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  1. #1
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    Sep 2012
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    Can Someone "Dumb" This Down For Me?

    I have another thread about a contractor coming out, doing a pretty comprehensive full system check. He was extremely knowledgeable and very informative. Not wanting to look like a complete idiot, I didn't ask some questions I should have.

    He did fully answer my questions, so the blame is on me.

    Anyway, he calculated that I needed to replace my 3 ton outside unit with a 2 ton. He also said I should replace my 75,000 BTU gas furnace with a 60,000 BTU gas furnace.

    Can someone (in simple terms) explain why going with smaller units is best? To us laymen, bigger is always better, but I know that really isn't the case, just don't understand the real reasoning.

    Thanks!

  2. #2
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    Aug 2009
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    Quote Originally Posted by Zomby View Post
    I have another thread about a contractor coming out, doing a pretty comprehensive full system check. He was extremely knowledgeable and very informative. Not wanting to look like a complete idiot, I didn't ask some questions I should have.

    He did fully answer my questions, so the blame is on me.

    Anyway, he calculated that I needed to replace my 3 ton outside unit with a 2 ton. He also said I should replace my 75,000 BTU gas furnace with a 60,000 BTU gas furnace.

    Can someone (in simple terms) explain why going with smaller units is best? To us laymen, bigger is always better, but I know that really isn't the case, just don't understand the real reasoning.

    Thanks!
    In shortest terms - cycle time.

    Detailed - If the unit is larger than the demands from the home, it will cycle on and off. This, ultimately, is not as efficient for a few reasons.

    First, being humidity control. When the unit turns off, the moisture on the evaporator coil, ultimately ends up evaporating back off into the home. Too short of a cycle time will also not give the moisture time to even accumulate enough to drip to the drain pan and actually be removed. It is much better to have a smaller system run constantly than a larger system cycle on and off.
    Second, being weather. If the absolute hottest day of the year calls for a 3 ton unit, then yes, on that single day, you will need a 3 ton unit running continuously. However, that is at the expense of excessive cycling the other large percentage of the cooling season. If, instead, it is sized for say, the 90% highest loads of the year, then yes, on those hottest days, it may not be able to cool all the way to your setpoint - however, it stretches the efficiency for the other larger portion of the cooling system, in the end, greatly helping the energy costs.
    Third, comfort. An oversized system will cycle, and the controlling aspect of that is the swinging temperature at the thermostat. A house whose temperature is swinging up and down by 3-4 degrees twice an hour as the A/C cycles on and off is not as comfortable as one that maintains a much more steady temperature by running longer between cycles (drifting slower 3-4 degrees up and down say, once an hour or two).

    There are 2 ways to avoid the cycling. Have a system matched closer to your heat load for most the season, or have a system with a variable capacity. truly variable systems do cost quite a bit more, but 2-stage units are not all that much more expensive. They are not quite as common at the lower end of the tonnage, but there are some 3 ton 2-stage systems (which can stage down from the full 3 tons, to about 2 tons). Then, it can run MOST of the time at the 2 ton load, and if the load slightly exceeds 2 tons, it will occasionally switch up to 3 tons for a while and then back to 2. This is much smoother than cycling between 3 tons and 0 tons.

  3. #3
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    Sep 2012
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    Quote Originally Posted by CraziFuzzy View Post
    In shortest terms - cycle time.

    Detailed - If the unit is larger than the demands from the home, it will cycle on and off. This, ultimately, is not as efficient for a few reasons.

    First, being humidity control. When the unit turns off, the moisture on the evaporator coil, ultimately ends up evaporating back off into the home. Too short of a cycle time will also not give the moisture time to even accumulate enough to drip to the drain pan and actually be removed. It is much better to have a smaller system run constantly than a larger system cycle on and off.
    Second, being weather. If the absolute hottest day of the year calls for a 3 ton unit, then yes, on that single day, you will need a 3 ton unit running continuously. However, that is at the expense of excessive cycling the other large percentage of the cooling season. If, instead, it is sized for say, the 90% highest loads of the year, then yes, on those hottest days, it may not be able to cool all the way to your setpoint - however, it stretches the efficiency for the other larger portion of the cooling system, in the end, greatly helping the energy costs.
    Third, comfort. An oversized system will cycle, and the controlling aspect of that is the swinging temperature at the thermostat. A house whose temperature is swinging up and down by 3-4 degrees twice an hour as the A/C cycles on and off is not as comfortable as one that maintains a much more steady temperature by running longer between cycles (drifting slower 3-4 degrees up and down say, once an hour or two).

    There are 2 ways to avoid the cycling. Have a system matched closer to your heat load for most the season, or have a system with a variable capacity. truly variable systems do cost quite a bit more, but 2-stage units are not all that much more expensive. They are not quite as common at the lower end of the tonnage, but there are some 3 ton 2-stage systems (which can stage down from the full 3 tons, to about 2 tons). Then, it can run MOST of the time at the 2 ton load, and if the load slightly exceeds 2 tons, it will occasionally switch up to 3 tons for a while and then back to 2. This is much smoother than cycling between 3 tons and 0 tons.
    Wow! What an excellent explanation. I actually understand it Thank you!

