Pulling 8 Gauge Wire, 1/2" Inch Conduit, for 150 Feet Difficulty?
There are rules on how much you can stuff in a conduit. Part of it is based on heat, but part is simply based on basic physics - you can only squeeze so much stuff into a small space. The rule in Chicago (or anywhere that uses (required or not) conduit) is not 1/2&quot; conduit, it is conduit that is big enough for the job. Here is a table (there are many available, Google is your friend):
Conduit Fill Table
It clearly shows you can get 3 or 4 #12 or #14 circuits into a 1/2&quot; conduit. But you can only get 3 #8 wires into a 1/2&quot; conduit, and that is if there is nothing else in the conduit.
The only practical options are:

Remove everything else and run just the #8 in the 1/2&quot; conduit to a subpanel and connect everything that was connected directly instead to that subpanel. Note though that the #8 wires may not be big enough for a proper subpanel for both your heater and all the existing circuits, and according to the table I linked, you can't bump up to #6. But it does avoid new conduit.
Run a new conduit. To avoid being maxed out immediately, go up to at least 3/4&quot;, maybe even 1&quot;, so that you can add more circuits and/or upsize the wires later.

As noted in another answer, #8 wire isn't big enough. You could get 2 #6 in there to run a heater, but then you couldn't run a subpanel (because you wouldn't have a neutral). End result: New, larger, conduit.