US20080283033A1

US20080283033A1 - Forced air furnace

Info

Publication number: US20080283033A1
Application number: US11/750,069
Authority: US
Inventors: Bertrand Michaud; Richard Breton
Original assignee: Stelpro Design Inc
Current assignee: Stelpro Design Inc
Priority date: 2007-05-17
Filing date: 2007-05-17
Publication date: 2008-11-20
Also published as: CA2630260A1

Abstract

A forced air furnace comprises an enclosure, an adjusting mechanism, an interface, an electric heating element and a fan. The enclosure has an air inlet and an air outlet. The adjusting mechanism is operative to adjust the air outlet between an open position and a partially closed position. The interface is located proximate to the air outlet and is used to connect the air outlet to a duct. The fan is powered by an electric motor. Both the fan and the electric heating element are located inside the enclosure. There is also described a method of installing a forced air furnace. The method comprises the step of adjusting a size of an exhaust area of an air outlet of the forced air furnace in order to produce a desired exhaust airflow through the exhaust area when the forced air furnace is connected to a ductwork.

Description

FIELD OF THE INVENTION

The present invention generally relates to the field of forced air furnaces. More specifically, the invention relates to a forced air furnace having an adjustable air outlet.

BACKGROUND OF THE INVENTION

Forced air furnaces are used both in residential homes and commercial buildings. These forced air furnaces are typically connected to a ductwork that conveys heated air to different rooms in the building. The length and complexity of these ductworks vary considerably depending on the building requirements.
The forced air furnaces are designed such that, in operation, their internal pressure, airflow for a given motor speed, and airflow direction are fixed. Only the fan motor speed may be adjusted during installation. Indeed, depending on the ductwork, which provides a back pressure on the forced air furnace, the fan motor speed may require adjustment so that it is capable of pushing air at a required flow rate in all the rooms of the building. In order to accommodate different installations, with different ductworks, conventional forced air furnaces design provides a few pre-set fan motor speeds. These pre-set fan motor speeds translate into different discrete flow rates for a given ductwork. However, it may happen that the ideal flow rate in a building, considering air speed and temperature difference, may not be available from the forced air furnace. Nevertheless, the installer having no further choice, selects the fan speed producing the closest airflow to what is required. The system is therefore roughly balanced and the fan motor speed is set once and for all during installation. Because of this limited adjustment capability, some forced air furnace installations are less than ideal. Indeed, the forced air furnace may provide too little or too much air in the rooms, the air may be coming in the room too fast or not warm enough, etc. In the end, the lack of possibilities of conventional forced air furnaces generates a lack of comfort for the building occupants. Further to not having a properly calibrated air heating system in some installations, situations may arise where, at a required fan motor speed, the fan motor is either too lightly or too much loaded, drawing much current and approaching its maximum recommended operating amperage. Such a situation may happen when, for example, the forced air furnace, using a powerful fan motor, is installed in a simple ductwork. In these situations, the fan motor operates outside its ideal operating range, which may be detrimental to its durability. Furthermore, because more current is drawn by the fan motor, energy cost is higher.
There is therefore a need for an improved forced air furnace capable of being better tailored to the specific requirements of certain building installations.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a forced air furnace that overcomes or mitigates one or more disadvantages of known forced air furnaces, or at least provides a useful alternative.
The invention provides the advantages of allowing a more accurate calibration of a forced air furnace.
In accordance with an embodiment of the present invention, there is provided a forced air furnace comprising an enclosure, an adjusting mechanism, an interface, an electric heating element and a fan. The enclosure has an air inlet and an air outlet. The adjusting mechanism is operative to adjust the air outlet between an open position and a partially closed position. The interface is located proximate to the air outlet and is used to connect the air outlet to a duct. The fan is powered by an electric motor. Both the fan and the electric heating element are located inside the enclosure.
In accordance with another embodiment of the present invention, there is provided a method of installing a forced air furnace. The method comprises a step of adjusting a size of an exhaust area of an air outlet of the forced air furnace in order to produce a desired exhaust airflow through the exhaust area when the forced air furnace is connected to a ductwork.

BRIEF DESCRIPTION OF DRAWINGS

These and other features of the present invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

FIG. 1 is a perspective view of a forced air furnace in accordance with an embodiment of the present invention;

FIG. 2 is a perspective view of a forced air furnace in accordance with another embodiment of the present invention;

FIG. 3 is a perspective view of a forced air furnace in accordance with another embodiment of the present invention;

FIG. 4 is a perspective view of a forced air furnace in accordance with another embodiment of the present invention;

FIG. 5 is a schematic top view of a typical forced air furnace installation;

FIG. 6 is an excerpt of an example of a performance table produced by forced air furnaces manufacturers; and

FIG. 7 is a diagram of an adjustment method for the forced air furnace of FIG. 1 in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The forced air furnace of the present invention is provided with added adjustment mechanisms, allowing better tailoring of an airflow rate, airflow speed, air temperature and fan motor amperage. This added adjustability eventually translates to a better comfort for occupants of a building.
FIG. 1 generally depicts a forced air furnace 10. The forced air furnace 10 comprises an enclosure 12 in which an air inlet 14 and an air outlet 16 are provided. An interface 18 is also made in the enclosure 12, all around the air outlet 16. This interface 18 is used to connect the forced air furnace 10 to a duct 17, shown in dotted lines in this figure for clarity reasons. The enclosure 12 contains an electric heating element 20, such as a resisting element, and a fan 22 powered by its own electric fan motor 24. An outlet adjusting mechanism 26 is placed in the air outlet 16 to vary the effective size, or exhaust area, of the air outlet 16. This way, the outlet adjusting mechanism 26 is capable of adjusting the size of the air outlet 16 from an open position to a partially closed position.
Similarly to known forced air furnaces, air is drawn by the fan 22 through the air inlet 14, and propelled on the heating element 20. The heating element 20 may be an electric heating element as is well known in the art. The air is heated by the heating element 20 and is then channeled towards the duct 17 through the air outlet 16. Up to this point, the forced air furnace 10 is very similar to known forced air system. The difference, thought, is in the presence of the outlet adjusting mechanism 26, which partially blocks the exhaust area of the air outlet 16, thereby effectively partially closing the air outlet 16.
The outlet adjusting mechanism 26 of the air outlet 16 may take different forms. For example, the adjusting mechanism could be an iris-like mechanism, one or many louvers, a pivoting or sliding door, various sizes of plates placed across the exhaust airflow, etc. A person skilled in the art could envision many such different outlet adjusting mechanisms. In an embodiment of the present invention, louvers 28 running across the air outlet 16 are used as the outlet adjusting mechanism. There are six louvers 28 shown in FIG. 1. Although the louvers 28 may be controlled independently, the louvers 28 may also be paired symmetrically starting from a center of the air outlet 16. Pairing the louvers 28 ensures that the air is directed symmetrically towards a center of the duct 17. Hence, there are a first pair of louvers 30, a second pair of louvers 32 and a third pair of louvers 34. Each pair of louvers 30, 32, 34 may be adjusted independently at varying angles, ranging from an angle of 90 degrees with respect to a direction of an outgoing airflow to 0 degree, or aligned, with the direction of an exhaust airflow. