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  1. #1
    Join Date
    Jan 2008
    Posts
    78

    Cooling time & temp ?

    Folks, reading some threads way too early on the 4th trying to figure out my AC issues

    How long should it take to drop the temp say 10 degrees? In our old place (condo) the place would get comfy in an hour or so....the new place (house), takes hours...(if it ever gets to temp)

    Also--what is the temp diff that a system should achieve...I keep hearing only 20 degrees, but if that was the case an a/c in AZ would only get you down to 90 which can't be the case.

    Can anyone recommend a reasonably priced meter I could buy locally to check the air temp at the diffusers?

    Thanks in advance

  2. #2
    Join Date
    Jan 2006
    Location
    Marietta Georgia
    Posts
    43
    auto parts stores have themometers you can stick in supply plenum or a supply duct. typically 15-20 degress between your return vent and supply vents. IF your condenser outside is running, feel the large line usually covered in black insulation, the copper line should be cool.( maybe 50 degrees or so) if hotter or colder, or if you just want a cooler house then call a pro.

  3. #3
    Join Date
    Jan 2008
    Posts
    78
    Thanks Jhar for the tips...but in my case the pro IS the problem. I have a 2 month old system, they've been out 3 times and always seem to find another problem. I'm trying to gather as much info as possible to assist the diagnosis.

  4. #4
    Join Date
    Jan 2008
    Location
    California
    Posts
    108
    Check the ductwork too - poor recovery can be a result of a loose connection, attic air infiltration.

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,612
    In hot weather, it could be 1/2 a day to drop 10. If the peak of the afternoon, you may not drop much at all, the entire capacity of the A/C will be going to maintaining with little or no reserve to drop the house. Any unit that can do much of a drop in hot weather is oversized. If your old house could do 10 in an hour, GROSSLY oversized!

  6. #6
    Join Date
    Dec 2007
    Posts
    4,770
    also, it is not the difference in the outside temp versus the inside temp.

    it is the difference in the temp between the air entering the indoor unit and the air leaving the indoor unit.

  7. #7
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,793
    Ask your contractor what indoor design temp he used.
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    How many times must one fix something before it is fixed?

  8. #8
    Join Date
    Jan 2008
    Posts
    78
    Quote Originally Posted by BaldLoonie View Post
    In hot weather, it could be 1/2 a day to drop 10. If the peak of the afternoon, you may not drop much at all, the entire capacity of the A/C will be going to maintaining with little or no reserve to drop the house. Any unit that can do much of a drop in hot weather is oversized. If your old house could do 10 in an hour, GROSSLY oversized!
    loonie...I'm not trying to be a wise guy, but is it really the norm so that you spend 20k on an ac system that can't cool the house down during the hotest part of the day..that doesn't make sense to me. why is this the norm? I lived for years with a wall unit that would get my condo down to 70 degrees when it was 90 out no problem.... why wouldn't you design a system that could actually get the house to 70 even when it was 90 out???? our central ac in our old place may have been oversized...but I was comfortable...I'd be curious to learn more...maybe I want too much from our unit.

  9. #9
    Join Date
    Jul 2008
    Location
    Middle TN
    Posts
    24
    Quote Originally Posted by njhusky View Post
    loonie...I'm not trying to be a wise guy, but is it really the norm so that you spend 20k on an ac system that can't cool the house down during the hotest part of the day..that doesn't make sense to me. why is this the norm?
    I think he meant that while it could not DROP the indoor temp 10 degrees during the hottest day it should be able to MAINTAIN the indoor temp on those days.

    Are you setting back the temps to "save money" and then wanting it to cool from 80 down to 70 in just an hour or two when it is really hot out? That is not the way most systems are designed I don't think. Try not setting back the t-stat and let it run - is it able to maintain your desired temp? If so, then you could try setting it back a little bit and see how well it recovers on a hot day. The "Energy Star" set back program calls for a 7 degree set back and not the 10 degrees you mention. Of course, they also want it set for 78 degrees when you ARE at home .

