Basement – Extra vents required for Direct-vent furnace

If the furnace and water heater are gas powered, it would be to provide adequate airflow for their combustion so that you don't get carbon monoxide forming in the utility room.

Here's a link with some information. An excerpt:

The heating appliances are fully isolated from the living space with partitions. Figure 1 provides two vents to the outside, one within a foot of the ceiling and one within a foot of the floor. If vertical ducts are used to bring combustion air to the appliance each vent should be sized at one square inch of free vent area per 4000 BTUH of the appliance input rating.

(This is based on a half-remembered long-ago conversation with a service person working on my furnace and heater, which are in the center of my mostly-unfinished basement: I mentioned framing in a utility room around them and he said not to for that reason).

I am a little concerned that you are overthinking this situation. Naill C gave you some good advise in your last question. There are very few houses not built to be virtually air tight with an air exchange system that do not have enough natural air flow to support open combustion appliances.The simple act of opening a door to enter or exit the house allows large amounts of fresh air in during cold weather when windows are closed. Unless you have made extreme attempt to block all air infiltration, there is probably pleanty of combustion air passing through your louvered/slat door. I totally understand your desire to minimize heat loss from the rest of the house. Before any external venting will improve the situation, the utility room will have to be somewhat sealed from air flow from the living space of the house. In new construction a vent of a calculated size is brought in from the out of doors, screened and baffled, and strategically located just above the floor by the appliance. The mechanical room is almost air tight, insulated and lined in a fire rated material. Since this space is indoors and only calculated amount of outside air is allowed in, freezing is not normally a concern if water utilities are also in the room. Drawing air from an attic can be risky. Obviously some think it a good idea, however there are drawbacks. If the attic is not well ventilated the air may not be sufficient. I simple vent register in the ceiling to the attic will let as much heat up as it pulls air down. In fact, any open path into the attic can be dangerous. A register, vent pipe etc. can act as an easy path or chimney for fire or natural gas to spread quickly into the attic. If you decide to proceed in isolating and venting your open combustion appliances, pick one of the proven techniques and run it by your local building code officer for local compliance. Better safe than sorry.