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  1. #1
    Join Date
    Oct 2007
    Location
    Houston Taxes
    Posts
    28

    Rooftop Unit VAV controls

    From my previous job, I only dealt with Hot and chilled water valves so it was much easier to program. Now, at my new job here, I dealt with Gas Heat, it is totally different. As the heat is enable, the discharge air temperature can go up to 120degree F after a short period of time, this is a VAV system. For Hot and Chilled water valves, we control VAV units base on Supply Air Temp but with Gas heat system, and if we control by Supply Air Temperature, unit will put back to cooling mode if the temperature goes up that high.
    Does anyone have better ideas how to control this better? A Return Air Temperature is also provided. Isn't it better if we use return air to determine heating and cooling mode and then control heating and cooling base on Supply Air Temperature?
    Any suggestions are much appreciated
    Natalie

  2. #2
    Join Date
    Jan 2008
    Location
    Quebec
    Posts
    71
    Some gaz unit can modulate their supply air instead of boosting the temperature... maybe that would be an option, if replacing the burner controls is possible.

    I saw a unit (Engineered Air) with a 15:1 modulating ratio. (after low fire boost, as well)

    Controling the return air will not be a good way as the boxes close when they don't need cool air so you will try to heat cool air that boxes don't want... And there's always someone that need cool air. So your supply will be too hot.

    You could also install a modulating electrical heater (SCR) to deal with mid-seasons and use gaz only when it's really cold outside, with a minimum damper position high enough to worth it.

  3. #3
    Join Date
    Feb 2007
    Location
    Portland OR
    Posts
    36

    VAV unit heating

    It looks like the unit is set up for on/off heating control and not modulating.
    We have some Intellepacs that have the same set up. We only use the heat for morning warm up and once the building is up to temperature it will not come on again. I would suggest programing a morning warm up routine and disable the cooling and economiser during the warm up cycle. The VAV boxes should be able to control to set point during the rest of occupied time. The perimeter VAVs should have aux heat so you should not need any heat after morning warm up.

  4. #4
    Join Date
    Oct 2007
    Location
    Houston Taxes
    Posts
    28
    Thanks for your reply.
    Yeah, that is what I am going to do for now.
    Many thanks

  5. #5
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,150
    I would also make sure you program in a setting so if your supply temp gets above 135 deg then cycle the heat off immediately and then allow it to cycle back on after a 5-10 min delay and with the supply temp below 85.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  6. #6
    Join Date
    Jul 2007
    Location
    Sacramento, CA
    Posts
    250
    You can program it to Outside Air Temperature(depending on where you are) to shut the gas valve and heat off when the OSA reaches let say 72 degrees or somewere around there and then allow cooling to take over from there and then after it drops below the setpoint, let the heating take over or set up a differental in the setpoint so that he heat does not come on until the OSA is 4degrees below the setpoint before the heat enages. I would limit the supply air temperature to 112 degrees or what you unit is capable of handling.
    You can also have a setback temperature to maintain a certian temperature if the unit is off at night to keep the building warm and decrease the cost of morning warm up or customer complaints that the building is to cold in the morning.
    Hope this give you some more ideas.

  7. #7
    The way your gas heat works (even for modulating) is:
    1. Once enabled the unit goes into high fire until it reaches 120.
    2. Once 120 is reached the heat is allowed to modulate.
    3. There are probably settings on you modulating gas heat controller that need to be set using dip switches.

    Therefore you could do the following:

    1. Reset your supply air heat setpoint based upon outside air temperature.
    2. When heat is enabled do not disable the heat for an adjustable time setpoint of 5-minutes. After the time delay if your unit supply air temperature is still greater than you supply air temperature setpoint + a differential and the modulating gas analog output is < 10% then disable the heat.

    Just a couple of thoughts as to how you could control this application. 5-minutes may be to long, you could also reset this setpoint based upon outside air conditions. Knowing that when it is warmer outside, say 55 versus 20 the amount of time required to reach 120 will be less.

    Just my thoughts.

  8. #8
    Join Date
    Mar 2007
    Posts
    20
    This sounds more like a VVT application not a VAV. Heat generally would be used for morning warm up and in that case there is no problem with 120 Deg air. you want the return to come up to setpoint quickly anyway.

    If there is No reheat on exterior zones in a cold weather climate that is just a bad design.

  9. #9
    Join Date
    Oct 2007
    Location
    Houston Taxes
    Posts
    28

    DX Cooling Controls

    As I understand, VAV unit is controlled to the supply temperature sensor to maintain 55 Deg at Discharge air. Most of our units here have DX Cooling. I installed a VAV program into one of the units that serves our office area here and watch the supply air temperature as compressors turn on and off. I set compressors minimum run time and interstage delay for 5 minutes. I see that as one of the compressor comes on, the supply air temperature drop very quick down to the 45 degree which is about the temperature that is disable to cooling if there is not minimum runtime is set. I set the throttling range for the PId loop as 10 degree. And it does the same thing when compressors turn off. The temperature rises quicky so it put the unit back to cooling but because of the minimum off time is set so it just sit there and wait for the time expires. So basically, compressors runs pretty much base on runtime if a unit controls by Supply Air Temperature? I see that the compressors turn on and off all day long. Is that the way it is supposed to be? Any suggestions are much appreciated.
    Natalie

