My raised spa had it's jets installed today and when I operate the spa, the jets are blowing air even with the blower off. I have a knob on the spa coping fully closed so no air comes through the Venturi system. Near the spa blower I can hear the PVC pipe sucking in air, so I assume this is how air is getting sucked into the spa jets. Obviously I don't always want air being mixed in the jets so what can be done to fix this? Should some type of flapper with a spring be put in line on the PVC pipe the blower is attached to?