Geeking out on filter pressure…a simple DIY for measuring the pressure across your HVAC filter
We’ve warned of the primary danger of not-changing your HVAC filter often enough, and also of installing a filter with too-high of a MERV rating: excessive pressure drop across the filter. For maximum life of your air handler and good filtration, you’ll want to keep this “differential” pressure much less than 0.5 inches of water (because HVAC manufacturers specify maximum 0.5” water for all the components of the system, not just the filter). How would one know what the pressure drop is, except to measure it? Good point! A recent post by a homeowner on Greenbuildingadvisor.com received a good reply with photos from another homeowner who accomplished what he wanted to do: install a manometer across the filter to read the pressure drop.
He wanted to use this differential pressure to tell when to change the filter. To be sure, a dirty filter will cause pressure to drop more than a clean filter, but the responder did not think it made a difference in when he changed his filter. This was because some filters can get really gross, before a big change in pressure occurs, and some filters start out with a high differential pressure, even when they’re clean. However, it’s nice to know, with the ability to measure this pressure drop, if you are stressing your system too much.
The highest cost of this DIY project is the pressure gauge. Here is a Dwyer gauge ($88) rated from 0 to 1” water, which is more than enough capacity for this application. You’ll need some flexible tubing to connect to the gauge (this one has 1/8" NPT(F) connections), and some “static pressure probes” which will be inserted into the ductwork on either side of the filter. Then, just make your holes in the ductwork with enough clearance to insert the probes, and position them at 90 degrees to the airflow so that the air velocity doesn't influence the reading. For more information on equipment, differential pressure and maximum pressures, check out this article.
