Riser also serves as an escape space for the trapped air in the casting. By increasing the height of the riser it may be difficult for trapped air to escape easily.

Also increases the possibility of sand wash/inclusion

THE RISER MUST FREEZE LATER THAN CASTING i.e. time require for freezing molten metal in riser is must higher than time required for freezing casting, which is discrbed by Modulus or Shape function(m) which is a ratio of volume to surface area. Now here height is increase surface area is increase and lower time require for freezing molten metal in riser which can not fulfil the function of riser that trapped porosity in riser or feeding to the mould. because of modulus of riser is decrease.

So this is simple design criteria that riser must be compact according to chworinov rule.

Also as height increase net weight of casting product increase so yeild is decrease which increase cost of casting product and also high strength of sand is require to bear the load due to more moulten metal.

The answer is not as easy as the one found in textbooks based on solidification time of castings. The location of the riser is important. If the riser is placed on top of the thermal center (usually geometric center) of the casting, then the riser does not need to be tall. It just needs to have sufficient volume so that the last point to solidify is in the riser. If the riser is on the side of the casting, then it has two additional duties; 1. to produce a temperature gradient to ward the riser, and 2. to produce a metallostatic pressure to suppress the formation of pores. Pores are extrinsic in metals so their formation and their number density and size depends on the cleanness of the liquid metal. Therefore, there is not a correct geometrical riser height calculation because one riser height that produces no pores will lead to porosity problems when the liquid metal inside the casting has more damage. Unfortunately it is not an exact science. I hope this helps.