Energy Tech Laboratories DHG240 BtuhbInput ⋅, CFM = 1.08 ⋅ FbTempbRise, Checking Gas Input

035-17233-000-C-0702

FIGURE 20 : PRESSURE DROP ACROSS A DRY INDOOR COIL VS. SUPPLY AIR CFM

ADJUSTMENT OF TEMPERATURE RISE

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 ETL rating plate and the data in Table 2.

After the temperature rise has been determined, the cfm can be calculated as follows:

BtuhbInput ⋅ 0.8

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CFM = 1.08 ⋅ °FbTempbRise

After about 20 minutes of operation, determine the furnace temperature rise. Take readings of both the return air and the heated air in the ducts (about six feet from the furnace) where they will not be affected by radiant heat. Increase the blower cfm to decrease the temperature rise; decrease the blower cfm to increase the rise. Refer to Table 11 for blower motor and drive data.

BELT DRIVE BLOWER

All units have belt drive single-speed blower motors. The vari- able pitch pulley on the blower motor can be adjusted to obtain the desired supply air CFM. Tighten belts enough to prevent slipping. but do not over tighten. Belt deflection should be between 1/4" and 1/2" per foot. Refer to Table 15 for blower motor and drive data.

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 regu- lator 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 HOUR1

SECONDS FOR 1 REV.	SIZE OF TEST DIAL
	1/2 Cu. Ft.	1 Cu. Ft.
4	450	900
6	300	600
8	228	450
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

1Example: By actual measurement, it takes 13 seconds for the hand on the 1-cubic foot dial to make a revolu- tion with just a 300,000 Btuh furnace running. Read across to the column in the table above, headed “1 Cubic Foot”, where you will see that 278 cubic feet of gas per hour are consumed by the furnace at that rate. Multiply 278 X 1050 (the Btu rating of the gas obtained from the local gas company). The result is 292,425 Btuh, which is close to the 3000,000 Btuh rating of the furnace.

SECURE OWNER’S APPROVAL

When 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.

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