On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, wrote:
On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), wrote:

On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:
The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.

It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.


yeah i agree,

and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.

If the duct air temp is normal , the air flow is normal.

m



A differential pressure tester or manometer across the filter gives a
much better indication.

We have them at work, with alarms when they exceed their limit.

I've never seen one in residential. I just listen to mine, when I start to hear the filter noise rise I change them.

They used to make filters with some kind of a whistle that was supposed to alert you when they were dirty. I haven't seen one in a while.