Unfiltered outside air intake on return

I’ve read justification on this but this is the first time seeing it in this configuration. The home was a condo with the furnace in the attic. The outside air intake was pulling air in from a roof vent and depositing it inside the return air plenum. No damper was present. The filter was upstream in the upstairs ceiling (so no filter at incoming fresh air). My concerns:

  • Wouldn’t the unfiltered dust brought in from the outside negatively impact the air handler and create unhealthy indoor air?
  • Wouldn’t this reduce the efficiency of the system, having to cool down hot, outside air in the summer, and heat cold winter air?

As you’ve probably heard on the news, we had a significant fire in San Diego yesterday. What if this condo were near the fire and the occupants were running the AC? The house would fill with smoke.

Had one the same here Darin a few mos ago. Called my HVAC contact and he said its a newer code thing where structures zoned commercial (coa’s included) are required to have a certain percentage of fresh outdoor air being taken into the system to prevent buildup of fumes etc, which is grest for warehouses, but not for residential systems. He stated they always disconnect them for all the points you made already.

I have seen these high efficiency units draw air from a 3" PVC pipe that is located outside the foundation area… With a screen present. I agree some type of pre filtration would be in order but I am unclear if it’s required. Benefits include creating a positive air pressure within the house and having a fresh air supply (not from within) for the combustion air and intake during the cooling cycle. In Florida it’s a requirement for all new systems to have a fresh air supply high efficient or otherwise.

Glenn and Rick - thanks for the comments. In milder climates like here in San Diego this could have benefits I suppose. Seems the fresh air intake would be better located on the other side of the filter but maybe there is a good reason for piping it directly into the return plenum.

A properly designed system would have a damper and the filter would be downstream. Benefits of fresh air intakes are to replace exhausted and leaked air and to provide air changes, required when construction is ‘tight’.
However, it has been said that outside air is often cleaner and less contaminated than air inside a home, especially in newer houses that are still off gassing from all the fake wood, plastic flooring, glue, paint etc… YMMV SD had pretty nice air when I was there a few years ago :slight_smile:

Outdoor air is less healthy than indoor air?

Just askin…

Another question comes to mind; What is the air filter in the HVAC for?
It’s not to clean the indoor air!

it’s been my understanding the filter is to protect the equipment not the people…

That would be correct Mr. Jim! :wink:

CFM “IN” should be slightly (+) with reference to the interior.

A means of controlling this volume of air will prevent over pressurization and excessive energy loss. If you have 30% duct leakage to the exterior (like Most homes) This will draw excessive air through this piping and will significantly affect efficiency and performance.

If you do not have a means to shut off the damper to this air intake, you will continue to have air infiltration during the off cycle.

I would hope that that would set off the smoke alarm and reiterate the important fact that you should get your *** out of the house, now!

Most people aren’t aware of that.

That is true, and may be because of this, or similar. :wink:

Ultra Allergen Air Filter

The 3M Filtrete Ultra Allergen air filter reduces the following contaminants from your air:

  • Smog
  • Smoke
  • Pollen
  • Bacteria
  • Pet dander
  • Mold spores
  • Household dust
  • Dust mite debris
  • Virus carrying particles

Not only will the Ultra Allergen Filter use its electrostatically charged fibers to attract and retain micro-particles & airborne allergens **but it will also capture bacteria and particles that can carry viruses :shock:

Yes, if that equipment catches any of that stuff, you’re in for a costly service call. :stuck_out_tongue:

those were just fine as long as you triple the size of the return Plenum!

Create a pressure drop and you lose your compressor or heat exchanger.

Just got a call about a cracked heat exchanger this morning due to restricted return airflow I identified during the inspection.