I love these videos! If you wanted to make this model match a common reality, you would have VAV boxes being controlled by zone sensors in unrelated zones which have big heat absorbing bookcases in front of said zone sensors to make them think that those zones are hot, send that information to the VAV controllers and cause the VAV to try to cool the zone it is actually serving based on that unrelated zone's bad input. Gotta love remodels.
What happens when heating is on, and a zone requires cooling? For example, if you have a building where the zones towards the perimeter need heat, but the zones deeper into the building need to be cooled.
Question: I believe you are asking, “What happens when an individual temperature control zone is calling for cooling while the air handler is currently supplying heat?” Answer: Large buildings typically have central air handlers that are supplying air to dozens or even hundreds of individual temperature control zones. Each temperature control zone is handled by a VAV box. These large air handlers typically only supply cooling, but may vary their supply temperatures for summer and winter operation. Interior zones typically need cooling all year long. VAV boxes serving perimeter zones typically include some form of reheat (electric or hot water) to handle winter loads due to glass or exterior walls.
Sorry, I don't know why I'm just now seeing this, but I'll answer it for future reference. If the building has a building management system (BMS), you could make a sequence for the AHU to look at what the VAV boxes are doing. An easier way would be to put a pressure sensor in the ductwork and have the system maintain a certain pressure.
If by “motorized VCD” you are referring to a “motorized volume control damper”, that’s a pressure dependent airflow control device. It could be connected to a wall-mounted thermostat to provide variable volume airflow for room temperature control. This would be pressure dependent control because the air volume delivery would be dependent on available duct pressure. It also would not be able to control minimum or maximum flow limit control. This could result in stuffy conditions when the space temperature is satisfied or annoyingly high sound levels when loads are higher. Today’s typical “VAV box” is a pressure independent air terminal. It typically includes an inlet velocity sensor and a VAV controller that provides air flow limit controls. This ensures that minimum ventilation requirements will meet ASHRAE 62 guidelines and that maximum flow limit will help to manage airflow in the building while limiting sound levels. Terminal units also provide many options with regard to electric and hot water reheat. So to answer your question directly, a motorized VCD can be used in place of a VAV box, but it is generally a poor choice. Pressure independent control is standard these days and that requires a VAV box with an inlet sensor and a VAV controller.
Thank you for your response. Another question strikes, a VAV box is dependent of the pressure of the main duct so how it is a pressure independent control unit. What is the definition of a pressure dependent control unit and pressure independent control unit ??
Pressure dependent controls just drive a damper open or closed to control temperature. When the room is satisfied the thermostat sends no signal and the current damper position is maintained. The damper can move full open to full closed or mechanical limits can be set on the travel. Pressure independent controls require a velocity sensor. These VAV box and their associated controls provide airflow within minimum and maximum flow limit settings based on load requirements. These controls therefore automatically readjust the damper position to correct for any changes in inlet static pressure. For instance, the thermostat is commanding a controller to provide 500 cfm. The damper moves until the inlet sensor registers the velocity associated with 500 cfm. There’s a sudden change in room load in a nearby conference room, so when that box opens the duct pressure starts to drop. The box in the first room will automatically open to maintain 500 cfm to the first room. So pressure independent controls can automatically compensate for changes in duct pressure, so long as there is adequate pressure to meet all the airflow requirements. When airflow is insufficient and boxes are ‘starved’, pressure independent boxes simply drive wide open to provide whatever airflow is available.
The VAV box is usually being supplied with cooling air only (55 degrees F from the air handler) and is trying to satisfy the thermostat. So if the thermostat is set at 75 degrees and the room is 72 degrees, the VAV box closes the damper (to supply less 55 degree air) and if the room is 77 degrees, the VAV box opens the damper (to supply more 55 degree air).The VAV controller is not doing auto-changeover from cooling to heating. There are controllers that do auto-changeover, but they need to get a supply air temperature signal to know when to change from heating to cooling. This is not how typical VAV boxes work though.
Funny. I was just working on a Titus II VAV with AUTO CHANGEOVER today. Sort of. The controller is set to maintain a minimum airflow so it can not close completely. When the space starts dropping enough below the setpoint there is a pressure actuated switch that then brings on the heating contactors.With the airflow at minimum the electric heat can temper the 55 deg air to make it lukewarm at least. LOL This building I am working on appears to be a bastard with several different brands and types of pneumatic VAV boxes and t-stats. And some are direct acting and some reverse! Whoever set up that building needs a good swift kick in the ass. But that's Scottsdale, AZ for ya. The WORST engineering and building designs I have ever seen.
I love these videos!
If you wanted to make this model match a common reality, you would have VAV boxes being controlled by zone sensors in unrelated zones which have big heat absorbing bookcases in front of said zone sensors to make them think that those zones are hot, send that information to the VAV controllers and cause the VAV to try to cool the zone it is actually serving based on that unrelated zone's bad input. Gotta love remodels.
found your videos very informative. Great work. Please keep it up
Best brief explanation of a VAV
Good explanation
wow! great videos. i have to control 2 vavs. but i don't know how. thanks Titus!
