Zone blow by and efficiency

Discussion in 'General Discussions' started by cspann, Sep 26, 2009.

  1. cspann

    cspann New Member

    Hello all,
    I am a recent inductee into the Geothermal realm. In March 09 I replaced my 2.5ton Trane heat pump with a WaterFurnace 4 ton Envision (with de-super heater), which included a 4 zone system and April Aire humidifier. Part of the install included adding new duct work to bring heating and cooling to an office/laundry space that previously had no duct work ( used to rely on fans in the doorway to blow air into the office/laundry space). This added about 250 sq ft to the existing 2200 sq ft . So far ( 5 months into it) the best I've seen in energy savings ( looking at Kwhr usage) has been a reduction of about 10 percent. One thing I have noticed is a lot of blow by in two of my other zones. Most of the time I have one zone (office) which is calling for cooling most of the time, this is due to a small computer cluster I have in the office. when this zone calls for cooling I can feel a good amount of air blowing into two other zones ( basement and main floor). These two other zones are always well below their dialed in temperature ( 5 deg below set temp for main floor, 10 deg below for basement). The installer has been back twice; once to adjust the zone controller, (Water Furnace IntelliZone) and the second time to put is a smaller size dampener on the office duct line ( he physically blocked off the remained duct work area). So my question is; is this as good as I can expect? Thanks
  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Was the jump from 2.5T to 4T justified by a load calculation, even with the addition of 10% to square footage under air?

    In low stage a 4 ton needs 2.75 'tons' of airflow, which is about 900 CFM or a bit more.

    That there are 4 zones suggests one or more are quite small, and if just one small zone calls, 900 CFM has to go into it or elsewhere. Choices for 'elsewhere' include:

    1) barometric bypass damper
    2) Dump zone
    3) other zones, by having their dampers not completely close.

    It sounds from your description that #3 is occurring, either by accident or design. That some zones are being cooled 5-10 degrees below setpoint suggests #3 is happening to excess.

    A zoned system has to be designed to accommodate the system's minimum airflow, not ice the evaporator or subject the compressor to excessive head pressure, and keep the system blower's static pressure within limts. These constraints are sometimes at odds with homeowner's desired zone configuration.

    Intellizone offers additional flexibility and control over a typical zone board but it does not bend the above rules.
  3. teetech

    teetech Member Forum Leader

    You have a small zone "driving" your unit with 4 zones. As engineer said you need at least 900 CFM for the unit to operate low speed. That air (or cooling) has to go somewhere.

    Since this is an add on it could not be designed into the entire duct system. That is the pitfall of adding zones to existing duct work.

    Intellizone does not like bypass dampers and that can be risky if the TD across the coil is too high.

    Can you get more air to that zone?
  4. cspann

    cspann New Member

    First of all my apologies to engineer and teetech for not providing a timely response. I unfortunately was under the impression that the site would provide me an update when someone posted against my message.
    In regards to a load calculation.. I am not sure if the installer did one, but I did (using HVAC computer software) and it showed around a 4 ton system.
    So for the minimum of 900 CFM for my office zone, the one that it calling for air during summer which is my office , I am set up with duct work that is 8" X 12". this rectangular duct work eventually goes to flexible duct work on the second floor. I am unable to provide a good dimension of the flexible ducts since they are insulated and I have no idea of inner tube dimension. However for the "office zone" I do have four 9.5" x 4" supply vents and one 8" x 12" return vent. So my question would be what size duct work should I be looking at?
    The idea of a dump zone makes sense to me.. why not dump the extra pressure onto the 2nd floor, rather than blow by to the basement and main floor.
    FYI, The installer was back out after I contacted Water furnace customer service. Currently the plan is to upgrade the size of the ductwork to the office zone ( not sure what size they are going to). Also they will move my April Aire Humidifier from the supply side to the return side.
    Again bad for not checking updates. I appreciate the advice being provided.
  5. teetech

    teetech Member Forum Leader

    The upstairs is not a good place for computer cluster added heat.(unless you design for it) If possible I would try to move some of that heat down stairs or basement would be ideal.

    Let us know how you make out on this.
  6. gabby

    gabby Member Industry Professional Forum Leader

    Re: cspann

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  7. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Glad to hear you did a load calc.

    Described ductwork won't happily move 900 CFM. Doubling what you describe might suffice.

    Take 2-3" off the outer diameter of a flex duct to determine its inner diameter (or read the jacket label)

    However, way oversizing ductwork to a given zone isn't necessarily the right answer - could just move the problem to the rooms themselves...I like my office paperwork to stay on desk and table, not be blown all over the room by the HVAC system.

    A dump zone is a bit of a bandaid - it needs to be physically accessible to the main supply duct upstream of all zone dampers, and it has to blow into an area occupants won't mind being excessively heated or cooled since it will be forcing conditioned air into a space, volume, or zone not calling for it.
  8. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I'm curious to know how the references of your installer checked out. Did he indicate experience with many such installs. What was the reason for the 40% increase in unit size, did you have a manual J loss/gain calculation done?
  9. cspann

    cspann New Member

    Gabby thanks for the insight for notification, should keep me a bit more diligent on my replies.
    In regards to teetech and Engineer; Relocation of the office would be problematic for other reasons, since my current office space allows it to be physically secure. While I do like upping the size of the duct to handle the required minimal air flow I hear you on the wind tunnel effect.
    The weather has cooled off dramatically here in MD. I have noticed a decent improvement in my energy use last month and expect this to continue throughout winter as my heating demand is a lot more even throughout the house.
    I will update my posting once the installer has done his modifications ( increase in ductwork and relocation of dampener) I will try to remember to run in office only zone to see how often I will have to re-file my paperwork.
    AMI, installers references checked out good, he has done Zone systems before. My problem child is the additional heat load which I was adamant about during the initial assessment with him. I did not have a J loss/gain cal done ( at least to my knowledge)
  10. cspann

    cspann New Member

    13 Nov 2009 Update
    The Geo installer was back out to make modifications to the system. A team of three guys spent most of the day making the modifications which included:

    A lot of work around the intake hood and exhaust hood at the air handler. This included a re-layout of two of the four duct trunks?( zones). The office run was upped from an 8X12 to a 10X12 at the exhaust side of the air handler (eventually tapers back down to 8X12 at a angled transition).

    The humidifier was moved from the air exhaust side of the air handler to the air intake

    The moved my water heater away from the furnace; needed to be done for above modification

    The caulked around the basement dampener.

    So forcing the office zone to cool and setting the other zones for no demand I get decent air at the office, when air handler is at stage1. The really good thing is I feel no blow by in the downstairs ( I used to be able to feel a good amount of air at the downstairs vents even with that zone was not calling) I had some slight air on the main floor and none on the second floor.
    So ... encouraging. It might be worth caulking around the main floor dampener to get rid of the residual air that I feel

    Even better, As I last mentioned, things have cooled off here in MD. The majority of October was in heat mode. My energy use in OCt 2008 was about 2500 Kwhr, For Oct 2009 it was 1500KwHr. Now that is why I had geo installed.
  11. teetech

    teetech Member Forum Leader

    Hope it works out for you, next summer will tell.

    I know moving an office is not easily done but sometimes computers can be wired to another location as I have encountered this before.

    I bet that office zone does not call for much heat.

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