HEPA advice?

Old 07-12-12, 07:43 PM
Thread Starter
Join Date: Jul 2012
Location: USA
Posts: 10
Upvotes: 0
Received 0 Upvotes on 0 Posts
HEPA advice?

Not seeing a better subforum to put this in...

Can someone with better knowledge than me help inform me as to how HEPA stuff actually works? I have some problems with some pretty severe allergy issues (as in life threatening at times, and I lack health insurance and have to treat things with alternative medicine when it flares up) and at some point I need to look into cleaner air than I have right now to continue living where I am. Other than I think the pollutants are coming from outside because i'm usually worse when outdoors, and the house is about 9 years old and I think past the normal formaldehyde outgassing and similar phases one normally has, and in general seems to have pretty good indoor ventilation.

The problems that i've heard of is that there seems to be no such thing as a whole house air purifier... every single one "diverts 1/3 of the air through a 99.97% filter" in other words, once you add in the extra pressure needed to even push through the filter i'd be amazed if even 15% of the air actually goes through there. Is there any such thing that actually filters 100% of the air, for really real, even if it's less than total HEPA, or can be upgraded to total HEPA level later, or similar? Otherwise it would seem that I would be just as well off using individual room HEPA filters since it sounds like nonsense to say that a furnace filter can do (X) air changes in the house per day if it's mostly bypassing the filter all the time.
Old 07-13-12, 07:18 AM
Group Moderator
Join Date: Oct 2004
Location: WI/MN
Posts: 19,163
Received 1,273 Upvotes on 1,213 Posts
All of the air in the furnace needs to go through the filter just for the furnace's sake.

My parents bought a house in MN a few decades ago which had been previously owned by people will a highly allergic child and they installed a Honeywell electronic air filter in the furnace intake. It got removed later when the furnace was replaced but I never noticed a difference in the air in the house.
Old 07-13-12, 08:32 AM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 19,281
Received 7 Upvotes on 7 Posts

I think you are sort of missing the point on the "1/3 of the air through the filter" thing. Assuming the system/unit information you looked at is accurate....it's not pushing that same 1/3 of the air through the filter each time. Lets just say it's 100% effective at filtering to make the explanation easier.

The first time all the air in the house has been re-circulated you will have removed approximately 1/3 of the pollutants in the air. The second time through you will remove approx 1/3 of whats left. The third time though...etc etc etc.

Of course that's not totally accurate since the air will be constantly re-mixing as it makes its way from the output of the system back to the inlet, but it illustrates my point. It's basically diluting the amount of stuff in the air over time. But I doubt it would take very long in actual fact.

The June issue of CR had results on whole house and portable air systems. The best results came from the dealer installed 4" thick high efficiency filters installed on the home HVAC system. Other than the initial cost the seemed to be the cheapest to operate over time as well.

Of course...it really depends on what your allergies are. All the systems only remove particles, not vapors or gases for instance.
Old 07-13-12, 12:46 PM
airman.1994's Avatar
Join Date: Dec 2005
Location: VA
Posts: 5,491
Upvotes: 0
Received 8 Upvotes on 8 Posts
I would look into a media filter like space-guard which will be like 85% efficient and filter all the air to your HVAC system and use the HEPA room models for like a bedroom and living room where you spend most of your time.
Old 07-14-12, 12:28 AM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Received 39 Upvotes on 31 Posts
Electronic air filters can do an excellent job but rarely, if ever, are they properly installed or serviced in residential applications. Unlike a "conventional" filter the efficacy of the electronic filter decreases with usage and therefore they need to be cleaned often. I have a friend that cleans his only once a year (he only lives in this house half the year) but they really need to be cleaned every two weeks and once a week is not too often.

My ideal installation would have a thick media filter (four inches minimum) installed upstream of the electronic filter. The media filter needs to be sized not so much to the duct size but to allow maximum filtering with minimal pressure drop. This often means increasing the return air duct dimension with a transition piece before the media filter and then reducing it after the media filter.

Having a larger than "recommended" electronic filter is also a good idea and definitely using larger return air ducts from before the media filter to the inlet of the furnace/air handler is best. The purpose of the larger filters and ductwork is to reduce the velocity of the air making the filter more efficacious. An installation like this would normally have the media filter changed on an infrequent schedule, maybe as long as a year or two between changes while the electronic filter cells would be cleaned every two weeks.

Of course an installation like this will be costly and take up a significantly larger amount of space, something that few builders or homeowners are willing to sacrifice. ALL the ductwork from the return air grille to the furnace/air handler MUST be properly sealed to eliminate the negative pressure from drawing in dust. This includes the door over the filter compartments and the blower door on the furnace/air handler, all of which require gaskets and positive closure methods.

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: