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  1. #1
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    Lets talk load calcs!

    More specifically, design conditions. ACCA has us design our a/c's for 75* and 50% humidity. But what I would like to know is, how many of us go into homes, and the a/c is actually set at 75? Most people in my area, for whatever reason,have their units set between 70 and 72. When my wife is not looking i move ours to 73 but then she figures it out and moves it back to 71.

    Now when I do my designs, i still do the 75*, unless someone likes it really cold then I adjust, but how was it determined that 75 is optimal?

    Or is it just my region, because of humidity? In the deserts of Arizona, is 75 pretty common?

    I have just been thinking on this the last few days as I go in and out of peoples houses and see that everyone has there units set below 75.

    Heat I get, 68 is understandable. Most people in my area dont even set to 68, many I find are below that. (expect for them old people, had one lady had her unit set to 88* and still had blankets on.)

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  3. #2
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    Our design is 75/95 cooling, 70/-2 heating.

    I agree that it is seldom that I ever see a stat set at 75, but I bet for every 2 I see at 70 I see at least 1 at 78. I always figure my loads at design for this area. My thought is on average people move about every 7 years, or at least they use to. So if you size one for 68 and those people move out and the new family runs theirs at 78 you are now very oversized. I always tell people what design conditions are and how the equipment will react below, at, and above design, meaning below the system will cycle, at it will run constant and above the temp in the house will rise. I also explain that the amount of time at or above design is a very small part of the day and that there are factor built into the calculations that tend to oversize the system. I have yet to have anyone unhappy from system performance. I have also found that when you replace the oversized system with the correct size and put in a good thermostat instead of the Big Box purchased stat that they are happy at 75 or very close to it.

  4. #3
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    When I do calcs I ask the current homeowner where they set the stat and I size for that.
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    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Pecmsg - "NEST should only be used with 3-M filters to guarantee total destruction of the equipment!"

    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

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  6. #4
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    Thread Starter
    Quote Originally Posted by BNME8EZ View Post
    Our design is 75/95 cooling, 70/-2 heating.

    I agree that it is seldom that I ever see a stat set at 75, but I bet for every 2 I see at 70 I see at least 1 at 78. I always figure my loads at design for this area. My thought is on average people move about every 7 years, or at least they use to. So if you size one for 68 and those people move out and the new family runs theirs at 78 you are now very oversized. I always tell people what design conditions are and how the equipment will react below, at, and above design, meaning below the system will cycle, at it will run constant and above the temp in the house will rise. I also explain that the amount of time at or above design is a very small part of the day and that there are factor built into the calculations that tend to oversize the system. I have yet to have anyone unhappy from system performance. I have also found that when you replace the oversized system with the correct size and put in a good thermostat instead of the Big Box purchased stat that they are happy at 75 or very close to it.
    I do the same as well, that is usually when i get the ones that say "we like it at 68" as they have a blanket covering them on the couch.. I dont have hard numbers, but I bet 80% of all units I rip out are oversized.

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  8. #5
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    95/75 in my opinion is bogus. Cooling should be designed for near the max total heat expected and the best way is wet bulb depression. I balanced a hospital and the engineer had a RTU designed for 95 max dry bulb. The RTU was already installed when at a meeting the administrator told the engineer it should have been sized for 105 F. The engineer went back to his office and wrote a letter saying the RTU was designed for 105 F and he was right because he lowered the design wet bulb temperature. You can't pull that trick if you use wet bulb for design and state that on the submittal.

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  10. #6
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    I remember when I took a class by Jack Rise. He said "no one sets their stat at 75. The first thing of the day, my wife opens the curtains and shades". also. if you design below 75 cooling and sell the home, the next owner may set the stat at a higher temp. Be careful but sometimes you have to fudge the manual j. I think manual S would come into play here.
    Doug

  11. #7
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    Quote Originally Posted by Gunslinger View Post
    I remember when I took a class by Jack Rise. He said "no one sets their stat at 75. The first thing of the day, my wife opens the curtains and shades". also. if you design below 75 cooling and sell the home, the next owner may set the stat at a higher temp. Be careful but sometimes you have to fudge the manual j. I think manual S would come into play here.
    The next owner isnt your problem unless they have a say in the housing arrangements. You're selling comfort to your customer, not to subsequent owners.
    This forum requires that you wait 20 seconds between posts. Please try again in 50 seconds.

    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Pecmsg - "NEST should only be used with 3-M filters to guarantee total destruction of the equipment!"

    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

  12. #8
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    Quote Originally Posted by HVAC_Marc View Post
    The next owner isnt your problem unless they have a say in the housing arrangements. You're selling comfort to your customer, not to subsequent owners.
    I don't know Marc, that sounds pretty shortsighted to me. Yes you are selling to that customer but it has to fit the home. So you sell the system for a couple that want to keep it at 68. 2 months later they both die in a car crash and an elderly couple buy the house and are never comfortable because the humidity is too high. You just got a bad name for doing the right thing for the original customer and the new owner will not understand why they have to replace a 3 month old system you just put in. Maybe in 10 years when the equipment only has a life span of 3 years you can do that. In commercial situations I don't think you have a choice as the internal load can very so much from business to business, but in resi I don't think the load changes enough to be an issue. Again remember in most cases you are only at design or above for maybe 50 hours a year if that. I think the only time the ones that want to run at 68 will be too hot is at/near record heat levels. For instance where I am at I size for 95 but it will usually hit over 100 to over 105 at times. Very few times have I gotten a complaint about it not keeping up, and those are more often than not maintenance issues, dirty filter or coil.

