CA2495821A1 - Proportional control system for a motor - Google Patents

Abstract

The present invention provides a proportional control system for use in an HVAC unit within a ventilation system comprising a processing means (130) programmed with an air exchange - defrost cycle. One or more motor speed sensing means are positioned in the HVAC unit and operatively connected to the processing means (130) and one or more temperature sensing means (120) are positioned in the HVAC unit and operatively connected to the processing means (130) wherein the temperature and motor speed sensors determine the motor speed to be applied during the defrost cycle.

Description

PROPORTIONAL CONTROL SYSTEM FOR A MOTOR
The present invention also provides a proportional control system for use in an HVAC
unit within a ventilation system comprising a processing means prograrmned with an air exchange-defrost cycle. One or more sensing means are also positioned in the HVAC unit and operatively connected to the processing means and a damper mechanism positioned in the HVAC unit wherein the sensing means and the processing means determine the motor speed to be applied during the defrost cycle.
FIELD OF THE INVENTION
The present invention pertains to the field of control systems and more specifically to a proportional control system for a defrost cycle within an HVAC unit for a ventilation system.
BACKGROUND
The present invention generally relates to an apparatus for ventilation systems which have means for the transfer of sensible heat and/or water moisture between exhaust air (taken from inside a building) and exterior fresh air (drawn into the building). Such an apparatus may, for example, have means for the transfer of sensible heat and/or water moisture from warm exhaust air to cooler exterior fresh air, the systems using warm interior air as defrost air for defrosting the systems during cool weather.
Sensible heat and/or water moisture recovery ventilation systems are known which function to draw fresh exterior air into a building and to exhaust stale interior air to the outside. The systems are provided with appropriate ducting, channels and the like which define a fresh air path and an exhaust air path whereby interior air of a building may be exchanged with exterior ambient air; during ventilation the air in one path is not normally allowed to mix with the air in the other path.
A sensible heat and/or water moisture recovery ventilator device or apparatus, which may form part of a ventilation system, in addition to being provided with corresponding air paths may also be provided with one or more exchanger elements or cores, e.g. one or more rotary andlor stationary (i.e. non-rotary) exchanger elements or cores. Heat recovery ventilation devices may also have a housing or cabinet;
such enclosures may for example be of sheet metal construction (e.g. the top, bottom, side walls and any door, etc. may be made from panels of sheet metal). The heat exchanging core(s), as well as other elements of the device such as, for example, channels or ducts which define air paths, filtration means, insulation and if desired one or more fans for moving air through the fresh air and exhaust air paths may be disposed in the enclosure. Such ventilation devices may be disposed on the outside of or within a building such as a house, commercial building or the like;
appropriate insulation may be provided around any duct work needed to connect the device to the fresh air source and the interior air of the building. A stationary heat exchanger elements) may, for example, take the form of the (air-to-air) heat exchanger element as shown in U.S. Pat. No. 5,002,118. Thus, the heat exchanger elements) may have the form of a rectangular paraellpiped and may define a pair of air paths which are disposed at right angles to each other; these exchanger elements) may be disposed such that the air paths are diagonally oriented so that they are self draining (i.e. with respect to any condensed or unfrozen water).
During the winter season, the outside air is not only cool but it is also relatively dry.
Accordingly, if cool dry outside air is brought into a building and the warm moist interior air of the building is merely exhausted to the outside, the air in the building may as a consequence become uncomfortably dry. A relatively comfortable level of humidity may be maintained in a building by inter alia exploiting an above mentioned desiccant type thermal wheel for transfernng water from the stale outgoing air to the relatively dry fresh incoming air. During winter these types of heat exchangers may transfer up to 80% of the moisture contained in the exhaust air to the fresh supply air.
Advantageously a rotary exchanger wheel may transfer both sensible and latent heat between fresh air and exhaust air; in this case the exhaust air stream as it is cooled may also be dried whereas the incoming fresh air may be warmed as well as humidified. However, a problem with such heat recovery ventilation equipment

