Help! - new system is not cooling adequately

Discussion in 'HVAC Heating & Cooling' started by bldn10, Jul 14, 2016.

  1. bldn10

    bldn10 New Member

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    Location:
    Memphis TN
    I just renovated a house to sell and am closing next week. It is a 1 story house of 1800 sf w/ an added closet and hall upstairs of around 200 sf. The contractor and I decided that a 4 ton unit would suffice if I had extra insulation blown into the ceiling. We took up attic floor and blew in $900 worth. It is a Rheem furnace and condensor w/ ADP coil, R8 flex ducts running throgh unconditioned attic. But the system is just not cooling adequately. W/ the thermostat set on 75 it will be 75 generally throughout the downstairs in the a.m. but in the afternoon, as the temp climbs over 90, it cannot maintain 75 while running continuously. And it is not dehumidifying well either. Upstairs is a disaster - when it is 75 down it will be over 80 up. The contractor failed to put a return upstairs (which is usually closed off from downstairs) but added one at my suggestion. Waiting to see how it works. The attic has 4 gable windows but no other ventilation. I have a box fan in one of them. But the attic does get decent outside air as the other night it got down below 70 and the next morning it was 70 in the attic and 80 down, set on 80.

    He checked the outlet temps when it was 75 and they ranged from 63 to 70. To my knowledge he has not checked the outlet airflow. Is here a rule of thumb how much flow there should be at the outlets?

    I am freaking out because I am closing next week but cannot until this is fixed. All I can expect from the contractor is that this 4 ton system is operating as it should. How can that be determined? If it is and is not enough, I guess I will just have to bite the bullet and go to 5 tons.
     
  2. Tom Sawyer

    Tom Sawyer In the Trades

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    1 - 4 tons is likely two times oversized
    2 - did anyone bother to do a manual J loss/gain on the structure?
    3 - did anyone bother to do a duct design or did you just string flex duct?

    I'm betting that your duct design is the problem.
     
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  4. bldn10

    bldn10 New Member

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    4 tons is too much for 2000 sf in Memphis?
    I'm pretty sure no calculations were made.
    I'm sure they thought about how best to run the ducts but did not do a duct design, whatever that is. A guy who works for me - his father is an HVAC contractor - and the son thinks the trunk lines may be too small. That's why I'm asking about airflow. Thanks
     
  5. DonL

    DonL Jack of all trades Master of one

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    Where is your T-Stat ?

    Hot air normally rises.

    Did the installer set your fan/blower speed ?
     
  6. bldn10

    bldn10 New Member

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    Tstat in central hall downstairs. Actually probably a little warmer than house in general.
    He apparently did not because my guy changed it to HIGH.
     
  7. Tom Sawyer

    Tom Sawyer In the Trades

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    If no duct design was done I can guarantee that's your problem. You can't just string flex duct and expect good results. You need to know the CFM of the unit along with the total,developed length of the longest run as well as the loads served. I'd also bet the return air ducts are either undersized or you only have one big one somewhere in the middle of the house. And yes, if the house is reasonably insulated four tons is more than enough. With A/C bigger isn't always better and you can't deliver 48,000 btu of calling if the ducts aren't right.
     
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Is the unit running continuously? Given your typically high humidity levels, you should have a nearly continuous stream of condensate running out of the thing as well.
     
  9. WorthFlorida

    WorthFlorida The wife is still training me.

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    • Get your installer to put gauges on it. It does seem like ductwork issues provided the unit is properly charged and no leaks from the day of installation. With incorrect airflow the high & low pressure difference will be off even with a properly charged system.
    • When it is not cooling enough, after a few hours, take off the cover to the air handler and check to see if there is icing occurring on the coils. If there is icing it means one of two things, not enough air flowing over the coils or the refrigerant is low.
    • Since this was a construction site, replace the filter. As it happened to me, concrete and drywall compound dust clogged up the filter, hence not enough air over the coils.
    • Too much tonnage and the unit will not run as much, therefore, the second floor will be hot as stated above, heat rises and humidity will be higher making it uncomfortable.
    • The old adage of 1 ton (pre SEER 10 days) for every 500 square feet went out the door about 25 years ago.
    • My house built in 2007, two story, 2375 sq ft, A/C unit on the second floor is 3.5 tons at seer 16, replaced 12 months ago. That is in Orlando, FL Very comfortable.
     
