[Minimac]

Before doing anything, get the energy audit. Than you can prioritize the most cost effective improvements to make. A 15 year old water tank would be among the first items on my list. A 40 gallon tank with electronic ignition(no pilot) isn't any pricier than a thirty gallon and is cheap enough to be done quickly while deciding whats next. An insulating blanket for it is a good start then insulate your water lines, if they aren't already. Remember to flush the tank,at least annually(cleans out deposits), and you will get many years of service out of it, and there will be plenty of capacity for a growing family. I would forget about the waste water heat recovery, for now, you have bigger fish to fry. Tighten up the house as best you can. Plenty of caulk around windows, doors, and sill plates will do wonders. Insulate! Your biggest heat loss/transference is through your ceilings and roof. Being in Hotlanta, look at the various reflective products for the underside of the roof and increase whatever insulation is in the attic. Just that will buy you a lot of time to have your heating system properly sized. All D.I.Y. projects with a high cost return.A high efficiency rating doesn't always translate into cost savings.