[NiHaoMike]

Originally Posted by MN Renovator

Aren't you still dumping the extra moisture that is in the larger coil back into the air after the cycle is complete? You may be removing more at the beginning of the cycle with the reduced airflow but you aren't really removing the humidity from the building until the moisture has saturated the coil and it starts to drip off. These are things that I'm not sure really get measured in the lab tests and I think the latent heat that gets added back after the cycle isn't insignificant. ..and I think that for the average oversized system in most homes this is a detriment because short cycles make this an issue. In my case, I probably would never notice because I make sure that the shortest cycle that ever runs is at least 2 hours. Most systems are oversized and probably run 30 minutes and 20 minutes of that is saturating the coil and only 10 minutes of true moisture removal and so 20 minutes of cooling essentially was left as sensible only but since the moisture removal reduces sensible BTU removal the overall sensible efficiency is less and the latent removal not what it could have been.

Granted system design is extremely important and I think a slightly undersized system(rounding down from manual J instead of rounding to the next .5 ton size) is the key to the best efficiency but contractors aren't willing to do it and homeowners generally don't understand the benefit of better moisture control and efficiency of longer cycles. Looking at the expanded performance charts with systems that use TXVs and ECM motors, it seems to me that you can get decent latent removal with the most efficient sensible removal at 375-400 CFM per ton. 350 CFM per ton is usually used for the beginning 30 minutes of a cycle by many manufacturers to saturate the coil and they go to 400 CFM for most efficient sensible removal after that but if you run a long enough cycle, it seems to me that the math works better for the 375-400 CFM per ton range. ECM motors can move the 400CFM per ton without much increase in blower power, the increased load to the condenser might increase power to it but it would be a good use of that power.

You can play around with the fan delays to minimize that problem. And, of course, tricks to increase the cycle time like slowing down the thermostat and installing a VFD.