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  1. #27
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by 54regcab
    Doesn't it take about 10 minutes for a system to reach full efficiency? I belive the 7 minute delay just allows the coil to get cold a bit faster. I personally put a timer/relay on my A/C system to switch the blower from low to high speed @ 7 minutes and it helped a lot with dehumidification Delta T only decreases about 1 degree when high speed kicks in. Normally it won't hit the high blower speed unless it's over 95 outside or the thermostat setting has changed. Oddly enough our power bills don't seem to be any higher either.
    If your system is cycling off before 7 minutes, then it's oversized.

    Also, the timer approach is simply a convenience. The "better" approach is to get a dehumidistat so that the low-speed blower operation is activated on demand rather than whether it is needed or not.

    Lennox makes one of these ("EfficiencyPlus") and I had one in my old house. On a damp day with low-demand, the system would run it's entire cycle (typically 12-15 minutes) at 275 CFM/ton or so and do this all day, which is what prompted my question (e.g. why is this any different than closing some registers).

  2. #28
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,183

    Hmm

    The unit is in fact oversized by about 50%. I can't justify replacing a working system when a $5.00 timer does a decent job at controlling the humidity (I already had the 24V DPDT relay on hand). I looked into dehumidistats but I have yet to see a standalone digital model or one integrated with a thermostat at a reasonable cost (the analog ones just aren't accurate IMHO)
    When the existing equiptment (2000 model) wears out we will be getting a 2 ton instead of a 3, and a 40K BTU furnace instead of an 88K. Until then we'll just pay the increased utility costs. I really do wish more HVAC contractors would use HVAC-CALC instead of using "rules of dumb" to size equiptment. As a HO I would have no problem paying the same price for the smaller equiptment knowing that the contractor took the time to size it correctly.
    IMHO Chances are also good if a contractor went through the trouble to engineer the system correctly, it will most like be installed to actually get the rated efficiency

  3. #29
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by 54regcab
    The unit is in fact oversized by about 50%. I can't justify replacing a working system when a $5.00 timer does a decent job at controlling the humidity (I already had the 24V DPDT relay on hand). I looked into dehumidistats but I have yet to see a standalone digital model or one integrated with a thermostat at a reasonable cost (the analog ones just aren't accurate IMHO)
    When the existing equiptment (2000 model) wears out we will be getting a 2 ton instead of a 3, and a 40K BTU furnace instead of an 88K. Until then we'll just pay the increased utility costs. I really do wish more HVAC contractors would use HVAC-CALC instead of using "rules of dumb" to size equiptment. As a HO I would have no problem paying the same price for the smaller equiptment knowing that the contractor took the time to size it correctly.
    IMHO Chances are also good if a contractor went through the trouble to engineer the system correctly, it will most like be installed to actually get the rated efficiency
    Wow, you sound like you had the same experience I did in my old home. Contractor used a 2-ton when a 1.5 ton would have worked just fine (and based on the 2-ton's 50% duty cycle at 95 degrees, I'd bet even a 1-ton would have worked. It seems crazy to try to cool 1400 sq feet with that, but it's a townhouse with a load only on 40% of the exterior wall space).

    They put in a 66K/44K 2-stage furnace, too. Even on low, the thing would only cycle 2 times an hour with 5-6 minutes of burner operation. Not comfortable at all -- roast you out and then you endure cold spots for 20-25 minutes until the next cycle. Ugh.

  4. #30
    Join Date
    Feb 2004
    Location
    Wadsworth, OH
    Posts
    316
    Originally posted by ct2
    ok so if blocking the regesters is a bad thing (and I understand why it is) what does a zoned system do differently. The zoned systems that we make will either close one side or the other or supply air to both outlets ( on a two outlet plenum)

    but by closing off one zone you are diverting all of the airflow to the other, and by closing all of the regesters on one floor you would force the airflow to the other.....ooooo smaller ducts going to the regesters increases static and reduces airflow through the system--- right?
    Allow me to shed some light here....here is some data from our test lab. Rob Falke from the National Comfort Institute was doing some training in our lab last week and decided to measure the changes in system performance when closing 50% of the duct capacity. We operated the 5 ton Heat Pump on 2nd stg and closed a 10 x 16 trunk damper leaving open eight six inch branch ducts. (by design criteria that would be 100 cfm x 8 = 800 cfm total) What many people forget to realize is that when you zone or close registers you change all the dynamics of the air flow. Design criteria is no longer relative. By closing outlets we increase supply static which increases velocity which increases cfm delivery through the open outlets. We are allowing the system to react to a peak load in one zone.
    Here is the test results;

    -----------------------100% open-------50% open--------Diff
    Total delivered CFM-------1816-----------1412---------(-22%)
    Enthalpy change-----------4.86-----------3.82---------(-21%)
    D.B. Change---------------17.1-----------12.6---------(-26%)
    Total BTU---------------39,715---------24,272---------(-39%)
    Sensible BTU------------34,910---------19,214---------(-45%)
    Latent BTU---------------4,805----------5,085----------(+6%)

    This was a quick test done at the end of a long day...I will be utilizing the NCI performance testing forms and doing some further testing in the next few weeks.

    jr


    [Edited by jramunni on 08-09-2005 at 04:57 PM]

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