One of the occasional participants in conversations at my shop during the 60s was an old-line engineer who'd installed a home-made heat pump system about 20 years before the rest of the world caught on. His worked quite well (according to him) and inexpensively, because he tapped the water table (about 35 feet down in his neighborhood) and used recirculated water at the stable ~34F temperature as the "heat sink side" of the heat exchangers.

The minor loss from heating while sinking to 35F when the OAT was a little higher was more than made up when the air got below about 38 (due to better conduction in/out of the water), and dumping the heat while cooling was a lot more efficient anytime house cooling was needed.

Since Colorado and Nebraska have drained the Olgallala I'm not sure his system would still be working (water tables have dropped about 30 feet in some parts of the area) but it worked great back then.

And gnu is correct about the need to be aware of a lot of possible results of tight sealing of a house. Radon accumulation is an almost universal result, but usually seasonal venting is sufficient if done right. Humidity related difficulties can be really troublesome, both from too high or too low moisture - and a lot of damage can result before some of the effects get noticed unless you watch for them.

Fresh air is still usually a good thing, despite some "fresh air" being rather dirtier than I remember from days long ago.

John