2 having a desiccant type heat exchanger wheel, is the production of frost or ice in the air permeable heat exchange matrix of the thermal wheel.
During especially cold weather such as -10° F. or lower (e.g. -25° C. or lower), prior to expelling the relatively warm exhaust air, the equipment provides for the transfer of latent heat from the relatively warm moist exhaust air to the relatively cool dry (fresh) outside air by the use of a suitable desiccant type heat exchange wheel.
However, the cooling of the relatively moist interior air by the cold exterior air can result in the formation of ice (crystals). An uncontrolled buildup of ice within the matrix of a rotary exchanger wheel can result in decreased heat transfer, and even outright blockage not only of the exhaust air path but the (cold) fresh air path as well.
Accordingly a means of periodically defrosting such a system is advantageous in order to maintain the system's efficiency.
A defrost mechanism has been suggested wherein the fresh air intake is periodically blocked off by a damper and warm interior air is injected, via a separate defrost air conduit, into the fresh air inlet side of the fresh air path of the ventilation apparatus.
However, during the defrost cycle, the stale inside air is still exhausted to the outside via the exhaust air path; this is disadvantageous since by blocking only the fresh air inlet and continuing to exhaust interior air to the outside, a negative air pressure can be built up in the interior of a building relative to the exterior atmosphere.
Such a negative pressure may induce uncontrolled entry of air through any cracks and cranies in the structure of the building; the negative pressure may, in particular, produce a backdraft effect, for oil and gas type beating systems, whereby exterior air may be pulled into the chimney leading to the accumulation of gaseous combustion products in the building.
An alternate system has been suggested wherein both the fresh air inlet and exhaust air outlet are both blocked off such that warm interior air is circulated through the fresh air side of the heat exchanger element as well as through the exhaust air side of the heat exchanger element and is sent back into the building; see for example U.S. Pat.
No. 5,193,610.

3 Another problem with respect to ventilation systems comprising a heat exchanger element or core relates to the installation of an exchanger device in a building such as for example a house or other type of building. In order for the system to operate efficiently and effectively the outgoing exhaust air flow preferably at least substantially equals the incoming fresh air flow; i.e. the exhaust and fresh air flows are preferably balanced so as to minimize or eliminate under-pressure or over-pressure in the house relative to the outside atmospheric pressure; a certain degree of overpressure may, however, be tolerated.
Presently, such ventilation systems are balanced by means of balancing dampers and removeable flowmeters such as, for example, a pitot tube type flow measuring device comprising a manometer to measure pressure difference; these elements must usually be installed by the balancing technician at appropriate places in the duct work connected to the ventilation device.
Given the above, it would be advantageous to have a control system in order to defrost a ventilation system which does not require the use of motors at full speed during the defrost operation or the addition of a number of additional components to the ventilation system. It would also be advantageous to have a ventilation system with a defrost system that is controlled by the outside temperature and the speed of the motors.
It would also be advantageous to have a defrostable ventilation apparatus which is of simple construction.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.

4 SUMMARY OF THE INVENTION
An object of the present invention is to provide a proportional control system for use in an HVAC unit within a ventilation system comprising a processing means programmed with an air exchange - defrost cycle. One or more motor speed sensing means are also positioned in the HVAC unit and operatively connected to the processing means and one or more temperature sensing means are positioned in the HVAC unit and operatively connected to the processing means wherein the temperature and motor speed sensors determine the motor speed to be applied during the defrost cycle.
Another object of the present invention is to provide a proportional control system for use in an HVAC unit within a ventilation system comprising a processing means programmed with an air exchange-defrost cycle. One or more sensing means are also positioned in the HVAC unit and operatively connected to the processing means and a damper mechanism positioned in the HVAC unit wherein the sensing means and the processing means determine the motor speed to be applied during the defrost cycle.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a side perspective of one embodiment of the present invention.
Figure 2 is a side perspective of a heat exchanger.
Figure 3 is a perspective, broken away enlarged and partially schematic view of a portion of the heat exchanger shown in Figure 2.
Figure 4 is a side perspective of one embodiment of the present invention.
Figure 5 is a flow chart showing the establishment of the high speed defrost cycle.
Figure 6 is a side perspective of an HVAC unit with a mechanical damper used in one embodiment of the present invention.
Figure 7 is a side perspective of an HVAC unit with a mechanical damper used in one embodiment of the present invention.

