insulation
When the liquid line is routed through regions where temperature losses are expected, no insulation is
required, as this may provide additional sub-cooling to the refrigerant. When routing the liquid line
through high temperature areas, insulation of the line is appropriate to avoid loss of sub-cooling through
heat gain.
Liquid Line Guidelines
In order to ensure liquid at the TXV, frictional losses must not exceed available sub-cooling. A commonly
used guideline to consider is a system design with pressure losses due to friction through the line not to
exceed a corresponding 1-2°F change in saturation temperature.
If the velocity of refrigerant in the liquid line is too great, it could cause excessive noise or piping
erosion. The recommended maximum velocities for liquid lines are 100 fpm from the condenser to a
receiver tank to discourage fluid backup, and 300 fpm from receiver tank to the evaporator to minimize
valve induced liquid hammer.
Liquid Line Accessories

I/O MANUAL Horizontal/Modular/Vertical/Rooftop AHU 2015pg. 20
COMMERCIAL AIRE PRODUCTS 501 Terminal Road, Fort Worth TX 76106 Tel: [phone]
Liquid line shut off valves and filter driers are factory provided. Filter driers must be field installed on 2-6
ton units. The total length equivalent of pressure losses through valves, elbows and fittings must be
considered when adding additional components in the field. It is a good practice to utilize the fewest
elbows that will allow the mating units to be successfully joined.
Suction Line Sizing
The suction line is more critical than the liquid line from a design and construction standpoint. More
care must be taken to ensure that adequate velocity is achieved to return oil to the compressor at
minimum loading conditions. However, reducing the piping diameter to increase the velocity at minimal
load can result in excessive pressure losses, capacity reduction, and noise at full load.
Suction Line Routing
Pitch the suction line in the direction of flow (about 1 foot per 120 feet of length) to maintain oil flow
towards the compressor, and keep it from flooding back into the evaporator. Crankcase heaters are
provided to keep any condensed refrigerant that collects in the compressor from causing damage or
wear. Make sure to provide support to maintain suction line positioning, and insulate completely
between the evaporator and condensing unit.
It is important to consider part load operation when sizing suction lines. At minimum capacity,
refrigerant velocity may not be adequate to return oil up the vertical riser. Decreasing the diameter of
the vertical riser will increase the velocity, but also the frictional loss.
A double suction riser can be applied to the situation of part load operation with a suction riser. A
double suction riser is designed to return oil at minimum load while not incurring excessive frictional
losses at full load. A double suction riser consists of a small diameter riser in parallel with a larger
diameter riser, and a trap at the base of the large riser. At minimum capacity, refrigerant velocity is not
sufficient to carry oil up both risers, and it collects in the trap, effectively closing off the larger diameter
riser, and diverting refrigerant up the small riser where velocity of the refrigerant is sufficient to
maintain oil flow.
At full load, the mass flow clears the trap of oil, and refrigerant is carried through both risers. The
smaller diameter pipe should be sized to return oil at minimum load, while the larger diameter pipe
should be sized so that flow through both pipes provides acceptable pressure drop at full load.
Suction Line Insulation
The entire suction line should be insulated. This prevents condensation from forming on the line, and
reduces any potential loss in capacity associated with heat gain.
Suction Line Guidelines
For proper performance, suction line velocities less than a 4,000 fpm maximum are recommended. The
minimum velocity required to return oil is dependent on the pipe diameter, however, a general
guideline of 1,000 fpm minimum may be applied.
In a fashion similar to the liquid line, a common guideline to consider is a system design with pressure
losses due to friction through the line not to exceed a corresponding 1-2°F change in saturation
temperature.
At points where small pipe size can be used to provide sufficient velocity to return oil in vertical risers at
part loads, greater pressure losses are incurred at full loads. This can be compensated for by over sizing
the horizontal runs and vertical drop sections. This will however require additional refrigerant charge.
Circuits with variable capacity scroll compressors require suction riser traps every 15 feet.
Suction Line Accessories