Results 1 to 13 of 13
  1. #1
    Join Date
    Apr 2013
    Posts
    8

    New to site - imfamous problem of oversized unit

    I recently added heat and air to an upstairs living area (2 bedrooms), previously there were ducts (very small) from the main system downstairs. The rooms never cooled adequately, so we always blocked off the vents and used 2 upright portable AC's. Both of the portable units were both 1 ton each. The smaller room (150 sq ft) cooled fine but the larger room (~400 sq ft) never cooled especially when the summers in Oklahoma had 50 days straight above 100 degrees. Both rooms face the south and have 3 windows each and have lots of surface area (sloped walls).

    When sizing the unit the contractor did not do any calculations, but rather based his recommended size on other houses in the neighborhood and somewhat on the portable units I had been using knowing that the central air unit would be much more efficient. He suggested a 2 ton unit. I am an engineer by trade and education so I researched a lot and determined that I would ask for an upsize of 20% and go to a 2.5 ton. I knew this was not preferred based on the footage versus the tonnage.

    Now, as expected the unit cools great, but the humidity hangs around the 60-70% range. I know this is due to the lack of runtime which is reducing the dehumidification process. Typical runtime is approx 10 minutes. So like many others who have posted, I am looking for ways to improve my situation and would like some advice. I understand to correctly fix the problem I should replace the unit, but that is not in the cards right now.

    I believe these are my options:

    • Add a dehumidifier to the system. $$$
    • Use a dehumidifier in the bedrooms. $
    • Slow the airflow (motor speed) (already did this with slight improvement)
    • Increase the cooling load to include an uncooled/unheated sunroom and possibly the garage – not sure how to place a thermostat when cooling areas that are not connected?$


    The AC contractor I used is a very reputable company and has been great to work with, so no real issues there especially since I asked for the larger unit.

    Thanks in advance!

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Unfortunate that your researched in the wrong place. You never oversized for the load with refrigerant based equipment. In commercial the systems ate 2 stage and manage the load diversity or they are chilled water so you always have the same indoor dewpoint and change airflow and water flow to load.

    The dehumidifier its the best bet. Sharing airflow with another zone could work. Some thermostat can have remote sensors you can average as a compromise.

    Sent from my DROID RAZR using Tapatalk 2

  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    Quote Originally Posted by pramos67 View Post
    When sizing the unit the contractor did not do any calculations, but rather based his recommended size on other houses in the neighborhood and somewhat on the portable units I had been using knowing that the central air unit would be much more efficient. He suggested a 2 ton unit. I am an engineer by trade and education so I researched a lot and determined that I would ask for an upsize of 20% and go to a 2.5 ton. I knew this was not preferred based on the footage versus the tonnage.

    Now, as expected the unit cools great, but the humidity hangs around the 60-70% range. I know this is due to the lack of runtime which is reducing the dehumidification process. Typical runtime is approx 10 minutes.
    Blue + Green = poor humidity control

    Neither of you did the CORRECT calculations. Your contractor guesstimated, and you as an engineer should have insisted on applying the correct discipline of engineering to the task at hand. And that discipline of engineering would be mechanical, which the ACCA Manual J residential heat load calculation method, Manual S residential equipment selection procedure, and Manual D residential duct design and installation, are soundly based upon.

    You are now left with a) standalone or whole house dehumidifier; b) reduction in equipment capacity; or c) increase sensible heat load on the house. "B" is the infamous "pay more to do it right the second time vs. the first time"; "C" is a silly option; "A" is adding more equipment to compensate for a poor design of the HVAC. None optimal, but "A" may be the only feasible one.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Depending on the model of portable unit, the net output is usually lower than nameplate. If they were single.exhaust pipe, they masked the room negative and drew in hot humid outside air.

    Sent from my DROID RAZR using Tapatalk 2

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,428
    I assure you you can best support your local utility by setting the t'stat to 66'F.

    Bill shouldn't be more than $200 per month for the A/C portion.

    Thank$ for the $upport.! ..!!
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,423
    This is what happens when you have an A/C contractor that has no idea what he is doing and an engineer that thinks because he is an engineer he knows what he doing.

    I think its time to start over. Sorry!
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  7. #7
    Join Date
    Apr 2013
    Posts
    8
    Not that it matters now, but the portable units were both dual hose units.

    Yes, this is going to be tough and costly lesson learned. The hard thing to admit is that I had used load-calc prior to asking for the 2.5 unit. The calculated load for btu's cooling was 21,962. I added the 20% due to the fact of one of the interior walls joins with the attic and in August this wall was nearly 100 F. The calculated load with the 20% was 2.196 - my mistake was up-sizing instead of opting for a 2 ton. Now, that I look at it a 1.5 ton may have been more appropriate for the space.

    I am looking at the option of decreasing the capacity of the equipment. I know it is not the ideal situation, but could my situation improve with just replacing the condenser and leaving the evaporator the same?