5 DETAILED DESCRIPTION OF THE INVENTION
Defiyiitio~s The term "Sensing means" is used to define components capable of measuring various factors independent or dependent of a ventilation system. The sensing means are positioned in an area of the HVAC unit where exterior air enters the ventilation system. The sensing means measures one or more factors of the air and these measurements are then sent to a processing means. The sensing means may also encompass means to measure the motor speeds of impellers or fans commonly found in HVAC units. The motor speed may be measured through the voltage applied to the motor or through the motor revolutions.
The term "Processing mans" is used to defined an electronic circuit which obtains measurement readings from the sensing means operatively associated with a ventilation system and subsequently evaluates the required power to be applied to the motors.
The term "Motors" is used to define a motor used to activate a blower or an impeller commonly found in a ventilation system. The motor is controlled by the processing means to evacuate stale air which is passed through a heat exchanger prior to being evacuated outside of a building.
The term "Contact Area" is used to defined an area where the presence of undesired materials is accumulated. The contact area is an area within a heat exchanger of a ventilation system. The undesired materials of the present invention is the presence of frost on a surface.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

6

7 PCT/CA2003/001221 The present invention provides a system that enables to control the speed of motors in relation to the measurement of factors independent of the motors. A processing means, sensing means and motors are operatively associated to a ventilation system.
The sensing means monitor atmospheric factors such as the air temperature and the HVAC unit motor speeds wherein such measurements are transmitted to the processing means. The processing means through these measurements determines the appropriate motor speeds to potentially reduce the noise and wasted energy emitted from the use of the motors during the removal of frost accumulated on a contact area.
The further variation of the motor speed may be determined through the sensing means measurement.
Sensifag Meafas In one embodiment of the present invention, the sensing means may be defined as components able to measure the air temperature, atmospheric pressure, relative humidity or any other atmospheric factor known to a worked skilled in~ the art. The sensing means may also be define as components capable of measuring various characteristics dependent of a ventilation system such as the air flow, impeller or fan motor speeds, the air pressure or the temperature of any components within a ~ ventilation system or any other characteristics of a ventilation system as would be known by a worker skilled in the relevant art. The components utilised to measure these characteristics may be defined as diodes, transistors, thermocouples, thermistors, semiconductors or any other appropriate measuring devices as would be known by a worker slcilled in the art.
Processing Mearas In one embodiment of the present invention, the processing means may provide sensor excitation and signal conditioning circuits for each sensor system, a digitizer, for converting analog sensor signals to digital values, a microcontroller, having non-volatile program memory, volatile working memory, and persistent memory for adaptive parameters. The processing means may also receive user input to control the operation and produce outputs including audible and visible alarms. The processing means may be battery powered, and is preferably intrinsically safe, meaning that, even with a fault condition, it will not be capable of igniting a combustible gas in the environment. This intrinsic safety is achieved by the avoidance of energy storage elements configured to provide spark energy to ignite a flame, and through the use of flame arresters.
In another embodiment, the processing means may store a program in read only memory (ROM). The processing means may operate by using temporary storage in registers and random access memory (RAM). Sensor calibration data, as well as environmental factors and data about the ventilation unit may be periodically stored and updated in electrically erasable programmable read only memory (EEPROM).
In one embodiment, the processing means has two states namely an active state and a defrost state. Both the active and defrost state are implemented by the processing means for a specific amount of time and at specific motor speeds respectively.
These two states can be varied by the processing means dependent on the enviromnent in which the HVAC unit is installed. The active state can be prolonged or diminished as the defrost state can also be prolonged or diminished. The motor speeds can also be increased or diminished based on the environment in which the HVAC unit is installed.
lVlOt03 S
In one embodiment, the motors are devices which provide the necessary mechanical power for the flow rate within a ventilation system such as an electrical motor or any other motor suitable for a ventilation system as would be known by a worker spilled in the relevant art.
Contact Area In one embodiment of the present invention, the contact area may be defined as the an area within a heat exchanger located near the exhaust of a ventilation system.
The materials used to manufacture heat exchangers may be composed of steel, metal, plastic, or any other material as would be known by a worker skilled in the art for the

