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

Josh Luthman josh at
Tue Aug 31 14:49:53 UTC 2021

Is this conversation really taking place on NANOG?

Don't backfeed power.  Got it.  Stupid people are going to be stupid, we
won't solve it here.

Josh Luthman
24/7 Help Desk: 937-552-2340
Direct: 937-552-2343
1100 Wayne St
Suite 1337
Troy, OH 45373

On Tue, Aug 31, 2021 at 10:41 AM 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?
> 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.
> 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.
>  -mel
> > On Aug 31, 2021, at 7:15 AM, Mark Tinka <mark at> wrote:
> >
> > 
> >
> >> On 8/31/21 16:06, Mel Beckman wrote:
> >>
> >> I think you’re forgetting about the all-important blower fan in a
> gas-fired furnace.
> >
> > Well, I was referring to a pure electric furnace, not one that uses a
> blower over a gas-fired one :-).
> >
> > In that case, the blower is not a major draw on power.
> >
> > But again, we don't have those things here, so :-).
> >
> >
> >> That said, the reason the code requires furnaces to be hardwired is to
> ensure that the blower interlock system can’t be bypassed. An electrical
> interlock ties a heat recover ventilator to circulation air blower
> operation of a forced-air furnace system. This ensure that the blower
> circulates supply and return air within the structure. A plug-in power
> source leads to the possibility that this interlock could be accidentally
> defeated, resulting in an overheat within the flame box.
> >
> > Makes sense.
> >
> > Does this, then, mean that if the blower itself were to fail, the gas
> burner would not light?
> >
> > Mark.
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