Frequently Asked Questions

BUILDING OWNERS

ARCHITECTS

ENGINEERS

MECHANICAL CONTRACTORS

GENERAL CONTRACTORS

BUILDING OWNER'S FAQ

Q. Is there a better way to serve the perimeter of an underfloor system than using underfloor fan powered boxes?

A. Yes, we are strong advocates of treating the perimeter via overhead ductwork whenever possible. The advantages are numerous to this strategy including:

• The ability to lower the raised floor height which improves walk ability and reduces overall building cost
• The ability to have much better control of perimeter cooling
• Improved control of underfloor temp and static pressure as the 14 “blockade zones” are removed from the underfloor
• Electrical and data cabling can be freely run underfloor
• A much easier project to build and operate
• Lower first cost
• No conflicts with underfloor fan powered boxes and furniture/walls
• Easier to service the equipment
• Better heating efficiency (ceiling VAVs take space return air and warm it; the current underfloor design probably draws air from the carpet level, then re-heats this air in heating mode)
• No perimeter floor grilles conflicting with furniture

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Q. What is a column unit?

A. A column unit is a term used to describe Air Zone's vertical floor-by-floor air handling units for raised floor and overhead distributed designs. Some of our smaller CFM units are close in size to a building column!

Back to top

Q. I have heard a lot of concern about raised floor leakage. Is this something that I need to be concerned about?

A. Tate has done the best job in the industry of addressing this issue. Tate Access Floors, Inc. is proud to provide three best practices guides to aid in the proper design and construction of an underfloor air distribution (UFAD) system. Trade specific guides have been created for the Architect, General Contractor, and Commissioning Agent extracted from lessons learned through Tate’s experience working on a wide range of UFAD projects. These guides will help to ensure the proper sealing and reduction of underfloor air leakage in an access floor air plenum. Furthermore, Tate believes a holistic approach to design and construction should be used and recommends regular consultation be held with key individuals on the construction team throughout the entire design process. You can find these documents at http://www.tateaccessfloors.com/resources/plenum_guides.aspx.

Back to top

Q. How much energy does a UFAD system really save over conventional overhead distribution?

A. That answer depends on a number of variables including whether economizer systems are being used and the type of airside distribution system. The energy savings results from lower TSP at the air handler vs conventional overhead systems. A quick analysis will show that if we ignore the energy savings due to increased economizer hours because our supply temperature is 62 degrees vs. 55 degrees (in an overhead system) and we assume that our floor-by-floor column units have been located correctly in the space layout so underfloor ductwork is not necessary, and the external static pressure at the unit is .3 or less, then we can safely say that underfloor systems will save between 6-10% in total KW hours consumed by the building (all other things being equal).

Back to top

Q. There was a day last week that the AHU were providing warmer air than design so the floors themselves required more air to cool the space. One of the air columns did not meet the .05” SP. The Hz of the ACF was set to max out at 40 Hz. Is that correct? I’m sure the fan runs quieter at lower Hz but I thought maybe it wouldn’t be set to max out at 40 Hz if in the rare instance it would need more power it could get it.

A. The air column units are set at the factory for both the upper and lower limits, and this is necessary to protect the equipment. On this project the lower limit was established and set to prevent the electric heater from unwanted overload cut-out. The upper limit is established to protect the motor from operating in an overload condition. Regarding the warm day that was experienced, a problem of this type is not solved by providing more airflow; the mix of the airflow is the important factor. If any space is warmer than desired, the primary damper should be programmed to open more and thus provide more cold air in the fan chamber. The fan volume is established by the pressure under the floor and this can be anywhere between 0.02 and 0.06; if all of floor grills are opened fully, it may be difficult to reach 0.06 floor pressure because all access flooring will leak 20 to 30% of the total airflow into the occupied space. This leakage is not negative because the leakage is from the under floor plenum onto the occupied space and leakage exist on all projects. The temperature sensor under the floor must be properly set to maintain quality space temperature; normally the under floor sensor is set at approximately 62 degrees. If the sensor is currently set at 62 degrees then lower the set point to 60 degrees. Again, increasing the airflow will not solve a warm space condition; the mix of airflow from the primary and by-pass air must be changed to provide a cooler space.

