Improving air quality of a ducted reverse cycle air conditioner
As I run my ducted AC I watch the PM2.5 count rise, 50, 70, 100.
The filter on my ducted reverse cycle system doesn't do anything to reduce the smoke. In fact I've noticed the PM2.5 count increase when I run it! I assume leaks in the roof mean air will be sucked in to replace it, and given research find the average USA duct system leaks 20% that's quite likely.
I tried running my Phillips 2000 Air Purifier for the 80m2 space but the PM2.5 kept going up. In the end we turned the AC off and moved into my son's room as we just couldn't get the count down.
I got excited when I found a better filter that deals with particles 10 times smaller. Plus it's approved by Sensitive Choice . However the local filter shop said it's not great and don't have it. After lots of research I believe it has a chance, although there may be better temporary solutions.
The filter shop did suggest I try an extra carbon filter, however as the smoke rolled in and I ran the AC the PM2.5 count kept climbing to 100. It didn't help. I have since learnt that carbon captures gases and smells, not particles.
The filter most of us have in Australia is a cleanable polyester cloth. As best I can tell it's at best a MERV 1-4.
My air purifier has a HEPA filter that was developed as part of the Manhatten Project. All HEPA are MERV 17-20. It's the best however it also takes a lot of pressure to get air through. I can't just use one as I looked up my ducting fan as it's rated to 125 Pascal's, while a clean HEPA is 250 Pascal's and a dirty one 500-750 Pascal's.
I found a duct filter by QAir that has a HEPA filter but at $2,500 it's a bit pricey.
With HEPA ruled out I found that MERV filters have a much lower static pressure drops. I found various numbers, typically around 25 Pascals clean and 50 Pascals dirty. Although one was as high as 125 Pascals.
Research found doubling the intake reduced the static pressure by 3-4.5 times. And along this line I found what I needed, a HVAC installers suggestions that you should aim for a face velocity of 1m/s. There MERV 13 had a static pressure drop off 6 Pascal's!By the face velocity low you can expect a much lower static pressure. The typical face velocity in testing of filters is 2-2.5 m/s so by going to 1m/s you can be rest assured it will be ok.
I learnt that Lennox MERV 16 filters are the gold standard when it comes to filters. They also cost $234 for two so I went with a $62 Green Label HVAC Air Filter 24x25x1, AC Furnace Air Ultra Cleaning Filter MERV 16 - Pack of 2 which I later found a review suggests are fake.
I'm worried about destroying the motor by adding a better filter. Typically the better the filter the more static pressure to push air through it. I've heard it costs $700 to replace a burnt out fan in Canberra.
My Mitsubishi PEA-RP71 is rated to 125 Pascal's of pressure based on my research. This is in line with your typical USA HVAC which gives me confidence I can apply their principals to my problem.
My AC can move 27 m3/min or 1,620 m3/hr, and replaces the air 7.5 times per hour.
Based on my existing intake of 350mm x 700mm the face velocity of the filter should be around 1.84m/s. My testing found 2.045m/s on the original filter.
My current filter it's 0.245m2 in size. My guess is it's MERV 1-4.
Using the rule of a face velocity of 1m/s I used Air Flow Conversion Calculator to estimate that I need an intake of 0.45m2.
Using the two filters I have increased my intake to 0.77m2 which is 3 times larger than the original. You can just make out the original filter in the picture above. I plan to drop the filter 20 cm soon as its not getting much airflow around the edges.
After adding the two MERV 16 filters and removing the original the airflow at the outlets has dropped by 2%, and 30% at the intake. I suspect my intake measurement is off as it was hard to measure. I will repeat the measurements after I drop it 20 cm. The intake averaged 1.4m/s, so lower than the original filter but not the 1m/s target.
To test it worked I had to use my Phillips 2000 air purifier to measure the PM2.5. I put it on low which is not enough to do 3 bedrooms at 80m2. I first took baseline readings with the AC off. I'm guessing the initially drop is either the Phillips 2000 or dust I created settling. Next I turned on the AC using the original filter and we see it jump and then settle back down. Sadly my phone stopped capturing data and I didn't notice until I finished for awhile here. Finally I installed the two MERV 16 filter and we see the readings drop further to around 8 PM2.5.
The output is a nice number, although the drop of 40% is not great. I will repeat the test when the air in is worse. If I get the same result either I'm sucking in outside air, sucking in unfiltered air, or the filter isn't working.
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| My filter |
The filter on my ducted reverse cycle system doesn't do anything to reduce the smoke. In fact I've noticed the PM2.5 count increase when I run it! I assume leaks in the roof mean air will be sucked in to replace it, and given research find the average USA duct system leaks 20% that's quite likely.
