Can you use a magnehelic to verify room/bldg pressure? I find that if i light a match and open the door there is a slight draft to the outside so that tells me that im slightly positive- good. However a have a compuund magnehelic and if i zero it outside, cover/plug the low side port the pressure always drops slightly below 0" ? Ill spare the long history but the reason im checking such things is because the bldg I take care off uses 2 AHUs serving vavs that run off of about 1.8" static. There is hardly any flow to a few of the boxes and inlets at the boxes that are open to the ceiling plenum return where the filters go the air actually blows back at you rather then pulling in. FAQ- rotation is correct on everything, the dampers modulate as they should, there are no restrictions in the duct to push the air back- Ive actually taking runs off to verify the latter, all diffusers are open? Very strange.