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  1. #1

    Temperature difference between return and vents

    My temperature difference from the return and the registers vary. On cool days (and night) I get a 16 to 20 degree difference. During the middle of the day, the temperature difference drops to around 10 degrees (78 at the return and 68 at the registers). I'm hoping it could be as simple as a leak in the ducts coming from the return. These are in the attic where it would be cooler in the evenings and HOT during the day. Could something else cause this problem?

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,369
    Duct leakage is a possible cause, particularly on the return side.

    Also, old, torn, or missing duct insulation can also cause you to lose capacity from your system via attic heat gain to the duct supply air. If you have the old gray jacketed flex duct, and it's been up there for some time, it's possibly falling apart. Over time the jacket deteriorates in the intense attic heat, exposing the insulation and duct liner underneath, which over time causes it to break down, and you start getting leaks.
    • 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.

  3. #3

    Confused Duct checked

    I had the ductwork checked and he found no leaks. He suggested that our heat pump was too small. We have a 1 story ranch with about 2400 sq ft. Our heat pump is 3 ton. He recommends 1 ton for every 600 sq ft. Is this right?

  4. #4
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by dsanders55 View Post
    I had the ductwork checked and he found no leaks. He suggested that our heat pump was too small. We have a 1 story ranch with about 2400 sq ft. Our heat pump is 3 ton. He recommends 1 ton for every 600 sq ft. Is this right?

    Did this system ever cool the house properly?

    Oftne hard to find return duct leaks,maybe they missed some.

    The temp drop changing is not an indication of the system being undersized.Sounds more like duct leakage,or poor duct insulation.

    Tons per square foot is a "Rule of Thumb/Dumb.


    A Manual J load calc is what is required,call around asking until you find someone that will do one.

  5. #5
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    A Manual J load calc is what is required,call around asking until you find someone that will do one.

    Why is a manual 'J' important? Well, if you measured the square footage of your home, as you did apparently, you got 2400. Using the techs formula for tonnage, you need a 4-ton system. Okay, now let's take the roof off you house. It's still 2400 square feet but you're trying to heat or cool the entire world. How big a system do you need now? A Manual 'J' calculation will reveal that information. What about a skylight in a room, high ceilings, picture windows, windows of equal size and construction that face different sun exposures? Manual 'J' takes it all into consideration. Accurate, repeatable results are where it's at.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

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