  10. #10
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,150
    Compressors do turn on and off and that is all you can do unless you have unloaders, a digital scroll, or a VFD on the compressor, and even at that they will eventually cycle on/off; however, short cycling will prematurely kill the compressor. It it better to keep a compressor running for a longer period of time than to just cylce it on and off. When you say the DAT drops quickly how quick is quickly? What is your return air temp? It sounds like you may have either have a lack of load or a lack of airflow.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  11. #11
    Join Date
    Oct 2007
    Location
    Houston Taxes
    Posts
    28

    DX Cooling in VAV unit

    Hi Crab Master.
    Thanks for your quick respond. It depends on Outside Air Temp (effects the load but not controls) and space load too...Unfortunately we don't have a Return Temp in the unit because the unit is supposed to be a CAV but it has a Supply Temp Sensor installed, I pushed a VAV program into the unit to watch the Supply Air Temperature sensor. Oh, I forgot that supply fan VFD is always 100% since there is not VAV boxes connect to the unit,(duct static pressure is low), that could make it different.
    With the Supply Air VFD at 100%, once the compressor turns on, temperature drop from 74 degrees (almost as the same as Space Temp) to as low as 45 to 52 degrees within 2 minutes, and the same way when the compressors turn off, supply air temperature rises up to 74 degrees within a couple minutes.
    Is that normal?
    We do use Digital Scroll Compressors and Chilled Water Valves, we don't have problem with these at all.
    Thanks


    Quote Originally Posted by crab master View Post
    Compressors do turn on and off and that is all you can do unless you have unloaders, a digital scroll, or a VFD on the compressor, and even at that they will eventually cycle on/off; however, short cycling will prematurely kill the compressor. It it better to keep a compressor running for a longer period of time than to just cylce it on and off. When you say the DAT drops quickly how quick is quickly? What is your return air temp? It sounds like you may have either have a lack of load or a lack of airflow.

  12. #12
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,150
    "As I understand, VAV unit is controlled to the supply temperature sensor to maintain 55 Deg at Discharge air."

    That is the design intention of the VAV system; but if you consider the reason for this I believe you'll gain a better understanding of how to control to it. The reason for this design is to be able to always provide cool air to any zone that needs cooling. If the zone doesn't need cooling then the box damper will go to a minimum position and minimize the amount of cooling into the space, or if the zone needs heating the box will be at a minimized position and the reheat will operate to reheat the incoming cool air in order to warm up the space. This is "old school/inefficient" way of thinking IMO. Here you always need cool air in case any zone needs cooling as there is no other cooling source but at the air handler, where as heating you have heating sources at both the air handler and the vav box. What if no zone needs cooling? Why continue to provide cool air that no zone needs or that will only need to be re-heated by every zone? With DDC controls, if your network has the capacity, you can look at every single zone and only bring on the cooling when any zone needs cooling. However with DX cooling you need to make sure you have enough airflow to allow for DX cooling. You can either bring all your airflow from your boxes in and then add them up to verify you have enough airflow, or make sure all boxes minimum position is adequate for first stage mechanical cooling. For more stages of mechanical cooling you'll have to utilize the total airflow either by adding all the boxes or with an airflow station. Now all that being said you may also need to consider the de-humidification factor, depending upon your location.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


  13. #13
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
    Posts
    4,150
    Quote Originally Posted by natalie_p View Post
    Hi Crab Master.
    Thanks for your quick respond. It depends on Outside Air Temp (effects the load but not controls) and space load too...Unfortunately we don't have a Return Temp in the unit because the unit is supposed to be a CAV but it has a Supply Temp Sensor installed, I pushed a VAV program into the unit to watch the Supply Air Temperature sensor. Oh, I forgot that supply fan VFD is always 100&#37; since there is not VAV boxes connect to the unit,(duct static pressure is low), that could make it different.
    With the Supply Air VFD at 100%, once the compressor turns on, temperature drop from 74 degrees (almost as the same as Space Temp) to as low as 45 to 52 degrees within 2 minutes, and the same way when the compressors turn off, supply air temperature rises up to 74 degrees within a couple minutes.
    Is that normal?
    We do use Digital Scroll Compressors and Chilled Water Valves, we don't have problem with these at all.
    Thanks
    Again you may have an airflow problem - are the filters clean, is the air being recirculated/short cycled from the supply right back to the return, is the evaporator coil clean, is the fan wheel clean? Assuming your return air is staying around 74 degrees then you are getting a 22 to 29 degree temperature drop. Anything above 25 deg for an extended period of time is too much say 5 minutes. 20 deg td is ideal. Also as you get a greater TD is becomes more noticeable by the tenants and will create more complaints.
    You are going to have less of a problem with a digital scroll or a chilled water valve because you can vary the amount of cooling that is running (0 thru 100% for chwv and approx 20 to 100% with digital scrolls). With a standard DX hermetic compressor you get either 0% or 100% capacity.
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
    BIG Government = More Dependents
    "Any 'standard' would be great if it didn't get bastardised by corporate self interest." MatrixTransform
    http://threedevilskennel.com/ - not my website.
    Versatile Hunting Dog Federation - www.vhdf.org/


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