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

[Mark Tinka <mark@tinka.africa>]

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.

Mark.