While this is a VERY basic solution, you can actually use tables in word to
create what will look like (in the printed form) a basic floor plan. The
same principle would work in Excel by the way. You just create a large table
with several columns and rows to look like graph paper. Format the table to
hide all grid lines. Then (very tediously) add colored lines to the borders
of individual cells as needed to make an outline of your exterior walls.
Then repeat the process for the outline of each room or device (much the same
way you would do with a pencil on graph paper). You can add text to the
cells for labels, merge cells as needed to create a larger "room" and
generally get creative. In a pinch, this will work and frequent users of
Word already have the necessary skills without needing to learn Word's
primitive drawing environment.
For an inexpensive and easy to learn software option, give Smartdraw a look.