[Greenbuilding] Repurposing Heating-Cooling ducts to Fresh Air only

[conservation architect]

I am working on a master plan for a deep energy retrofit of a fairly large 2 
story frame house.  This building was completely renovated in 2007.  At that 
time a ducted HVAC system was installed.   In spite of gutting the house and 
installing fiberglass insulation in old framing cavities with new wiring and 
plumbing, it is still very drafty.  The cracks in exterior siding of walls 
and the ceiling are still the source.  We are renovating an attic which is 
an opportunity to upgrade the roof plane thermal barrier.  We are planning 
to foam wall over old siding and incrementally build a continuous thermal 
foam barrier with future projects.  Though quite drafty, are intentions are 
to eventually be very tight.  At that time, we will need active fresh air 
ventilation.

The HVAC ducts have the typical flaws.  I could feel the cool air  drafting 
out of joints  in a hot attic.  Even though this setup is only 6 years old, 
I am encouraging the owner to switch to zoned mini split system to avoid all 
the duct heat losses.

I am wondering if I could repurpose the old heating and cooling ducts to 
fresh air delivery only.  My thought is that the old ducts would be 
oversized relative to the fresh air volume needed to be delivered. 
Therefore, they could flow at lower pressure and therefore less duct 
leakage.  The ducts already connect to all parts of the house.  Perhaps the 
registers could be balanced to deliver the right amount of air.  Given that 
these ducts already connect to finished grills, that could save over 
installing a new system.  If the duct leakage is a problem, we could 
potentially run a duct sealing procedure to get to 5% duct leakage.  This 
might be less costly than installing a new system.

However, is there a case to be made to install a simpler new system?

Your thoughts are always appreciated.

Eli