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  1. #1
    Join Date
    Jan 2001
    Posts
    41

    Power Consumption - 3.5 ton vs 5 ton

    We currently have a 5 ton Carrier AC unit for the 2,200 sq ft first floor of our house in Oklahoma. I have always thought it was oversized as the humidity inside the house during the summer months is higher than it should be. A reputable HVAC company ran a load calc recently and determined that we should have a 3.5 ton unit. We are trying to weigh the benefits, cost and comfort, of replacing our existing unit with a smaller one.

    I know that a 3.5 ton unit will have to run longer per cycle, but all things being equal, will the cost to run the smaller unit be about the same as it is for the 5 ton unit, which runs shorter cycles? I know the smaller unit should be able to create a more comfortable environment, but I'm wondering about the operating cost comparison of the two.

  2. #2
    Join Date
    Aug 2007
    Location
    ky
    Posts
    181
    Theoretically for a same seer unit there would not be much of a difference.

    Supposed you has a XX,XXX Btu/H of heat gain .
    It would take a shorter amount of time to get rid of that heat gain using 5 ton of cooling (Short cycling) using higher electric consumption for shorter time
    or it would take longer to cool down using 3.5 ton unit using less electric but at longer time.

    Both would get rid of the xx,xxx Btu/h heat gain.

    but in practice . the 3.5 ton unit would removed more moisture, making you feel more comfortable and able to set the thermostat 1-2 degree higher.
    which translate to shorter run and lower electric consumption

    You should see a drop in the operating cost.
    How much , i can't tell
    just don't expect a staright drop of 15% in electric usage (if they are the same SEER )
    you will use less electric in 2 possible way.
    1) New unit has better SEER thus more efficient.
    2) New smaller unit removed humidity better . thus a slightly higher thermosat setting .


    Good luck

  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Fewer cycles should use slightly less energy... but energy consuption at the compressor will be minimal. The big energy savings will be from the inside blower. You ductwork is likely only sized for 4 Tons, so with a 3.5 Tons of airflow, the blower will operate at a lower static pressure. Lower pressure = less energy for the same total volume of air moved.

    You may notice hte smaller unit is much quieter since there is less air movig in the ductwrok and register. Energy consumption will drop noticeably if you go to a vairable speed air handler, if you have a regualr 4 speed PSC blower now.

    Also, with a vairable speed air handler, you can reduce the airflow to dehumidify if the thermostat is set-up for controlling humidity levels. Lower airflow makes the indoor coil colder, which condenses more water out of the air.

  4. #4
    Join Date
    Jul 2008
    Posts
    551
    Quote Originally Posted by motoguy128 View Post
    ....Lower pressure = less energy for the same total volume of air moved.

    .....

    That only holds true for ECM motors because PSC can't maintain the same total volume of air.

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by badtlc View Post
    That only holds true for ECM motors because PSC can't maintain the same total volume of air.
    True, power consumption on PSC's are not linear at lower speeds. I think they tend to be most efficient at full load and the highest speed. So you would need to consider the longer run time of a 3.5 ton system vs. shorter runs times at 5 tons.

    But if we're assuming he has a 5 ton PSC blower now, and the new system uses a 4 Ton PSC. The 4 Ton system installed on the same ductwork at med hi speed for 3.5 Tons, will liekly use less power than the 5 ton system at hi speed even when about 25-30% longer runs times are factored in.

    Also, I suspect that the ductwork is undersized for 5 tons, and/or has too little filter area, and the 5 ton unit isn't even capable of reaching 5 tons. It's probably closer to 4.5 Tons.



    Either way, if the system is more than maybe 5 years old, it's probably worth considering at least replacing the condenser only if there's still a ARI match with that air handler. I know a 4 To nand 5 Ton often use hte same line size, but I'm not sure if the same line size or a slightly oversized lineset can be used with a 3.5 Ton.

    Also, using a 5 Ton coil with a 3,5 Ton condenser can be done, but won't provide as good latent capacity.... which is the primary objective.


    Ultimately, I think the smaller unit will consume less power in normal use, because lower humidity levels will allow you to keep the temprature higher at the same comfort level.

    More detail is needed on the existing system and proposed system to determine what direct energy savings the equipment itself would deliver.

    Most improtantly, a properly sized unit WILL be more confortable throughout the whole cooling season. My parents have an oversized system on their condo in MI. They always complain about the humidity levels.

