York Learn About the D7CG Nomenclature and Performance of Air Conditioners

530.18-N8Y

evaporator coil may vary greatly, measuring the pressure drop across a wet coil under field conditions would be inaccurate. To assure a dry coil, the scroll compressors should be de-energized while the test is being run.

4.Knowing the pressure drop across a dry coil, the actual CFM through the unit can be determined from the curve in Figure 15.

0.7

	0.6					072
					060
(IWG)	0.5		036
					048
DROP	0.4
PRESSURE	0.3
	0.2
	0.1
	0
	750	1250	1750	2250	2750	3250	3750

NOMINAL CFM

FIG. 15 PRESSURE DROP vs SUPPLY AIR CFM

The temperature rise (or temperature difference between the return air and the heated air from the furnace) must lie within the range shown on the UL or CGA rating plate and the data in Table 2.

If the CFM is above or below the specified value, the supply air motor speed may have to be changed by moving the motor speed tap to another terminal.

CFM =	Btuh Input x 0.8
	1.08 x oF Temp. Rise

WARNING: Failure to properly adjust the total system air quan- tity can result in poor system performance.

5.After readings have been obtained, remove the tubes and seal the holes with the 5⁄16" dot plugs removed in Step 1.

NOTE: DE-ENERGIZE THE SCROLL COMPRESSORS BE- FORE TAKING ANY TEST MEASUREMENTS TO ASSURE A DRY INDOOR COIL.

CHECKING GAS INPUT

NATURAL GAS

1.Turn off all other gas appliances connected to the gas meter.

2.With the furnace turned on, measure the time needed for one revolution of the hand on the smallest dial on the meter. A typical gas meter usually has a 1/2 or a 1 cubic foot test dial.

3.Using the number of seconds for each revolution and the size of the test dial increment, find the cubic feet of gas consumed per hour from Table 18.

If the actual input is not within 5% of the furnace rating (with allowance being made for the permissible range of the regulator setting) replace the orifice spuds with spuds of the proper size.

NOTE To find the Btu input, multiply the number of cubic feet of gas consumed per hour by the Btu content of the gas in your particular locality (contact your gas company for this information - it varies widely from city to city.)

TABLE 18 - GAS RATE - CUBIC FEET PER HOUR

Seconds	Size of Test Dial
for One	1/2 cu. ft.	1 cu. ft.
Rev.
10	180	360
12	150	300
14	129	257
16	113	225
18	100	200
20	90	180
22	82	164
24	75	150
26	69	138
28	64	129
30	60	120
32	56	113
34	53	106
36	50	100
38	47	95
40	45	90
42	43	86
44	41	82
46	39	78
48	37	75
50	36	72
52	35	69
54	34	67
56	32	64
58	31	62
60	30	60

Example: By actual measurement, it takes 38 seconds for the hand on the 1-cubic foot dial to make a revolution with just a 100,000 Btuh furnace running. Using this information, locate 38 seconds in the first column in the table above. Read across to the column headed “1 Cubic Foot”, where you will see that 95 cubic feet of gas per hour are consumed by the furnace at that rate. Multiply 95 x 1050 (the Btu rating of the gas obtained from the local gas company). The result is 99,750 Btuh, which is close to the 100,000 Btuh rating of the furnace.

SECURE OWNER’S APPROVAL: When the system is functioning properly, secure the owner’s approval. Show him the location of all disconnect switches and the thermostat. Teach him how to start and stop the unit and how to adjust temperature settings within the limitations of the system.

18 Unitary Products Group