Evap size and EER

[Doug30293]

Here is my thinking. We typically assume about 400 CFM per cooling ton across the evaporator. But let's say we increase the evaporator size and reduce the blower to 300CFM. The air discharge temp will be lower but the overall cooling capacity from the evap is the same.

However, less CFM means less heat load from the blower itself. Thus, the overall system efficiency should increase. Am I missing something?

[passandscore]

It is not uncommon for people to step up the size of the indoor coil to say 5 ton while utilizing a 4 ton condensing unit in order to lower the humidity.

“Some contractors may use an undersized condenser unit to help reduce humidity. The problem with doing so is that the condenser, in many cases, just isn’t large enough for the heat load the house produces. The condenser may lower the humidity, but it may not sufficiently cool the house for human comfort, or the unit will have to run for a prolonged period of time to cool the home. For these reasons, I disagree with the practice of installing an undersized condenser.”

see article:
http://www.achrnews.com/articles/mismatched-coils-can-really-mess-things-up

[Doug30293]

Thanks for your input. The idea of installing an undersized condensing unit doesn't make sense to me either. The high efficiency residential units I've seen have oversized condenser coils. So it seems better to use a 5 ton condenser coil with a 3-4 ton compressor to get better heat rejection.

A fixed output compressor is just that - fixed output. So the energy savings has to come from other components. Reduced fan loads and better heat transfer through both coils seems like the obvious (cheapest) choice from the designer's point of view.

Any mixing of components requires complete analysis of the process. That is not likely to happen at the installer level.