    Someone could probably do the math and tell us which is cheaper - using a properly sized system to maintain the desired temp or using an over sized system to let you set it way back, but get cooled down quick when you get home. The larger system will use more power when running while the right sized system has to run most all day. In AZ no worries about too much humidity from an over sized system.

    As was mentioned you measure the drop from air coming out of register / vent compared to the air going into the air return (at least for us non-pros to measure it). I have a little non-contact IR thermometer I use but you can use anything as long as it is fairly accurate. Radio Shack used to sell these, and may still carry them. If it is 80 in your house then the vents may be blowing 60 degree air (probably higher due to loss in the ducts - which is why the pros check it at the handler). Once you have cooled it down to 75 then the air may be 55 degrees, get it to 72 and it may be 52 degrees. This is why it can take a while to drop the temperature if you set it back on a really hot day. Like leaving the windows down on your car VS leaving them rolled up on a hot, sunny day. The AC in your car can get you cool a lot quicker if the windows were left down since it is much cooler in the car.

    Dennis

  10. #10
    Join Date
    Jan 2008
    Posts
    78
    dwayne

    thanks...I understood what loonie meant about maintining temp...seemed silly to me to keep tha ac on when I am out for 10 hours..

    good point on the az comment, so is what you are telling me that if you design a system to drop the temp 30 degrees, it my not cyclel properly on avg days? but my system is a 2 stage, so that shouldn't be an issue.

    what are std design conditions...how much temp drop over how much time? how much max temp drop

  11. #11
    Join Date
    Jul 2008
    Location
    Middle TN
    Posts
    24
    *I* am no expert, I am here getting help with my system too .

    Where most of us live we have to have (or should have) a properly sized system to remove humidity and keep us cool. Here, if you sized a system to drop the temp 30 degrees on a hot day in a short of amount of time I would have mold and mildew problems and very short cycles

    Since you live where it is dry then you could have an over sized system and it would be OK, I guess?

    Again, someone in the biz could run the numbers and see how it stacks up, but I would think the properly sized system running at your set point all day (or maybe set back a few degrees) would use less power than having a monster system than could set back 30 degrees and the blast the temp down when you got home. Everything you save setting back is burned up trying to catch up in a hurry with a huge system.

    If you have a two stage system then maybe you could save power by a modest set back and have the small compressor keep the temp at the set back point, then call on the big compressor when you get home. The key would be not setting it back so far that the system can't get you comfy in the time you want.

    Dennis

  12. #12
    Join Date
    Feb 2008
    Location
    Western NC
    Posts
    2,504
    Its an air conditioner.....pick a temp and set it.

    If you want to do a setback, do a moderate one...like 3 degrees.

    What you are failing to see here is all the heat that is absorbed. What I mean by that is all your furniture, appliances, walls, carpets, etc etc will absorb more heat and therefore it takes longer to remove said heat.

    Air Conditioners are typically only DESIGNED for 85-90* weather. Yes they are tested well beyond their limits but that is all they are designed for.


    If you feel that something is wrong, then there more than likely is. Get the system looked at by a trained professional. This is not a DIY site. You need to contact your local contractor to come out and service the system. And you need to make sure they will service it properly!!!
    I fully support the military and the War on Terrorism.


    If you don't know, then don't do. If you don't know and still do, then be prepared to pay someone else a lot to undo what you did and then do it right.

    If you do know, then do. But do it right. Otherwise, you may not be doing it long.

  13. #13
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,612
    Air conditioners are great maintainers. As veteran says, pick a temp and leave alone in hot weather. My A/C is a bit oversized. In 90+ steamy weather, I can hit hold and 70 and when I get home from work, it will be 70. BUT, if I leave on program where it rises to only 74 during the day, when I get home it will be 73 or 74. It won't have dropped at all. It may take 5-6 hours to get down to 70.

    A properly sized A/C should maintain 75 inside on your hottest day of the year based on the industry standard. For those who want it colder inside, the designer would need to plug that into his figures and get you a bigger unit.

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