I have a 1995 build house in southern Michigan. The basement is unfinished, and the furnace and water heater are not enclosed. I replaced the old 80% furnace with a 97% efficiency furnace, but due to challenges with the layout of my basement, they set it up to use indoor air rather than outdoor as for combustion. I also recently replaced my water heater with another atmospheric vent water tank heater. I was not offered the option of a direct vent, but in retrospect wish that’s what I had purchased. 

I have a makeup air vent going from outdoors through an insulated flex pipe to near the furnace and water heater. I am now appreciating how much cold outdoor air that brings into the house, and presumably hot humid air in the summer. I would like to make my house more efficient and comfortable, but hopefully in a way that is not extremely expensive or burdensome. Here are some of the as I see them; I would like opinions on which I ought to choose and whether I’ve missed any: 

Live with the furnace and water heater as they are. 

Enclose furnace and water heater with a wall around that corner of the basement to minimize the impact that the makeup air has on comfort in the rest of the house. 

Have the furnace air intake routed so that it pulls combustion air from outdoors ($450 per quote from company) and the new water heater replaced with a direct vent (~$2500) so I can eliminate the makeup air vent. 

Anything else? 

Thanks