The Homemade Heat Pump Manifesto

[MN Renovator]

"Look at it this way. If your condenser needs to be about 5.5x the size of your evaporator to get great efficiency, how good is the efficiency of the units that have the same size either end? My ASHP has a condenser about twice the size of the evaporator. As far as efficiency goes it sucks, but it does the job and is affordable. Would people buy a unit that was three times bigger and vastly more expensive if it was more efficient? Mostly not. It's a design compromise between cost and efficiency. A tradeoff we DIY's don't have to really make. We can afford the extra time, effort and outlay to make the most efficient system possible."

I'd have to argue against that a little bit. The bigger that both coils are, the more BTUs you can transfer for the same size compressor as long as the size of the coils doesn't significantly increase the amount of power that it takes to pump the refrigerant, but that almost isn't a factor for the size these are. I've been looking all over at AHRIdirectory.org data and time and time again I see a higher EER, SEER, and HSPF for a given nominal size when the evaporator coil model number is a size up from the nominal size of the condensor. Some examples are quite extreme if you combine different efficient elements of a system. Take an 18k or 24k condensor, put it into a furnace or air handler that is a variable speed unit, throw the 18k with a 24k coil or the 24k with a 30 or 36k coil and the values can be 12.2 EER and 14.5 SEER when the condensor is a 13 SEER unit. The combination ratings show capacity 101% and power 88%. The combination I used was for Carrier equip, a 80% 313*AAV024045 furnace 44k nat. gas input variable speed blower with a 113A 18k or 24k condenser and an oversized coil. If anyone wants to see what I was playing with.


The big issue with oversizing an evaporator coil is humidity control, you basically need to saturate the coil with condensation before it beads out into the pan and drains out off the evaporator. If the evaporator is huge and your system takes 30 minutes to get enough on the coil to starts removing humidity and the system stops, that soaked coil evaporates all of that moisture back into the house. This is why you MUST not oversize for cooling if you buy an A/C in an area where there is grass outside your house(non-desert), especially with a high SEER system or you run cycles that are too short to be useful. If humidity was zero concern you could probably have a monster indoor coil and probably get an extra SEER point out of it if you had the amount of air flow exact and the system was charged perfectly.

Hope this helps!