In planning, I was considering an earth tube for the hrv, but with the potential moisture issues, filtering, and cost, just don't think it is worth the investment.  But I was thinking, I am going to have an insulated, unheated attached garage, with some south facing windows. The garage should be warmer in winter and cooler in summer than the outdoors. Wouldn't it be a good idea to run the HRV ducts throught the garage to "temper" the air before delivering the air to the HRV. Should be an inexpensive way to reduce load.

As long as they are totally sealed from exhaust, I don't see a problem, some inspector might

Someone I know did an earth tube and had it running to a small hydronic coil with a matching one near the HRV. Seems to have tempered the air to the house quite nicely. Took away the chill in winter and heat in the summer.

Dang it, forgot about the old garage penetrations thing.but if I put it above the sheetrock? Then it should not be an issue.

Your garage doesn't have enough available heat content to "temper" the incoming air through a duct and what it does have it already got from the house. This won't do anything worthwhile for you, energy wise. If you are looking to avoid chilly incoming air, you can use a heat coil to warm it after the HRV. My heating coil comes off the heatpump. If I had a greater need for cooling, I might have played around with a rudimentary earth loop, but as it is, it isn't worth it.

What ICFHybrid said- an uninsulated piece of duct is a piss-poor heat exchanger, and pulling the heat from a mass as low as a garage is useless.

Earth tubes can work thermally because the thermal contact between the dirt and pipe is so much better and the thermal mass of the subsoil is quite high, but even there you're typically looking at on the order of 100' or more of 6" pipe, not 25' across a garage.

In general earth tubes make the most sense in dry or very-dry climates, but are fraught with issues when applied to regions with high summertime dew points (or even moderate summertime dew points.) Bang for buck it's one of the least cost effective approaches to energy efficiency in most situations.