Back to top

Q. We are in a climate where we use 100% OA economizer. How can I incorporate the use of a column unit in that approach?

A. Economizer systems can be used with our column units very easily. The economizer shaft typically would be brought to the mechanical closet where our column unit is located. Normally a Mixed-Flow Air Column Unit would have an OA inlet into the mechanical room with perhaps 10 to 15% OA; this air is blended with the primary air being supplied at the top of the Column Unit and with the RA being induced. The mixed air condition is developed based on the primary and return air being supplied to the Column Unit. Upon the demand for lower air discharge from the unit, the primary air damper will open to the maximum and the induced return air is then at a minimum; likewise upon demand for a warmer discharge air temperature, the primary will modulate to the minimum position and the induced air will be at the maximum airflow position. If an economizer cycle is required an OA duct sized for the required total airflow should deliver the required airflow into the mechanical room, a return air damper would be needed and it would need to close when the 100% OA damper is open; the primary air damper would close to zero airflow and the induced return air actually becomes 100% OA.

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Q. How low can a raised floor height be constructed and still use UFAD?

A. We believe that UFAD systems can go as low as 6" FFH (4-5/8" of clear space) to 8" FFH provided our column units are spaced in frequent points on the floor plate to ensure good air distribution. This low FFH is very applicable for general office where electrical plug and play conduit is only 7/8" diameter and voice and data cables are not intensive. The floor height needs to be greater for applications where cable trays are being used.

Back to top

Q. What has experience shown regarding different supply temperature requirements floor by floor in a UFAD system?

A. Research in UFAD buildings that have been operating for several years show that the temp supply on each floor in a UFAD system will need to vary particularly on a low rise building. For example, on a 3-story building, the supply temperature required for proper occupant comfort under the raised floor will differ from floors 1 (with the raised floor on a slab on grade), floor 2 (being sandwiched in the middle of the building), and floor 3, which is receiving radiant energy from the roof. As a result, engineers are finding our floor-by-floor air columns are a great solution to solve this problem as this way individual floor temperature needs can be met.

Back to top

Q. How large of a mechanical closet do I need to contain the column unit? Where is the optimal location for them to be placed?

A. For the mixed flow unit, the walls can be located 6" away from the three sides, and all service would be from the front of the unit through a single or double set of doors. For the environmental air tower, the walls can be located 6" away on two sides of the unit. We recommend a minimum a clearance of 2' on the adjacent coil side and 2' from the front of the unit to the doors.

Back to top

Q. I thought underfloor systems were not supposed to have ductwork and yet I have seen many projects with it and it seems difficult to build – is there anyway to avoid underfloor ductwork?

A. Yes! Underfloor ductwork or plenum dividers are not desirable and should be avoided if at all possible. If more injection points are delivered with our column units, then underfloor ductwork can be in most cases completely avoided. Column units should be located on the building core edges or in the tenant spaces between the core and perimeter.

Back to top

ARCHITECT'S FAQ

Q. Is there a better way to serve the perimeter of an underfloor system than using underfloor fan powered boxes?

A. Yes, we are strong advocates of treating the perimeter via overhead ductwork whenever possible. The advantages are numerous to this strategy including:

• The ability to lower the raised floor height which improves walk ability and reduces overall building cost
• The ability to have much better control of perimeter cooling
• Improved control of underfloor temp and static pressure as the 14 “blockade zones” are removed from the underfloor
• Electrical and data cabling can be freely run underfloor
• A much easier project to build and operate
• Lower first cost
• No conflicts with underfloor fan powered boxes and furniture/walls
• Easier to service the equipment
• Better heating efficiency (ceiling VAVs take space return air and warm it; the current underfloor design probably draws air from the carpet level, then re-heats this air in heating mode)
• No perimeter floor grilles conflicting with furniture

Back to top

Q. What is a column unit?

A. A column unit is a term used to describe Air Zone's vertical floor-by-floor air handling units for raised floor and overhead distributed designs. Some of our smaller CFM units are close in size to a building column!

Back to top

Q. I have heard a lot of concern about raised floor leakage. Is this something that I need to be concerned about?

A. Tate has done the best job in the industry of addressing this issue. Tate Access Floors, Inc. is proud to provide three best practices guides to aid in the proper design and construction of an underfloor air distribution (UFAD) system. Trade specific guides have been created for the Architect, General Contractor, and Commissioning Agent extracted from lessons learned through Tate’s experience working on a wide range of UFAD projects. These guides will help to ensure the proper sealing and reduction of underfloor air leakage in an access floor air plenum. Furthermore, Tate believes a holistic approach to design and construction should be used and recommends regular consultation be held with key individuals on the construction team throughout the entire design process. You can find these documents at http://www.tateaccessfloors.com/resources/plenum_guides.aspx

Back to top

Q. How much energy does a UFAD system really save over conventional overhead distribution?

A. That answer depends on a number of variables including whether economizer systems are being used and the type of airside distribution system. The energy savings results from lower TSP at the air handler vs conventional overhead systems. A quick analysis will show that if we ignore the energy savings due to increased economizer hours because our supply temperature is 62 degrees vs. 55 degrees (in an overhead system) and we assume that our floor-by-floor column units have been located correctly in the space layout so underfloor ductwork is not necessary, and the external static pressure at the unit is .3 or less, then we can safely say that underfloor systems will save between 6-10% in total KW hours consumed by the building (all other things being equal).

