Is it time to replace the filter?
I asked this at More Energy Myths a few weeks ago but haven’t seen a response so I will post it here.
Four and five inch media filters are pricey and there is a device by General Aire (http://www.generalfilters.com/search/G99-Air-Filter-Gage_PT60.html ) to help you determine when it’s time to replace the filter. It’s mounted between the filter and the blower. You zero in the gauge at the time you install a new filter and then the indicator gradually swings towards the “replace” zone as the filter loads and vacuum increases. Although this might be helpful, it seems to me if you used an actual vacuum gauge, say the 0-30 IN-Hg you have laying around from your shade tree mechanic days, you could make a more accurate evaluation. But what Hg reading should you be looking for?