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  1. #1
    Join Date
    Dec 2010
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    Rock Falls IL
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    33

    Why is equipment sizing such and ongoing problem?

    I am not in the HVAC industry but became very interested as I was researching my future gas furnace purchase. I learned much here and it has become an interesting hobby to visit here and learn. I would like to request feedback from all of you on what seems to be such a widespread problem of oversized equipment that has been installed. It amazes me all of the problems (and various complaints) inquired about here after an install. (i.e. short cycling, noise, hig utility bills, too humid...) This is certainly not directed at anyone, either the homeowners or the technicians that installed. This seems to happen so often. Whose shoulders should the selection and installation of the improperly sized equipment rest on? It would seem that the manufacturers should focus on this issue and provide the training needed to install the [I]properly [I] sized equipment. My compliments to all of the very knowedgeable professionals here that do so much to help the consumers of your trade. Guys, never underestimate the value of the information you all provide here. You have steered so many in the right direction.
    Best wishes to all,
    joe
    Last edited by joeselectric; 12-27-2010 at 12:25 PM. Reason: even more mispelled words!

  2. #2
    Join Date
    Dec 2010
    Location
    Down by the river
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    1,664
    oversized equipment leads to poor humidity control, which leads to mold growth and poor home health down the road. It also is wasted power. They are starting to set higher standards for power consumption and if your house needs a 2.5 ton vs someone who would just drop in a 3 ton, thats alot over the life of the system. Also as general building practice change, better insulation,windows,and methods for tighter structure, one has to stop the rule of thumb heat and a/c designers. ie I have known some to design 5-600sq ft a ton, well I have designed some system's that were like 9-1000sq ft per ton. Can you see what would happen if you had a 3000sqft house done by rule of thumb vs a real precise load calculation. Now yes there is a differance between a 3000sqft house with 8-9' ceilings and one with 14-18ft ceilings.

  3. #3
    Join Date
    Dec 2010
    Location
    Rock Falls IL
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    33
    Southern, Yes, I see. There is just so much to all of this. Is not doing a proper load calc. almost like throwing darts in the dark?
    Thanks for the reply!
    Joe

  4. #4
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    Dec 2010
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    Down by the river
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    no it's more like throwing them straight up in the air, you still don't know where they will land but 1 or 2 is going to bite ya.

  5. #5
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    I have wondered why oversizing is so common and have come to some conclusions.

    1. Contractors don't want to leave any chance of undersizing, as this almost guarantees a tear-out and replacement at the contractor's expense. In new construction, they just seem to put in whatever they have in stock, as long as it's big enough. If it's not new construction, there is some guesswork in a load calculation, so it's "safer" for the contractor to error on the large size.

    2. Everyone understands undersizing, where the unit can't heat or cool the house. However, most people don't understand that you can have too much heat or cooling capacity. This requires a little more thought and understanding of how things work, somewhat more than the typical homeowner's ability. Most people think bigger is better. When I try to explain to people why this is not true, I often get the "deer in the headlights" look from them.

    Put 1 and 2 together, and you have a the oversizing mess we're in.

  6. #6
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    Dec 2010
    Location
    Rock Falls IL
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    33
    gary, that makes sense. So consumers have expectations that no matter how cold or hot it gets the temp in the living space had better be exactly what it is set to. Has anyone been succussful in finding a homeowner that can be educated to understand that in extremes of weather the system won't always maintain set point and be okay with that?
    Thanks!
    joe

  7. #7
    I have not had a homeowner that is satisfied unless that thermostat setpoint is reached on a 100 degree day with high humidity. Most contractors still oversize slightly for just this reason. Nothing more irritating than ripping out a system because it wont get below 78 on a 100 degree day. Hopefully, in the future, you will see more and more 2 stage equipment installed, this helps greatly.

  8. #8
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,485
    Quote Originally Posted by joeselectric View Post
    gary, that makes sense. So consumers have expectations that no matter how cold or hot it gets the temp in the living space had better be exactly what it is set to. Has anyone been succussful in finding a homeowner that can be educated to understand that in extremes of weather the system won't always maintain set point and be okay with that?
    Thanks!
    joe
    Well, I educated myself, does that count?

