Results 1 to 13 of 13

Thread: Cost of 4-5 degrees more cooling

  1. #1
    Join Date
    Oct 2005
    Location
    CT
    Posts
    354
    Post Likes

    Cost of 4-5 degrees more cooling

    What’s the cost of cooling an extra 4-5 degrees in 90+ degree weather? Here is a tale of two systems, one at home and one at work. At my office, the system is oversized, cycling often even at the design temperature and cannot get the indoor humidity level below 64% on a 90-95 degree day. To be comfortable, we seem to have to set the indoor temp to 74-75. Our home system was sized nicely by the kind of HVAC pro we all hope for. At around the design temp she is running 100% of the time and is delivering a nifty 43% humidity. We find we are comfortable at home at a much higher 78-79 set temperature. I am thinking that I must be paying a lot for the 4-5 degree difference, no? It seems like an oversized system costs more 3 ways: 1.) More to buy (A 3 ton costs more up front than a 2 ton) 2.) More to run when it fires as 3 ton uses more juice than a 2 ton and 3.) Much more to run as you have to make it run more often to give you the 4-5 degrees cooler temperature to be comfortable. We are about 25 cents/kw here….ouch! I wonder if most folks realize the effect humidity has on their comfort and cost of their cooling when they don’t control it. Bet many, if not most, don’t even have an indoor humidity gauge. Of course if one’s system is oversized, I guess all one can do is go out and buy yet another piece of equipment, a (central) dehumidifier!!!!

  2. #2
    Join Date
    Jan 2012
    Location
    Broomall, PA
    Posts
    4,118
    Post Likes
    Properly installed whole house dehumidifiers use less energy than the AC unit. Controlling humidity with the whole house when the AC isn't running (milder, humid, rainy days, or at night-as Teddy Bear constantly preaches) is much more comfortable even at a higher set point. <50% RH and 75-77° is pretty comfortable to me, and most people.
    If you're above design day temp, you may have to just bite the bullet if you want comfort.

    The bigger question is how long does that unit need to run to get it down another 4-5°, if you even can. Then grab a meter and do some math. I don't have exact numbers, but I don't think my electric bill goes up much more than $20.00 for the month when we get a heat wave.
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  3. #3
    Join Date
    Oct 2005
    Location
    CT
    Posts
    354
    Post Likes
    Thread Starter
    Quote Originally Posted by STEVEusaPA View Post
    Properly installed whole house dehumidifiers use less energy than the AC unit. Controlling humidity with the whole house when the AC isn't running (milder, humid, rainy days, or at night-as Teddy Bear constantly preaches) is much more comfortable even at a higher set point. <50% RH and 75-77° is pretty comfortable to me, and most people.
    If you're above design day temp, you may have to just bite the bullet if you want comfort.

    The bigger question is how long does that unit need to run to get it down another 4-5°, if you even can. Then grab a meter and do some math. I don't have exact numbers, but I don't think my electric bill goes up much more than $20.00 for the month when we get a heat wave.
    Thanks for weighing in, Steve! The home unit actually manages those 70 degree 90% humidity days fairly well. One of our best purchases was a humidity gauge for outside and inside humidity measurement (inside in several locations). When we have the inside humidity down to 50% or less, we are careful not to open up windows and let in that 90% air. In the olden days, all we looked at was the temp inside and outside and we kind of got tricked into opening when we should not have. (Folks don't always appreciate how much % humidity influences comfort.)

    The office could probably use the dehumidifier you describe, but everyone goes home after 7-8 hours and even the 60 humidity is not terribly uncomfortable, a little clammy maybe but no too bad.

  4. #4
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    9,866
    Post Likes
    Quote Originally Posted by sgbroimp View Post
    What’s the cost of cooling an extra 4-5 degrees in 90+ degree weather? Here is a tale of two systems, one at home and one at work. At my office, the system is oversized, cycling often even at the design temperature and cannot get the indoor humidity level below 64% on a 90-95 degree day. To be comfortable, we seem to have to set the indoor temp to 74-75. Our home system was sized nicely by the kind of HVAC pro we all hope for. At around the design temp she is running 100% of the time and is delivering a nifty 43% humidity. We find we are comfortable at home at a much higher 78-79 set temperature. I am thinking that I must be paying a lot for the 4-5 degree difference, no? It seems like an oversized system costs more 3 ways: 1.) More to buy (A 3 ton costs more up front than a 2 ton) 2.) More to run when it fires as 3 ton uses more juice than a 2 ton and 3.) Much more to run as you have to make it run more often to give you the 4-5 degrees cooler temperature to be comfortable. We are about 25 cents/kw here….ouch! I wonder if most folks realize the effect humidity has on their comfort and cost of their cooling when they don’t control it. Bet many, if not most, don’t even have an indoor humidity gauge. Of course if one’s system is oversized, I guess all one can do is go out and buy yet another piece of equipment, a (central) dehumidifier!!!!