Back to top

Q. How low can a raised floor height be constructed and still use UFAD?

A. We believe that UFAD systems can go as low as 6" FFH (4-5/8" of clear space) to 8" FFH provided our column units are spaced in frequent points on the floor plate to ensure good air distribution. This low FFH is very applicable for general office where electrical plug and play conduit is only 7/8" diameter and voice and data cables are not intensive. The floor height needs to be greater for applications where cable trays are being used.

Back to top

Q. How large of a mechanical closet do I need to contain the column unit? Where is the optimal location for them to be placed?

A. For the mixed flow unit, the walls can be located 6" away from the three sides, and all service would be from the front of the unit through a single or double set of doors. For the environmental air tower, the walls can be located 6" away on two sides of the unit. We recommend a minimum a clearance of 2' on the adjacent coil side and 2' from the front of the unit to the doors.

Back to top

ENGINEER'S FAQ

Q. How do I calculate space cooling and heating loads in a building with UFAD?

A. In his book Underfloor Air Distribution Guide, (ref pages 158-162) Fred Bauman points out that while cooling and heating loads are calculated in a similar fashion as that of overhead distribution, there are differences. The most important is that because UFAD systems rely on stratification, it is not correct to assume a completely mixed room. Stratification will divide the room into an upper and lower zone. In general, once the room air has risen above this stratification height, it will not reenter the lower zone, and herein lies the fundamental difference from the fully mixed room assumed in an overhead system. As such, convective heat gains from sources above the stratification height will be exhausted directly into the ceiling return and not included in the airside calculations. The solution is to use the assumption of two distinct mixed zones, which should yield results that the total CFM required is basically the same as an overhead distributed system.

Back to top

Q. Is there a better way to serve the perimeter of an underfloor system than using underfloor fan powered boxes?

A. Yes, we are strong advocates of treating the perimeter via overhead ductwork whenever possible. The advantages are numerous to this strategy including:

• The ability to lower the raised floor height which improves walk ability and reduces overall building cost
• The ability to have much better control of perimeter cooling
• Improved control of underfloor temp and static pressure as the 14 “blockade zones” are removed from the underfloor
• Electrical and data cabling can be freely run underfloor
• A much easier project to build and operate
• Lower first cost
• No conflicts with underfloor fan powered boxes and furniture/walls
• Easier to service the equipment
• Better heating efficiency (ceiling VAVs take space return air and warm it; the current underfloor design probably draws air from the carpet level, then re-heats this air in heating mode)
• No perimeter floor grilles conflicting with furniture

Back to top

Q. What is a column unit?

A. A column unit is a term used to describe Air Zone's vertical floor-by-floor air handling units for raised floor and overhead distributed designs. Some of our smaller CFM units are close in size to a building column!

Back to top

Q. When I design overhead systems, I usually duct the space return air into the floor-by-floor AHU to minimize the pressure differential between the mechanical closet and the occupied space. Do I still need to do this with underfloor?

A. No! The external static pressure in an underfloor system is typically .3 in w.g. (.05" underfloor and .25" return) compared to 1.5" or more external static pressure in an overhead system.

Back to top

Q. I have heard a lot of concern about raised floor leakage. Is this something that I need to be concerned about?

A. Tate has done the best job in the industry of addressing this issue. Tate Access Floors, Inc. is proud to provide three best practices guides to aid in the proper design and construction of an underfloor air distribution (UFAD) system. Trade specific guides have been created for the Architect, General Contractor, and Commissioning Agent extracted from lessons learned through Tate’s experience working on a wide range of UFAD projects. These guides will help to ensure the proper sealing and reduction of underfloor air leakage in an access floor air plenum. Furthermore, Tate believes a holistic approach to design and construction should be used and recommends regular consultation be held with key individuals on the construction team throughout the entire design process. You can find these documents at http://www.tateaccessfloors.com/resources/plenum_guides.aspx

Back to top

Q. How much energy does a UFAD system really save over conventional overhead distribution?

A. That answer depends on a number of variables including whether economizer systems are being used and the type of airside distribution system. The energy savings results from lower TSP at the air handler vs conventional overhead systems. A quick analysis will show that if we ignore the energy savings due to increased economizer hours because our supply temperature is 62 degrees vs. 55 degrees (in an overhead system) and we assume that our floor-by-floor column units have been located correctly in the space layout so underfloor ductwork is not necessary, and the external static pressure at the unit is .3 or less, then we can safely say that underfloor systems will save between 6-10% in total KW hours consumed by the building (all other things being equal).

