How far to drain water system in cold?

I have a remote cabin in WA state, which I visit infrequently in the winter.
When I know it's going to be cold (below freezing for several days), I "drain" the water system and turn off the well pump/pressure tank. By draining, I mean I ensure all the water is out of the pipes in the main cabin, and also partially drain the pressure tank and hot water tank (in a basement under the main cabin) as well.
My thinking is that completely draining the two basement tanks is safer, but it takes long enough that instead I have been draining 5-10 gallons out of each to allow for expansion if the water freezes up. Is this sufficient to prevent the freezing from rupturing the units or doing some other permanent damage? The pressure tank is about 50 gallons capacity, the water heater is 30.
Tnx
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It depends on how cold it is going to get. If it is just below freezing for a few days, then your plan is probably OK. However, if it gets cold enough to freeze your water heater solid, then you are running the risk of ruining things.
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Water freezes,then it expands, drain the tanks.
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ransley wrote:

Slight correction: water expands, then it freezes. As the temperature drops, water shrinks until it reaches its maximum density at 3.7C, then expands until it reaches 0C. Then some of it freezes. As more heat is removed more of the water becomes ice. The water and ice combination do not drop below 0C until all of the water turns to ice.
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wrote:

A long term - one week - hard freeze, like in the low to mid 20's, is going to cost you a couple of tanks. You have to remember that the expansion will go upward somewhat but as soon as the surface of the water freezes the upward movement reduces and expansion equalizes and horizontal expansion catches up with the vertical - I'm rambling. Sorry. Why not leave the water heater on and wrap the expansion tank in heat tape and insulate it. You can buy heat cable at a harware store for such applications. Forget the plastic wrapped, ten footers. This stuff is bought by the foot and a controller is purchased and attached to the end. It sure is a PITA, isn't it!? I have the same issues and I do what I listed above. Fortunately, my espansion tank is below ground, insulated and only 30 gallons. Fifty is HUGE !!!
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