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 <60F rise across the furnace. That's near the max claimed in the spec.
Another thing is that I don't heat part of the house. Three registers are closed. Last time I did the experiment, heating the whole house cost me almost 50% more in gas.
So, I don't want to heat the whole house. I need the better air filters. Should I worry about overheating the furnace? Is there anything simple I can to about it? Like turn up the blower speed?
I do have an electronic filter that's designed to replace the 1" paper filter. I'd have to cut it down to fit. Looks like it has about as much material as the original fiberglass filter, so should have lower pressure drop.
Is that a viable alternative? It's not clear how they compare at removing pollen. It's hard to decide based on the vague marketing hype.