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

Owen DeLong owen at
Tue Aug 31 18:46:36 UTC 2021

> On Aug 31, 2021, at 07:41 , Mel Beckman <mel at> wrote:
> Mark,
> 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?

For central heating, you’re absolutely correct. However, there used to be (don’t know if they are still sold in the US) wall-mounted single and/or double-sided gas radiant heaters that distributed hot air out of the top via convection and radiated heat from a vertical heat exchanger as well.

> 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.

I have no such valve (external air) in my house. I suspect this is applicable primarily in industrial/commercial HVAC.

The blower interlock is also slightly different in how it operates on my system.

When the thermostat calls for heat, the electronic ignition starts up. When it reaches ignition temperature, the gas solenoid is activated and the burner lights off. If ignition is not detected within a set period of time, the safeties will shut the system down to error mode.
Assuming ignition is detected, the blower is engaged. If the blower fails to start or the temperature in the flame box exceeds a certain value, then the safeties will shut the system down to error mode.

> 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.
> 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.

Yep… Unless you have an old-fashioned fireplace, in which case, you can have heat without electricity.


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