How to choose a furnace filter for pressure drop
I finally came down with Covid and also, we are having the dryest/warmest winter on record (think forest fires next summer) so I got to thinking about using MERV13 filters in my furnace. I have a fairly new single stage 95% gas furnace which currently has a 1″ MERV11 filter.
I got out my manometer to test pressure drop. TSP was .712 IWC, which is much higher than the usual limit of .5″ IWC for most furnaces. Most of the pressure drop was in the filter: .572 IWC
I could, and will, replace the MERV11 with a MERV8 or less just to protect the furnace but my long term question is: I am assuming that a fatter (2″ or 4″) filter would have less pressure drop. Should I rebuild the filter slot to accommodate the larger filter?
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Replies
Lot of this depends on the manufacturer. A 4" frame definitely has more room for material so it can be made to have lower pressure drop but lot of the stuff in the box store is designed to meet only the same target. This tends to mean that both 1" an 4" units are about the same pressure drop. You have to do a bit of searching to find the better ones. This shows the pressure drop VS flow of one manufacturer, most are not as helpful:
https://dwyer-inst.com/PDF_files/Priced/DF13_cat.pdf
If you are in fire zone, I would definitely make more room for filtering. You might one to even bump it up to 5" so it can take a standard Lennox Merv16 or a combination of 4" filter and 1" carbon.
Larger filter will also mean longer life so saves a bit of maintaince, especially true if you have a manometer and you can measure when the filter actually gets clogged.
P.S. Some budget digital manometers are not great at measuring 1" WG range, especially the case for the ones that can do 3psi.
The Lennox Merv16 (5”) is the one I’m using for HRV intake. Flow numbers vs static are excellent for the X6672 filter. It has a carbon layer but don’t expect any miracles with fire smoke.
Air Flow Rate (CFM) 659 initial resistance (IWC) .10
Air Flow Rate (CFM) 989 initial resistance (IWC) .17
Air Flow Rate (CFM) 1319 initial resistance (IWC) .24
Air Flow Rate (CFM) 1649 initial resistance (IWC) .32
Air Flow Rate (CFM) 1600 initial resistance (IWC) .31