EXAMPLE:
By actual measurement, it takes 19 seconds for the hand on a 1 cubic foot dial to make a revolution with a 192,000 Btuh furnace running. To determine rotations per minute, divide 60 by 19 = 3.16. To calculate rotations per hour, multiply 3.16 • 60 = 189.6. Multiply 189.6 • 1 (0.5 if using a 1/2 cubic foot dial) = 189.6. Multiply 189.6 • (the Btu rating of the gas). For this example, assume the gas has a Btu rating of 1050 Btu/ft.3. The result of 199,000 Btuh is within 5% of the 192,000 Btuh rating of the furnace.
ADJUSTMENT OF TEMPERATURE RISE
The temperature rise (the difference of temperature between the return air and the heated air from the furnace) must lie within the range shown on the CSA rating plate and the data in Table 25.
After the temperature rise has been determined, the CFM can be calculated as follows:
CFM = Btu Input • | 0.8 |
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 6 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 (See ’SUPPLY AIR DRIVE ADJUSTMENT’).
NOTE: Each gas heat exchanger size has a minimum allowable CFM. Below this CFM, the limit will open below this rating.
BURNERS/ORIFICES INSPECTION/SERVICING
Before checking or changing burners, pilot or orifices,
CLOSE MAIN MANUAL
1.Open the union fitting just upstream of the unit gas valve and downstream from the main manual
2.Remove the screws holding each end of the manifold to the manifold supports.
3.Disconnect wiring to the gas valves and spark igniter(s). Remove the manifold & gas valve assembly. Orifices can now be inspected and/or replaced.
To service burners, complete step 4.
4.Remove the heat shield on top of the manifold supports. Burners are now accessible for inspection and/or replacement.
NOTE: Reverse the above procedure to replace the assem- blies.
Make sure that burners are level and seat at the rear of the gas orifice.
HIGH&LOW GASADJUSTMENT
LO
HI
ON
INLET |
|
PRESSURE | OFF |
| |
TAP | OUTLET |
| |
| PRESSURE |
CONNECTORS
MV
C
HI 
FIGURE 27 - TYPICAL GAS VALVE
42 | Unitary Products Group |
