Sensible, Latent and Total Cooling
A general question about the balance between Sensible and Latent capacities. My understanding is that the Latent load is used to remove the humidity and the sensible is used to cool the air.
Assume I have a calculated load of 23,500 BTUH sensible and 4,300 BTUH latent for a total of 27,800 BTUH.
Lets say I use a system rated 26,500 BTUH Sensible and 8,100 BTUH Latent for 34,600 Total to condition this house.
When the system starts most of the energy will be used to remove moisture until wet bulb temp gets close to coil temperature.
After a while when things settle down and steady running conditions are achieved and I am no longer having to dump large amounts of moisture on the coil, does any of the Latent capacity transfer/help the Sensible cooling capabilities?
In other words, is the Latent cooling rating only available to remove moisture or can it help the Sensible cooling?
I'm confused by the question.
The rating of your system is based on specific conditions of the incoming air to the unit. The actual share of latent vs. sensible depends on the actual conditions of the incoming air. As the air in the space dries out and cools down, there will be less latent load and more sensible cooling, but the total Btu/hr stays pretty much the same.
At least that's how I understand it.
A BTU is a BTU is a BTU.
The amount of sensible or latent cooling will be determined by air conditions as they enter the coil.
So if the coil only sees a latent load of 4,000 BTUH and the Unit is rated at 34,000, then 30,000 BTUH will be available for sensible cooling?
This could be a learning experience for all of us.
Originally Posted by pendetim
The answer to your burning question, the cooling coil adjust to any cooling load. If there is no latent cooling load at the coil temp, the coil removes more sensible btus. Latent cooling only occurs when the coil temp is below the dew point of the air being cold.
Homes have variable sensible/latent cooling loads. On hot, humid day with occupants inside, the calculated loads to cool to 75^F, 50%RH, are as you stated. Operating the selected a/c that is properly setup removes the btus at the s/l ratio stated. Colder coils remove less sensible and more latent heat. 75% sensible/25% latent is typical. +-10% is the range of adjustment by changing air flow.
As the sun sets, the sensible cooling loads on the home may decline to near zero. While latent cooling load declines very little. The latent load is from the occupants and the infiltration/ventilation fresh air which has a fairly daily constant level of moisture content. 2,000 btus per hour from the occupants and 4,000 btus per hour from 100 cfm of 70^F dew point outdoor air are typical.
The thermostat senses temperature which represent sensible cooling load. The dew point of the air represents the latent part of the load. Most commonly we use the %RH to describes the latent (moisture level). In addition some use the wet bulb temp at a specific temp to describe moisture levels.
Typical outdoor design conditions.
95^F, 79^F WB, 50%RH, 73^F dew point is typical outside conditions.
Desired inside design conditions.
75^F, 63^F WB, 50%RH, 55^F dew point is typical.
The fun starts when the sensible load declines while the latent remains high. The t-stat shuts off the a/c at 75^F. The %RH/dew point is rising without the sensible cooling load. The a/c is unable to remove latent without removing sensible heat.
So as the sensible load declines, the %RH rises. The a/c removes more latent as the %RH rises. The end result is a higher %RH than desired. To compensate, we setup the a/c to remove more moisture than the design condition requires. So during peak cooling, we have 45%RH, to help during the low cooling load.
This helps but when the cooling declines further, we resort to over cooling or supplemental moisture removal from a dehumidifier. Anytime the outdoor dew point is above the desired indoor dew point without any cooling load, a dehumidifier or reheat are the only method of maintaining the desire %RH.
More info than you wanted, I am sure. But most of us do not understand the relationship and difficulty of maintaining ideal indoor temps and %RH while having occupants and adequate fresh air ventiltion in the home. Most homes are short on fresh air and long on moisture during the shoulder cooling season.
A recent article in the Contracting Bussiness, stated that this an opportunity for our a/c contractors to better serve their clients providing ideal temps/%RH by inculding supplemental humidity removal (a dehumidifier).
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"
TB - You make my fingers cramp up just reading your response.
Great answer - You have a way of converting engineering into everyday language. I hope you teach somewhere.
Thanks, Teddy Bear, Excellent detailed response! It makes sense and confirms what I suspected based on the Thermodynamics courses I took many years ago, saturated and superheated steam properties, the ammonia and steam tables etc... all that good stuff.
Originally Posted by teddy bear
This is definitely NOT to much information. I am trying to understand what is going on in my home so I can make informed decisions around equipment selection.
Thanks Again, Tim
You will regain 50% of your unused latent capacity as sensible capacity.
In that example, you would regain 50% of 8100-4300, which would give you 3800 unused latent capacity, and 1900 btus of that would be used as sensible. Bringing up your total capacity to 28,400 BTUs.
Assume I have a calculated load of 23,500 BTUH sensible and 4,300 BTUH latent for a total of 27,800 BTUH. Lets say I use a system rated 26,500 BTUH Sensible and 8,100 BTUH Latent for 34,600 Total to condition this house.
This of course is only during those design condition days.
TB excellent explanation
With digital thermidistats/humidistats not being vary expensive these days why not simply set one up to change the blower speed as conditions warrant? When humidity is over 50% put the blower on low speed, when it's below 50% put the blower on high. I would think that it would be easy to implement with a SPDT relay.