Growing up, I remember my father walking into little used rooms---the guest room, the study---and shutting closed the vents in each room. It turns out he was trying to restrict the flow of warm air into these barely inhabited rooms to divert the heat into rooms that we were in.
If your home runs on a central heating system, this is a great way to bring more heat into your home without having to actually raise the thermostat. In fact, you may find that once you close the vents on several rooms, you may be able to lower the temperature if the other rooms get too warm.
But when weekend guests do show up, do your best to not only open the vents to warm up their room, but also do one simple thing to help keep the air clean, too. Be sure to wipe down the vents with a damp towel to pick up all the collected dust and debris that’s rested on top of the vents. If left undusted, the heat can re-circulate the dust right into the air. Achoo!