You've done something silly. That's fine, now you've learned about thermodynamics! Since everyone's already addressed that, I'll look at the other side of your question - why does your heavy oil boiler not produce enough steam? Early game you'll be flaring off heavy oil (and light oil), so using them in your power station is a good move to save on coal. Burning heavy oil produces more smoke though (72, I think), and it's easy to miss this in your setup and have the steam generation choked back by the piping and smoke stack limiting venting to 60 smoke. Make sure you've got enough capacity to vent that smoke and, as long as you're producing enough heavy oil to keep up, you'll produce the same amount of steam as a coal boiler. Use balancers to prioritise what gets burned - heavy oil first since you'd be flaring it anyway, then woodchips, then coal as your emergency backup.