A/C airflow problems??

Discussion in 'HVAC Heating & Cooling' started by ojmisc, May 6, 2008.

  1. ojmisc

    ojmisc New Member

    Messages:
    5
    I've got a 3 ton Trane a/c unit that produces about 20 degrees difference from return to supply. I measured this by inserting a thermometer at the return and then setting same thermometer into the various supplies throughout the house. Once my A/C system comes on it basically stays on till evenings when everything cools down. I have a 1/3 HP blower motor set up to use the highest speed possible for cooling. For heating system works fine. Contractors just want to replace the HVAC system with a new one. My a/c system is working just not putting out the cold air that it has inside. My system's return is 128 sq inches - while the supply is 364 sq inches - Is this imbalance my problem? Can I use a faster blower fan to overcome this problem? What to do? :confused:
  2. PEW

    PEW DIY Senior Member

    Messages:
    487
    20 degrees sounds in the ball park. Tell us a little more about sf, insulation, construction, past performance.
  3. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    I got a free condenser off of free cycle.
    It turns out they never cleaned it and it was filled with leaves and dirt.

    If you do wash it out, just remember to keep the garden hose sprayer set to a more straight spray pattern. (not a fan pattern) and make sure to spray straight into the unit to prevent bending the fins.
  4. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    *******************************************************
    it would seem as if the return is not providing the supply with ENOUGH volume of cooled air. Normally a duct system will only put OUT as much air as it can bring IN, so in your case it would seem as your return is only giving approx 33% of the air flow the supply is capable of handling.

    btw, how many supply runs (size) come off this 364 sq in. supply main?
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    We are missing a few items in the equation here.

    Is this system "as designed and built" or have you changed anything? Who determined that 3 tons is the right size for your house? Is the indoor coil and air handler or blower properly matched? Is that blower motor the right one for your unit? Did you mismatch a 13 Seer condenser with a 10 Seer coil?


    All the information for setting up the system is in the installation and service manuals for your equipment.

    A good HVAC tech could evaluate all your issues...sizing, blower set up, return air design, etc. Rigth now all we can do is guess at it.
  6. ojmisc

    ojmisc New Member

    Messages:
    5
    The house is roughly 30x50 two levels - basement and main living space - HVAC is center in the middle. Most of the vents have been closed in the basement. The house had a fire back in 95 and was rebuilt professionally - all HVAC components including airways were updated. The house built in 1956 is a hybrid house - metal construction on top of a wood frame in the basement -insulated in the walls and attic.

    I moved in 02. Previous owner had already modified professionally the HVAC system to try and bring in more cool air into the kitchen (without success) I added a small two story extension of 300 sq in total - with own air supply.

    Last Summer on the first hot day - started the AC up and just started getting hotter and hotter - system wasn't even keeping up. I had it - called in the 4 different professionals - they only wanted to sell me new units and not service what I had. My gut told me that the system was overall working due to the temp differences between supply and return and cold air was at the registers it just wasn't "getting out". I put this story to the techs and asked why a "new" system would do any better since my old system seemed to be working.

    Only one tech mentioned the potential air imbalance between supply and return - I even got a boost in cooling when I closed off by 50% two supplies just outside my distribution box with their dampers!

    MY distribution box is basically a rectangle on top of my HVAC system. The two original supplies are 16 x 8 and run about 25 feet each way they get reduced in sizeabout half way down. The run into my addition is about 25 feet also and is 9 x 12. There is also a 8 inch round that was run into my kitchen for the extra cooling that doesn't work.
    The return is at the top of the steps about 15 ft away and feeds a 16 x 8 run into a 24 x 20 return box attached to the HVAC system - there is also a 8 inch round that's taped into the basement for additional return air.

    The system is fully charged with freon, ducts were cleaned, indoor air coil/heat exchanger was physically removed and fully cleaned (original owners used wrong size filter - what a mess). A clean blower motor is 1/3 HP - 1075 RPM, 4 speed - with fastest speed for cooling.

    The system is a Trane all sized, picked and installed by professionals when house was redone - I'm not sure if all the components "match" - assume so (oh-oh).

    Can my distribution box somehow not channel the air into the supplies? Do I just cut a large hole into my "return box" to increase air into the HVAC system itself? I'm really confused on how to fix this.

