Furnaces break every day. When they do, you need to hire someone to fix it, otherwise known as a HVAC technician. If you've always felt you were mechanically inclined and helping folks with their heating and air issues, you can study to become one of these contractors.
HVAC stands for Heating, Ventilating, and Air Conditioning. Basically, these technicians deal with all machinery that regulates the hot and cool air inside commercial and residential buildings. The field is considered a worldwide enterprise and is highly regulated by several standards organizations, so there's a decent amount of training and certification involved to become a part of this industry.
If, instead, you decide you wish to sell furnace and boiler equipment, then you have more business training to look forward to. You'll need to be familiar with business practices in the area, how to manage employees, and be familiar with local, state, and federal tax laws. Selling the products wholesale strictly to commercial clients may make things easier in regards to tax laws.
In either case, you'll need to contact the manufacturers of the units and parts to see if you can purchase systems in bulk. You'll also need a place to store equipment, like a warehouse or storefront. Smaller pieces like filters and pumps may be easier to store while large items like boilers, ducts, or even industrial systems may be difficult to fit. Plan your building accordingly.
Plan on studying the industry so you can advise your clients on the proper pieces they need. Know the differences between home furnaces run on gas, oil, propane, and wood. Help troubleshoot electrical circuits. Be familiar with water pressure and how hot the water needs to be for the furnace to work properly.