No drain? I can't get enough info from your picture, but I'd start by reading the hot water heater manual. If you don't have it maybe you can download it from the maker's web site or call the heater maker's support dept.

BTW I used air to blow out the lines last ear and I'm a convert. I used a high volume low pressure powered dinghy pump (not a workshop compressor) to move more air and assure the pressure doesn't get too high.