I'm glad you like them.
Great info
this is great video for learning.
What happens when heating is on, and a zone requires cooling? For example, if you have a building where the zones towards the perimeter need heat, but the zones deeper into the building need to be cooled.
Question: I believe you are asking, “What happens when an individual temperature control zone is calling for cooling while the air handler is currently supplying heat?”
Answer: Large buildings typically have central air handlers that are supplying air to dozens or even hundreds of individual temperature control zones. Each temperature control zone is handled by a VAV box. These large air handlers typically only supply cooling, but may vary their supply temperatures for summer and winter operation. Interior zones typically need cooling all year long. VAV boxes serving perimeter zones typically include some form of reheat (electric or hot water) to handle winter loads due to glass or exterior walls.
@@TitusHVAC Yep, that's what I meant. Thank you for your response!
But still why larger box instead of straight duct
would the vav box controller send a signal to the air handler fan to ramp up more air if needed? Thanks.
Sorry, I don't know why I'm just now seeing this, but I'll answer it for future reference. If the building has a building management system (BMS), you could make a sequence for the AHU to look at what the VAV boxes are doing. An easier way would be to put a pressure sensor in the ductwork and have the system maintain a certain pressure.
What is the difference between Vav and motorised VCD in HVAC? Can we use a motorised VCD instead of a VAV box or vice verse
If by “motorized VCD” you are referring to a “motorized volume control damper”, that’s a pressure dependent airflow control device. It could be connected to a wall-mounted thermostat to provide variable volume airflow for room temperature control. This would be pressure dependent control because the air volume delivery would be dependent on available duct pressure. It also would not be able to control minimum or maximum flow limit control. This could result in stuffy conditions when the space temperature is satisfied or annoyingly high sound levels when loads are higher.
Today’s typical “VAV box” is a pressure independent air terminal. It typically includes an inlet velocity sensor and a VAV controller that provides air flow limit controls. This ensures that minimum ventilation requirements will meet ASHRAE 62 guidelines and that maximum flow limit will help to manage airflow in the building while limiting sound levels. Terminal units also provide many options with regard to electric and hot water reheat.
So to answer your question directly, a motorized VCD can be used in place of a VAV box, but it is generally a poor choice. Pressure independent control is standard these days and that requires a VAV box with an inlet sensor and a VAV controller.
Thank you for your response. Another question strikes, a VAV box is dependent of the pressure of the main duct so how it is a pressure independent control unit. What is the definition of a pressure dependent control unit and pressure independent control unit ??
Pressure dependent controls just drive a damper open or closed to control temperature. When the room is satisfied the thermostat sends no signal and the current damper position is maintained. The damper can move full open to full closed or mechanical limits can be set on the travel.
Pressure independent controls require a velocity sensor. These VAV box and their associated controls provide airflow within minimum and maximum flow limit settings based on load requirements. These controls therefore automatically readjust the damper position to correct for any changes in inlet static pressure.
For instance, the thermostat is commanding a controller to provide 500 cfm. The damper moves until the inlet sensor registers the velocity associated with 500 cfm. There’s a sudden change in room load in a nearby conference room, so when that box opens the duct pressure starts to drop. The box in the first room will automatically open to maintain 500 cfm to the first room.
So pressure independent controls can automatically compensate for changes in duct pressure, so long as there is adequate pressure to meet all the airflow requirements. When airflow is insufficient and boxes are ‘starved’, pressure independent boxes simply drive wide open to provide whatever airflow is available.
how VAV controller can change the mode from cooling to heating, we all now that controller should work in cooling direct and in heating reverse?
The VAV box is usually being supplied with cooling air only (55 degrees F from the air handler) and is trying to satisfy the thermostat. So if the thermostat is set at 75 degrees and the room is 72 degrees, the VAV box closes the damper (to supply less 55 degree air) and if the room is 77 degrees, the VAV box opens the damper (to supply more 55 degree air).The VAV controller is not doing auto-changeover from cooling to heating. There are controllers that do auto-changeover, but they need to get a supply air temperature signal to know when to change from heating to cooling. This is not how typical VAV boxes work though.
Funny. I was just working on a Titus II VAV with AUTO CHANGEOVER today.
Sort of.
The controller is set to maintain a minimum airflow so it can not close completely.
When the space starts dropping enough below the setpoint there is a pressure actuated switch that then brings on the heating contactors.With the airflow at minimum the electric heat can temper the 55 deg air to make it lukewarm at least. LOL
This building I am working on appears to be a bastard with several different brands and types of pneumatic VAV boxes and t-stats. And some are direct acting and some reverse!
Whoever set up that building needs a good swift kick in the ass.
But that's Scottsdale, AZ for ya.
The WORST engineering and building designs I have ever seen.
@@hackfreehvac For what application in AZ did you need to heat the space?
@@allymacky4039 Office spaces.
@@hackfreehvac Oh man. Temps get that low in AZ too? What area/what time of year?
Romans8:31
if God is for us who can be against us?