8 construction of heat exchangers. The contact area must also be of a rigidity wherein the accumulation of material such as frost will not shear or break the contact area.
In one embodiment of the present invention, the material used to manufacture the contact area or the heat exchanger of the ventilation system may be material with a relatively high conductivity of electricity in order to allow the use of a sensing means that measures the conductivity of a material when frost is accumulated on the contact area.
In one embodiment and with reference to Figure 1, a ventilation system encompasses a proportional control system for a motor. The fresh air intake 10 with an attached impeller 20 pushes air through a heat exchanger 30. The heat exchanger 30 has a square shape and can be made of plastic. The heat exchanger 30 utilised in this embodiment will be further described below in greater detail. Once air passes through the heat exchanger 30, the air is then circulated within the ventilation system through the air exhaust 40. Stale air is removed from the ventilation system through the stale air intake 50 with an attached impeller 60. The stale air is then passed through the heat exchanger 30 and evacuated outside the building through the stale air exhaust 70.
In one embodiment and with reference to Figure 2, a commonly used heat exchanger for use in a ventilation system is shown. The heat exchanger 30 enables air to pass in the direction 80 or the direction 90. The outside air enters the heat exchanger 30 in the direction 80 and the stale air enters the heat exchanger 30 in the direction 90.
25 With further reference to Figure 3, the heat exchanger structure comprises a plurality of plastic extrusions 100 with closely spaced parallel passageways 104 separated by square extruded channel members 102 extending perpendicular to the direction of the passageways 104. Although only two of the extrusions 100 and a pair of channel members are shown in Figure 3, for the sake of simplicity in the drawings, it should 30 be understood that there are many extrusions and channel members in the typical heat-exchanger.