Back to top

Q. There was a day last week that the AHU were providing warmer air than design so the floors themselves required more air to cool the space. One of the air columns did not meet the .05” SP. The Hz of the ACF was set to max out at 40 Hz. Is that correct? I’m sure the fan runs quieter at lower Hz but I thought maybe it wouldn’t be set to max out at 40 Hz if in the rare instance it would need more power it could get it.

A. The air column units are set at the factory for both the upper and lower limits, and this is necessary to protect the equipment. On this project the lower limit was established and set to prevent the electric heater from unwanted overload cut-out. The upper limit is established to protect the motor from operating in an overload condition. Regarding the warm day that was experienced, a problem of this type is not solved by providing more airflow; the mix of the airflow is the important factor. If any space is warmer than desired, the primary damper should be programmed to open more and thus provide more cold air in the fan chamber. The fan volume is established by the pressure under the floor and this can be anywhere between 0.02 and 0.06; if all of floor grills are opened fully, it may be difficult to reach 0.06 floor pressure because all access flooring will leak 20 to 30% of the total airflow into the occupied space. This leakage is not negative because the leakage is from the under floor plenum onto the occupied space and leakage exist on all projects. The temperature sensor under the floor must be properly set to maintain quality space temperature; normally the under floor sensor is set at approximately 62 degrees. If the sensor is currently set at 62 degrees then lower the set point to 60 degrees. Again, increasing the airflow will not solve a warm space condition; the mix of airflow from the primary and by-pass air must be changed to provide a cooler space.

Back to top

Q. We are in a climate where we use 100% OA economizer. How can I incorporate the use of a column unit in that approach?

A. Economizer systems can be used with our column units very easily. The economizer shaft typically would be brought to the mechanical closet where our column unit is located. Normally a Mixed-Flow Air Column Unit would have an OA inlet into the mechanical room with perhaps 10 to 15% OA; this air is blended with the primary air being supplied at the top of the Column Unit and with the RA being induced. The mixed air condition is developed based on the primary and return air being supplied to the Column Unit. Upon the demand for lower air discharge from the unit, the primary air damper will open to the maximum and the induced return air is then at a minimum; likewise upon demand for a warmer discharge air temperature, the primary will modulate to the minimum position and the induced air will be at the maximum airflow position. If an economizer cycle is required an OA duct sized for the required total airflow should deliver the required airflow into the mechanical room, a return air damper would be needed and it would need to close when the 100% OA damper is open; the primary air damper would close to zero airflow and the induced return air actually becomes 100% OA.

Back to top

Q. How low can a raised floor height be constructed and still use UFAD?

A. We believe that UFAD systems can go as low as 6" FFH (4-5/8" of clear space) to 8" FFH provided our column units are spaced in frequent points on the floor plate to ensure good air distribution. This low FFH is very applicable for general office where electrical plug and play conduit is only 7/8" diameter and voice and data cables are not intensive. The floor height needs to be greater for applications where cable trays are being used.

Back to top

Q. When is a program going to be commercially available to determine load calculations in an UFAD project?

A. Soon - check the Center for Built Environment Web site for details. http://www.cbe.berkeley.edu/underfloorair/Default.htm

Back to top

Q. What has experience shown regarding different supply temperature requirements floor by floor in a UFAD system?

A. Research in UFAD buildings that have been operating for several years show that the temp supply on each floor in a UFAD system will need to vary particularly on a low rise building. For example, on a 3-story building, the supply temperature required for proper occupant comfort under the raised floor will differ from floors 1 (with the raised floor on a slab on grade), floor 2 (being sandwiched in the middle of the building), and floor 3, which is receiving radiant energy from the roof. As a result, engineers are finding our floor-by-floor air columns are a great solution to solve this problem as this way individual floor temperature needs can be met.

Back to top

Q. How large of a mechanical closet do I need to contain the column unit? Where is the optimal location for them to be placed?

A. For the mixed flow unit, the walls can be located 6" away from the three sides, and all service would be from the front of the unit through a single or double set of doors. For the environmental air tower, the walls can be located 6" away on two sides of the unit. We recommend a minimum a clearance of 2' on the adjacent coil side and 2' from the front of the unit to the doors.

