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There are 2 ways you can accomplish this. The first way is using compressed air.
Open all the drains in the system , including the hot water tank. (should be a drain plug under exterior cover) You can purchase a air-fitting adapter for the city water connection so you can blow the air out from this location, places like camping world should carry this adapter. Run the on board water pump until no water comes out the faucet at the same time remove the plug for the hot water heater. Connect the air adapter to the city water hookup, and close all the drains except the one for the hot water heater. Blow air through until all the water is removed from the hot water heater. Next Close the bypass valve on the hot water heater, normally this is located in the interior you may have to remove a panel to access the valve. After the bypass is closed open the faucet furthest from the water pump, blow air until only water comes out shut the faucet and repeat procedure working furthest away from water pump, make sure to do the shower. Open the toilet valve and allow the air to force out any water there. Close all faucets the system is winterized.
The second way is with Nontoxic Anti freeze. You can pour a few gallons directly into the fresh water tank and run the water pump until the antifreeze is visible (pinkish color) Make sure the bypass valve for the water heater is open so you are not filling the hot water heater with anti freeze. You can also install a connection at the water pump inlet and pump the antifreeze with the water pump. IN this case make sure the fresh water tank is empty and the hot water tank is drained and bypassed..
One other note, IF the unit is being plugged in for storage make sure you shut off the breaker for the hot water heater. What happens is the heating element can burn up if there is no water in the tank and the power is left on..
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