Lennox International Inc 06/11 50672801 Optimizing System Refrigerant Charge, Cfm =, Action

3. Determine if fan needs adjustment

Optimizing System Refrigerant Charge

This section provides instructions on optimizing the system charge. This section includes:

SOptimizing procedure

SAdjusting indoor airflow

SUsing subcooling method

SApproved matched components, targeted subcooling (SC) values and add charge values

SNormal operating pressures

STemperature pressures

OPTIMIZING PROCEDURE

1.Move the low−side manifold gauge hose from the vapor line service valve to the true suction port (see figure 17).

2.Set the thermostat for either cooling or heating demand. Turn on power to the indoor unit and close the outdoor unit disconnect switch to start the unit.

3.Allow unit to run for five minutes to allow pressures to stabilize.

4.Check the airflow as instructed under Adjusting Indoor Airflow to verify or adjust indoor airflow for maximum

efficiency. Make any air flow adjustments before continuing with the optimizing procedure.

5.Use subcooling method to optimize the system charge (see figure 21). Adjust charge as necessary.

ADJUSTING INDOOR AIRFLOW

Heating Mode Indoor Airflow Check

(Only use when indoor unit has electric heat)

Indoor blower airflow (CFM) may be calculated by energizing electric heat and measuring:

STemperature rise between the return air and supply air temperatures at the indoor coil blower unit,

SMeasuring voltage supplied to the unit,

SMeasuring amperage being drawn by the heat unit(s).

Then, apply the measurements taken in the following formula to determine CFM:

Amps x Volts x 3.41

CFM =

1.08 x Temperature rise (F)

Cooling Mode Indoor Airflow Check

Check airflow using the Delta−T (DT) process using figure 20.

ADJUSTING INDOOR AIRFLOW

									DT
Temp.
of air		80	24	24	24	23	23	22	22	22	20	19	18	17	16	15
entering	Dry−bulb	78	23	23	23	22	22	21	21	20	19	18	17	16	15	14
indoor
		76	22	22	22	21	21	20	19	19	18	17	16	15	14	13
coil ºF
		74	21	21	21	20	19	19	18	17	16	16	15	14	13	12
A		72	20	20	19	18	17	17	16	15	15	14	13	12	11	10

7019 19 18 18 17 17 16 15 15 14 13 12 11 10

Wet−bulb ºF 57 58 59 60 61 62 63 64 65 66 67 68 69 70

1. Determine the desired DT

ature using dry bulb (A) and wet bulb (B). DT is the intersect- ing value of A and B in the table (see triangle).

2. Find temperature drop across coil

bulb entering and leaving air temperatures (A and C). Tem- perature Drop Formula: (TDrop) = A minus C.

ference between

		B
C	TDrop
53º	19º
	air flow	air flow	B
DRY			64º
BULB
All temperatures are		INDOOR
		COIL
expressed in ºF

A 72º

DRY

BULB

WET

BULB

the measured TDrop and the desired DT (TDrop–DT) is within +3º, no adjustment is needed. See examples: Assume DT =

15 and A temp. = 72º, these C temperatures would necessi- tate stated actions:

Cº	TDrop	–	DT	=	ºF	ACTION
53º	19	–	15	=	4	Increase the airflow
58º	14	–	15	=	−1	(within +3º range) no change
62º	10	–	15	=	−5	Decrease the airflow

4.Adjust the fan speed crease/decrease fan speed.

Changing air flow affects all temperatures; recheck tempera- tures to confirm that the temperature drop and DT are within +3º.

Figure 20. Checking Airflow over Indoor Coil Using Delta−T Formula

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