Is this an up flow system,(vents in the ceiling) or down flow, (vents in the floor?) As always, theres a place to add filters , before the fan on the suction side! No telling how long this been running without the filter and already contaminate the freshair system.. This is supposes to be checked out by the building inspector before you buy any house,Just talk it over with the Realestate Agent , have the full system cleanned and send them the bill or talk to your big brother the BBB! Thank-you-very-much!