Goodman Mfg GMS8, GDS8, GHS8 IX. GAS Supply and Piping, Twinning, High Altitude Derate

24 VAC HUM

1.24 VAC Hum - A ¼” piggy back quick connect terminal on the pressure switch provides 24 VAC humidifier control. This terminal will be powered any time the pressure switch is closed. To connect 24 VAC hum, connect 24vac line of humidifier to piggyback on the pressure switch. The com side of the humidfier to C on the terminal strip on the control board (or to com side to 24VAC transformer). Do not connect 115V humidifier to these terminals.

TWINNING

A twin terminal located on the thermostat terminal strip can be used to twin two furnaces. This twin terminal synchronizes the circulation blowers of the two furnaces. Furnaces must be same model and heat / cool speeds the same. In addition, both furnaces must be powered by separate circuits originating from the same power leg in the main electrical box (hot and neutral must be in phase). Furnaces can be twinned to provide either single or 2-stage heat. If used as 2-stage heat the temperature rise from the register to return will be reduced during low fire operation.

To twin furnaces, ensure models are the same. Connect twin terminals on both furnaces and ensure heat and cool speeds are the same on both furnaces. For single stage operation, wire thermostat as normal to one of the furnaces. Conncect W from furnace 1 to one of the coil terminals on an isolation relay. Connect the other coil terminal to C of furnace 1. Connect R of furnace 2 to the common terminal of the isolation relay. Connect normally open (NO) to W of furnace 2.

For 2-stage operation connect twin of both furnaces. Connect one furnace to 2-stage thermostat as normal, connecting W1 on thermostat to W on this furnace. Connect W2 from thermostat to the coil terminal on isolation relay. Connect the other coil terminal to C of furnace 1. Connect common of the isolation relay to R on furnace 2 and normally open (NO) to W of furnace 2. With 2- stage configuration, furnace 1 will provide low stage heat (circulator blower of furnace 2 will run at heat speed). During a call for High heat both furnaces will operate.

NOTE: Check operation of twin by turning thermostat to “fan on” or placing a jumper wire from R to G (furnace connected to thermostat). If twinning wire is working properly, both blowers will turn on at the same speed. If twinning does not work properly, check proper 24-volt polarity and ensure both furnaces are powered from the same power leg. If the 24-volt polarity is incorrect, switch common and 24-volt leads on furnace that is not working. Check operation.

Furnace 1 Furnace 2

TWIN Y W R G C	TWIN Y W R G C
	NO
	C

Isolation Relay

Thermostat

Furnace 1 Furnace 2

TWIN Y W R	G C	TWIN Y W R G C
		NO
		C
R	W	Isolation Relay
Y	G	W2

2-Stage Thermostat

IX. GAS SUPPLY AND PIPING

GENERAL

The furnace rating plate includes the approved furnace gas input rating and gas types. The furnace must be equipped to operate on the type of gas applied. This includes any conversion kits required for alternate fuels and/or high altitude.

CAUTION

TO PREVENT UNRELIABLE OPERATION OR EQUIPMENT DAMAGE, THE INLET GAS SUPPLY PRESSURE MUST BE AS SPECIFIED ON THE UNIT RATING PLATE WITH ALL OTHER HOUSEHOLD GAS FIRED APPLIANCES OPERATING.

Inlet gas supply pressures must be maintained within the ranges specified in the following table. The supply pressure must be constant and available with all other household gas fired appliances operating. The minimum gas supply pressure must be maintained to prevent unreliable ignition. The maximum must not be exceeded to prevent unit overfiring.

Inlet Gas Supply Pressure

Natural Gas	Minimum: 5.0" w.c.	Maximum:10.0" w.c.
Propane Gas	Minimum: 11.0" w.c.	Maximum:13.0" w.c.

NOTE: Adjusting the minimum supply pressure below the limits in the above table could lead to unreliable ignition. Gas input to the burners must not exceed the rated input shown on the rating plate. Overfiring of the furnace can result in premature heat exchanger failure. Gas pressures in excess of 13 inches water column can also cause permanent damage to the gas valve.

At all altitudes, the manifold pressure must be within 0.3 inches w.c. of that listed in the Product Data Book applicable to your model for the fuel used. At all altitudes and with either fuel, the air temperature rise must be within the range listed on the furnace nameplate. Should this appliance be converted to LP, refer to the instructions included in the factory authorized LP conversion kit LPT* .

HIGH ALTITUDE DERATE

IMPORTANT NOTE: The furnace will naturally derate itself with altitude. Do not attempt to increase the firing rate by changing orifices or increasing the manifold pressure. This can cause poor combustion and equipment failure.

When this furnace is installed at high altitude, the appropriate High Altitude orifice kit must be applied. The furnace is supplied with orifices sized for natural gas at altitudes up to 4,500 feet

IO-242C 17 05/05