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  1. #1
    Join Date
    Jan 2009
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    386

    Need Help with AC

    This is the first part (from my other recent thread):

    I had posted an earlier thread about what to expect with a service call for the A/C not cooling properly. I had a tech come out and look at the system a little while ago, and here is what he said:

    - I asked him about static pressure, and told him the numbers I had gotten from an energy audit: He said
    * 0.35" W.C. across a clean blue fiberglass filter was fine (shouldn't it be more like 0.05" - 0.10" if the system is designed properly?).
    * static depended on the system and house, not the 0.5" W.C. allowable on the name plate
    * said high static was common
    * he didn't have the tool with him to measure static pressure, and he said that they don't do that for residential, only commercial.

    - I told him about lowering the fan from High to Med/Hi (see other thread). He said he never would have checked if I hadn't mentioned it

    - I asked about face velocities and asked him if the return drop might be too small. He said that that's way over his head (exactly what the last tech said).

    - He originally set the blower speed for heat to medium low, which is definitely not enough air flow for the 80K output furnace. I asked him why, and he said you couldn't have the heat and cool on the same blower tap (I don't see why, because you would just splice the wires). I asked him about the heat exchanger cracking, and he said that there were plenty of "safeties" in place to prevent that from happening. I said I didn't want it tripping on the high limit, and he just said yeah. He just left the blower lead for heat unplugged.

    - He said the system was a little low on freon, and he said he added a little under a pound. He said there was a 21 degree difference at the outside unit, and super heat was 12 degrees. I have no clue if that's good or bad, but he sounded like it was good.

    - I asked him about it taking longer to cool the house down with the much reduced airflow. He said that would cool the house down faster.

    It's been over 45 minutes since he left, and the temperature hasn't even cooled down 0.1 *F inside (it's 75.3 inside currently) - it's only 82 *F outside. What should I be doing now? I'm afraid you all are going to say get another tech.
    So, I called the company back up last night, and they sent two more techs out to the house today (I believe one of them was an apprentice). I talked to him about the high static at the blower, and he measure 0.8" W.C. I mentioned that with the small return drop (20"x12") the face velocity at the filter was over 900 FPM. He said that's fine, and actually looked at his "duct-u-lator" chart and told me that 10"x18" was fine for a 3.5-ton system, so my drop was plenty big. He also measured 0.35" pressure drop across the filter. He said that had nothing to do with size of the return drop.

    He said that my system was undersized. He was using his "rule of thumb" and said I should have a 5-ton for 2200 sq. ft. He said I should get a load calc done, and I should, but the price he quoted me was astronomical. He also said that when contractors size duct work, they do it for the size of the A/C system. Because of this, without doing any research or measuring of anything, he said the duct work was too small.

    Anyways, I feel like I'm really screwed at this point, because the system is now operating worse than it was when I called them out. Before, I was getting a 23 *F temperature split at the air handler with the blower on Med/Hi (1592 CFM). However, yesterday, the tech said that the system was just under a lb. low and supposedly charged it with some more R-22. He set the blower at Med (1376 CFM), and I'm seeing a drop of around 23 *F. With the drops being the same, I would have been removing more heat from the air before the charge and with it set on Med/Hi. Now that he's added more freon, I'm guessing I won't be able to achieve that 23 *F drop at Med/Hi, will I?

    I really don't know what to do at this point, as I've got a bill for 2.5 hours and 1 lb. R-22 from 2 visits, and my system is performing even more poorly than it was before. I feel like I was better off before calling them out. It may be undersized, but for now, I at least want it running the way it was before I called them.

    I really appreciate the help you guys have given on other threads. What should I do now?

  2. #2
    Join Date
    Mar 2005
    Location
    Houston, Texas
    Posts
    11,847
    Motoguy will be along shortly, to answer these, and many more of your questions.

  3. #3
    Join Date
    Jan 2009
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    386
    Quote Originally Posted by the dangling wrangler View Post
    Motoguy will be along shortly, to answer these, and many more of your questions.
    Can I get some serious answers in this thread, please? I don't want it to get closed.

  4. #4
    Join Date
    Apr 2008
    Location
    Northwest Arkansas via Chicago Area via Straight Up from There on Lake Superior
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    1,411
    Quote Originally Posted by skizot View Post
    ...

    Anyways, I feel like I'm really screwed at this point, because the system is now operating worse than it was when I called them out. ...

    I really don't know what to do at this point, as I've got a bill for 2.5 hours and 1 lb. R-22 from 2 visits, and my system is performing even more poorly than it was before. I feel like I was better off before calling them out. ...
    Just a HO, but you are "screwed" if you pay for that quality of service. You talk to the owner? Is this a new system where you feel obligated to deal with this company? If not, time to bail on them, IMHO.

    Also, I guess I could tell my story again about telling my auto mechanic what to fix versus what isn't working ..... Nah.

