Well, Tom, I've operated here in Vegas for 9 years without a ground.  Soil 
here isn't good, and only about two feet to this real hard rock stuff. 
yep, I should have a ground, but I've really never noticed any problems, 
and I run a kilowatt at times.  I had a ground rod back in Kansas, and I 
had the main rig grounded, and one day I decided to tie everything 
together, all station accessories.  I had a terrible ground loop and rf 
into all kinds of stuff, so you just never know.  I wouldn't use the 
heating duct, it will be full of joints not bonded and with a little 
corrosion, lots of diode effects and I think that is worse than no ground at 
all.  As always, your mileage may vary.
73
Butch Bussen
wa0vjr
open Node 3148
Las Vegas