    But seriously, I've seen this when I talked to my contractor. Initially he wanted to put in larger equipment. After we talked a while, and he realized that I had done my homework, he looked at my sizing and agreed. But then he understood that I knew my system might struggle to reach setpoint on the occassional -0F or +110F day. Implicit in his agreement was that I take the risk for undersizng.

    BTW, I didn't hold it against him for wanting to oversize. He's experienced and knows his installations, but he also knows that expectations matter, so the "safe" approach is to avoid any possibility of the system not reaching setpoint on those extreme days, however infrequent they might be. I can understand that point of view.

    His recommendation: 100000 BTUH, 3.5-ton
    What's really going in: 80000 BTUH, 2.5-ton
    (the 80000 is already in, and is probably slightly oversized)

  9. #9
    Join Date
    Dec 2010
    Location
    Rock Falls IL
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    33
    I am understanding this better. The cotractors are almost in a no win situation in many cases. If they properly size then you have those extremes when it doesn't reach setpoint. Oversize and maybe all of those typical problems show up. I also learned just in time and have the proper size for my climate and load. Good idea for the contractor to have the homeowner sign that he knows the capabilities of the system.

  10. #10
    Join Date
    Jun 2007
    Location
    Round Rock
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    3,560
    Quote Originally Posted by simplyrollin View Post
    I have not had a homeowner that is satisfied unless that thermostat setpoint is reached on a 100 degree day with high humidity. Most contractors still oversize slightly for just this reason. Nothing more irritating than ripping out a system because it wont get below 78 on a 100 degree day. Hopefully, in the future, you will see more and more 2 stage equipment installed, this helps greatly.
    Bingo. Give this man a prize. This is exactly why they are oversized. I have a lot of customers that live in very high end homes. $2 million +. As many have told me that they don't give a squat about manual J or how much energy they use or whatever else I want to tell them. All they want is for the thermostat to reach whatever temp they want when they want it. I have one customer right now that wants his house 65ish or less no matter what the temp is. If we can't do it. Rip the equipment out and give him his money back. If we have a problem giving him his money back, around $50K, he'll sue us.
    I like DIY'ers. They pay better to fix.

  11. #11
    Join Date
    Oct 2010
    Location
    Houston, Tx
    Posts
    2,093
    This is a good topic as I myself, never having done a heat load for a system size, have often wondered how two experienced professionals using the exact same man j or software for the exact same home in the exact same location will come to two entirely different system sizes and yes, I have asked on here if this is the case and the pro's themselves have said that yes, more common than not for this to ocur.

    My guess is the educated guess' are not so educated after all, too many variables and what one person sees and thus makes an educated guess on conflicts with how another sees it and makes his educated guess with.

    Need more scientific fact based entries and not these educated guess', IMO.

    I hope something of concrete comes about sooner rather than latter because I will feel so bad if I get it wrong come my time to do so and I am not impressed with the fact that a Man J is so if-sh in result nor do I stand by it whole heartedly from what I understand of it and judging by what I have read of everyone's contradicting stories concerning system size.

    Luckily I just installs so I'm not too conflicted as I am more worried about getting it right in the event I need to perform a home analysis.

  12. #12
    Join Date
    Jun 2009
    Location
    Albuquerque NM
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    2,485
    Quote Originally Posted by joeselectric View Post
    I am understanding this better. The cotractors are almost in a no win situation in many cases. If they properly size then you have those extremes when it doesn't reach setpoint. Oversize and maybe all of those typical problems show up. I also learned just in time and have the proper size for my climate and load. Good idea for the contractor to have the homeowner sign that he knows the capabilities of the system.
    You got it, man!

    That's why I got a guy who was good at installation and let me take the responsibility (and risk) for sizing. However, that's not a viable solution to the problem. Very few homeowners are obsessed with proper HVAC sizing like I am.

  13. #13
    Join Date
    Dec 2010
    Location
    Northern Virginia
    Posts
    55
    I am new to the forum, but have a few things to add on this subject. If a contractor’s design engineer performs a true "Manual-J" load calculation, then the new tighter construction and higher insulation values of modern building products would be accounted for. There are also geographic considerations to take into account. Any given region has a historically based set of design temperatures to go with it. At some point during the extremes of the winter or summer, you are going to exceed these design parameters. You can't account for everything, or you will be over sizing. It's hard to drive this one home with homeowners. Just like trying to convince them that refrigerant doesn’t get consumed or that the system doesn’t work faster if you bury the needle on the thermostat.

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