Normalize, normalize, normalize.
http://www.datamodel.org/NormalizationRules.html

First, yes, you're right.... your table structure is not normalized, and
you're seeing just one of many problems of dealing with unnormalized data
structure -- namely, the ridiculously complex expression that you need to
use to get an average.

What you need is a child table with four fields in it:
   tblHourTable
           HourID  (primary key -- autonumber is fine)
           LinkingField  (foreign key back to main table)
           HourNum  (long integer to hold values 1 through 12)
           HourValue   (field to hold the value for each hour)

The linking field should hold the value of the primary key field for the
record to which the data are related in the main table.

Then your expression becomes:

=Sum(Nz([HourValue],0))+[Factored_DT])/Sum(Abs(Not [HourValue] Is Null))

To correct for the possibility of the entire denominator being a value of
zero:

=IIf(Sum(Abs(Not [HourValue] Is
Null))<>0,Sum(Nz([HourValue],0))+[Factored_DT])/Sum(Abs(Not [HourValue] Is
Null)),"what value you want when denominator is zero")

As has been pointed out, you really need to normalize this data.  One
workaround is to write a custom VBA function and use it in your query.

You could also try something like the following untested expression.
Although the length of the expression may get close to the allowable length
for an expression in the query grid.

IIF(Hour1 is Null and Hour2 is Null and ... and Hour12 is Null, Null,