I am trying to understand a lot of conflicting information that I have found. I have read on some sites that an AC should drop 1 deg per hour and others that say it should take 18-20 min to drop a deg. Is there an industry standard? I know that all homes and conditions are different, but I am just looking for a general idea under normal operating loads (not peak heat). I have also heard that your AC should cycle 2-3 times per hour under normal conditions, but if the swing temp of a thermostat is +- 1 deg then how would you have more than 1 cycle per hour if it takes 1 hour to drop a degree? Even if the thermostat is +- 0.5 deg it would still run for 1 hr to drop the house by 1 deg.
How fast should AC drop 1 deg
Jeremy05
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The reason for all the confusion and fear about the sizing the a/c system is that many believe that the a/c will not be able to maintain 50%RH if the a/c is oversized.
Their fears are justified.
The reality is that most a/c contractors do not have a way of knowing what the max a/c cooling load is in your home. We can estimate the load by following the ACCA Manual J procedure which makes an attempt at trying to accurately calculate your peak cooling loan, both sensible and latent (moisture) based on your local climate. There is some safety factor as Designer Dan states.
Personally, I like my a/c to be able to be off or set-up 5^F when I am not normally in my home. Because of the mass of my home, I am willing to wait a hour or more for recovery. Also I like extra cooling capacity for a hot day with typical extra guests to maintain reasonable t-stat settings.
Also consider that as equipment ages and climates may change slightly over time, some consider a little extra capacity may be wise.
These are the obvious points that should be discussed at the time of purchase and "what if" should be considered.
Here are few more issues. Mainly is the decision of how much fresh outside air is going pass through your home on a hour to hour basis. Wind and exhaust devices are the major air movers. Most do not realize that a 22 mph wind is the equal to 50 Pascals blower door test. This could be an air change in less than an hour. Either we size for this or we tolerate for the extreme. Also few realize that sensible peaks at solar gain while the latent load is based on the local weather system with a fairly stable outdoor dew point 24 hours a day. Occupants are in the home evenings mainly with more moisture load.
More recently, because of continued air tightening of the home, mechanically filtered fresh air ventilation has been suggest as being beneficial to the occupants by purging indoor pollutants and renewing oxygen when occupied. During calm winds, many homes are only getting a fresh air change 12-18 hours. Pollutants high and oxygen low, this may affect health and comfort. Some physically compromised occupants show signs of Environmental Illness. Experts suggest a fresh air change in 3-5 hours when occupied.
When you go out to bid a conditioning system, how much of this do you want to discuss with the home owner on a low bid project?
As consumer and an independent contractor is better to attempt a full disclosure discussion for long term satisfaction.
I am making this a two part post because of the length this post.
For many, this rehash. More to follow.
Any comments welcome.
Regards Teddy Bear
Their fears are justified.
The reality is that most a/c contractors do not have a way of knowing what the max a/c cooling load is in your home. We can estimate the load by following the ACCA Manual J procedure which makes an attempt at trying to accurately calculate your peak cooling loan, both sensible and latent (moisture) based on your local climate. There is some safety factor as Designer Dan states.
Personally, I like my a/c to be able to be off or set-up 5^F when I am not normally in my home. Because of the mass of my home, I am willing to wait a hour or more for recovery. Also I like extra cooling capacity for a hot day with typical extra guests to maintain reasonable t-stat settings.
Also consider that as equipment ages and climates may change slightly over time, some consider a little extra capacity may be wise.
These are the obvious points that should be discussed at the time of purchase and "what if" should be considered.
Here are few more issues. Mainly is the decision of how much fresh outside air is going pass through your home on a hour to hour basis. Wind and exhaust devices are the major air movers. Most do not realize that a 22 mph wind is the equal to 50 Pascals blower door test. This could be an air change in less than an hour. Either we size for this or we tolerate for the extreme. Also few realize that sensible peaks at solar gain while the latent load is based on the local weather system with a fairly stable outdoor dew point 24 hours a day. Occupants are in the home evenings mainly with more moisture load.
More recently, because of continued air tightening of the home, mechanically filtered fresh air ventilation has been suggest as being beneficial to the occupants by purging indoor pollutants and renewing oxygen when occupied. During calm winds, many homes are only getting a fresh air change 12-18 hours. Pollutants high and oxygen low, this may affect health and comfort. Some physically compromised occupants show signs of Environmental Illness. Experts suggest a fresh air change in 3-5 hours when occupied.
When you go out to bid a conditioning system, how much of this do you want to discuss with the home owner on a low bid project?
As consumer and an independent contractor is better to attempt a full disclosure discussion for long term satisfaction.
I am making this a two part post because of the length this post.
For many, this rehash. More to follow.
Any comments welcome.
Regards Teddy Bear
Joined
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12,358 Posts
My previous post outlined the problems we had as we improve the general quality of our homes and learn more about indoor air quality and comfort issues.
Next comes the changes that we make to our equipment and how to apply to these improved structures.
First we continue to do the best we can with ACCu's Manual J calculations and their other manuals recommendations.
After the equipment is selected based on calculations, we install the equipment in the home. The equipment is started and function checked.
