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Thread: VAV Boxes

  1. #1
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    VAV Boxes

    I have a building we take care of. The building is equipped with series vav boxes w/ electric reheat on the exterior and cool only boxes on the interior. They just fitted out a new space. They installed cooling only boxes with electric reheat. We were not involved with it, due to cost.

    So finally get the call, the new space is having temperature issues and their electric bill is through the roof. So I find the one thermostat was installed with a Celsius knob. That was a little funny and the temperature issue.

    The problem I see it the electric bill.......I think it would cost alot more to satisfy your heat load with a cooling only box w/ reheat, then it would be with a series box w/ reheat. The main ahu's run cooling all day(no daytime warmup), due to it being a medical office facility. We only reset the discharge air 8 degrees and it based on OA.

    So I explain everything to the company's operations manager. He calls me back and tells me he needs something in writing, about the differences in vav boxes. The construction manager says a vav is a vav, there is no difference.

    I'm not the best with typing out explanations......so I'm looking for a link or something to refer them too.

  2. #2
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    Here's a PDF with a ton of VAV info. It's a little "techy" but it does explain the different types of systems and their pros and cons. Electric reheat is covered around page 100.
    http://www.taylor-engineering.com/do...ide_5-2-07.pdf

  3. #3
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    Can you turn down the minimum's on the dampers That way the series fan pulls warmer air from the space ? When you say they have temperature issues what are the temps doing?

  4. #4
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    When the VAV goes into heating mode, the primary air should go to "heating minimum", which is just enough to satisfy the ventilation requirements of the space. Even if that minimum were 10% of maximum flow, it's still potentially 55-63 degree air you are dumping into a space with a requirement for heating. Not very efficient, but that's the breaks. It is more efficient than cooling only because you are mixing 10% 55-63 degree primary air with 90% plenum air which could be anywhere from 68-74 degrees. This allows your main supply fan to unload because the VAVs are closing. You end up with 70 degree air hitting your electric heat coil. If you're heat coil was sized for a 50 degree delta T, you would be supplying your space with 120 degree air using a series box.

    The alternate scenario is a cooling only box with electric heat. Because there is no fan to make up the difference between the "heating minimum" setpoint and the airflow requirements of the space and electric heater coil, you must take that air you just spent money to cool down to 55-63 degrees and heat it directly, without offsetting it with plenum air. If your heater is capable of a 50 degree delta T, the best you're going to get is 113 degree air, and your main supply fan is never going to unload. The VAVs must stay open because they have to supply all of the air to keep the heater from tripping out on thermal and/or burning up.

    Hope all of that makes sense. It's probably the exact same thing you were trying to explain to your stubborn customer.
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  5. #5
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    Rob, thanks. That was a good explanation. I pretty much copy and pasted it, but changed a few words around to fit the building.

    Btw- The temperature complaint was the one area was always too hot. That stat had a celsius knob on it.......they had it cracked to 32 degrees. LOL.

  6. #6
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    Too many sites with fan less VAVs with electric reheat, could never understand why anyone would do this except for the cheaper factor. But someone always ends up paying with electric bills or lack of comfort.

    The C means cool right?

  7. #7
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    One more thing that might be available to you is resetting the DA based upon actual load from the space? Many VAV controls now provide a terminal load -100 = Full heating demand and 100 = Full cooling demand. "If" all the boxes are calling for Heat then you might be able to reset the DA even higher?

    Now should there be a server room or another constant cooling load with a VAV and using a strategy such as this obviously it would not work. Then other means of cooling the server room should be explored to satisfy the majority at the lowest operational cost.

  8. #8
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    Quote Originally Posted by Chris_Worthington View Post
    One more thing that might be available to you is resetting the DA based upon actual load from the space? Many VAV controls now provide a terminal load -100 = Full heating demand and 100 = Full cooling demand. "If" all the boxes are calling for Heat then you might be able to reset the DA even higher?

    Now should there be a server room or another constant cooling load with a VAV and using a strategy such as this obviously it would not work. Then other means of cooling the server room should be explored to satisfy the majority at the lowest operational cost.
    I think server rooms and other critical loads should always have a standalone system with redundancy. It's always a problem when they're cooled by the common house-air .

    It's important to note that the electric meter is on the buildiing, not the air handlers, VAVs, or compressors. Depending on what sort of system you have and what kind of shape your air-side controls are in, it may be better to reset static pressure instead of discharge air temp. Maybe even more so in a system with parallel or series fans, because you can already get a lot of turndown on the main fans already. You can reset based on critical zone(s), average space temperature, outside ambient, and many other things. Ya gotta look at the entire system.