  10. bldn10

    bldn10 New Member

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    There are 4 low returns downstairs and those ducts are rather large, probably 14". The unit is on one end of the house so there are some long runs. I believe the "large" trunks are 12".
    Yes, until it reaches desired temp, and when the temp is rising outside, it runs continuously, producing a constant stream of condensate. However, it does not appear to be dehumidifying all that well, as it typically is between 55 and 57%.
    They checked the refrigerant and we have run it w/o a filter for awhile just to test it. Have not seen any icing. One of the first things I did was make sure the coil was not clogged up, and it did not appear to be. I was kinda hoping that would be it. :)
    It was unseasonably cool yesterday and I couldn't evaluate how it was doing.
     
  11. Tom Sawyer

    Tom Sawyer In the Trades

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    Gauges and a couple of thermometers will give you superheat and sub cool which will tell you if the airflow is proper. Also, if all the ducts are flex duct be aware that there is tremendous friction loss with flex.
     
  12. jacobsond

    jacobsond DIY Junior Member

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    Another customer with a hacked in system and no load calculations done. Very sad and all to common. Dont sign off until its working correctly. Might have to get a HVAC pro in there do inspect. Be willing to pay for that. You have a system worth $1000s of $$$ get it right.Be willing to pay a few hundred to have it evaluated by someone else. Was there a startup report given? There should have been. Dont except until there is.
     
  13. Tom Sawyer

    Tom Sawyer In the Trades

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    Often times the thrill of saving a buck turns into the nightmare of spending twice as much to straighten out the mess. You get what you pay for. There's a reason some trades require a license.
     
  14. WorthFlorida

    WorthFlorida The wife is still training me.

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    You probably have a few things dogging the problem. One is HVAC contractors need to start thinking of more of air conditions system needs than heating. The average temps are rising and more homes are getting central AC. In my book, and I had to argue with a contractor building me a 2 story home in Syracuse NY, that the returns have to be up high on the wall at least on the second floor. Even for heating you want to pull excess heat off the ceiling to warm a home. But that doesn't fix your problem. Your man will now need to get what Tom Swayer mentioned, superheat and sub cool readings.
     
  15. DonL

    DonL Jack of all trades Master of one

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    You have 4 returns for "a 1 story house of 1800 sf" ?

    How many filters do you have ?
     
  16. bldn10

    bldn10 New Member

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    "HVAC contractors need to start thinking of more of air conditions system needs than heating"
    Especially in this house - I retained the working radiator system - the CH will rarely even be used!
    2 of the returns branch off the same line but the other 2 are independent. 1 filter between the return plenum and furnace.
    At noon today I checked the room temps w/ T-stat on 75 and showing 75, and they were good except for upstairs. Outside temp 90-91. I turned it down to 70 to see what it would do and came back after 3 hours. It was still showing 75 and a few rooms were 76-77. It was 81 in the upstairs closet so the new return was not the solution. Bottom line today is that w/ outside temp rising from morning high 80s to afternoon low 90s, it ran continuously for 3 hours and could not lower the house 1 degree. I left a message for the contractor that all he has done so far has failed. If he doesn't give me a solid plan by Mon. I will bring in someone else to evaluate the system.
     
  17. WorthFlorida

    WorthFlorida The wife is still training me.

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    You may be running cooling and heating at the same time.
    Is the new a/c air handler also your heating source or supplemental heat? You call it a furnace but gave no model number. Is it electric? It has happened, when the A/C is running the electric elements could be energized? Electric coils for air handlers are usually added during the install of 5K or 10K watts. A miss wired thermostat may have the relay in the closed position or the contacts have welded together. The controller board for all of this may also be bad. The heating element is after the evaporator coil. It cools first then reheats.

    Check the thermostat that it is optioned for the correct type of heat, gas or electric. Also, if you have a heat pump/AC unit, electric heat is also added for supplemental heat for the very cold days. It is also a thermostat option.
     
    Last edited: Jul 15, 2016
  18. Tom Sawyer

    Tom Sawyer In the Trades

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    If the electric elements were on, ther would be virtually no AC.
     
  19. bldn10

    bldn10 New Member

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    Memphis TN
    Gas furnace, pretty sure it is not lighting up. I wish.
     
  20. DonL

    DonL Jack of all trades Master of one

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    If the fan is on High and air is moving I would not expect icing. Icing gives you low air flow.

    A lot of the new systems are pre-charged. You need to adjust the charge for the length of tubing you use for the install.

    How far the evaporator is from the condenser makes a difference on the charge it needs.

    You may have a gas leak, or the system was not charged properly.

    If they did not use pressure gauges doing the install, I would get someone that knows what they are doing.

    Good Luck.
     
    Last edited: Jul 16, 2016
  21. DougB

    DougB Member

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    Is there a sight glass in the system? When the compressor starts you should see bubbles, then liquid. If there are streaks on the sight glass, that is oil, and the system is low on refrigerant.

    Also the suction line out of the A-Coil should be cold, or a little frosty.
     
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