As most of us know, there is a setup required to adjust the amount of sensible cooling and latent cooling load. We have an adjustable air blower that is set to specific amount of air flow to compensate for variable air flow resistance to provide the specific amount of sensible cooling verses latent cooling. The ratio is based on your climate. We will deal a "green grass climate" in this post. Most of us understand that the sensible highest during the peak of the radiant solar gain and least after sunset and rising by mid morning typical sunny weather, while the latent load is steady and dependent on outdoor dew point. For many years, most recommended setting up the air flow to remove more moisture than required to maintain 75^F, <60%RH as a max %RH. More recently, we increasing the latent removal rate keep the home <50%RH during peak sensible cooling to dry the structure enough to help maintain evenings when low or no sensible cooling.
The <50% RH concept helped keep the home <60%RH evenings and rainy days if very little +55^F dew point fresh air passed through the home evenings and rainy days. But when the home gets the suggested fresh air change in 3-4 hours to purge pollutants and renew oxygen, the indoor %RH rises dramatically above 60%RH. After a few rainy days, the home is damp mold grows and dust mites thrive. Next we recommended over cooling up to 3^F to remove more moisture. Even with the well setup a/c, we do not have enough sensible cooling load for the a/c to remove 2-4 lbs. per hour of moisture necessary to maintain 50%RH. The next step is use reheat to provide enough sensible cooling to remove the moisture from the fresh air and occupants. If we follow ASHRAE fresh air recommendations, a fresh air change in 3 hours and have design occupancy, reheat to remove moisture 2-3 KWHs of reheat plus running the a/c which would be equal to reheat. During the last energy crunch in the late 70s, many energy codes frowned on reheat except for hospitals/museums/etc.
In the last 30 years, dehumidifiers have been designed to remove up to 9 lbs. of moisture per KWH and provide 3 KWHs of free reheat. These dehumidifiers are an investment though, but worth the saving and improve in the home a steady<50%RH at a desired temperature of 75^F,<50%RH. Consider a fresh air change in 3-5 hours at the desire temperature and %RH. This is comfortable and healthy.
All of this accomplished by installing a well set up mid-range single speed a/c and mating a whole house dehumidifier like the Santa Fe Ultra, Broan, Trane, etc. with the optional fresh air feature. This includes Merv 13 filtering of the house and fresh air for the home.
I should ad that adding 2 or VS speed improves humidity removal but is unable to provide real 50%RH reheat or supplemental dehumidification.
This a simple explanation.
Post any confusing issues.
Regards Teddy Bear
Next comes the changes that we make to our equipment and how to apply to these improved structures.
First we continue to do the best we can with ACCu's Manual J calculations and their other manuals recommendations.
After the equipment is selected based on calculations, we install the equipment in the home. The equipment is started and function checked.
As most of us know, there is a setup required to adjust the amount of sensible cooling and latent cooling load. We have an adjustable air blower that is set to specific amount of air flow to compensate for variable air flow resistance to provide the specific amount of sensible cooling verses latent cooling. The ratio is based on your climate. We will deal a "green grass climate" in this post. Most of us understand that the sensible highest during the peak of the radiant solar gain and least after sunset and rising by mid morning typical sunny weather, while the latent load is steady and dependent on outdoor dew point. For many years, most recommended setting up the air flow to remove more moisture than required to maintain 75^F, <60%RH as a max %RH. More recently, we increasing the latent removal rate keep the home <50%RH during peak sensible cooling to dry the structure enough to help maintain evenings when low or no sensible cooling.
The <50% RH concept helped keep the home <60%RH evenings and rainy days if very little +55^F dew point fresh air passed through the home evenings and rainy days. But when the home gets the suggested fresh air change in 3-4 hours to purge pollutants and renew oxygen, the indoor %RH rises dramatically above 60%RH. After a few rainy days, the home is damp mold grows and dust mites thrive. Next we recommended over cooling up to 3^F to remove more moisture. Even with the well setup a/c, we do not have enough sensible cooling load for the a/c to remove 2-4 lbs. per hour of moisture necessary to maintain 50%RH. The next step is use reheat to provide enough sensible cooling to remove the moisture from the fresh air and occupants. If we follow ASHRAE fresh air recommendations, a fresh air change in 3 hours and have design occupancy, reheat to remove moisture 2-3 KWHs of reheat plus running the a/c which would be equal to reheat. During the last energy crunch in the late 70s, many energy codes frowned on reheat except for hospitals/museums/etc.
In the last 30 years, dehumidifiers have been designed to remove up to 9 lbs. of moisture per KWH and provide 3 KWHs of free reheat. These dehumidifiers are an investment though, but worth the saving and improve in the home a steady<50%RH at a desired temperature of 75^F,<50%RH. Consider a fresh air change in 3-5 hours at the desire temperature and %RH. This is comfortable and healthy.
All of this accomplished by installing a well set up mid-range single speed a/c and mating a whole house dehumidifier like the Santa Fe Ultra, Broan, Trane, etc. with the optional fresh air feature. This includes Merv 13 filtering of the house and fresh air for the home.
I should ad that adding 2 or VS speed improves humidity removal but is unable to provide real 50%RH reheat or supplemental dehumidification.
This a simple explanation.
Post any confusing issues.
Regards Teddy Bear
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