We live in an old home with an old AC unit in it. On these 100 degree days, the coolest it can get the house is 78 during the hottest time of the day. I have the money saved up for a new unit, but I'm going to let this one run as long as it can.

One question I have though is about the location of the air handler. The air handler is in an interior closet of the house, right next to a water heater. It's a fairly big closet with lots of wasted space. However, when I open the doors to that closet, it's a good 10 degrees hotter in there than it is in the hallway outside. There are vents in the door to the hallway,and also vents in the side of the closet to the next room over. There is also a vent in the ceiling leading to the attic. I assume those are all there to help with air flow, and air draw into the handler.

So my question is...Since the closet is hot, is the handler just sucking in hot air which is decreasing the ability of the AC to cool it?

1) Would it be a good idea to leave the closet door open during the hottest time of the day to allow cooler air into the closet, or is that a bad idea because I would them be releasing hot air into the hallway
2)Should I block off the vent in the closet that leads to the attic. I'm sure a ton of hot air is pouring in from the attic, but I have to believe someone put it there for a reason. There is no duct work attached to it. just an open vent in the ceiling. There seems to be plenty of other vents in the floor of the closet to allow air flow.

Post some pictures.

Where does the "air handler" draw air from? In other words where is your AC filter?

Are you saying the "mechanical closet" ceiling is open to the attic and the opening covered with a louvered vent? Also open to the hallway & another room thru similar vents?

The air handler is sitting on a riser. The filter is in the bottom, and it sucks air from below the riser up through the bottom. There is a drain in the floor under that riser where the condensation line drains, and the relief valve for the water heater is plumbed if it were to ever open.

Here is a picture of the vent in the ceiling. It's not louvered meaning I can't open and shut it. Just a grate covering an opening to the attic.

Lastly, I included pictures of the vents on the wall in the room opposite of the closet. These vents are on the bottom of the wall and open under the riser where the handler is.

Your furnace is pulling air in from the return air vents, not the closet that it's in. That same unit can go horizontal in an attic, there will minimal difference in performance from your 85 degree closet to the 140 degree attic.

Wow, insufficient return air area is problematic but all those grates are overkill. I assume you just have a small filter in the bottom of the air handler. So quick look says return air under the riser, but how does the riser seal against the closet door, especially with the pipe hanging over the edge. If now flush and sealed against the door some of the return air will be drawn from the closet above the riser, and with vent the attic also.

It's a decent size filter. Going from Memory so I may be off, but it's like 20" x 24" x 6" thick.

The riser has walls on three sides. One wall has the three large air grated you see in the picture. One wall has nothing, and the wall opposite the one with the three grates has one small grate. However, the side of the riser facing the closet doors is wide-open (There is a piece of sheet metal on the right half, but the left half is open). Certainly wide open to the closet as when you close the closet door it doesn't remotely seal that opening. There is a good 18-24" between the riser and the closet door. I included a pic. I assume it is open so the drain in the floor can be serviced. That PVC pipe you see in the picture is the drain line from the pan the water heater sitting in. Hopefully it never sees any water. The condensation line from the AC handler go through the top of the riser and ends at the drain.

It seems you have a GAS HVAC and thus it needs combustion air ventilation, as you described its closet.
CA air is not part of the closed HVAC conditionaed air system, thus is not to be tampered.

Improving your closed HVAC air system as it cycles through attic ductwork could be improved with foil-face up batt insulation (top of ducts) + losts more attic insulation of any type + exterior wall shading (esp. at glass) of any type.

Be sure to purchase the biannual HVAC Maintenance Service deal to insure every dollar spent on energy is making the most COOL possible with a "well oiled machine". Have them check your humidity levels and possible duct leaks.

If there is a vent in the door it would have been a source for return air but the above riser needs to be separate from the below riser especially with the grill plate directly into the unconditioned attic. The grill and water heater overpressure drain look like afterthoughts. Grill for combustion air to solve pilot wont stay lit issues and the drain by lazy installer. The riser needs to extend against the door and seal to prevent you drawing return air from the essentially unconditioned closet. I also have seen cases where there was no sheetrock below the riser and it was allowing return air being drawn between the studs out of the attic area. Your return air needs to be only from your conditioned space and your equipment closet is not because of the attic grate.