An unusual problem for sure and my HVAC contractor (or anyone else in the business) has not been able to figure out a solution. I thought I would reach out to a larger group of experts that may know a bit more.

Simple goal: to have the fresh air intake be turned on ONLY when the AC/Heat/Fan modes are engaged. NOT a preset time per hour, and NOT run the fan to make up time that the system was not running. Very simply, when the HVAC fan is running, the fresh air damper opens.

Currently using a the April Aire controller/damper connected from an outside air source to the return plenum. A Lennox Pure Air system with the 20x25x5 carbon Marv16 filter was also included in the new HVAC system, as were new ducting.

The reasons for this are a bit more complex, for the outside air in the front of house, especially during the evenings is contaminated from a nearby septic system venting to atmosphere. These noxious chemicals accumulate in concentration at a lower section in the front of our house and infiltrate the home through the window vents, facia board gaps and entry door. Although the attempt of using the fresh air system was to "pressurize" the house, it is clearly insufficient to overcome the leaking of the system in the attic (BTW, that passed the HERS test, and I was told ALL HVAC systems "leak").

The chemical off-gassing has not yet been deemed above legal standards, although the concentrated effect that makes it way into our house makes your nose run, and irritates your throat and lungs. Just uncomfortable to breath. When the evenings were warm, outside the home is just horrible. Without legal council, and lots of $$$$, we are screwed. So it may be just better to resolve the issue through strategic ventilation. Mold testing shows no mold.

By using the fresh air intake ONLY when the system is running, it minimizes the negative pressure caused by the HVAC and reduces the bad air being drawn in. The furnace is an Amana AMES800804BU. The board has connections for an external humidifier (24V) and electronic air cleaner (120v), however after review of the circuit, it would appear these are always on connections and not on when the system fan is running. Connecting a relay to the FAN call from the thermostat will work for fan mode, but will do nothing for AC or HEAT where the fan runs at a different speed.

I have several decades of experience with mechanical engineering and machine design.

Thank you.

  • Have you considered a "hot roof" (where the attic is part of the air-sealed/conditioned/insulated space instead of being vented)? There is something to be said for having your HVAC ducts inside the space they're conditioning...also, have you considered air-sealing the culprit penetrations at the front of the house, for that matter? Jan 23, 2021 at 21:18
  • Also, is this in an area where municipal sewer services are not provided...? Jan 23, 2021 at 21:31
  • Hillside area on a Private street. Everyone nearby is on the sewer except the one house. Ground fails to percolate, but their septic system and leach lines installed 40 years ago. Air sealing has been done as best as possible, however the windows have the drain vents that you can feel the air blow in when the HVAC is on.
    – Robert_S
    Jan 23, 2021 at 22:10

1 Answer 1


The easiest way is to install the fresh air intake on the return side of your system. That way it sucks in when running and then is filtered, simple and inexpensive. If you want a self powered system again it is easy enough to put a pressure switch or vane flow switch in the duct so the system only turns on when the air handler is running. This is simple stuff. Your hvac should know about both systems as they are common as safety’s.

  • It is installed this way as a passive system, to the return plenum. Good idea about the vain air flow switch to control the damper. Thank you. Not sure why the HVAC guys did not know of this.
    – Robert_S
    Jan 23, 2021 at 22:13
  • I have read that the EAC control only engages during any fan call, but uncertain for my model furnace. If this is true, it would simply be run the EAC output to the step down transformer and then to the damper, disregarding the controller completely. Or perhaps run it to the AprilAire controller and set for 60 minutes per hour. This way, when temp or humidity extremes are there, the outside air is not mixed.
    – Robert_S
    Jan 23, 2021 at 22:14
  • The only issue I see is connecting to the EAC, it is not energized during a FAN only mode. Also, running the EAC controlled 24v output to the Aprilaire controller will only disable the damper, but the fan will start based on the timing cycle for it gets it voltage from the thermostat.
    – Robert_S
    Jan 23, 2021 at 22:33

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