    I have a 1500 SF single story ranch with crawlspace in the soupy middle of Missouri and my current 9 SEER 14 year old Goodman actually doesn't cycle too often when the temps are above 90 or so. Is the reason it doesn't cycle because the unit is so old and it is pumping its guts out to cool the house?

  4. #4
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    Quote Originally Posted by Zomby View Post
    Wow! What an excellent explanation. I actually understand it Thank you!

    I have a 1500 SF single story ranch with crawlspace in the soupy middle of Missouri and my current 9 SEER 14 year old Goodman actually doesn't cycle too often when the temps are above 90 or so. Is the reason it doesn't cycle because the unit is so old and it is pumping its guts out to cool the house?
    That's not something that can be answered on here - there are far too many variables to consider that would have to have a site visit, which I think you already had. in the end, you are the customer, so if you want a 3 ton, you can have a 3 ton installed - though if there are questions, it might be worth asking about the 2-stage units, and see what the tech says. If he shy's you away from them, then there may be some knowledge deficiencies there. Never hurts to ask though.

  5. #5
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    Quote Originally Posted by CraziFuzzy View Post
    That's not something that can be answered on here - there are far too many variables to consider that would have to have a site visit, which I think you already had. in the end, you are the customer, so if you want a 3 ton, you can have a 3 ton installed - though if there are questions, it might be worth asking about the 2-stage units, and see what the tech says. If he shy's you away from them, then there may be some knowledge deficiencies there. Never hurts to ask though.
    Got ya. Thanks again for your help!

  6. #6
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    This is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.

    Further infractions may result in loss of posting privileges.
    Last edited by beenthere; 10-04-2012 at 04:24 PM. Reason: Non Pro * Member

  7. #7
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    Quote Originally Posted by Zomby View Post
    Wow! What an excellent explanation. I actually understand it Thank you!

    I have a 1500 SF single story ranch with crawlspace in the soupy middle of Missouri and my current 9 SEER 14 year old Goodman actually doesn't cycle too often when the temps are above 90 or so. Is the reason it doesn't cycle because the unit is so old and it is pumping its guts out to cool the house?
    1st, with a properly sized AC, you do not use setbacks. Leave it the same tmerpature all day, and only turn it down right before bedtime for comfort, then around 9-10AM you cna return it ot daytime temperature.

    In you location (jsut a little further south than me) when its' over about 96F, a perfectly sized system should be running constantly by about 3PM on a sunny humid day and keep running non-stop until about 9-10PM. Above 95F, it may fall behind 1-2F around 5PM and won't recover until 11PM or so. Operating this way, it will use maybe 20% less energy than the same 3 ton unit that cycles on and off.

    Also, if you house only need 2 tons on cooling in central Missouri, I bet it only needs 45k BTU's of heating. But 60k BTU's is still a nice improvement. Its hard to undersize heating.


    Look at it this way, if your 3 ton system (possibly with undersized ductwork too, which will reduce capacity) cycled on for 9 minutes and off for lets say 6 minutes on a day in the mid 90's, it was only using on average 2 tons of capacity, or less being older (dirty coils, lower compressor efficeincy) and possibly low on airflow. A 2 ton system in those conditions, would run continously and use at least 20% less energy above the SEER rating.

  8. #8
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    May 2004
    Location
    south louisiana
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    good answers.

    crazifuzzy wrote:
    There are 2 ways to avoid the cycling. Have a system matched closer to your heat load for most the season, or have a system with a variable capacity. truly variable systems do cost quite a bit more, but 2-stage units are not all that much more expensive. They are not quite as common at the lower end of the tonnage, but there are some 3 ton 2-stage systems (which can stage down from the full 3 tons, to about 2 tons). Then, it can run MOST of the time at the 2 ton load, and if the load slightly exceeds 2 tons, it will occasionally switch up to 3 tons for a while and then back to 2. This is much smoother than cycling between 3 tons and 0 tons.


    this is what I see happening in most houses.
    hvac contractor worries about the 10% of the year &
    oversizes for that, so they don't get call backs.
    or they install vs..to satisfy both actual requirements 90% of the
    time, and to eliminate need for call backs.
    higher cost to homeowner, but better than oversized all the time.

    OP, the only dumb question is the one you don't ask.
    everyone learns somewhere. having conversation with
    your hvac company/tech is a good thing imo.

    best of luck
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  9. #9
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    Good stuff fuzzy. One thing I would also ask is what indoor and outdoor temperatures he is sizing your unit to handle. Be sure that the indoor temperature is something that will be comfortable to you and your family. Also the last manual j I looked at called for an outdoor design temperature of 94. As you know this last summer was much above that. We ran a lot of things that weren't keeping up when it got to 105 for those 10 days.

