About every 6 mths a similar post comes up, and my response is pretty much the same.
So here goes....
There basically two types of "convection" ovens. The first is the plain-jane N. American Blodgett/ Baker's Pride/Vulcan ovens, the second type are the ultra-expensive Euro ovens like Rational, which are normally called "Combi" Ovens. For this post I will refer to Convection ovens and Combi ovens.
There are two types of convections, gas, and electric. With gas ovens you have the gas burners enclosed in a box on the bottom, called a "fire box". The fan sucks up the heat through the back wall and pumps it out in the middle of the oven. With Electric, the heating elements are arranged around the fan. Many municipalities will allow electric ovens to be installed with a hood or ventilation, as this oven produces neither exhaust or grease-laden vapours--which is why you see 'lectric convections in many supermarket bake-offs. But each municipality is different. You will need a hood and ventilation for gas convections.
There are two laws you have to acknowledge, the first is that heat always rises. What this means is that a gas convection oven will always be hotter in the upper 1/3 of the chamber. The second is that the fan always rotates clockwise, thus the right hand side of the oven will get more hot air blown at it than the left. The combis are much more intelligent with the fan blowing one way for 5 mins, then stopping, then reversing direction for 5 mins, then stopping and reversing direction again.
So, with a gas convection your top r/h side side will always be hotter, items will bake faster and take on colour faster. Same with the electric.
The convection has only one temperature zone, there is no way to adjust bottom heat or top heat, and many ovens DO NOT have a damper-- as simple vent you can open or close-- to control the humidity to some extent in the oven. Some do, but not many.
I have never seen a convection oven with true steam.
What "Real/Genuine" baker's ovens have, is a steam generator-- a separate box that takes water and electricity and pumps steam into the oven with the touch of a button.
What many convections have (as an additional factory option) is a squirt gun--a $30 solenoid valve that squirts water at the squirrel cage fan in the oven. The fan flings tiny droplets of water over the oven walls. The heat from the oven converts the water to steam. HOWEVER this lowers your oven temp by as much as 100 degrees and it takes the oven about 10 mins to "catch up" to the set temperature you had it at in the first place. The steam is good for bread and buns, but I have never in my career used steam for cookies.
You will always have to rotate baking trays in a convection, and every time you open the door, you blast out your heated air, making your kitchen all nice and toasty, and make your oven work harder to catch up. It is suicidal to bake meringues, small eclairs or cream puffs or lemon meringue pie in a convection, and if you bake on paper, the fan will lift the corners of the paper and shove the items into the center of the pan, baking everything into a blob. Batters like muffins and cakes will be lopsided on account of the fan blowing in one direction.
Don't get me wrong, convections are great workhorses for kitchen use, and they are pretty darn good for bread. For baking and pastries there are better options available, and the best, in my opinion, is a deck oven.
A deck oven has separate controls for top and bottom heat. Thus, if you want a deep-dish quiche with a dark gold bottom crust and pale gold top crust, you can set different "zones" for top and bottom heat. For cookies you can use a gentle heat setting but still at 360F overall temp, and decks come with dampers to adjust the humidity in the oven. The doors on decks are not as large as convections, so when you open a door, you don't have the fan pumping out heat.
You won't find deck ovens at restaurant eqpt. suppliers. You have to go to bakery eqpt dealers. A rest. eqpt. dealer will try to sell you a convection first, and if you insist on a deck, will try to sell you a pizza oven--which is only good at temps above 475F and pretty much useless for pastry work. You need to check out bakery eqpt. dealers.
Hope you find this information usefull....