I would imagine that the first lesson or at least an early one learned was that a great idea may not sell while mediocre ones do. The difference can be nothing more than people skills and politics, neither of which are taught in school. Instead, these skills are either already acquired or learned via the school of hard knocks. Selling is many times more about the people than the product.
First, Central Heating may not seem like an exciting field, but I've found that it's what we do with or in a field that makes it exciting. I never thought manufacturing could be exciting until I joined Screaming Circuits. Central heating is a mess. Whether in a home, or office, I don't know that I've ever heard anyone speak complementary about heating or cooling.
The wrong amount of air always seems to go to the wrong place, at the wrong time, at the wrong temperature. I believe that the technology is available to solve those problems now, but I don't know of too many people working on it. Best of luck to you Simon.
As far as lessons learn, one of the first was pricing. It seems pretty fundamental now, but I think a lot of entrepreneurs fall into the trap (I bet Kickstarter is full of people that don't understand how pricing works). It may seem intuitive that if a part costs a dollar, it should increase the cost by a dollar, but that wouldn't pay for all of the costs associated with choosing, purchasing and placing that part into your design.
Simon is clearly finding the real world -- and being at the top of a small startup -- is changing his perception of engineering.
Can other readers remember lessons learned in their early days, fresh-faced out of education and the theoretical world of engineering?