My 95.5% Gas furnace came with a fiberglass air filter with little restriction and no filtering action.
I substituted a Filtrete allergy reduction filter and went about my business. Been running that way for 2 years.
In my filter search today, I found a gauge that tells you when to change the filter by monitoring negative pressure near the blower inlet. It says it can be calibrated for 0-.4"WC.
I did some measurements on my system. With the filter removed and the filter door open, I measured -0.1"WC. I measured with my Filtrete installed and with a lesser MERV9, I think, filter then a washable permanent electrostatic filter. All measured -.55"WC. With the filter access door closed, it goes up to -0.6"WC. That's outside the calibration range of the filter monitor gauge, so it got me thinking. I don't have a fiberglass filter to compare.
I also measured some temperatures. With indoor temperature at 65F, the air coming out of the closest vent stabilizes at 119F. It's about
12' from the furnace and the ducts are insulated, so it should be somewhere near the furnace output temperature.
Looks like I'm getting