I live in South Carolina - it's currently 19 degrees out and during the summer it can be up to 100+ degrees, I need a new unit but I'm not sure if a 2 stage unit is necessary - I have a single stage unit now - and it works okay, but I have some return issues - If I get another return installed and another vent installed in my living room (largest room in the house) will a single stage unit be sufficient? The price difference is about $1,000... is it worth it?

Who says you need a new unit if the old one is working OK? For such a small house, 2 stage won't make that big a difference and your payback would be fairly slow. You typically need more AC in SC than heat, unless you are along the NC border. Is this a gas unit or heat pump?

Heat pump, split system - the unit I have is 16 years old and looking pretty bad - plus I wanted to take advantage of the tax credit. I keep hearing about better comfort and quieter - I want comfort - don't care about quiet - I have a lot of cold spots - but with another return and an extra vent, you think that would be fine for a single stage unit?

Heat pump, split system - the unit I have is 16 years old and looking pretty bad - plus I wanted to take advantage of the tax credit.

With the tax credit it is possibly a worthwhile investment. However, your old unit has a pretty low AFUE/SEER so anything would be an improvement 1 or 2 stage. Make sure to get a unit that will function at a lower temperature or you will be buying a lot of straight electric heat. Critical to any HVAC system is the air flow. If you don't have decent circulation at relatively low velocities, you will never be comfortable in the house, cold in winter, hot in summer. Personally, I'd go with a 1 stage and spend the difference on improving the ventilation. As far as tax credit goes, unless you have to pay taxes, don't be sold on that issue. I am retired, get along quite well with SS and a pension and haven't paid taxes in 3 years due to exemptions and standard deductions. Tax credit is no good for me.

When you say improve the ventilation do you mean putting the extra return in bedroom and vent in living room? Unfortunately I'm 30 and have many years of working and paying taxes ahead of me

Yes, adding vents and returns will be of great benefit. Oh well, I'm 69 and enjoying retirement! Good luck with your project. If my answers have helped you you can "tip" me by checking "YES is this helpful." Thanks, brian