  10. #10
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    Quote Originally Posted by ryan1088 View Post
    Good stuff fuzzy. One thing I would also ask is what indoor and outdoor temperatures he is sizing your unit to handle. Be sure that the indoor temperature is something that will be comfortable to you and your family. Also the last manual j I looked at called for an outdoor design temperature of 94. As you know this last summer was much above that. We ran a lot of things that weren't keeping up when it got to 105 for those 10 days.
    You still shouldn't oversized for a 10 day heat wave once every 5, 10 or in that case 70+ years since it was that hot. I know I didn't mind my system falling behind by 1-2F in the late afternoon. THe humidty was still low and it was saving me money. In extreme hot weather, you can always turn it down 1-2F overnight, and "precool" the space. It does help if oyu have a lot of thermal mass. SOmetiems just closing a couple blinds will make enough difference.

    Its not uncommon to hit -10F where I am. I still would size for 0F because it will only be below that for 6-8 hours on those coldest days.


    I think soemtimes you have ot look and see how close you are to a particular tonnage or furnace size and then take a 2nd look at your design conditions including shading and see if it makes sense to be perfectly sized, a little undersized, or go up one size.

  11. #11
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    Jul 2012
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    Western KY
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    Quote Originally Posted by CraziFuzzy View Post
    In shortest terms - cycle time.

    Detailed - If the unit is larger than the demands from the home, it will cycle on and off. This, ultimately, is not as efficient for a few reasons.

    First, being humidity control. When the unit turns off, the moisture on the evaporator coil, ultimately ends up evaporating back off into the home. Too short of a cycle time will also not give the moisture time to even accumulate enough to drip to the drain pan and actually be removed. It is much better to have a smaller system run constantly than a larger system cycle on and off.
    Second, being weather. If the absolute hottest day of the year calls for a 3 ton unit, then yes, on that single day, you will need a 3 ton unit running continuously. However, that is at the expense of excessive cycling the other large percentage of the cooling season. If, instead, it is sized for say, the 90% highest loads of the year, then yes, on those hottest days, it may not be able to cool all the way to your setpoint - however, it stretches the efficiency for the other larger portion of the cooling system, in the end, greatly helping the energy costs.
    Third, comfort. An oversized system will cycle, and the controlling aspect of that is the swinging temperature at the thermostat. A house whose temperature is swinging up and down by 3-4 degrees twice an hour as the A/C cycles on and off is not as comfortable as one that maintains a much more steady temperature by running longer between cycles (drifting slower 3-4 degrees up and down say, once an hour or two).

    There are 2 ways to avoid the cycling. Have a system matched closer to your heat load for most the season, or have a system with a variable capacity. truly variable systems do cost quite a bit more, but 2-stage units are not all that much more expensive. They are not quite as common at the lower end of the tonnage, but there are some 3 ton 2-stage systems (which can stage down from the full 3 tons, to about 2 tons). Then, it can run MOST of the time at the 2 ton load, and if the load slightly exceeds 2 tons, it will occasionally switch up to 3 tons for a while and then back to 2. This is much smoother than cycling between 3 tons and 0 tons.
    Nice!!!

  12. #12
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    Jun 2001
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    Moore, Oklahoma, United States
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    4,190
    Quote Originally Posted by energy_rater_La View Post
    good answers.

    crazifuzzy wrote:
    There are 2 ways to avoid the cycling. Have a system matched closer to your heat load for most the season, or have a system with a variable capacity. truly variable systems do cost quite a bit more, but 2-stage units are not all that much more expensive. They are not quite as common at the lower end of the tonnage, but there are some 3 ton 2-stage systems (which can stage down from the full 3 tons, to about 2 tons). Then, it can run MOST of the time at the 2 ton load, and if the load slightly exceeds 2 tons, it will occasionally switch up to 3 tons for a while and then back to 2. This is much smoother than cycling between 3 tons and 0 tons.


    this is what I see happening in most houses.
    hvac contractor worries about the 10% of the year &
    oversizes for that, so they don't get call backs.
    or they install vs..to satisfy both actual requirements 90% of the
    time, and to eliminate need for call backs.
    higher cost to homeowner, but better than oversized all the time.

    OP, the only dumb question is the one you don't ask.
    everyone learns somewhere. having conversation with
    your hvac company/tech is a good thing imo.

    best of luck
    In the above situation I'd do a 2 ton single stage and just let the system fall behind for a few hours on a hot afternoon. For a 2 stage to get it's rated SEER, design conditions should cause the unit not to keep up on HIGH speed, the unit should fall behind when conditions exceed design. Unfortunately the rated SEER is rarely realized since the 2nd stage is viewed as "reserve capacity".

  13. #13
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    Quote Originally Posted by 54regcab View Post
    In the above situation I'd do a 2 ton single stage and just let the system fall behind for a few hours on a hot afternoon. For a 2 stage to get it's rated SEER, design conditions should cause the unit not to keep up on HIGH speed, the unit should fall behind when conditions exceed design. Unfortunately the rated SEER is rarely realized since the 2nd stage is viewed as "reserve capacity".
    Correct. I love and hate this topic. There are alot opinions and, of coarse, we all like ours the best. I'm on the 3 ton 2 stage train myself but your point is absolutely correct. I'll trade a little SEER For that extra capacity when I "want" it. Not that I need it.

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