while read loop

you should try

awk '{print $2}' home/dir/file.txt | while read num
do
  if [ "$num" = "0" ]; then
    echo "Number is equal to zero"
  else
    echo "number is not equal to 0"
  fi
 done 

for a mixed awk/bash solution.

As other have pointed out, awk redirection occur later.

It looks like you're trying to redirect the output of home/dir/file.txt | awk '{print $2}' to the while loop;

first I guess that the correct path should be /home/dir/file.txt (however this is just an assumption);

second /home/dir/file.txt | awk '{print $2}' doesn't redirect the content of /home/dir/file.txt to awk, while < /home/dir/file.txt awk '{print $2}' does;

third, you're redirecting the output of the command as a file, but it's a string, and you should redirect it as it: <<< "$(< /home/dir/file.txt awk '{print $2}')".

Also, alternatively, you could pipe the output of the command directly to the while loop instead: < /home/dir/file.txt awk '{print $2}' | while read num.

while read num
do
    if [ "$num" = "0" ]; then
    echo "Number is equal to zero"
else
    echo "number is not equal to 0"
fi
done <<< "$(< /home/dir/file.txt awk '{print $2}')"

or

< /home/dir/file.txt awk '{print $2}' | while read num
do
    if [ "$num" = "0" ]; then
    echo "Number is equal to zero"
else
    echo "number is not equal to 0"
fi
done

Since I can't comment, just wondering, why people have not used IFS

while IFS=" " read var num year string
do
  if [[ $num -eq 0 ]]; then
    echo "Number is equal to zero"
  else
    echo "number is not equal to 0"
  fi
 done < home/dir/file.txt