On Sat, 5 Oct 2019 16:37:21 -0700 (PDT), trader_4
wrote:

On Saturday, October 5, 2019 at 5:11:17 PM UTC-4, wrote:
On Sat, 5 Oct 2019 09:08:32 -0700 (PDT), trader_4
wrote:

On Saturday, October 5, 2019 at 11:07:44 AM UTC-4, danny burstein wrote:
[lots snipped]

My furnace has a conventional AC blower motor - not sure if it's PSC or
some other type. It's either 1/3 or 1/2 hp and doesn't have a large
capacitor either built into it or wired up near by. My 40-year-old
furnace otherwise has NO electronics other than the gas-valve soleniod.
No draft inducers, sensors, vent fans, etc.

Anyone ever run such a motor from one of these small generators?

Just thinking of picking up a generator to run my furnace
(intermittently) in case of any extended winter power outage.

While I have no personal experience, I spoke with one
of the long-term, highly regarded, drinks #6 oil for
breakfast, HVAC folk near our old home and asked him
about backup generators.

He said they had been running into problems with the
various flame/safety sensors, or possibly the ignitors,
or both (?) which monitored teensey, tiny, current
levels, and that these often shut down the furnaces
and boilers 'cuz they'd get wrong readings.

He added that it was super critical to get good
and solid grounding for everything involved.

Again, no personal experience, but it kind of
makes sense.


Very interesting. On some of these they use a flame sensor that relies
on just a steel rod, whereby somehow the flame itself generates a
small voltage. Seems that would rely on sensing a voltage between that
rod and ground, ie EGC. And if the generator is not properly grounded to
the house ground, to the furnace, I can see how that would be a problem.
And don't ask me about the physics whereby that flame sensor works....


house ground,

If they are using the MBJ in the panel as a current path to the
thermocouple I can see that but it is not as much the "ground" (the
grounding electrode system) as the grounding conductor to the furnace
frame. OTOH the control circuit may just be using the furnace frame as
the current path in which case we are really talking about the bonding
of the thermocouple base.

What I had in mind was an interruption in the EGC between the furnace
burner and the portable generator. You wouldn't think that would be
easy to do, ie if you're using a standard extension cord it should be
hard to impossible. But that might account for the problem.

But then on second thought, probably not even that. The sensing circuit
for the flame rod would be between the flame rod and the ground of the
circuit board, which is tied to the furnace frame. So, I take it back,
if some people are having issues like described, IDK what the mechanism
could be. Like I said, I ran several furnaces on a generator that were
high eff, modern ones, they worked with no problems.

I don't always trust HVAC techs on electrical issues. They are more
like plumbers. If there was an issue I might consider the fact that
*some* small inverter generators do not have main bonding jumpers.
(like a 2000 Honda). I am still not sure why that would affect the
thermocouple but how knows? It may have more to do with the thermostat
not getting voltage (24vac) to start the blower and the furnace sees
that. This is the wild card in all of this. The last time I had a
furnace, the 24v transformer was on the blower circuit after the
disconnect at the furnace (actually in the same box on the side of the
plenum). That was a circa 1971 house in Md. Gas appliances with no
electronics. The gas valves were fed directly from the thermocouple
through a point on the blower relay in the furnace and the thermostat
in the water heater as I recall. It was 35 years ago tho. There was no
electrical connection at all to the water heater. It just had a pilot
light.