58
Table 25 — Pressure Reading
NOTE: Readings below those noted indicate a potential problem
with the installation and may be the result of low voltage to com
-
bustion blower motor, a loose or damaged blower wheel, restric
-
tion in the vent piping, inadequate vent system, or negative pres
-
sure in the building (indoor applications only).
Manifold Pressures
The gas valve manifold pressures may be measured at the gas
valve test ports, see Fig. 88. See Tables 26 and 27 and for typical
gas heat data points and modulating gas heat data points.
Modulating Gas Heat
If the OAT is below OAT heating set point, heating will be initiat
-
ed. The modulating heat valve(s) will control to a LAT of 72°F.
Units with modulating gas heat may also have 'Override' control.
Override control (sometimes called high fire override) is used to
quickly raise the space temperature. For example, if the space is
cold early in the morning, the unit will sense the low space tem
-
perature and go into high fire override even if the outdoor tem
-
perature is not that cold. After the space temperature is in the nor
-
mal range, the unit will then go back to the modulating output
mode.
Units equipped with the optional modulating gas heat control will
have several components in addition to the standard gas valve.
These components includes the following:
MODULATING GAS VALVE
For an example of a modulating gas valve, see Fig. 89.
SIGNAL CONDITIONER (SC30)
See Fig. 90. The signal conditioner has three functions:
1. Convert the 62X,R controller's DC output signal of 0 to 10
VDC to a 0 to 20 VDC signal to provide compatibility with
the modulating gas valve.
2. Controls the normally open relay used to switch inducer
speed and gas valve stage. Closes or opens the relay based on
the 62X,R controller's DC output to the SC30. Relay closes at
approximately 5.3 VDC input. Relay opens at approximately
4.7 VDC input.
3. Provides an adjustable timer (0 to 30 secs) and adjustable
VDC (5 to 15 VDC) output to the modulating gas valve at
start up (powering of the SC30). Adjustment potentiometers:
• R18 - 0 to 30 sec timer
• R22 - 5 to 15 VDC output voltage
Fig. 88 — Gas Valve Manifold Pressure Ports
Fig. 89 — Modulating Gas Valve
Table 26 — Typical Gas Heat Data Points
Type of Gas: Natural Gas Pressure at Inlet (burner off): 13.5 in. wg max. Supply voltage at JB: Marked VAC ± 5
%
Table 27 — Additional Data Points for Modulating Gas Heat
SERIES
AMBIENT
(Min. in. wg)
HIGH FIRE
STEADY STATE
(Min. in. wg)
HMA
-3.50
-1.70
HMB
-3.50
-1.90
HMG
-2.80
-1.70
LO
°
ON
C
°
OFF
SECOND STAGE (HI FIRE) MANIFOLD PRESSURE
ADJUSTMENT (3/32 IN. ALLEN KEY)
FIRST STAGE (LO FIRE) MANIFOLD
PRESSURE ADJUSTMENT
ELECTRICAL
CONNECTION
BLOCK
MANUAL SHUT-OFF
CONTROL KNOB
1/8 IN. NPT INLET
PRESSURE TAP
MODULATOR VALVE
(MODULATING SYSTEMS ONLY)
HEATER OPERATING DATA
LOW FIRE
HIGH FIRE
Gas Pressure at Train Inlet
13.3 in. wg
13.0 in. wg
(See rating plate for minimum)
Gas Pressure at Burner Manifold
1.1 to 1.3 in. wg
3.3 to 3.5 in. wg
Gas Input Rate
25 to 35% Maximum Input
Rated input less 5% Max.
CO
2
in Flue Gas
1.8 to 3.2%
6.2 to 7.2%
CO in Flue Gas
<300 ppm
<50 ppm
Flue Gas Temperature at Discharge
>200°F
420 to 520 F
Ambient Temperature: –40 to 70°F
Draft Fan at Discharge
–0.80 to –2.00 in. wg
–1.75 to –2.40 in. wg
MODULATING SYSTEM
OPERATION
ANALOG SIGNAL TO SC30
VDC TO MOD VALVE
MANIFOLD PRESSURE
(if applicable)
Initial Call for Heat
0 VDC
5 VDC
10 VDC
12.5 to 13.0 VDC
0.5 to 1.0 VDC
9.5 to 10.0 VDC
18.0 to 19.0 VDC
1.1 to 1.4 in. wg
0.4 to 0.6 in. wg
0.9 to 1.2 in. wg
3.2 to 3.5 in. wg
