Originally Posted by
mxarey
I get-it that I don't want to oversize. But what I'm worried about is dehumidification when the temperature is moderate. The latent load is still about 3,500 at 80 degrees, but the sensible load drops a lot. If I'm reading the equipment specs right, I'm gonna have to run the compressor and blower on low speed just about all the time to handle the humidity, but if I do that it looks like I'll deliver too much sensible cooling. However, if I use a 3-ton unit with a big inside coil, the latent capacity at low speed is 2.5 the latent capacity of the 2-ton but the sensible capacity only goes up by a factor of 1.17. Seems like it would have to run a lot less to handle the humidity and would be less likely to overcool.
But on a total capacity basis, 3 tons looks way oversized relative to the Manual S 1.15x limit. And just going back to rules of thumb, 3 tons would be a bit under 1200 square feet per ton, a pretty high number by traditional standards, but 2 tons would be 1700 square feet per ton, which is way out there.
Anyone have any experience with this sort of situation? All advice gratefully appreciated.
Mark
You have figured out the great mystry of comfort in modern homes. Most a/c designers do not understand this problem.
The senisble cooling load of homes in green grass climates varies from 80% on a design day to 0% during wet, cool day. On the otherhand, the latent load is 20% of the total cooling load on a desigh day to 100% during wet cool weather. Yet our a/c systems operate with 75% sensible/25% latent cooling ratio. The tools used are slowing capacity and over-cooling. The new varible speed systems, while they work better are not the solution. You will end up with a cold damp home during cool wet weather. Also a/c designers are avoiding the necessary fresh air ventilation that all homes need to purge indoor pollutants, renew oxygen, and remove odors. In addition to the latent load in infiltrating/ventilation fresh air, the latent cooling from the occupants occurs throughout the cooling load range. 100 cfm of 70^F dew point fresh air plus the moisture from 4 occupants is 7,000 btus per hour of latent cooling with very little sensible cooling.
I will attach data from the a home with a variable speed a/c slow the inability to maintain <50%RH. The owner spared no expense to get have the best, yet had high humidity in the home and cold temps during certain weather.
I will follow later with data.
The solution is simple, add a whole house dehumidifier that compliments the home's load profile and a/c preformance to maintain the customer's desires. Typically, occupants want 70-78^F, <50%RH. A properly setup single speed a/c with a sized WH Dehu will maintain <50%RH during all of the ranges including no a/c during unoocupied times.
The total installed cost is comparable to the VS a/cs. In addition all quality homes should have mechanics to provide the amount of fresh filtered make-up air to purge indoor polutants and renew oxygen at a minimum when occupied. Whole house dehus like the Ultra-Aire unit provide fresh air ventilation as an option.
Again congrats on figuring out the variableness of the sensible cooling loads, while having a steady latient load.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"