ACCESSORY CONNECTIONS
The furnace control will allow power-switching control of various accessories.
ELECTRONIC AIR CLEANER CONNECTION
Two 1/4” (6.4 mm) spade terminals (EAC and NEUTRAL) for electronic air cleaner connections are located on the control board. The terminals provide 115 VAC (1.0 amp maximum) during circulating blower operation.
HUMIDIFIER CONNECTION
Two 1/4” (6.4 mm) spade terminals (HUM and NEUTRAL) for humidifier connections are located on the control board. The terminals provide 115 VAC (1.0 amp maximum) during heating system operation.
A mounting hole is provided on the control panel next to the furnace control board for mounting a humidifier transformer if required.
SINGLE STAGE THERMOSTAT
If a single stage thermostat is used, the high/low input staging will be controlled by the furnace control board, using the staging jumper. If the staging jumper is set to the 10, 15, or 20 position, the furnace will switch from low fire to high fire after 10, 15, or 20 minutes and will stay in high fire until the thermostat is satisfied. If the staging jumper is left in the OFF position and a single stage thermostat is used, the furnace will only operate at low fire.
SECTION VI: TWINNING
In applications where more heating capacity or more airflow capacity is needed than what one furnace can deliver, twinning can be used to make two furnaces operate in tandem. When two furnaces are installed using the same duct system, it is very important that the two furnace cir- culating air blowers operate in unison. If one blower starts before the second blower, the duct system will become pressurized and the blower on the second furnace will turn backwards causing the second furnace to overheat, resulting in damage to the furnace. Twinning is used to make two furnaces operate in tandem, using one duct system, one room thermostat and causing both furnaces to turn on and off simulta- neously.
Before installing the relay and wiring, disconnect electrical power to both furnaces. Failure to cut power could result in electrical shock or equipment damage.
The relay must not be installed in any location where it could be exposed to water. If the relay has been exposed to water in any way, it must not be used.
TWINNING DUCT SYSTEM
Twinned furnaces must only be applied on a common duct system. A single air supply plenum must be used for both furnaces and coil(s). Separate plenums and supply ducts systems cannot be utilized. A sin- gle return air plenum, common to both furnaces must be used. It is sug- gested that a return platform be utilized, with bottom air entrance into each furnace. If a side entrance returns system is used, the common return duct must be divided equally so as to supply each furnace with an equal amount of return air.
Both furnaces must be identical models in both heating capacity and CFM capacity. Both furnaces must be operated on the same motor speed tap. See typical application, Figure 20.
505309-UIM-D-0511
If furnace staging is desired with two furnaces on a common duct, where the gas burner on the first furnace operates on W1 and the gas burner on the second furnace operates on W2, then the use of an air- mixing device in the plenum to mix the air from both furnaces is strongly recommended. The mixing device must be installed before any ducts that supply air to occupied spaces. Twinning causes both indoor fans to operate simultaneously. If a mixing device is not used, any ducts that are connected down stream from the furnace that operates on W2, will be supplying cold air in the Heating mode to the occupied spaces unless W2 is energized.
Vent Pipe
| Electrical | Gas Supply |
| (Both sides) |
| Supply |
| |
1 Coil for 
Each Furnace
| COMMON | Supply |
| Air |
| SUPPLY |
| |
| PLENUM | |
FIGURE 20: Typical Twinned Furnace Application
When two furnaces are twinned, typical system total airflow will be approximately 85% of additive individual furnaces, i.e., two 2000 CFM units will yield a total 3400 CFM.
If a return duct is connected to only one furnace (with a connection between the two furnaces) an imbalance in the airflow will occur and the furnace furthest from the return plenum will overheat.
GAS PIPING
Furnace gas supplies must be provided as specified with these instruc- tions. Since the furnaces are side by side, with no space between, gas supplies must enter on the right and left respectively. All gas piping must be in accordance with the national fuel gas code, ANSI Z223.1, latest edition, and/or all local code or utility requirements.
TWINNING
In applications where more heating capacity or more airflow capacity is needed than what one furnace can deliver, twinning can be used to make two furnaces operate in tandem, using one duct system and one room thermostat. When one duct system is used for two furnaces, it is necessary that the two blowers operate in unison. The twinning function of the board in this furnace ensures that both blowers turn on and off simultaneously, and operate on the same blower speed.
The control in the furnace has the twinning feature. With this feature, a wire is connected between the TWIN terminal on one furnace board to the TWIN terminal on the second furnace board. The board then com- municates the blower status from one furnace to the other along this wire. This communication makes the second furnace blower come on at the same time, and on the same speed, as the first furnace blower. To ensure stable communication, the common terminal of each control must be connected.
