Coultl, the trick to a tank heater without a pump is to 1) mount the heater so that the cold water has to "fall down" into the heater, and 2) the "heated" water line that you tap into is higher than the "cold" water line. Or you can weld a nipple on one of the other frost plugs (not the block drain plug) and this becomes your low "cold" line. daBenz was like this before I took the heater out. No big deal to leave the old heater screwed in. As PEH implies: set the cab to defrost on high temperature and the heater will also defrost the windows.

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daBenz - 1970 220D