I am working my way through ACCA's latest manual LLH (Low Load Homes). The postulate of continuous fan increases indoor humidity in the cooling season arises in Section 8. This principle has been…Continue
"As I have read through everyone's excelent responses, it has caused me to reassess my assumptions. Tim O'Brien made a observation that is rightly relevant when he provided,
"I believe most A/C system evaporators running at part load…"
"Bradford - I have seen single family homes with ducted central AC or heat pump hold enough water upon shutdown that the living space relative humidity increased >10% when the fan was left on. I'm sure there is some diversity among the…"
"That is why I like VRF systems, David. We agree especially in the humid south and of course with cycling in general of on-off single-capacity systems.
Inverter drives are the current ideal and I use them whenever I can on larger systems (which…"
>I do not see that amount of moisture on that size of coil as a problem, let alone a 3 SF residential coil with limited outside air.
Probably not. But to Danny's point, a lot of folks in residential building science &…"
"Got it and understood thank you. I was figuring if there was a problem, the water was not leaving the unit.
That amount of moisture though is negligible though, at least in large, high-latent air handling units.
For example, a 10,000 cfm 100% OA air…"
"Danny - you are correct. An evaporator with no airflow over it will dry out slowly. Keeping the airmover on and some air velocity over the evaporator increases the liquid water evaporation rate exponentially.
The inside of the evaporator plenum…"
"@Bradford, Danny is referring to residual moisture that remains adhered to the coil after the compressor cycles off. Due to adhesion, a meaningful amount of bulk water will remain on the coil rather than dripping off into the catch pan. Eventually,…"
"All cooling coils which remove moisture (which is most of them!) require a drain pan below them to capture this. So much is known and ideally this would be either stainless steel or an approved thermoplastic or ABS for corrosion protection. The…"
"Depending on loads and how the stat is managed, there's usually one extended off-cycle per day, long enough for coil to fully dry. But if you still want continuous blower (for other reasons), at least employ an air handler, stat, or 3rd party…"
"Although I haven't personally measured coil moisture, it stands to reason that what you describe in your last paragraph may not be a small impact. Having looked at a lot of AC cycle graphs over the years, I would posit that the "off…"
I am working my way through ACCA's latest manual LLH (Low Load Homes). The postulate of continuous fan increases indoor humidity in the cooling season arises in Section 8. This principle has been shouted from the roof tops of many building science venues for years. And up until this point, I have embraced that Kool-Aid with the masses. The subject matter is discussed in an Allison Bailes article found here from 2012. …See More
"Well said David Butler. One more thing I would add is if you have some flexibility with the loop design, you can frequently downsize the equipment as its capacity increases with lower EWT. In other words, if you are stuck between say a 2 and 3 ton,…"
You will need to do a bit of gymnastics here to pull that off.
As you know, the delivered cooling capacity is dependent on (1) Entering air temp, (2) Entering humidity or wet bulb, (3) airflow and with air to air equipment (4) outdoor…"
HVAC design, Manuals J, S, T, & D, Duct leakage, Air flow, ENERGY STAR new home requirements, and more. Since heating & cooling are such a big part of home energy use, understanding HVAC is critical.See More
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