Blower speeds, call to dehumidify
I have a 3 ton AC with a variable speed blower furnace in central Ohio.
C33-49C-2-6 .073 orifice
Beenthere and others have largely stated ~350/ton is a good max AC speed to control humidity. What is a safe speed on a call to dehumidify? 756 as option 1 below seems low volume.
885 CFM first 7.5 minutes
1080 CFM max speed to end of cooling call
756 CFM call to dehumidify
984 CFM first 7.5 minutes
1200 CFM max speed to end of cooling call
840 CFM call to dehumidify
The manufacturer has the info to most accurately answer your question but normally I'd look for a minimum airflow on a continuous basis, such as dehumidifying, as 80% of the minimum cooling speed. So at 350-CFM/ton = 1050 x .8 = 840 CFM. Any slower and I'd be concerned about ice forming on the evaporator coil.
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I don't see an option 3... get a thermostat that has humidify control and use the Dehumdifying terminal on the control board that pretty mcuh all vairable speed furnaces and air handlers have. Then you jsut dehumidify on-demand only. IF humidity is high, it slows to 80% airlfow. All other times, you get normal airflow for better efficiency and full system capacity.
So for example, lets say you have a setback in the evening before bedtime. The unit might comes on and run for lets says 6 minutes to pull down the humidity then continue to run for 10 more minutes ot dorp the temprature maybe 2F.
In hotter weather when you have lots of heat load, you migh not even need to slow the fan speed. It might run at normal the whole time or slow the fan for jsut a few minutes at the beginning.
The two examples given include a Comfort Sense 7000 slowing the blower on a call to dehumidify. That call is 70% of max cooling speed.
Originally Posted by motoguy128
Originally Posted by motoguy128
Lets get serious, 20-30 mins. of cooling is required to load the cooling coil/pan to the point that moisture starts going down the drain. At the end of the cooling cycle, all the moisture on the coil/pan evaporates back into the home. We need a good hour run to remove 2 lb. of moisture per ton of capacity from the space. I suggest setting the air flow to max the latent removal. So during a long run, you get the house dried down a little. During the evenings the runs are short and do not remove much moisture.
Seldom is the home too dry. During low/no cooling loads and outdoor dew points +60^F, supplemental dehumidification is required to maintain <50%RH in a home.
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"
Crickets? No opinion on a safe blower speed at 70% call to dehumidify?
Would slowing the blower via call to dehumidify be best left off in favor of a constant lower speed of say 1050 - 1080 CFM?
947 CFM first 7.5 minutes
1155 CFM max speed to end of cooling call
808 CFM call to dehumidify
Too many variables to say. Ideally, you run it as low as you can and still keep the evaporator from icing up. This ideal speed is going to vary based on the evaporator itself, and incoming air temp.
If your going to use a thermostat that can slow the blower. I would have them set the blower to a CFM that matches fairly close the the SHR that the home has(probably the 1155 you posted). Then set the blower ramp profile to go to full speed right a way. And let the thermostat decide if the blower needs slowed down or not.
Its more efficient that way on the warmer days when it runs long enough that a slow ramp up is not needed because the humidity is already low enough.
I'm considering using a different company to install a txv, set airflow and charge the way it is supposed to be done. We'll see. I had a humidity problem last year but that was largely related to a major leak in the supply trunk left by the company I'm going to fire.