  6. #6
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,759
    You should see a slightly lower operating cost.

    Shorting cycling raises your electric bill.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    Join Date
    Jan 2001
    Posts
    41
    The current 5 ton unit is a Carrier Performance series 13 SEER, and the load calc shows that the duct work is sized for up to a 4 ton unit.

    Here's the part that is hard to swallow and makes the decision to change more difficult. The unit was installed in our house that we had built three years ago. If it had been 7-10 years, it would be more of a no-brainer. We haven't received a quote yet, but we're just not sure if it's going to be worth the money at this time.

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    [quote=red;3111382]The current 5 ton unit is a Carrier Performance series 13 SEER, and the load calc shows that the duct work is sized for up to a 4 ton unit.

    [quote]

    So the builder or original HVAC installer installed the wrong size outdoor unit. They probably had some extra 5 Ton units on hand, and though they were "doing you a favor" by installing a bigger unit.... like a free upgrade from a V6 to a V8 in a car... of course the dealer doesn;t pay for the extra gas it burn over the next 10 years.... just like builder don;t have ot live in your damp house all summer.

    It sounds like you need to track them down and tell them to install a 4 Ton or 3.5 Ton unit.

    The good news, is that if you install a 3.5 Ton Performance 15 outdoor unit, you might get 30% or $1500 off, if the indoor unit is a good match.

    You have to decide if the $$$$ is worth it. Maybe the original installer will cut you a deal. Tell him your unhappy with how it's performing. Maybe even get Carrier's regional rep involved.

    A 5 ton unit on 4 ton ductwork could have some issues down the road. from too low of airflow. Keep that air filter clean!!!

  9. #9
    Join Date
    Sep 2006
    Posts
    1,253
    red,

    What all of this boils down to is the cost of your comfort. And you are the only one who can determine the value of it.

    It sounds like you have a contractor that can do you a good job of resolving your issue. One thing to keep in mind is the first floor system of your home may provide less cooling than Manual J might suggest.

    While the Manual J calculation will get very close on the whole house heat gain, it will not necessarily split it between the first and second floor accurately. In my personal case my second floor system provides about 2/3 of the cooling and the first floor about 1/3. Manual J had it almost exactly opposite but hit the total load accurately.

    The point of all this is you may want to spend some time with your contractor investigating this issue. You may find you can reduce the size of that first floor unit even more.

  10. #10
    Join Date
    Nov 2006
    Location
    Mechanicsburg, PA
    Posts
    310
    Manual J should separate it well between the first floor and second floor as the second floor has a heat gain for the ceiling and the first floor does not. The first floor has a floor loss which is normally minimal.
    One thing which will save more money which I would like to add to the other posts is you loose a certain amount of cooling ability to de-humidifying. As the humidity level drops the cooling ability increases.

  11. #11
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    I don't think I would change it now but that's just me....

    The cost of the fix will never be recovered over the life of the system.

    Comfort wise and for much less $$$$ have a contractor add additional dump ducting to get the static in line. Then add a dehumidifier or two to keep the humidity in check. The system will last a long time and will cost much less than a complete swap out.

    I'd ask the orig. installer if they would correct it but I wouldn't hold my breath.

  12. #12
    Join Date
    Feb 2004
    Location
    Pacific NW
    Posts
    570
    three years ago. If it had been 7-10 years, it would be more of a no-brainer. We haven't received a quote yet, but we're just not sure if it's going to be worth the money at this time

    Yep, at 3 years, 13 seer, that certainly does turn the question into the no-brainer of the year - keep what you have but do make ONE VERY MINOR revision (alluded to in some previous posts) that will also reduce operating costs.

    Your 'reputable HVAC company ' sounds like they sent you a salesman not anyone who kinows anything or even what a psychometric chart is. Go to their office if truly reputable, ask to talk to a competent tech who can show you on a psychometric chart what the simple fix he can make will do for you, then hire him to do that 15 minute job. There are some very simple and low cost solutions to your problem keeping all your present equipment, DIY rules prevent revealing them here.

  13. #13
    Join Date
    Aug 2007
    Posts
    273
    Get a seperate stand alone dehumidifer which will reduce humidity when the a/c is not working. I use this approach living in washington dc where it can get prety humid in the summer months. You can run the extra humidifer for a long time for the cost of a new unit!

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