Carrier 58CTA Installation And Service Instructions Manual Download

Page: 37 / 58

Low NOx models in downflow or horizontal positions MUST
use Table 14 (21,000 BTUH per burner). See input listed on
rating plate.

a. Obtain average yearly gas heat value (at installed altitude)

from local gas supplier.

b. Obtain average yearly gas specific gravity from local gas

supplier.

c. Find installation altitude in Table 13 or 14.

d. Find closest natural gas heat value and specific gravity in

Table 13 or 14.

e. Follow heat value and specific gravity lines to point of

intersection to find orifice size and low-and high-heat
manifold pressure settings for proper operation.

f. Check and verify burner orifice size in furnace. NEVER

ASSUME ORIFICE SIZE. ALWAYS CHECK AND
VERIFY.

g. Replace orifice with correct size, if required by Table 13 or

14. Use only factory-supplied orifices. See EXAMPLE 2.

EXAMPLE 2: (0–2000 ft altitude)
For 22,000 Btuh per burner application, use Table 13.
Heating value = 1000 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 43*

Manifold pressure: 3.7-in. wc for high-heat

1.6-in. wc for low-heat

* Furnace is shipped with No. 43 orifices. In this example
all main burner orifices are the correct size and do not need
to be changed to obtain proper input rate.

3. Adjust manifold pressure to obtain low fire input rate. (See

Fig. 52.)

a. Turn gas valve ON/OFF switch to OFF.

b. Remove manifold pressure tap plug from gas valve.

c. Connect a water column manometer or similar device to

manifold pressure tap.

d. Turn gas valve ON/OFF switch to ON.

e. Move setup switch LHT (SW-1) on furnace control to ON

position to lock furnace in low-heat operation. (See Fig. 32
and Table 8.)

f. Manually close blower door switch.

g. Jumper R and W/W1 thermostat connections on control to

start furnace. (See Fig. 32.)

h. Remove regulator adjustment cap from low-heat gas valve

pressure regulators. (See Fig. 52) Turn low-heat adjusting
screw (5/64 hex Allen wrench) counterclockwise (out) to
decrease input rate or clockwise (in) to increase input rate.

NOTE:

DO NOT set low-heat manifold pressure less than 1.4-in

wc or more than 1.7-in. wc for natural gas. If manifold pressure is
outside this range, change main burner orifices.

i. Install low-heat regulator adjustment cap.

j. Leave manometer or similar device connected and proceed

to Step 4.

NOTE:

If orifice hole appears damaged or it is suspected to have

been redrilled, check orifice hole with a numbered drill bit of
correct size. Never redrill an orifice. A burr-free and squarely
aligned orifice hole is essential for proper flame characteristics.

4. Verify natural gas low-heat input rate by clocking meter.

NOTE:

Gas valve regulator adjustment caps must be in place for

proper input to be clocked.

a. Turn off all other gas appliances and pilots served by the

meter.

b. Run for 3 minutes in low-heat operation.

c. Measure time (in sec) for gas meter to complete 1 revolu-

tion and note reading. The 2 or 5 cubic feet dial provides a
more accurate measurement of gas flow.

d. Refer to Table 10 for cubic ft of gas per hr.

e. Multiply gas rate cu ft/hr by heating value (Btu/cu ft) to

obtain input.

If clocked rate does not match required input from Step 1,
increase manifold pressure to increase input or decrease
manifold pressure to decrease input. Repeat steps b through
e until correct low heat input is achieved. Re-install
low-heat regulator seal cap on gas valve.

5. Set low-heat temperature rise.

The furnace must operate within the temperature rise ranges
specified on the furnace rating plate. Do not exceed tempera-
ture rise ranges specified on unit rating plate for high-and
low-heat. Determine the temperature rise as follows:

NOTE:

Blower access door must be installed when taking tem-

perature rise reading. Leaving blower access door off will result in
incorrect temperature measurements.

Table 9–Altitude Derate Multipler for U.S.A.

ALTITUDE

(FT)

PERCENT

OF DERATE

DERATE MULTIPLIER

FACTOR*

0–2000

1.00