Good Filter for Confined HVAC System
guidoism
| Posted in General Questions on
How can I improve my air quality when the space for an air filter is so tiny? I recently bought a small house and the HVAC system doesn’t have a lot of room for a good filter. It had a 20x25x1 MERV 2 filter. I would like to improve the air quality situation — I would prefer MERV 11 or higher but honestly I’d just like something better than MERV 2 for now. I mostly care about removing dust from the air.
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The physical size doesn't affect a filter's ability to filter air well, but a smaller filter will clog up sooner. Your installation looks very leaky too, which means a lot of air will probably bypass the filter. Get a higher MERV filter that will fit in your filter slot, but be careful you don't create excessive back pressure which can be a problem for the furnace. Higher MERV filters tend to be more restrictive to airflow.
Check if you can rework your duct to allow a bigger pleated filter, such as a 2" or 4" deep filter. 1" is the residential standard, but bigger = less maintenance (longer life), and lower back pressure. I like the Aprilaire filter units that use a really big pleated filter that expands a little bit like a venetian blind, but they have pretty good sized housings and might not fit your unit if you have limited space.
Bill
I agree w/ Bill that the installation looks very leaky. A lot of duct work is hacked up pretty badly, and it's often obvious where they put in a "slot" for the air filter. I use foil tape (available at your local big box store) to seal around the filter each time I change it to make sure that the filter isn't bypassed.
MERV 2 seems terrible. Without running air pressure and flow reduction calculations, it seems like you could put in a much better filter next time, even if you can't readily widen the filter slot.
I recommend the Filtrete 1900 merv 13. I have a manometer installed measuring pressure across my 16x25x1 filter. I measure the initial pressure drop and replace it based on an increase in pressure drop. I tested just about every big box store filter in my size. The Filtrete was hand downs the winner in lowest initial pressure drop and had a pretty long life before there was a big increase. Some of the cheap Merv 4 and 5 filters were surprisingly really restrictive.
Anyone else running a high quailty filter should install a manometer. I use a Dwyer Mark 3 that's been around for decades and is extremely accurate. You just drill a hole before and after the filter and connect 2 tubes. I think it cost $40. Worth it if you can safely extend expensive $20+ merv 13 filters.
Here is a picture of my setup.
Based on limited data I had found online, I also selected Filtrete 1900 as being terrific for high filtration with low pressure drop. Great to hear confirmation that that was a good choice!
It's worth mentioning, GBA being a green building site and all, that using a manometer can let you maximize the life of your air filter. We do this commerically at customer sites, because datacenter air handlers run continuously, 24x7, and typically take four or more 24x24x4 air filters (these are a lot bigger than a typical residential filter). There is a lot of money to be saved by only replacing the air filters when needed, and not just on a schedule. Obviously fewer filter replacements also means less materials used, and less trash to throw out.
What you do is monitor the pressure drop across the filter, ideally using a two-port differential type manometer. These are often available used on places like Ebay for reasonable prices. I like the Dwyer Magnahelic units. You can use single port units too, they're just not as accurate. Take note of the initial pressure drop with a new filter, then keep track of the pressure drop as it increases (as the filter clogs up, the pressure drop across it will increase). You then replace the filter when the pressure drop across it gets close to the maximum allowable for your particular system. Doing this usually lets you stretch out the time between filter changes quite a bit, especially if you have a clean and tight home.
BTW, backer rod and the square foam weather stripping can be used to seal around air filters in ductwork. I don't like using tape, since bits of tape residue build up over time as you remove and retape at filter change time. Eventually you end up with a mess. It's better to rig up a small access door or panel with a weather strip seal around it. The Aprilaire unit I mentioned before incorporates this into it's cover.
Bill
Fun fact.
A clean filter actually doesn't do as good of a job filtering as one that is near the end of life. Most filter's performance improves as they fill up, which makes sense if you think about the collected dirt as another layer of super fine filter media.
Changing your filter often is good for minimizing pressure drops but not the best for air quality.
For example, I change the filter in my ERV after allergy season.
P.S. Replacing stock filters for a high performance one can get you into trouble. I had a furnace lock out on me after I installed a merv 13. Back to good old merv 8.
>"P.S. Replacing stock filters for a high performance one can get you into trouble. I had a furnace lock out on me after I installed a merv 13. Back to good old merv 8."
If you get a "deeper" filter (such as going from a 1" to a 4" thick filter), the effective area of the filter media greatly increases, which results in lower back pressure. This lets you use a higher efficiency filter without problems, but obviously requires a bigger filter slot.
It's worth mentioning that "high efficiency", in regards to filters, means the filter is more efficient at removing particulates from the air. "High efficiency" doesn't mean less back pressure, it's actually usually the opposite.
Bill
Note that you can also get more area just by increasing the angle of the pleats in a 1" deep filter, and that appears to be part of how the Filtrete 1900 achieves low pressure drop.
I also use my Dwyer manometer to measure the airflow cfm of my ductwork. I just had to move one hose to a hole after my ac coil and and you can easily get the return and supply static pressure. Once you have a CFM number without a filter its not hard to come up with a filter change interval. I replace mine when the CFM drops by about 200 which correlates to a .10 inches of w/c pressure drop.
The fun fact that a dirty filter filters better is true of conventional filters. Higher-tech filters like the Filtrete filters that use electrostically charged fibers stop filtering as well when they start to get loaded with dust, before they start to filter better as they get clogged.
https://www.researchgate.net/publication/8471333_The_Long-Term_Performance_of_Electrically_Charged_Filters_in_a_Ventilation_System
The same is true for activated carbon filters, which are doing a different function.
You could always raise the furnace and use a bottom return. This would let you put in a thicker filter media. The aprilaire units Bill mentioned are great, they even have tested pressure drop data, a rarity in residential filters! See attached for pressure drop data.
The 3m Filtrete line all have the intial pressure drop at various cfm's printed right on the filter now.
Deleted
There was some study done in California linked below. The data showed it had one of the lowest intial pressure drop of all the filters tested. They claim it had a lower pressure drop than even most of the 2 inch filters they tested.
I have done extensive testing with so many filters. It has a really low intial drop and in my case can last almost twice as long as the recommended 3 month change interval. I couldn't believe how restrictive some of the cheap low merv box store filters had.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://efiling.energy.ca.gov/GetDocument.aspx%3Ftn%3D223260%26DocumentContentId%3D27716&ved=2ahUKEwj_kKfwx6jvAhXImOAKHfRxDCkQFjAAegQIARAC&usg=AOvVaw0fMnRp45Ptfqv-MSM6_xR-
There was some study done in California linked below. The data showed it had one of the lowest intial pressure drop of all the filters tested in 1 inch size. They claim it had a lower pressure drop than even most of the 2 inch filters they tested.
I have done extensive testing with so many filters. It has a really low intial drop and in my case can last almost twice as long as the recommended 3 month change interval. I couldn't believe how restrictive some of the cheap low merv box store filters were.
I attached a Pdf of the California testing.