    Spot on! Additionally, the 3 ton system has a higher starting surge than the 2 ton system and it does it more frequently. And cycling, not run time, is what contributes more wear and tear on the equipment.
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    11,067
    Post Likes
    Lets not give up on the work a/c. Get a good tech and re-adjust the air flow rate blowing through the cooling coil. Slow the air to get an adequate dehumidification rate when the unit is running. This will dramatically increase the dehumidification when the unit is cooling and also slow the overall sensible cooling rate which lengthens the run time. This may get the work space very comfortable. If not we know a few more tricks before needing the dehumidifier. Adding a dehumidifier would allow turning off the a/c nights and maintain <50%RH. I do this with my FL home. No a/c all summer and maintain <60%RH in SW Fl without any a/c and +75^F dew points 24/7. Saves a lot of money while doing a better humidity job.

    You might check out using a better air filter as the first step in slowing air flow to get better dehumidification from your work a/c. Another point is that homes that have adequate fresh air ventilation will need a whole house dehumidifier during evenings to remove enough moisture evenings and rainy days. A steady trickle of fresh air purges indoor pollutants and renews oxygen. Supplemental dehumidification with a small whole house dehumidifier like the Santa Fe Ultra/Brpan/Trane units.

    Good to here from another 78^F, 45%RH fan. There are several of us on this site. After you get used to this, going outside in the summer is more comfortable.

    Keep us posted.

    Regards Teddy Bear.
    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"

  6. #6
    Join Date
    Oct 2005
    Location
    CT
    Posts
    354
    Post Likes
    Thread Starter
    Quote Originally Posted by teddy bear View Post
    Lets not give up on the work a/c. Get a good tech and re-adjust the air flow rate blowing through the cooling coil. Slow the air to get an adequate dehumidification rate when the unit is running. This will dramatically increase the dehumidification when the unit is cooling and also slow the overall sensible cooling rate which lengthens the run time. This may get the work space very comfortable. If not we know a few more tricks before needing the dehumidifier. Adding a dehumidifier would allow turning off the a/c nights and maintain <50%RH. I do this with my FL home. No a/c all summer and maintain <60%RH in SW Fl without any a/c and +75^F dew points 24/7. Saves a lot of money while doing a better humidity job.

    You might check out using a better air filter as the first step in slowing air flow to get better dehumidification from your work a/c. Another point is that homes that have adequate fresh air ventilation will need a whole house dehumidifier during evenings to remove enough moisture evenings and rainy days. A steady trickle of fresh air purges indoor pollutants and renews oxygen. Supplemental dehumidification with a small whole house dehumidifier like the Santa Fe Ultra/Brpan/Trane units.

    Good to here from another 78^F, 45%RH fan. There are several of us on this site. After you get used to this, going outside in the summer is more comfortable.

    Keep us posted.

    Regards Teddy Bear.
    Great thoughts, much appreciated! At home, we pull in 5% fresh air into the system from outside year around. (Not so at work, sadly.) That was the home installer's idea and an excellent one. Will look into the work ideas you presented. There, the air filters (2) there are 1" electrostatic (washable metal) jobs. The unit there is a Trane TUC1BO80A. The manual shows it is a 4 speed, so unless it is on lowest, your idea is worth a try for sure.

  7. #7
    Join Date
    Oct 2005
    Location
    CT
    Posts
    354
    Post Likes
    Thread Starter
    That turned out to be a good move from another standpoint: The factory setting was high and now set to medium-low one does not feel such a blast of air out of the round ceiling vents. The bookkeeper complained about that sometimes and now I doubt she will. As for the better humidity removal, will need a bit of time to assess that and will report back. Again, much obliged!

  8. Likes kdean1 liked this post.
  9. #8
    Join Date
    Oct 2005
    Location
    CT
    Posts
    354
    Post Likes
    Thread Starter
    Quote Originally Posted by sgbroimp View Post
    That turned out to be a good move from another standpoint: The factory setting was high and now set to medium-low one does not feel such a blast of air out of the round ceiling vents. The bookkeeper complained about that sometimes and now I doubt she will. As for the better humidity removal, will need a bit of time to assess that and will report back. Again, much obliged!

    Well, I am back with the results.
    Teddy Bear rocks! We are now a good 10 percentage points lower on the humidity scale at the office and able to run a higher temp for equal or better comfort which saves money. Fan speed is now at med-low, so there is even another step theoretically possible. Much obliged, TB!

  10. Likes STEVEusaPA liked this post.
  11. #9
    Join Date
    Oct 2013
    Location
    MA
    Posts
    1,050
    Post Likes
    Did anyone actually measure airflow though? Low airflow can hurt the reliability and longevity of the system. How do you know you didn't turn airflow down too much?

  12. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    11,067
    Post Likes
    Quote Originally Posted by ss120396 View Post
    Did anyone actually measure airflow though? Low airflow can hurt the reliability and longevity of the system. How do you know you didn't turn airflow down too much?
    Of all the things you measure, air flow is the toughest. +-error depending a bunch of things. After you are done adjusting air flow by measuring air flow, you need to check the for the right amount of moisture reduction from the air as air passes through the cooling coil. You need a minimum of 5-7^F reduction in return air dew point at a minimum in the return verses supply air dew point, to maintain <50%RH in the occupied space.

