Ruud RP13, Installation Instructions Manual

31

The air distribution system has the greatest effect 

on airflow. The duct system is totally controlled 

by the contractor. For this reason, the contractor 

should use only industry-recognized procedures.
The correct air quantity is critical to air 

conditioning systems. Proper operation, efficiency, 

compressor life, and humidity control depend 

on the correct balance between indoor load and 

outdoor unit capacity. Excessive indoor airflow 

increases the possibility of high humidity problems. 

Low indoor airflow reduces total capacity and can 

cause coil icing. Serious harm can be done to the 

compressor by low airflow, such as that caused by 

refrigerant flooding.
Heat pump systems require a specified airflow. 

Each ton of cooling requires between 320 and 

450 cubic feet of air per minute (CFM). See the 

manufacturer’s spec sheet for rated airflow for the 

system being installed.
Duct design and construction should be carefully 

done. System performance can be lowered 

dramatically through bad planning or workmanship.
Air supply diffusers must be selected and located 

carefully. They must be sized and positioned 

to deliver treated air along the perimeter of the 

space. If they are too small for their intended 

airflow, they become noisy. If they are not located 

properly, they cause drafts. Return air grilles must 

be properly sized to carry air back to the blower. If 

they are too small, they also cause noise.
The installers should balance the air distribution 

system to ensure proper quiet airflow to all rooms in 

the home. This ensures a comfortable living space.

These simple mathematical formulas can be used 

to determine the CFM in a residential or light 

commercial system.
Electric resistance heaters can use:
 

CFM = volts x amps x 3.413

  SHC x temp rise

Gas furnaces can use:
 

CFM =    Output Capacity in BTUH*

         SHC x temp rise

*Refer to furnace data plate for furnace output capacity.
SHC = Sensible Heat Constant (see table below)

An air velocity meter or airflow hood can give a 

more accurate reading of the system CFM.
The measurement for temperature rise should be 

performed at the indoor coil inlet and near the 

outlet, but out of direct line of sight of the heater 

element or heat exchanger. For best results, 

measure air temperature at multiple points and 

average the measurements to obtain coil inlet and 

outlet temperatures. 

START-UP

Start-Up

Start-Up

At initial start-up or after extended shutdown 
periods, make sure the crankcase heater is 
energized for at least 12 hours before the 
compressor is started. (Disconnect switch is 
on and wall thermostat is off.) 

Even though the unit is factory-charged with 

Refrigerant-410A, the charge must be checked 

to the charge table attached to the service panel 

and adjusted, if required. Allow a minimum of 

15 minutes of run time before analyzing charge.

CAUTION:

Single-pole contactors 

are used on all standard single-phase units 

through 5 tons. Caution must be exercised when 

servicing as only one leg of the power supply is 

broken by the contactor.

Checking Airfl ow

Airfl

 ow

ALTITUDE 

(FEET)

SENSIBLE HEAT 

CONSTANT 

(SHC)

ALTITUDE 

(FEET)

SENSIBLE HEAT 

CONSTANT 

(SHC)

Sea Level

1.08

6000

0.87

500

1.07

7000

0.84

1000

1.05

8000

0.81

2000

1.01

9000

0.78

3000

0.97

10000

0.75

4000

0.94

15000

0.61

5000

0.90

20000

0.50