[conradpdx]

Well, I know a few things you should consider. Soil type will influence the depth of the trench. I wouldn't necessarily hope for mud deep down. Depending on your local climate that could mean that things will freeze deeper than your "frost line". Also wet soil round the exchange coils could mean you might have condensation and humidity problems which could turn into greater problems with mold etc. inside the house.

Most systems either drain into an underground leech field or back into the house to be pumped away. If your house is supported on a basement foundation this wouldn't be a problem, you could easily slightly slope the exchangers so they drain into the house then pump/drain it there. If your houses foundation is above surface grade too moist of surrounding soil would make it so you'd need to dehumidify the air as it enters the house. If you use a leech field and the soil is constantly damp (even before the exchangers are installed) you'd get greater condensation on the tube walls which will amount to too much humidity entering the house, perhaps even in the exchangers themselves. Which (I doubt it'd happen at 5 ft. too much thermal mass over top) could perhaps cause freezing within the pipes which would be a disaster.

Even if you still plan on doing it yourself I'd at least spend a few hundered bucks and consult someone that designs these type of systems professionally, at least to determine the size and depth of your field. After all $500-1000 dollars to know you get right would be paid off really fast if you DIY'd this project. Especially if it saves you from having to add on to the exchange coils or fix slope/grade of system later.

Now let me also say I'm by no means an expert. I spent about a month exploring my possibilities before we went with the air to air exchanger. Luckily the climate here in Portland Oregon seldom gets below freezing so such a system is possible. I'm not saying that you can't do it, but I'd hate to see someone get in over their head or not think of all the possible outcomes I know for awhile there I was considering renting the backhoe and doing it myself as well, but it was one of those things that the more I thought about it the more I realized it'd be easy to get in over ones head on this project. And I do some fairly extreme DIY projects ( half wall structural cutouts, below grade egresses access points etc.).

It very well can be easier than I think too. After all before we went the heat pump route I wasted about $4,000 on electric radiant floor heating mats that I was convinced would do more space heating than they did.