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,641
    Quote Originally Posted by pramos67 View Post
    I recently added heat and air to an upstairs living area (2 bedrooms), previously there were ducts (very small) from the main system downstairs. The rooms never cooled adequately, so we always blocked off the vents and used 2 upright
    I believe these are my options:

    • Add a dehumidifier to the system. $$$
    • Use a dehumidifier in the bedrooms. $
    • Slow the airflow (motor speed) (already did this with slight improvement)
    • Increase the cooling load to include an uncooled/unheated sunroom and possibly the garage – not sure how to place a thermostat when cooling areas that are not connected?$


    The AC contractor I used is a very reputable company and has been great to work with, so no real issues there especially since I asked for the larger unit.

    Thanks in advance!
    How cold is the cooling coil of the a/c after 10 mins. of cooling. If you can get the coil down to 45^F, it will slow the cooling rate and remove enough moisture to get to 50% RH at 75^F at peak cooling. Also fan "auto" important to prevent quick re-evaporate the moisture on the coil back into the home.
    A small whole house dehumidifier like the Ultra-Aire 70H will maintain <50%RH throughout the space. The dehumidifier also help the rest of the home during rainy cool weather.
    The real heat is yet to come. Keep us posted.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Did you run hte protable units at a constant temperature or did you shut them down and thurn them on only when you came home in the afternoon.

    A lot of times people think their AC in not big enough, but they don't realize that depending on home construction, on a hot day it can take 2X the capaicty to cool a space off, than it does to maintain temperature. I've only lived in my home 2 years, but after downsizing both of my AC units, I'm learning about my homes behavior in hot and cold weather, that I can hold my indoor temp at 74F with just 3 Tons in 95F, sunny humid weather. But it would take as much as 6 tons to cool it off by just 1F per hour. Guess how low the humidity is if I had a 3 ton unit running constantly, vs. 6 tons cycling on and off 1/2 the time? Of course the smaller unit uses less energy as well. It takes 8-10 minutes for refrigerant pressures to equalize and reach peak effciency.

    There someone on here that cools their 2000sqft 2 story home with I think just 3 6k BTU window units and their montly bills are really low in summer.

    Think about your car's engine. How much fuel does it take just to warm up several hundred lbs of metal? How much to actually move the car? I have a very short commute. My fuel economy in 0-20F weather vs. 50-60f weather goes form 18mpg to 10mpg. SOme of that is effceincy because it runs richer when cold. But a lot of it is the energy just to heat up the metal and fluids.


    If you have a 2 or 3 ton indoor coil, you can match that to a 2 ton unit. Although it sounds like ideally you might go as small as 1.5 tons. A simple capacity requirement check shoirt of a full load clculation, is to measure run times of the unit is a steady state. So on a hot sunny day, let it hold 75F indoor all day. In the afternoon, time how long it's on and off for maybe 2 or 3 cycles. Then take the outdoor temp less indoor design (75F), and compare it to design temp. vs. indoor. Divide those 2 delta T's to get a ratio. Then take your duty cycle and divide it by that ratio. That is approx. what size would run continously. Keep in mind that reverse stack effect incrases, and we're still 1 month away from the most direct sunlight of the year.

    For example. I have two 2 ton units. It was 88F outside yesterday. The system were cycling about 1/2 of the time. So if design is 93F, then I take 4 tons / 3 / 0.73 = 2.7 tons. So could I actually have 1 single 3 ton zoned system? Yes. But it would require runnning ductwork somehow upstairs from the basement. Also, this number also factors in thermal mass, which Manual J does not. If you have brick or stone, they will absorb heat initially and the transfer over a extended period of time to the interior. So the equipment runs longer, but doesn't need as much peak capacity.

    Keep in mind that 2.7 tons peak during the hotest month will still use over $200 in electricity $ $0.13/kwhr.

  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,032
    If you can get the air infiltration rate down to say .4 or .375-ACH; then get a programmable ROOM TH that has a Temp-swing setting, you could possibly get 20 minute runtime cycles with longer off-times, with maybe an on temp of around 77 or 78F & an off temp of around 74F. Do some experimenting selecting the swing temp differential cycle time numbers... Use other (floor type or ceiling) fans to keep air circulating & dress cool.

    ACE hardware stores have the low cost ACE ATX 1500 4115176 or, LUXPRO PSP 511LCa; search the Internet for more information on them; both work well.

    Don't use a fan-off-delay, have the blower shut off with the outdoor condenser.

    They say the new smaller microchannel coils operate at a colder indoor coil temp therefore, would do more latent work & less sensible helping increase runtimes. I will only buy & would only sell (were I not retired) the new microchannel systems; they also require less refrigerant & reduce the required space for installation as much as 40%.