9 With further reference to Figure 3, each extrusion 100 comprises a solid top sheet 101 and a solid bottom sheet 103 with multiple vertical walls forming the passageways 104. Thus, crossed air flow paths are funned by the passageways 104, on the one hand, and the spaces 106 between the channel members and the hollow interiors of the members 102. These crossed flow paths are isolated from one another by the solid sheets 101 and 103.
The exhaust air preferably flows through the larger passageways 106, as indicated by the arrow 107, and the outside air flows through the passageways 104. This arrangement is preferred because the exhaust air may have entrained water droplets and condensation and ice may form in the exhaust air passageways so that the larger passageways will remain operative for heat transfer over a wider range of operating circumstances than if the smaller passages were used. Although condensation also will occur when hot, humid outside air is cooled in the heat exchanger, it is believed that the larger passageways will better suit the conduct of exhaust air.
The material of which the heat exchanger 30 is made preferably is polyethylene or polypropylene, or other plastic materials which also are impervious to deterioration under prolonged contact with water and flowing air.
Equivalent heat exchangers also can be used in the practice of the invention.
For example, isolating heat exchangers made of various metals can be used, as well as heat pipes whose ends are isolated from one another with one end in the outside air flow and the other in the exhaust air flow. Hydronic heat exchangers with liquid working fluids also can be used.
The plastic heat exchanger described above is advantageous over the usual metal heat exchanger, even though the heat conductivity of the plastic is considerably lower than that of the metal. The plastic lasts a very long time without corroding and is considerably less expensive than metal. Also, the plastic heat exchanger is less expensive to manufacture than metal heat exchangers. The added volume required for the plastic heat exchanger to exchange the same amount of heat as a metal heat exchanger is more than offset by the foregoing advantages.
The plastic heat exchanger is believed to be particularly advantageous when used with evaporative cooling because any scale which forms from the water spray can be broken free relatively easily by flexing the heat exchanger In one embodiment of the present invention and with reference to Figure 4, specific cycle times are pre-set for the ventilation system. The ventilation system will be in the active mode for 20 minutes and the defrost cycle will then be activated for 5 minutes.
The motor speeds of the stale air intake impeller 60 will be determined by the outside air temperature measured by the sensing means 120. The measurement by the sensing means will be sent to the processing means 130. The processing means 130 will then determine the speed of the stale air exhaust. For example, if the outside air temperature is -5° Celsius, the speed of the stale air intake impeller 50 will be activated at its lowest speed. The speed of the stale air intake impeller 50 will be increased proportionally based on the outside air temperature wherein the speed of the stale air intake impeller 50 will be at its maximum when the outside air temperature reaches or is lower than -25° Celsius. The maximum impeller speed will remain active during the defrost cycle until the outside air temperature increases higher then -25° Celsius wherein the speed of the stale air intake impeller 50 will be proportionally diminished to a minimum speed only once the outside temperature reaches to -5°
Celsius. For example, the speed of the stale air intake impeller will be at mid range when the sensing means measures an outside air temperature of -15°
Celsius. If the outside air temperature reaches higher than -5° Celsius then the defrost cycle will not be initiated. The active cycle of the ventilation system will be activated otherwise the ventilation system operates for an active cycle of 20 minutes and then activates a defrost cycle for 5 minutes at variable motor speeds of the stale air intake.
The defrost cycle will operate for 5 minutes during the first 24 hours of continuous active cycles and defrost cycles. If the continuous active~and defrost cycles persist for 24 hours, the defrost cycle is then increased to 6 minutes for the following 24 hours. As such, in one embodiment of the present invention, the defrost cycle can vary within 48 hours of continuous active 20 minutes cycles each followed by 5 minutes defrost cycles wherein the defrost cycle is increased to 6 minutes after 24 hours of continuous active and defrost cycles.
With further reference to Figure 4, the contact area 140 is generally located the area where defrost will be generated. During the defrost cycle, frost will melt creating water which may be evacuated from the HVAC unit 150 through a drain 160.
In one embodiment of the present invention, prior to commencing the defrost cycle, the defrost high speed limits are determined by the processing means. These high speed defrost limits are when maximum power is applied to the motors for the defrost cycle, i.e. to the stale air impeller. Various parameters are measured and considered by the processing means in order to determine the high defrost speed limits.
The processing means also has a table of speed indexes programmed within the processing means during the manufacturing process. The table of speed indexes may also be changed by a trained technician after installation of the proportional control. The table of speed indexes has various speeds which are used to apply different speeds to the motors or impellers in an HVAC unit incorporating the proportional control system of the present invention. A worker skilled in the relevant art would be familiar with the use of speed indexes to apply power to a motor or an impeller.
In one embodiment of the present invention and with reference to Figure 5, the processing means determines the defrost high speed limits. The first step 200 requires that the speed of the fresh air impeller be measured. At step 210, if the speed of the fresh air impeller is greater or equal than to a pre-determined speed set during manufacturing of the proportional control system, then the defrost high limit is equal to the value X at step 220. The value X is a value within a table of speed indexes as described above wherein the values within the speed indexes can be modified.
