4. For the furnace(s) to operate properly, the minimum
inlet gas supply pressure to each furnace for natural
gas operation is 5.0” w.c. and for propane (LP) gas
is 11.0” w.c. with the furnace(s) operating. Maximum
inlet pressure for either gas is 13.5” w.c. [½ psi]. For
higher gas pressures, a separate field supplied and
installed high pressure regulator sized for the total
Btuh input is required to reduce pressure to within
minimum and maximum range. The high pressure
regulator used must include full internal relief or a
separate relief valve is required to prevent gas pres-
sure exceeding the maximum 13.5” w.c. [½ psi] limit
to prevent damage to the furnace gas valve(s).
5. A 1/8” NPT tap is provided on the inlet of the gas
valve to each furnace. A fitting suitable for connection
to a pressure gauge capable of measuring gas pres-
sure should be connected to each furnace. Check
gas inlet pressure at each furnace with all of the fur-
naces operating at the same time. See Figure K9.
Airflow Considerations, Condensation and Full
Modulation Firing Rate Control
1. Airflow through the furnace is vertical downflow (see
Figure I10). In this configuration, condensate due to
operation of air conditioning system can form in the
furnace tubes and would drain through the open
furnace tubes near the base in the furnace control
vestibule. A condensate drain pan should be field
provided for these applications. A condensate drain
fitting is provided in the flue box. Some condensation
may occur in the flue collector box with 5:1 modulat-
ing control and it is recommended that a drain tube
be connected to the lower condensate drain pan
using a ¼” silicone drain line.
2. Furnaces equipped with modulating control are ca-
pable of minimum input rates as low as 20%. Below
the minimum modulation rate the furnace will cycle
on and off to maintain the discharge air setpoint.
Consideration must be given to the vent conditions
and particularly the circulating airflow rates to ensure
that the operating air temperature rise is above the
dew point temperature of the flue gases in all appli-
cations and below the maximum allowable air tem-
perature rise to limit the furnace tube temperature.
The selection software limits the furnace input to be
below the maximum temperature rise of 90°F [50°C]
and the discharge sensor limits the maximum supply
air leaving temperature. On units with 5:1 turndown
the DDC controller compares the inlet and discharge
temperature to and from the furnace and will override
the turndown to limit the minimum temperature rise
to 20°F [11.1°C] to reduce the possibility of conden-
sation in the furnace tubes. In the event of a heat
wheel failure where the furnace design temperature
rise may be insufficient, the low limit function will shut
down the unit to also reduce the possibility of con-
densation. Condensation of the flue gas is corrosive
and will result in shortened heat exchanger life and is
not permitted. The materials used for furnace tubes
and vent connectors are designed for non-condens-
ing operation during the heating cycle.
Gas supply line
Manual gas
shut-off valve
Ground joint union
with brass seat
To controls
Plugged 1/8” NPT test
gauge connection
Sediment trap
3”
min.
Gas supply line
1
Figure K8: Union installation
Max. 13.5” w.c.
Pressure
regulator
Gas
supply
Unit 1
Unit 2
Unit 3
Min. 5.0” w.c. natural gas
Min. 11.0” w.c. propane gas
Figure K9: Gas piping line, regulator and overpressure
relief valve must be sized to be within the minimum
and maximum pressure ratings of all furnaces or
appliances serviced
WARNING
1. All field gas piping must be pressure/leak tested
prior to operation. Never use an open flame to
check for leaks. Use a soap solution or other leak
detecting solution for testing.
2. Gas pressure to appliance controls must never ex-
ceed 13.5” w.c. [½ psi].
WARNING
1. When pressure testing at ½ psi or less, close the
manual shut-off valve on the appliance before testing.
2. When pressure testing gas supply line at ½ psi or
higher, close manual gas valve and disconnect
heater from supply line to be tested. Cap or plug
the supply line.
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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