Furnaces, Well Pumps and Electric Water Heaters Require a Transfer Switch
You can use extension cords from your portable generator to power any device with a plug, but anything that's directly connected to your home's wiring, including essentials like your well pump, furnace and electric water heater, requires a transfer switch.
A manual transfer switch is essentially a small circuit breaker box that you mount next to your main electrical panel. You match the capacity of the transfer switch to the wattage of your generator. Then you choose which circuits to connect to the transfer switch. The Gen Tran manual transfer switch shown here came prewired for six circuits and included the inlet box (generator connection) and the cord to connect the generator.
Using a transfer switch is the only safe way to connect your generator to house wiring because it requires you to disconnect the house wiring from the incoming power lines at the same time you switch to generator power. This prevents the possibility of “backfeeding” generated power into the power lines, which creates a potentially lethal hazard for power line workers.