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.
It has an (optional) blower motor on the back to help with circulating the heat, and also plugs in to allow easier starting, but if mains power isn't available it can still be started with 4 "D" cells, or, if you are willing to scrummage around underneath (where all the spiders live!), you can hold down an override switch and start it by clicking a piezo button (or, if you really don't like the hair on your arm, with a match). Even without the blower motor operating, it makes a surprising amount of heat, and also looks purdy...
W
P.S: Yes, we've clearly gotten away from the "Don't plug a generator in without a transfer switch, don't run a generator indoors, remember to test your smoke alarms every N, brush yer teeth, eat an apple, regularly exercise, drink plenty of water, etc., but sometimes it's nice to just have a chat - I miss NANOG...)
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@tinka.africa> wrote:
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>> On 8/31/21 16:06, Mel Beckman wrote:
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>> I think you’re forgetting about the all-important blower fan in a gas-fired furnace.
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> Well, I was referring to a pure electric furnace, not one that uses a blower over a gas-fired one :-).
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> In that case, the blower is not a major draw on power.
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> But again, we don't have those things here, so :-).
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>> 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.
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> Makes sense.
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> Does this, then, mean that if the blower itself were to fail, the gas burner would not light?
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> Mark.