A very hot airflow from radiator fans is a sign of a system running at - or over - it's maximum cooling capacity.
Water needs to be adequately cooled with the coldest ambient air temperatures. Therefore, the radiator should be getting as much "fresh air" as possible. Hot air from the radiator should also be driven out of the computer chassis by auxiliary chassis fans. Poor design and construction choice may result in insufficient radiator cooling, which may lead to overheating of the components. In the case of cooling capacity issues, the symptoms are usually a slow but steady increase in temperatures until the system reaches equilibrium or encounters stability issues.
The benefit of a custom water cooling loop is that the system can be expanded, and the cooling capacity extended almost without limitations.
The rule of thumb is to use at least one 120mm radiator (section) per each water cooled component plus one additional section.
For example, if you're liquid-cooling a CPU and a single high-performance graphics card, it is recommended you use at least one 240mm (2x 120mm) radiator for good performance. Ideally, you would get a 360mm (3x 120mm) radiator for the best results. Motherboard and memory water blocks usually have lower power output, and therefore they are not included in this equation.
Radiators also come in different sizes, but the 120mm type is the most common. For example, a 280mm radiator (2x 140mm), built for 140mm cooling fans, has a 33% larger cooling surface than the 240mm variant (2x 120mm), meaning it can dissipate substantially more heat given the same conditions. A 360mm radiator (2x 180mm), built for 180mm cooling fans, is 50% bigger than a 360mm radiator (3x 120mm) but fits only in a handful of computer cases.
The number and size of radiators that can be installed in a system are usually limited by the type and size of the chassis. That is why EK recommends the use of larger, water-cooling friendly computer chassis.