  13. #9
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    Quote Originally Posted by BNME8EZ View Post
    I don't know Marc, that sounds pretty shortsighted to me. Yes you are selling to that customer but it has to fit the home. So you sell the system for a couple that want to keep it at 68. 2 months later they both die in a car crash and an elderly couple buy the house and are never comfortable because the humidity is too high. You just got a bad name for doing the right thing for the original customer and the new owner will not understand why they have to replace a 3 month old system you just put in. Maybe in 10 years when the equipment only has a life span of 3 years you can do that. In commercial situations I don't think you have a choice as the internal load can very so much from business to business, but in resi I don't think the load changes enough to be an issue. Again remember in most cases you are only at design or above for maybe 50 hours a year if that. I think the only time the ones that want to run at 68 will be too hot is at/near record heat levels. For instance where I am at I size for 95 but it will usually hit over 100 to over 105 at times. Very few times have I gotten a complaint about it not keeping up, and those are more often than not maintenance issues, dirty filter or coil.
    I cant and wont predict the future of the house or occupants. The system I sell will fit the home and their needs. Even if it has to be some special low temp refrigeration doo-wad to keep them at 62 degrees all the time.

    People buying houses have to understand what they are getting and do their due diligence either themselves or by hiring people to tell them what they need to do when they buy that house.

    You dont get a bad name by doing right by your customer. You get a bad name by selling them something that doesnt fulfill their needs. Additionally, if you are lucky enough to get service from the future owners you then have the opportunity to explain the past owner's situation and why that system is like it is. You may get a sale out of it, you may not.

    If you want to put in a system that meets "all" needs you should just stand at the street with the 2T, 3T, 5T chart held up to your eye and charge it to beer can cold.

    Here, the standard to sizing is 90/10 ODB and 75AC/68H IDB. It doesnt happen that way. We get weeks of high humidity, 100+ degrees, and winter times of -20. We also tend to have very old leaky houses that have to sustain 35-75 mph winds during winter. I size to 100/-15 ODB and 70 IDB. Not one customer has ever complained of discomfort (and no coil leaks either - yay!).

    Not every customer can live with those conditions though.
    This forum requires that you wait 20 seconds between posts. Please try again in 50 seconds.

    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Pecmsg - "NEST should only be used with 3-M filters to guarantee total destruction of the equipment!"

    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

  14. #10
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    Quote Originally Posted by BNME8EZ View Post
    I don't know Marc, that sounds pretty shortsighted to me. Yes you are selling to that customer but it has to fit the home. So you sell the system for a couple that want to keep it at 68. 2 months later they both die in a car crash and an elderly couple buy the house and are never comfortable because the humidity is too high. You just got a bad name for doing the right thing for the original customer and the new owner will not understand why they have to replace a 3 month old system you just put in. Maybe in 10 years when the equipment only has a life span of 3 years you can do that. In commercial situations I don't think you have a choice as the internal load can very so much from business to business, but in resi I don't think the load changes enough to be an issue. Again remember in most cases you are only at design or above for maybe 50 hours a year if that. I think the only time the ones that want to run at 68 will be too hot is at/near record heat levels. For instance where I am at I size for 95 but it will usually hit over 100 to over 105 at times. Very few times have I gotten a complaint about it not keeping up, and those are more often than not maintenance issues, dirty filter or coil.
    I can't wait to try that logic on a sales call.

    "Ma'am, I can't oversize your system. Suppose you die in a car accident and your house is sold to somebody who would be more comfortable at 75 degrees? What then do you suppose would happen to my reputation, HMMM?"

    :grin:

    Sent from my SCH-I545 using Tapatalk

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  16. #11
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    The amount of moisture that can removed from the air in a house is a function of the discharge temperature of the supply air unless you have the equipment to remove, add and control the moisture content. If you assume a variety of conditions and compare against the psyc chart you may be surprised.
    For my houses I have oversized every cooling system and have been happy with all of them. If for some reason my house heats up I don't want to wait two days for it to cool off.

  17. #12
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    Quote Originally Posted by jbhenergy View Post
    More specifically, design conditions. ACCA has us design our a/c's for 75* and 50% humidity. But what I would like to know is, how many of us go into homes, and the a/c is actually set at 75? Most people in my area, for whatever reason,have their units set between 70 and 72. When my wife is not looking i move ours to 73 but then she figures it out and moves it back to 71.

    Now when I do my designs, i still do the 75*, unless someone likes it really cold then I adjust, but how was it determined that 75 is optimal?

    Or is it just my region, because of humidity? In the deserts of Arizona, is 75 pretty common?

    I have just been thinking on this the last few days as I go in and out of peoples houses and see that everyone has there units set below 75.

    Heat I get, 68 is understandable. Most people in my area dont even set to 68, many I find are below that. (expect for them old people, had one lady had her unit set to 88* and still had blankets on.)
    they are setting it that low as it is oversized and needs the extra run time to dehumidify --I have found when we do a proper callc and install those setpoints creep up a few degrees

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  19. #13
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    I agree with lkapigian. A 75 deg and 50% RH will probably be a good comfort level for most. Oversized units make the space feel chilly and don't dehumidify ideally.
    I still would oversize by the nearest fraction because of unforeseen events. The fudge factor can make the difference between a happy face and something else.

    There are some strange requests that can happen. A wealthy lady whose home was on a chiller wanted a very cold bedroom. Time has done away with the numbers but I believe it was below 60 degF.
    That doesn't mean calcs should be based on other than normal people.
    Comfort begins with the RH. I was in Fla last year and don't know how people live there. It would take me a long while to acclimate there.
    Something some overlook is in cooling a slight breezy feel is nice but not so much in heating. Overhead fans are not ideal because often they disturb the hot air higher up. If left alone it will be ok because it's an unoccupied area.
    I should have played the g'tar on the MTV. MK

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