Variable volume control assembly

Claim 2

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the upstream end having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 3

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pessure probe and static pressure probe to the control means.

Claim 4

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pessure probe and static pressure probe to the control means.

Claim 5

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 6

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 7

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the tapered orifice section and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 8

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the tapered orifice section and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 9

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the constant orifice section in the upstream end and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 10

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the constant orifice section in the upstream end and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 11

An improved variable volume control assembly as in claim 4 and 5 wherein the air measuring means is located in the inlet plenum and wherein the air flow measuring means is in the center of a collector-straightener means, the air flow collector-straightener means comprising a tubular member of perforated material having its openings parallel to the flow of air.

Claim 12

An improved variable volume control assembly as in claim 4 and 5 wherein the tubular collector-straightener is of solid material.

Claim 13

An improved variable volume control assembly as in claim 4 and 5 wherein the air measuring means is located in the inlet plenum and wherein the air flow measuring means is in the center of a collector-straightener means, the air flow collector-straightener means comprising a cone-shaped member of perforated material having its openings parallel to the flow of air with the smaller end being downstream.

Claim 14

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the upstream end having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube, and means for connecting the total pressure probe and static pressure probe to the control means and a vortex filter located in the outlet chamber of the box, the vortex filter comprising a piece of perforated material.

Claim 15

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the upstream end having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube, and means for connecting the total pressure probe and static pressure probe to the control means and a vortex filter located in the outlet chamber of the box, the vortex filter comprising a piece of perforated material.

Claim 16

An improved variable volume control assembly as in claim 9 wherein the vortex filter is V-shaped with the point of the V pointing downstream.

Claim 17

An improved variable volume control assembly as in claim 9 wherein the vortex filter is in a truncated V-shaped with the flat truncated surface pointing downstream.

Claim 18

An improved variable volume control assembly as in claim 9 wherein the free space of the perforated material is from about 25 to 70 percent.

Claim 19

An improved variable volume control assembly as in claim 10 wherein the free space of the perforated vortex filter is about 25 to 70 percent and wherein the tip of the V is in the outlet of the box.

Claim 20

An improved variable volume control assembly as in claim 11 wherein the free space of the perforated vortex filter is about 25 to 70 percent and wherein the flat truncated surface is in the outlet of the box.

Claim 21

An improved variable volume control assembly as in claim 9 wherein the ratio of the cross-sectional area of the inlet plenum to the cross-sectional area of the expansion chamber is from 1:1.25 to 1:2.

Claim 22

An improved variable volume control assembly as in claim 9 wherein the ratio of the cross-sectional area of the inlet plenum to the cross-sectional area of the expansion chamber is from 1:1.4 to 1:1.6.

Claim 23

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the tapered orifice section and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, and means for connecting the total pressure probe and static pressure probe to the control means and a vortex filter located in the outlet chamber of the box, the vortex filter comprising a piece of perforated material, and a ratio of the cross-sectional area of the inlet plenum to the cross-sectional area of the expansion chamber is from 1:1.25 and 1:2.

Claim 24

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the tapered orifice section and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening perpendicular to the flow of gas, and means for connecting the total pressure probe and static pressure probe to the control means and a vortex filter located in the outlet chamber of the box, the vortex filter comprising a piece of perforated material, and a ratio of the cross-sectional area of the inlet plenum to the cross-sectional area of the expansion chamber is from 1:1.25 and 1:2.

Claim 25

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 26

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 27

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 28

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, a total pressure probe located external of the tube and having its opening parallel to the flow of gas and facing upstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 29

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the tapered orifice section and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, and means for connecting the total pressure probe and static pressure to the control means.

Claim 30

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being tapered to form a tapered orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the tapered orifice section and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, and means for connecting the total pressure probe and static pressure to the control means.

Claim 31

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the constant orifice section in the upstream end and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, and means for connecting the total pressure probe and static pressure probe to the control means.

Claim 32

An improved variable volume control assembly for controlling the flow of conditioned air into a zone of the type comprising: a box having an inlet in one end for receiving conditioned air from a source of conditioned air and an outlet in the other end for delivering conditioned air to the zone; an inlet plenum for connecting the box with ducting from the source; damper means located within the box for controlling the flow of air through the box, the damper means dividing the box into an expansion chamber and an outlet chamber; air flow measuring means for measuring the flow of air through the box; motor means located externally of the box for controlling the opening and closing of the damper means; control means located externally of the box and responsive to the temperature in the zone and the flow of conditioned air through the box for controlling the operation of the motor means; wherein the improvement comprises an air flow measuring means comprising a tube having an upstream end and a downstream end, the orifice in the upstream end being constant to form a constant orifice section, an abrupt expansion of the orifice to form an expansion chamber in the downstream end of the tube, a total pressure probe located in the constant orifice section in the upstream end and having its opening parallel to the flow of gas and facing upstream, a static pressure probe located in the expansion chamber of the tube at the abrupt expansion of the orifice and having its opening parallel to the flow of gas and facing downstream, and means for connecting the total pressure probe and static pressure probe to the control means.