    Let's say you have large tonnage rooftops with recip compressors for example. When you reset discharge air, what are all of the starts and stops costing you on those compressors? In both mechanical wear & tear and kw usage. Now is the time of year here when recips are cycled to death. Let's say you have a central plant with 2 centrifugal chillers with older controls. From an energy standpoint, is it better to unload the chillers or back down the speed of the secondary pumps? Depending on if you reset static or discharge air, it could do either.
    Last edited by Tech Rob; 11-03-2012 at 02:51 PM.
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  9. #9
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    Quote Originally Posted by ascj View Post
    Rob, thanks. That was a good explanation. I pretty much copy and pasted it, but changed a few words around to fit the building.

    Btw- The temperature complaint was the one area was always too hot. That stat had a celsius knob on it.......they had it cracked to 32 degrees. LOL.
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  10. #10
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    Quote Originally Posted by Tech Rob View Post
    It's important to note that the electric meter is on the buildiing, not the air handlers, VAVs, or compressors. Depending on what sort of system you have and what kind of shape your air-side controls are in, it may be better to reset static pressure instead of discharge air temp. Maybe even more so in a system with parallel or series fans, because you can already get a lot of turndown on the main fans already. You can reset based on critical zone(s), average space temperature, outside ambient, and many other things. Ya gotta look at the entire system.

    Let's say you have large tonnage rooftops with recip compressors for example. When you reset discharge air, what are all of the starts and stops costing you on those compressors? In both mechanical wear & tear and kw usage. Now is the time of year here when recips are cycled to death. Let's say you have a central plant with 2 centrifugal chillers with older controls. from an energy standpoint, is it better to unload the chillers or back down the speed of the secondary pumps? Depending on if you reset static or discharge air, it could do either.

    It sounds like you're already approaching the energy usage as a system problem, and this is how it needs to be approached.
    Agreed and I do reset the CW SP as well when the demand is not there, but this is an RTU application.

    Resetting Static is HUGE when one can do it...... Why have a single/few callers and have a 1.5" static when there max is met with a 30% damper position or less? Lower static and a wide open damper = $ saved.

  11. #11
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    Quote Originally Posted by Chris_Worthington View Post
    Agreed and I do reset the CW SP as well when the demand is not there, but this is an RTU application.

    Resetting Static is HUGE when one can do it...... Why have a single/few callers and have a 1.5" static when there max is met with a 30% damper position or less? Lower static and a wide open damper = $ saved.
    This is because the overall system curve changes drastically when the dampers are open. The air distribution system is adversarial to the main air handler if all of the VAVs are mostly closed.

    Like you said; "when one can do it". It's not right for everyone. This just drives the point home that there's no one-size fits-all control scheme or magic pill for energy optimization.

    Resetting the CHW and CWP setpoints is kinda tricky, too. I've seen too many plant operators who thought that cranking the CHW setpoint up to 48 when the load was light = instant power savings. But when I showed them the difference in efficiency and kw/ton on a chiller running with a 60% pf versus one running with an 85% pf, they let me turn it back down. They turned down their air-side instead and maintained their dehumidification in the space. The power factor penalty is greater on a centrifugal running unloaded than a screw running unloaded. Should you run the tower fans full-bore and provide condenser water relief to the 750hp centrifugal motor? Or should you slow down the tower fans and maintain a constant condenser water setpoint? With most screw machines, this is the better way to go.
    "There is no greater inequality than the equal treatment of unequals."

    -Thomas Jefferson

  12. #12
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    The problem is the building that this owner has are tenant fit medical office buildings. Most of our medical building we have fcu or AHU's units feeding high heat load areas and the main AHU's just feed the other areas. This allows so much more in the lines of static and discharge reset.

    But back these building......we have three of them. Two of them were from the 80's and we recently replaced all the VAV controls and took control of the RTU's. Which has allowed me to really dial in the buildings. This building is only 6 years old. It has a Trane BMS with no Summit front end. It did at one point, but that's another story. No reset on static and DA is reset lightly based on OA not building load. We gave them a price on pulling the BCU and adding a Jace to bring it in with the other buildings. They elected to go with another contractor that claimed they could bring all their buildings into one front end...........it still hasn't happened.

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