    Thanks all.
  7. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    I shouldn't just "ballpark" numbers, but I will! 3 tons sounds a little undersize for 3000 square feet.
  8. ojmisc

    ojmisc New Member

    Messages:
    5
    Most if not all of the supply vents being used in the basement level have been closed. Essentially I'm trying to cool only the main living area - 1500 sq ft with a 3 ton system.

    How does putting in a bigger system - say 4 ton - change the amount of air that is output - through the system? Does the blower motor even run faster/higher RPM?

    My problem appears to be not "enough" cold air coming out - the air is cold at the supplies but just not moving out. How do I improve this?

    Thanks.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,348
    Location:
    New England
    There is an ideal air flow for the cooling capacity you have. It should be described in the installation manual. Depending on the controls, you can adjust the fan speed either by rewiring it, flipping some switches, or changing a pully and possibly the belt. You could also have severe leaks in your supply ducts, or they run through unconditioned space without adequate insulation, and you therefore are cooling the basement or attic, or whatever, depending on how they are run. Or, the existing system just isn't sized properly or isn't functioning properly for the space you wish to condition.

    On my air handler, I can adjust the maximum fan speed by flipping some DIP switches to the desired position.

    Running the air over the coils quicker means the a/c unit will not dehumidify the air as well, but may be needed to get the conditioned air where it needs to go. It is also possible (maybe even likley) that the ducts are not sized properly. Also, if there isn't a good return system, it will never work well. You can't blow air into a room unless there is a way for the existing air to exit...you can only pressurize it a little, then the air flow essentially stops.
  10. PEW

    PEW DIY Senior Member

    Messages:
    487
    Have you had a Trane dealer in to look over the system?

    Have you looked at the bottom of the evaporator? If the filter was not fit properly, it is possible that the bottom of the evap has picked up a load of crud and also blocking air flow. Have seen that many times.

    On the surface, the size of your return sounds on the small size. Also, have to agree with the others, 3 ton sounds small.

    Bottom line, you need someone qualified, and you trust, to review the entire system and make recommendations. Not something that can be done over the net.
  11. Mikebarone

    Mikebarone DIY Senior Member

    Messages:
    206
    Location:
    Phoenix, AZ
    just a thought...

    I have a friend that also has a Heat pump that is too small for his house. He has told me that for some time, that his electric bills are a lot higher then his neighbors, but he always blamed it on the fact that his unit runs all the time in the summer, and his house always is warm inside. I’m far from an A/C guy, but I went over there and notice, that while his unit was running, it was extremely hard to pull out his air filter, (that was almost brand new). It was like the unit was starving for, (intake) air. I’ve increased the air supply to the air handler by over 150 percent. Before increasing the intake air, he was also getting a difference of 20 degrees. Now that the unit is getting more air, the difference is down to around 17 degrees, but the extra out put in all of the rooms is a lot better.
    I also did a blower door test, (in reverse…blowing in) and after taping off all of his vents in the house, we set off a smoke bomb it the air handler. Well we were in the attic while his wife set off the bomb, and it looked like his attic was on fire. The main plenum, just above the air handler had so many leaks, it wasn’t even funny.
    I also installed a 1500 cfm booster fan in a 14 inch flex line in his attic, because the air handler is all the way on one side of the house, and there was poor delivery on the other side of the house.
    So far it’s working good, but we’ll see when it gets to the 115 degree mark.

    Good luck,

    Mike
  12. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,737
    Location:
    Central Florida
    The return sounds grossly undersized, and I'm not very happy with any of the duct sizing for that matter. I have a 2000 sq ft house with a 3 ton unit, a fancy-shmancy Carrier thing that uses Puron to save the ozone layer. My return air grills total 696 sq in, made up of 2 runs: a 20x25 (which seems to be the most common size I see in comparable homes) in the center of the house, and a separate 14x14 in the master bedroom, apparently installed to allow closing this space off tightly from the rest of the house. Total outgoing duct area is comparable. The large duct capacity allows for low airflow velocity and very low noise.

