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  1. #14
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    1,687
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    I have balanced a couple of low pressure VVT systems with diversity. It isn't easy. A school district in my area has started using that system and believe it or not they are realizing real energy savings along with installation cost. These systems operate with a very low supply static pressure so damper adjustments can bite into your available static pretty quick. You are better off proportioning airflows as a total to each space rather than trying to proportion each diffuser and after that address any objectionable flows. Some engineers don't like that but you don't have a lot of wiggle room on these systems. I tell them right up front what I'm going to do and why but also tell them If you insist I'll do it your way but you own it. The controls have to be capable of modulating the VVT'S in accordance with heat load and they are doing that here through the stat. The biggest trick is because individual areas need more static than is available with all VVT'S at max proportional set point the system operates best if you measure the airflow at the available static then calculate the static needed to deliver design airflow and set the static there the areas will heat or cool in sequence pretty much like a VAV with diversity. The irritating part of that for me is you have to be careful to not let the static get high enough to reduce airflow more than you want.

  2. #15
    Join Date
    Nov 2004
    Posts
    215
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    Wayne,
    Great info! Thanks for being so detailed ! The info come in handy if I run into one with diversity .

  3. #16
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    1,687
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    airdata,
    When you do have to balance one discuss the way you intend to do it with the engineer. Full load conditions will start off with the static at a minimum but as the areas begin to cool or heat the static must be allowed to increase which backs the fan up slightly on its curve. That cools the remote areas but is unusual for systems with static pressure control.
    The first one I did had me going until I finally figured out what they were trying to do. I tried to get them to increase airflow on the RTU but they refused. They have also gone to six residential type heat pumps in each penthouse on their bigger jobs which according to the engineer in the last school cut their energy usage by 45%.
    Trust me I understand why you haven't seen one of these yet because I did balance for 20 years before I saw the first one and early on I thought they were crazy.

  4. #17
    Join Date
    May 2018
    Posts
    14
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    I've only ran into it once. It was a VVT system with bypass dampers on all of the RTUs that had multiple zones. The system was only a few years old and was a complete nightmare. They are the poor man's VAV and wreak havoc on the equipment. Anyways, you put the units in full cooling, opened zone dampers 100% and balanced the CFM out of the diffusers according to the engineering specs. They had specific static pressure set points for each RTU, min and max zone damper positions to set in the automation and specific outdoor air minimum positions for the RTUs. The RTUs monitored CO2 and had demand control ventilation with power exhaust. The bypass dampers would modulate to maintain the static set point. It is crucial that the system is engineered and designed properly. The unit needs to provide the designed airflow to each zone under any given load and most importantly, the bypass needs to be sized correctly. In my situation, the engineers didn't design the bypass duct large enough so it spent 100% of its time around 80% open trying to drop the static down to set point since the zone dampers spent the majority of the time around their minimum positions. This caused the unit's to constantly cycle on high limit during the winter and low pressure during the summer

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