When water freezes, I think I remember that it expands about 5% - don't hold me to this, I'm working from memory and I am old. If you have drained 90%+ of the water and still have a 1/2" or so in the bottom of the heater, IMHO it will be perfectly fine. Ditto for the fresh water tanks - you probably still have a little in the bottom below the hose barb. And the lines if blown out will be just fine, even if there is a small dip someplace. As long as the water can't push against anything solid, it won't do any damage.
And the petcock at the bottom of the Seaward heaters is probably not exactly at the bottom either. So if you think that you have it all out, you probably still have a small amount left.
20 years of draining my Seaward through the petcock has resulted in success every year.
I drain my water tanks and then blow back into the tanks to purge the lines from the tanks to the pump. I blow out the pump- very important. Then I blow out all of the lines to each faucet. I used to use my dinghy pump for this but now I have a CO2 bottle that I pressurize the system to 15# and just walk around opening each faucet in turn. Again, 20 years, no problems.
And the petcock at the bottom of the Seaward heaters is probably not exactly at the bottom either. So if you think that you have it all out, you probably still have a small amount left.
20 years of draining my Seaward through the petcock has resulted in success every year.
I drain my water tanks and then blow back into the tanks to purge the lines from the tanks to the pump. I blow out the pump- very important. Then I blow out all of the lines to each faucet. I used to use my dinghy pump for this but now I have a CO2 bottle that I pressurize the system to 15# and just walk around opening each faucet in turn. Again, 20 years, no problems.