Back to top

Q. I thought underfloor systems were not supposed to have ductwork and yet I have seen many projects with it and it seems difficult to build – is there anyway to avoid underfloor ductwork?

A. Yes! Underfloor ductwork or plenum dividers are not desirable and should be avoided if at all possible. If more injection points are delivered with our column units, then underfloor ductwork can be in most cases completely avoided. Column units should be located on the building core edges or in the tenant spaces between the core and perimeter.

Back to top

MECHANICAL CONTRACTOR'S FAQ

Q. What is a column unit?

A. A column unit is a term used to describe Air Zone's vertical floor-by-floor air handling units for raised floor and overhead distributed designs. Some of our smaller CFM units are close in size to a building column!

Back to top

Q. There was a day last week that the AHU were providing warmer air than design so the floors themselves required more air to cool the space. One of the air columns did not meet the .05” SP. The Hz of the ACF was set to max out at 40 Hz. Is that correct? I’m sure the fan runs quieter at lower Hz but I thought maybe it wouldn’t be set to max out at 40 Hz if in the rare instance it would need more power it could get it.

A. The air column units are set at the factory for both the upper and lower limits, and this is necessary to protect the equipment. On this project the lower limit was established and set to prevent the electric heater from unwanted overload cut-out. The upper limit is established to protect the motor from operating in an overload condition. Regarding the warm day that was experienced, a problem of this type is not solved by providing more airflow; the mix of the airflow is the important factor. If any space is warmer than desired, the primary damper should be programmed to open more and thus provide more cold air in the fan chamber. The fan volume is established by the pressure under the floor and this can be anywhere between 0.02 and 0.06; if all of floor grills are opened fully, it may be difficult to reach 0.06 floor pressure because all access flooring will leak 20 to 30% of the total airflow into the occupied space. This leakage is not negative because the leakage is from the under floor plenum onto the occupied space and leakage exist on all projects. The temperature sensor under the floor must be properly set to maintain quality space temperature; normally the under floor sensor is set at approximately 62 degrees. If the sensor is currently set at 62 degrees then lower the set point to 60 degrees. Again, increasing the airflow will not solve a warm space condition; the mix of airflow from the primary and by-pass air must be changed to provide a cooler space.

Back to top

Q. What has experience shown regarding different supply temperature requirements floor by floor in a UFAD system?

A. Research in UFAD buildings that have been operating for several years show that the temp supply on each floor in a UFAD system will need to vary particularly on a low rise building. For example, on a 3-story building, the supply temperature required for proper occupant comfort under the raised floor will differ from floors 1 (with the raised floor on a slab on grade), floor 2 (being sandwiched in the middle of the building), and floor 3, which is receiving radiant energy from the roof. As a result, engineers are finding our floor-by-floor air columns are a great solution to solve this problem as this way individual floor temperature needs can be met.

Back to top

GENERAL CONTRACTOR'S FAQ

Q. How much energy does a UFAD system really save over conventional overhead distribution?

A. That answer depends on a number of variables including whether economizer systems are being used and the type of airside distribution system. The energy savings results from lower TSP at the air handler vs conventional overhead systems. A quick analysis will show that if we ignore the energy savings due to increased economizer hours because our supply temperature is 62 degrees vs. 55 degrees (in an overhead system) and we assume that our floor-by-floor column units have been located correctly in the space layout so underfloor ductwork is not necessary, and the external static pressure at the unit is .3 or less, then we can safely say that underfloor systems will save between 6-10% in total KW hours consumed by the building (all other things being equal).

Back to top

Q. How low can a raised floor height be constructed and still use UFAD?

A. We believe that UFAD systems can go as low as 6" FFH (4-5/8" of clear space) to 8" FFH provided our column units are spaced in frequent points on the floor plate to ensure good air distribution. This low FFH is very applicable for general office where electrical plug and play conduit is only 7/8" diameter and voice and data cables are not intensive. The floor height needs to be greater for applications where cable trays are being used.

Back to top

Q. I thought underfloor systems were not supposed to have ductwork and yet I have seen many projects with it and it seems difficult to build – is there anyway to avoid underfloor ductwork?

A. Yes! Underfloor ductwork or plenum dividers are not desirable and should be avoided if at all possible. If more injection points are delivered with our column units, then underfloor ductwork can be in most cases completely avoided. Column units should be located on the building core edges or in the tenant spaces between the core and perimeter.

Back to top