Thanks for your reply. The air conditioner was serviced a year ago and the furnace a couple weeks ago. Never had vents cleaned except the dryer vent once. I do supply furnace filters to encourage and require changing once a month. I wouldn't usually allow self repairs but I know the vent cover comes right off and don't see that much harm can be done to try a simple vac. But Murphy's law says the tenant will find a way to break it, right?