Actually you can get by with as little as 12in. in a residential setting.
See table 300.5 in the 2005 NEC (column 4).
For direct burial circuits at 240 volts you would have to go to 18in.
however.

I complete agree however that the water lines need to be below frost depth.
Three feet at least.

The best answer is to move it into the home or to an addition attached to
the home.

"w_tom" wrote in message
...
18 inches is not sufficient for any pipes with water. Pipes
18 inches is not sufficient for electric wires feeding the
building. Why then would it be acceptable for heat pipes?