Drain your tank, but also, drain the P-trap under
the sink.  There is a small bolt in the bottom
with a flat, flared head that you can unscrew and
it will drain the water from the trap.  If you
don't and it freezes, you can crack the plastic
and then have a leak under the sink next year.

If this has never been done, the small bolt might
be hard or impossible to unscrew....  How do I
know all this?....

You can (barely) see what I'm talking about in
the NW corner of the hemispherical P-trap in the
pix at GoWesty:
http://www.gowesty.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=253-069-183A&Category_Code=cp-ce-h20

                                Richard A Jons
                                Boulder, Colorado