    I suggest start with the moisture removal process. Checking the a/c function, refrigerant charge, sub cooling, superheat, and condenser function assures cooling capacity. Next would be moisture removal of the indoor cooling coil. Design air flow and air duct leakage are important. Excess indoor moisture sources like water leaks, fish tanks, plumbing stacks, and roof leaks all play. (Hope I have not forgot anything).

    Some of us go to home and take shot at the most likely problem to avoid an "all day" service call. My short cut "for a high indoor %RH" is to start by checking moisture removal of the indoor cooling coil by measuring "the reduction in dew point as sir flows through the cooling coil".

    Most homes do not get adequate fresh air change 3-4 hours to remove indoor pollutants and ideal indoor oxygen levels during mild temps and calm air. In green grass climates over night and rainy days outdoor dew points are 60-75^F dew points. The amount of moisture is 2-6 lbs. per hour depending on the amount of fresh air needed and the outdoor dew point. Here is a source of moisture that must not eliminated. Not to worry, P properly setup a/c will remove +-3 lbs. of moisture per hour per ton. A 3 ton a/c removes 9 lbs. per hour of run.

    During evenings and rainy days in "Green Grass Climates", supplemental dehumidification is required. Small whole house dehumidifiers like the Santa Fe Ultra/Broan/Trane units with the ventilation option proved Merv 13 air filters and control to provide adequate fresh air and <50%RH when connected to the a/c.

    I am including a US Dew Point map of a typical summer evening/rainy day showing the latent loads.

    Hope I got major issues. Looking forward any discussion.

    Keep us posted.

    Regards Teddy Bear

    AttachmentName:  US DEWpoints 2022-08-22 103348.png
Views: 145
Size:  451.0 KB
    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"

  13. #11
    Join Date
    Oct 2005
    Location
    CT
    Posts
    354
    Post Likes
    Thread Starter
    Quote Originally Posted by ss120396 View Post
    Did anyone actually measure airflow though? Low airflow can hurt the reliability and longevity of the system. How do you know you didn't turn airflow down too much?
    Wouldn't a serious low air flow issue likely bring a freeze up situation?

  14. #12
    Join Date
    Nov 2006
    Location
    Athens, Ohio
    Posts
    9,866
    Post Likes
    Quote Originally Posted by sgbroimp View Post
    Wouldn't a serious low air flow issue likely bring a freeze up situation?
    It may not manifest until the filter gets dirty or some doofus installs a 3M Filtrete filter. The ONLY way to know air flow is to measure it.
    *********
    https://www.hvac20.com/ High efficiency equipment alone does not provide home comfort and efficiency. HVAC2.0 is a process for finding the real needs of the house and the occupants. Offer the customer a menu of work to address their problems and give them a probability of success.

    Find contractors with specialized training in combustion analysis, residential system performance, air flow, and duct optimization https://www.myhomecomfort.org/


    Site member map HERE!

  15. Likes R600a, STEVEusaPA liked this post.
  16. #13
    Poodle Head Mikey's Avatar
    Poodle Head Mikey is offline Membership Chair/ARP Committee / Professional Member*
    Join Date
    Sep 2002
    Location
    The water's not gonna clear up in Washington DC until we get the pigs out of the creek.
    Posts
    33,969
    Post Likes
    No; there are other options.

    A refrigeration-system oriented mechanic could fine tune your too-large system to make it 'smaller' / remove more moisture from the conditioned space.

    PHM
    -------



    Quote Originally Posted by sgbroimp View Post
    What’s the cost of cooling an extra 4-5 degrees in 90+ degree weather? Here is a tale of two systems, one at home and one at work. At my office, the system is oversized, cycling often even at the design temperature and cannot get the indoor humidity level below 64% on a 90-95 degree day. To be comfortable, we seem to have to set the indoor temp to 74-75. Our home system was sized nicely by the kind of HVAC pro we all hope for. At around the design temp she is running 100% of the time and is delivering a nifty 43% humidity. We find we are comfortable at home at a much higher 78-79 set temperature. I am thinking that I must be paying a lot for the 4-5 degree difference, no? It seems like an oversized system costs more 3 ways: 1.) More to buy (A 3 ton costs more up front than a 2 ton) 2.) More to run when it fires as 3 ton uses more juice than a 2 ton and 3.) Much more to run as you have to make it run more often to give you the 4-5 degrees cooler temperature to be comfortable. We are about 25 cents/kw here….ouch! I wonder if most folks realize the effect humidity has on their comfort and cost of their cooling when they don’t control it. Bet many, if not most, don’t even have an indoor humidity gauge. Of course if one’s system is oversized, I guess all one can do is go out and buy yet another piece of equipment, a (central) dehumidifier!!!!
    PHM
    --------
    The conventional view serves to protect us from the painful job of Thinking

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •