Furnace - odd electrical current behavior
I have a fairly conventional Comfortmaker gas furnace, 80% AFUE, about 15-20 years old. I'm rebuilding my backup power system and decided to add the furnace to it. As part of this project I popped a clamp-ammeter onto the line from the furnace breaker and started it up to see what kind of current it needed.
What I saw was puzzling.
When the furnace started, it started both the blower and draft fan immediately (this is normal for this furnace). At this point my ammeter read 6.2 amps.
When I heard the gas click on and light, the current DROPPED to 1.6 amps! I didn't hear any change in the sound of the blower or draft fan.
The current stayed at that level for a good 15-20 seconds, then went back up, but this time to something more like 5.7 amps. Then over the next few minutes it gradually declined to a bit more than 5 amps.
I'm far from an expert, but this makes no sense to me. I expected the current to increase when the gas valve opened and the ignitor came on, but it *dropped* instead.
So what gives with the 1.6 amps?
And why would the current rise again later? Why would it fall a bit over time?
Sure would be nice if it would run at 1.6 amps all the time!
Thanks for any insight anyone can offer.