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 <m...@beckman.org> 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@tinka.africa> 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. >