  5. #5
    Join Date
    Mar 2005
    Location
    Houston, Texas
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    Quote Originally Posted by skizot View Post
    Can I get some serious answers in this thread, please? I don't want it to get closed.
    Sir, I was as serious ,as cancer.

  6. #6
    Join Date
    Jun 2008
    Location
    Cincinnati, Ohio
    Posts
    2,176
    Skizot,

    The only way to tell if the system is undersized is by doing a load calculation. Knee-jerk reaction tells me you should be pretty close to proper equipment sizing. (I'm basing this assumption purely off of my own home, 2150 sq. ft ranch with a boatload of windows) I have a 3.5 ton system in my home and it cools properly.

    As far as the duct sizing goes, I'm just a guy behind a computer screen. I would probably find another service company if you are not satisfied with your present one. I know, you didn't want to hear that.

    Since you have said your cooling system's performance has worsened since the tech's slowed the blower down, I would call them out and have them reset it to the previous speed.

    phb

  7. #7
    Join Date
    Jun 2007
    Location
    The Twilight Zone
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    2,964
    I would post your geographic location.

    Maybe a pro from this site is close to you, or knows someone that is close to you and can do a thorough check-out of your system.

    Start a new thread with your location in the title.

    Good luck.

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Can you post the results of the energy audit in regard to static and fan speed?

    If .8 ESP is from the energy audit and is correct,the static will change when you select another/different speed.


    Static/ESP is in relation to the cfms ,which change as you change speeds.

    Plus the furnace data says it includes the filter,but I doubt it includes .35 for the filter you are using.


    The test points are critical to get the actual static/ESP,plus all footnotes,etc., on the fan chart must be taken into account.

  9. #9
    Join Date
    Jan 2009
    Posts
    386
    Thanks for the replies so far.

    Quote Originally Posted by phbsales View Post
    Since you have said your cooling system's performance has worsened since the tech's slowed the blower down, I would call them out and have them reset it to the previous speed.
    I would do that, but they supposedly added almost a pound of R-22 to the system. I won't get the same 23 *F split if they put the blower speed back on Med/Hi, will I? I assume since they overcharged the system, the coil won't get as cold as it used to (operating with that 1 lb. less freon).

    I have not paid for any of it, yet. They've just left me with two tickets that I signed. I just want it returned back to the way it was.

  10. #10
    Join Date
    May 2009
    Posts
    51
    I've a story to tell about my install that you may find interesting. It's not directly related, but based on some of your duct dimensions, it could be helpful. I'll post it when I get home from work.

    Also, 10x18=180sqin ==> 1.25sqft

    At 3.5 tons, approximately 1400 cfm flow, that results in 1120 fpm through that duct. I'm pretty certain that's not good. Further, at 12x20, you would see about 840 fpm. You had already calculated this, and while better, again it's not 'good'.

  11. #11
    Join Date
    Jan 2009
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    386
    So can anyone explain to me why the ductulator says a 10" x 18" return drop is fine for a 3.5-ton unit? That's a velocity of 1400 / ((10 * 18) / 144) = 1120 FPM. I don't see how techs are out using these ductulator charts, installing systems that have a face velocity of over 1100+ FPM across the filter. That's probably one of the reasons why I have such a problem with pressure drop across my filter, and probably have other problems with the ductwork as well, if it was all designed using these ductulator charts.

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,741
    No where does a ductulator say 18x10 duct is fine for 1400CFM.

    A Ductulator doesn't tell you how to size duct.

    Just like a tape measure, doesn't tell you how to frame a wall.

    It is a tool to determine what size will give you what FPM, at what FR.
    The designer/installer, must know what FR and FPM is acceptable.
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  13. #13
    Join Date
    Jan 2009
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    Quote Originally Posted by beenthere View Post
    No where does a ductulator say 18x10 duct is fine for 1400CFM.

    A Ductulator doesn't tell you how to size duct.

    Just like a tape measure, doesn't tell you how to frame a wall.

    It is a tool to determine what size will give you what FPM, at what FR.
    The designer/installer, must know what FR and FPM is acceptable.
    Tell that to the guy that was here today then. He pulled out some slide wheel, he called it a ductulator, and he turned the wheel and read back that 12"x18" (sorry, I said 10"x18" before) was fine for a 3.5-ton system. He also said that would give 1000 FPM velocity. I calculated the velocity to be 933 FPM . Before he even pulled it out, he measured the return drop in the garage, which is 20" x 12" and he said that that was more than enough for a 3.5-ton system. That is far from the truth, as that's 840 FPM, given 1400 CFM. So, what would he have been using that he was calling a "ductulator", and where those numbers came from, if there's no charts/slide wheels out there that will give you the size of the return drop needed for a certain CFM.

    It's simple math to calculate the velocity. I'm sure they make charts/slide wheels that show this.

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