  11. #11
    Join Date
    Apr 2013
    Posts
    8
    I haven't measure the temp of the coil at a specific time yet. I have been looking at the whole house dehumidifier like the one you referred to. Would this be installed inline with the upstairs system or could be it be tied into both? I like the idea of being able to help the rest of the house. It's difficult for me to really gauge the system lately because desired indoor temp is close to the outdoor temp. For instance, the outdoor temp right now is 73 F/100 % humidity and the indoor temp is 72F with humidity level at 64%. About an hour ago the system kicked in and with the temp was 73 and humidity was 70%.
    Quote Originally Posted by teddy bear View Post
    How cold is the cooling coil of the a/c after 10 mins. of cooling. If you can get the coil down to 45^F, it will slow the cooling rate and remove enough moisture to get to 50% RH at 75^F at peak cooling. Also fan "auto" important to prevent quick re-evaporate the moisture on the coil back into the home.
    A small whole house dehumidifier like the Ultra-Aire 70H will maintain <50%RH throughout the space. The dehumidifier also help the rest of the home during rainy cool weather.
    The real heat is yet to come. Keep us posted.
    Regards TB

  12. #12
    Join Date
    Apr 2013
    Posts
    8
    We had the portable units for about 5 years and initially we tried to only use them when we needed them, but the upstairs would reach close to 100 F and the AC's just could not get the temp down to something comfortable. We eventually let both units run all day on "Auto". The smaller room cooled quite nicely, but the larger room never could get the room to 76 F. This was during our HOT July and August where the temps were in the 100's for nearly 50 days straight.

    Thanks for the info regarding the capacity check - I will perform this over the weekend when I can monitor it closely and the temps are supposed to be in the 80's.
    Quote Originally Posted by motoguy128 View Post
    Did you run hte protable units at a constant temperature or did you shut them down and thurn them on only when you came home in the afternoon.

    A lot of times people think their AC in not big enough, but they don't realize that depending on home construction, on a hot day it can take 2X the capaicty to cool a space off, than it does to maintain temperature. I've only lived in my home 2 years, but after downsizing both of my AC units, I'm learning about my homes behavior in hot and cold weather, that I can hold my indoor temp at 74F with just 3 Tons in 95F, sunny humid weather. But it would take as much as 6 tons to cool it off by just 1F per hour. Guess how low the humidity is if I had a 3 ton unit running constantly, vs. 6 tons cycling on and off 1/2 the time? Of course the smaller unit uses less energy as well. It takes 8-10 minutes for refrigerant pressures to equalize and reach peak effciency.

    There someone on here that cools their 2000sqft 2 story home with I think just 3 6k BTU window units and their montly bills are really low in summer.

    Think about your car's engine. How much fuel does it take just to warm up several hundred lbs of metal? How much to actually move the car? I have a very short commute. My fuel economy in 0-20F weather vs. 50-60f weather goes form 18mpg to 10mpg. SOme of that is effceincy because it runs richer when cold. But a lot of it is the energy just to heat up the metal and fluids.


    If you have a 2 or 3 ton indoor coil, you can match that to a 2 ton unit. Although it sounds like ideally you might go as small as 1.5 tons. A simple capacity requirement check shoirt of a full load clculation, is to measure run times of the unit is a steady state. So on a hot sunny day, let it hold 75F indoor all day. In the afternoon, time how long it's on and off for maybe 2 or 3 cycles. Then take the outdoor temp less indoor design (75F), and compare it to design temp. vs. indoor. Divide those 2 delta T's to get a ratio. Then take your duty cycle and divide it by that ratio. That is approx. what size would run continously. Keep in mind that reverse stack effect incrases, and we're still 1 month away from the most direct sunlight of the year.

    For example. I have two 2 ton units. It was 88F outside yesterday. The system were cycling about 1/2 of the time. So if design is 93F, then I take 4 tons / 3 / 0.73 = 2.7 tons. So could I actually have 1 single 3 ton zoned system? Yes. But it would require runnning ductwork somehow upstairs from the basement. Also, this number also factors in thermal mass, which Manual J does not. If you have brick or stone, they will absorb heat initially and the transfer over a extended period of time to the interior. So the equipment runs longer, but doesn't need as much peak capacity.

    Keep in mind that 2.7 tons peak during the hotest month will still use over $200 in electricity $ $0.13/kwhr.

  13. #13
    Join Date
    Aug 2009
    Location
    Jurupa Valley, CA
    Posts
    1,787
    The problem of an oversized unit's short cycle times isn't JUST the high humidity - short cycle times cause instabilities in room conditions as well. The indoor fan turning on then off 10 minutes later prevents full mixing in the space, causing intermittent drafts. The air temp in the area will be high, low, high, low, repeatedly, leaving to an overall drop in comfort beyond that of just the higher humidity.

    You know the problem is an oversized unit, the proper way to fix that would be to install a properly sized unit. There are options on what to do here. You can likely have the 2.5 ton outdoor unit with a 2 ton, and that will stretch your 10 minute runtimes out to maybe 14 minutes - an improvement, but still not ideal.

    The best options as far as comfort, and not having to do much with the indoor unit, is to ask about a 2-ton 2-stage unit. This will allow it to run much more continuously, and may end up cycling between stage-1 (about 1.5 tons) and stage-2 (2 tons) to maintain your temp. The cycle times between stages will vary, but that cycling will not have as much of an effect as going from 2.5 tons to 0 tons.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event