    I don't think 3 tons is too small for a well-built 3000 sq ft house, but as others have pointed out, you can't size just based on total sq ft. Now that I've upgraded my windows and doors, insulation, etc., my 3 ton unit is way oversized, but since everything in it is variable-speed and computer-controlled, it's able to operate efficiently at very low speed. If it were a conventional fixed-orifice, fixed-speed fan, etc., system, it would cool too fast in short cycles, and fail miserably at dehumidification.

    You definitely need a good AC design guy to check the system out, but I'll bet you could improve things by opening up the return.
  13. ojmisc

    ojmisc New Member

    Messages:
    5
    How does one find a good design HVAC person? The salesmen that have come over only want to sell me something new. What's a good "key word" to look for in the yellow pages when I'm searching for this HVAC design tech?
  14. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,737
    Location:
    Central Florida
    "Architect" comes to mind. They tend to be arrogant and pricey, but you might get lucky and find one who either is, or can recommend, somebody who specializes in that area. Your situation is a little complicated by the fact that it's an existing building, but a good engineer can measure or properly estimate all the heat-loss and dehumidification criteria to calculate the overall load, which will answer the sizing question. Then it's usually left to the mechanicals sub (Trane, in your case) to specify the equipment and design the ductwork. If you felt that the local Trane dealer(s) haven't performed, you might try e-mailing or calling Trane customer service. Trane was recently acquired by Ingersoll-Rand, so there may be some confusion in the ranks right now.

    Also, "Contractors, General". A good GC would be a good alternative to an architect; they're always fighting each other, but both could recommend an HVAC design firm.

    Another tip: often your local power company will conduct a free (and basic) energy audit, and recommend contractors to address any problems they might find, or to dig deeper. Read over Mike Barone's post again to see how to find and address one of the most common problems -- ductwork leaks. His observation that the return air filter was being sucked in very hard is a good sign of trouble. Mine are both overhead, and fall out when I open the grill, since the pressure differential across the filter is so low.

    You could do a lot of the troubleshooting yourself with some basic equipment, but it wouldn't be real cheap.

    Where are you located?
  15. ojmisc

    ojmisc New Member

    Messages:
    5
    Thanks Mikey.
    I guess I'll bug Trane first.
    I've done a lot of DIY stuff but in this case I'm just not sure what to do. I've done the cheapest things first and progressed on - filter changed, looked for any type of obvious leaks/blockages, had ducts cleaned, had air handler removed and cleaned (thought this was the answer - because it was beyond clogged).
    The majority of the duct work is covered with drywall. I see about half of one of the main supplies, coming out of the distribution box and then the connections of off that supply that goes into each room. These connections run about 15 feet off of the 8x16 supply and look to be of various sizes either 5, 6 or maybe 8 inch rounds - they would be a bear to replace!
    A return connected right into my return plenum can easily hold up a bill. The vaccum at my main return can do this too - the technician that have come by were not alarmed. I liked Mikebarone's posted answer - I want to better collect the hot air in each bedroom and his method can accomplish this and maybe solve my main problem too - need the verification.

    I'm in Northern Virginia.
    Thanks again.
  16. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Yes, besides the actuall tonnage rating of the coil and condenser, the system has to move an appropriate amount of air. Depending on your local climate, the amount of heat and the amount of cool needed are often quite different. Arizona and Montana might be two extremes of that situation. That is why modern furnaces and air handlers have multi speed motors. You might need a small amount of heat, and maybe a slower blower speed. And you might also need a much higher amount of cooling, and a higher blower speed.

    Any given furnace or air handler will have a Maximum CFM air flow rating. Trane for sure makes for example 70,000 btu furnaces in many models, in terms of the max cfm and tonnage of cooling which they can support.

    SO, this is still the big unknown in your equation, until someone who is really well versed in this field checks it out for you.
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,348
    Location:
    New England
    Before you go off half-cocked...a proper heat load calculation must be performed on YOUR place. Depending on the construction, insulation, window sizes, orientation, and placement, there is no valid one-size per square feet calculation that will do you justice. If the calculation determines the ideal size is inbetween available sizes, you always want to go to the lower sized unit, not the larger one. An a/c unit works best if it is running all the time. A big one will make the space cold but clammy, since it only runs a short time and can't dehumidify. Now, I suppose in the desert that has some merit, but not for longevity since things work better when they aren't turning on and off all the time.
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