Otherwise, the defrost lugh limit is the value Y at step 230 if the fan speed of the fresh air intake is less than the pre-determined speed. The value Y can also be found in the speed index table. The processing means completes the determination of the high speed defrost limit at step 240. The defrost low limit is always set at the speed programmed during manufacturing for the continuous speed of the fresh air intake impeller during operation of the HVAC unit. As such, the defrost high limit can be two values. The use of different defrost high speed limits enables to better control the defrost cycle in different climates found around the world. In another embodiment of the present invention, the processing means may alternatively measure the speed of the fresh air impeller when the impeller is activated in the override mode in order to determine the high speed defrost limit. The speed of the motor in the overnde mode can also be found in the speed index table.
In another embodiment of the present invention, the defrost high limit can be various values since the processing means can be programmed to apply various defrost high limits based on the measurement of the fresh air impeller speed at various stages of use. A worker skilled in the relevant art would understand how to modify the defrost high limits based on various motor speed as measured for the fresh air intake impeller.
Once the defrost high limits are established, the speed required to conduct the defrost operation is calculated through the use of a formula. The calculation of defrost speed is based on various factors such as the actual temperature, temperature at which the defrost cycle is commenced and a maximum temperature setting for maximum speed for the defrost cycle. The equation is then used to calculate the defrost speed. The formula is as follows:
Defrost speed = INT(K Mult x (Defrost High Limit - Defrost Low Limit)) +Defrost Low limit For example, the following values can be applied to the equation parameters stored in the processing means:
Def Temp -5°C Temperature at which the defrost cycle is initiated Max Temp - -20°C - Minimum temperature at which defrost is at maximum power Temp = Actual Temperature Value K Mult = (Abs(Temp)) / (Abs(Max Temp)) Once the result is calculated, the defrost speed is applied to the stale air intake impeller in order to remove the defrost from the contact area.
In one embodiment of the present invention and with reference to Figure 6, a ventilation system encompasses a proportional control system for a defrost cycle to be applied to a motor. The fresh air intake 10 pushes air through a heat exchanger 30.
The air is pushed in the heat exchanger 30 through the use of an impeller not shown.
The heat exchanger 30 has a square shape and can be made of plastic. Once air passes through the heat exchanger 30, the air is then circulated within the ventilation system through the air exhaust 40. Stale air is removed from the ventilation system through the stale air intake 50 with an attached impeller not shown. The stale air is then passed through the heat exchanger 30 and evacuated outside the building through the stale air exhaust 70. A damper mechanism 250 is also installed in the HVAC unit 5. The use of such a damper is required since under this embodiment, the defrost cycle uses a bypass system composed of a re-circulated air inlet 260. A certain time lag is caused when closing the mechanical damper. During this period cold air could be introduced at high speed without recovery directly into the dwelling. To prevent this situation, a mechanism was designed to compensate for the time lag caused by the activation of the mechanical damper. This mechanism uses the fresh air inlet temperature sensor (not shown) to detect if the mechanical damper 250 has closed or not. By continuously sensing the probe, it is possible to determine if the damper 250 has closed completely by sensing any rise in the air inlet temperature. Under this embodiment, the defrost cycle starts by shutting down the fresh air intake impeller and activating the bypass damper 250. During this period the stale air impeller (not shown) at the stale air intake 50 is set to run at the calculated defrost speed as described above. The defrost cycle will operate in this fashion until the processing means senses a rise in the air inlet temperature. Once such a rise in temperature as occurred since the damper has been completely closed as shown in Figure 7, the stale air impeller is stopped and the fresh air intake impeller is activated at the defrost speed calculated. The damper 250 closes the fresh air inlet but still enables the fresh air impeller to vacuum air into the HVAC unit from the re-circulated air inlet. A
worker skilled in the relevant art would be familiar with the various types of dampers that can be used to achieve this operation. The use of a damper 250 mechanism also enables a fail safe mechanism in case the damper mechanism fails.
In another embodiment of the present invention, an impeller could be installed in the re-circulated air inlet. As such, the damper could completely close the fresh air intake. The installation of an impeller in the re-circulated air inlet would not require the use of the fresh air intake impeller.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A proportional control system for use in an HVAC unit within a ventilation system comprising:
a) a processing means programmed with an air exchange - defrost cycle;
b) one or more motor speed sensing means positioned in the HVAC unit and operatively connected to the processing means; and c) one or more temperature sensing means positioned in the HVAC unit and operatively connected to the processing means wherein the temperature and motor speed sensors determine the motor speed to be applied during the defrost cycle.

2. The proportional control system of claim 1 wherein a first temperature sensing means is positioned the fresh air inlet intake of the HVAC unit.

3. The proportional control system of claim 1 wherein a first speed sensing means is positioned on the fresh air intake impeller.

4. The proportional control system of claim 1 wherein a second sensing means is positioned in the stale air intake of the HVAC unit.

5. The proportional control system of claim 1 wherein a damper mechanism is installed in the air inlet intake of an HVAC unit.

6. A proportional control system for use in an HVAC unit within a ventilation system comprising:

a) a processing means programmed with an air exchange-defrost cycle;
b) one or more sensing means positioned in the HVAC unit and operatively connected to the processing means; and c) a damper mechanism positioned in the HVAC unit;
wherein the temperature sensing means determine the motor speed to be applied during the defrost cycle.

7. The proportional control system of claim 6 wherein a first temperature sensing means is positioned in the air inlet intake of the HVAC unit.

8. The proportional control system of claim 6 wherein a motor speed sensor is incorporated within the processing means.

9. The proportional control system of claim 6 wherein a damper mechanism is installed in the air inlet intake of an HVAC unit.