I tried running my Phillips 2000 Air Purifier for the 80m2 space but the PM2.5 kept going up. In the end we turned the AC off and moved into my son's room as we just couldn't get the count down.
A better filter?
I got excited when I found a better filter that deals with particles 10 times smaller. Plus it's approved by Sensitive Choice . However the local filter shop said it's not great and don't have it. After lots of research I believe it has a chance, although there may be better temporary solutions.
Carbon Filter Maybe?
MERV to the rescue
The literature on filters is sparse in Australia so I ended up using mostly American research. Over there they use the MERV rating, and a G4 equates to a MERV 6. To filter small partials you need at least a MERV 13, and ideally a MERV 16. This chart suggests G4 can deal with around 50% of PM 2.5 particles which is an improvement over my current one.The filter most of us have in Australia is a cleanable polyester cloth. As best I can tell it's at best a MERV 1-4.
My air purifier has a HEPA filter that was developed as part of the Manhatten Project. All HEPA are MERV 17-20. It's the best however it also takes a lot of pressure to get air through. I can't just use one as I looked up my ducting fan as it's rated to 125 Pascal's, while a clean HEPA is 250 Pascal's and a dirty one 500-750 Pascal's.
I found a duct filter by QAir that has a HEPA filter but at $2,500 it's a bit pricey.
With HEPA ruled out I found that MERV filters have a much lower static pressure drops. I found various numbers, typically around 25 Pascals clean and 50 Pascals dirty. Although one was as high as 125 Pascals.
Research found doubling the intake reduced the static pressure by 3-4.5 times. And along this line I found what I needed, a HVAC installers suggestions that you should aim for a face velocity of 1m/s. There MERV 13 had a static pressure drop off 6 Pascal's!By the face velocity low you can expect a much lower static pressure. The typical face velocity in testing of filters is 2-2.5 m/s so by going to 1m/s you can be rest assured it will be ok.
MERV 16 Filters
I learnt that Lennox MERV 16 filters are the gold standard when it comes to filters. They also cost $234 for two so I went with a $62 Green Label HVAC Air Filter 24x25x1, AC Furnace Air Ultra Cleaning Filter MERV 16 - Pack of 2 which I later found a review suggests are fake.
MERV 13
I have found some simulations that found a MERV 13 removes 70% of outdoor PM2.5 while a MERV 16 removed 76%. For the diminishing returns they suggest MERV 12-13 is a good choice.Will I burn out my fan?
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| My AC unit, sadly fan cannot be seen |
My Mitsubishi PEA-RP71 is rated to 125 Pascal's of pressure based on my research. This is in line with your typical USA HVAC which gives me confidence I can apply their principals to my problem.
My AC can move 27 m3/min or 1,620 m3/hr, and replaces the air 7.5 times per hour.
Based on my existing intake of 350mm x 700mm the face velocity of the filter should be around 1.84m/s. My testing found 2.045m/s on the original filter.
My current filter it's 0.245m2 in size. My guess is it's MERV 1-4.
What Size Filter?
A Larger Filter
Using the rule of a face velocity of 1m/s I used Air Flow Conversion Calculator to estimate that I need an intake of 0.45m2.
Using the two filters I have increased my intake to 0.77m2 which is 3 times larger than the original. You can just make out the original filter in the picture above. I plan to drop the filter 20 cm soon as its not getting much airflow around the edges.
Results
Testing
Test 1: It work!
To test it worked I had to use my Phillips 2000 air purifier to measure the PM2.5. I put it on low which is not enough to do 3 bedrooms at 80m2. I first took baseline readings with the AC off. I'm guessing the initially drop is either the Phillips 2000 or dust I created settling. Next I turned on the AC using the original filter and we see it jump and then settle back down. Sadly my phone stopped capturing data and I didn't notice until I finished for awhile here. Finally I installed the two MERV 16 filter and we see the readings drop further to around 8 PM2.5.
Test 2
Lets see out the in/out looks
The output is a nice number, although the drop of 40% is not great. I will repeat the test when the air in is worse. If I get the same result either I'm sucking in outside air, sucking in unfiltered air, or the filter isn't working.
Conclusion
I have used it on and off for 4 days now and its working well, readings are typically under 10. I can now run my AC without killing my family.Updates
- 17-Jan-2019: Ran on fan only as readings in my son's room was 19 PM2.5. After 10 minutes it was 7 PM2.5. Need to put the Phillips air purifier off auto and onto a higher fan speed.
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