Should I run my ducted AC to filter the smoke out of the air?

Is it a good idea to filter the smoke by running your ducted AC or heater on fan mode?

TLDR; No, but with an $11 air purifier running in each room mine is fine

I have seen some advice that you can run your ducted AC and the filter will help with the smoke.

Smoke is 0.4 - 0.7 according to Wildfire Smoke A Guide for Public Health Officials. Looking at data from the Monash air quality station of readings over 100 PM10 I calculated on average 25% of that was in the 2.5-10 PM range. So it seems to also include large particles. Can a standard filter capture that?

When I tried it didn't work and in fact made the air much worse.

Filter Efficiency

I have now had a chance to test the standard filter I suspect we have in Canberra, the Dynabreeze filter.  I Iave done some testing and found it reduced PM2.5 by 3%. But if its filtering 3% why didn't it work?

Duct Leakage

I'm not sure yet however I suspect the filter didn't work due to duct leakage. They estimate ducts leak around 20% in one study. If the leak is before the fan then you are sucking in outside air. If the leak is after the fan then you are creating a negative pressure in the house which again sucks in outside air.

In Out Test

I have tested the air going into the return and coming out of my ducts with and without a standard filter. I tested this as I thought no filter would be better due to less negative pressure and hence less air being sucked in. The difference was minor however I found the following.

IN/OUT	In	Out	Change	Outside
AC NO FILTER	6	5	-16.67%
AC NO FILTER	5	6	20.00%
AC NO FILTER	8	9	12.50%
AC FILTER	5	7	40.00%
AC FILTER	25	27	8.00%	29
AC FILTER	23	23	0.00%	29
AC FILTER	22	22	0.00%	29

If correct and its increasing the PM count by 2 with the filter and its doing 7 ACH, then I expect an increase of 14 PM2.5 per hour until it reaches the outside level.

Having said that the difference is quite small in absolute terms. I need to repeat the test when the outside air is bad again.

Update 29-JAN-2020: I have tested it again today and I didn't find much of a difference in the reading. This may be due to the outside reading being low of 29.

Maths

Another test I did on putting a MERV 16 filter onto my intake found it reduced the PM2.5 by 40% at the outlets. This should have been 90% based on other testing I did. I image the missing 50% was due to duct leakage. I was getting an extra 5 PM2.5 when it was 75 in Civic, so my guess is a duct leak of 7% on the intake side would explain this.

The same leak in a typical filter would result in increasing the inside level by 5 PM2.5 every air change. As mine does 7 ACH that's an increase of 35 PM2.5 in one hour. It would level out at the outside level within a few hours. I'm surprised how such a small leak can have such a big impact.

I do suspect my leak would be less in the less restricted typical filter.

I have heard of someone who tested the air at the outlet and found it to be higher.

I did go in the roof and found air sucking in on the join from the intake to the blower box. I tried to seal it with duct tape, but it turns out duct tape is the worst option to use in ducts. Go figure. I brought some insulated tape instead and will go back up there soon.

Conclusion

So my advice is if you don't have a PM counter and have the typical filter above, be cautious about running your deducted AC. However if you have an air purifier then it's likely not a big issue.