Reminder: Never connect a generator to home wiring without transfer switch

Mark Tinka mark at
Tue Aug 31 14:54:55 UTC 2021

On 8/31/21 16:41, Mel Beckman wrote:

> But you said “Gas-fired furnaces or heaters should not have an impact because the only electrical requirement is to fire up the pilot light.” There is no gas-fired furnace I know of that doesn’t require a blower fan. How else does the heat get out of the furnace?

I was conceptualizing it the same way we have on-demand or tank-based 
gas water heaters here, where the heat is not distributed by a blower.

Like I said before, we don't generally use furnaces in this part of the 
world to heat homes. So I don't have experience with that :-).

> To answer your question, you need to understand that this safety system has two components. The first component, the furnace interlock relay, is designed to interlock the blower with the forced-air system, which also includes an outside air supply valve. When the blower is energized, a circuit inside the furnace gets power. The blower and furnace operate continuously when this circuit is energized, and the supply valve opens and closes as needed to ensure the air doesn’t get stale.
> The safety second component is the limit switch, which primarily turns the blower fan on and off, but also has a safety role. When the temperature in the air supply plenum gets too hot, the limit switch turns off the furnace burner (or boiler, in a water-based system) to prevent damage, and possibly a fire, from overheating.
> The actual state mechanics are thus not as simple as “if the blower fails the furnace won’t light”. And it’s because of these complex state mechanics that furnace electricity is hard wired.

You learn something new everyday.

Thanks, Mel.

> Without AC power, no furnace can operate in a power outage. So that’s certainly not “no impact” from a utility failure. But the many thousands of deaths that occurred in homes and offices before these safety systems were put into the code is why you need a generator transfer switch if you want heat (or A/C) in your home during an outage.

Makes sense.

My solar PV-based water heating system requires an AC reference to make 
solar power to energize the water tank element. If there is a loss of 
AC, it shuts off (not unlike a regular UL 1741-compliant PV inverter). 
To workaround that, I asked the vendor to add a UPS input, where I can 
attach a UPS that the system can use as a reference, to keep making 
solar power in the event of loss of grid mains. Of course, that was 
before I had whole-home backup, but it did the job during the gap.


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