There are a lot of components that make up the central air conditioning unit, and in order to troubleshoot what is wrong with your air conditioner, you need to understand a wide variety of components that make up the system including refrigeration, electricity, electrical components, electric mechanical controls and AC(air conditioning) components. Since the system can be complex it is probably better to have a trained HVAC specialist look at and diagnose the unit rather than attempt repairs yourself, but you should also be familiar enough with your ac unit so you understand how to maintain it.
So you think you want to clean that oil furnace or oil boiler yourself. You have watched the service guy do it and really didn't think that it looked all that hard. You stick the hose of your shop vac into the chamber and all the soot is broadcast all over the place! Yikes, what did he have that you don't in there? When cleaning an oil fired appliance you need to remember that the dust created by the unit is very fine and will just go right through the filters that are standard to most shop vac's. You will have to go to the store and find some special equipment to catch all that fine black dirt. It is very easy to find if you know what to look for. One final thing for you guys. If you are planning to use the shop vac that your wife uses for her everyday cleanups, you may want to rethink this. After you get done with it and have black soot all over the nozzle and such you will be sleeping in the doghouse if your wife goes to use it and ends up with black on something good of her's. You have now been warned. Don't blame me for anything that happens to you as a result of not heeding this warning.
First, let us say we have been installing furnaces since 1989 with next to NO repair problems. This includes the 92% versions. We ran a new house HVAC business for 10 years installing not only the furnace, air conditioner or heat pump but all the ductwork, wiring, in other words, a complete turnkey installation. We did this on between 150 - 200 new homes a year for this 10 year period. The size of the flue pipes ( diameter ) runs between 2" and 3". The size of the furnace and the length of the runs of pipe, as well as the number of elbows, will dictate which diameter of the pipe is needed. Here again, if the installer AND the estimator don't understand this, you can count on problems! In other words, if 3" pipe will be needed SOMEONE has to be sure there is enough clearance to allow for installation AND the 1/4" pitch.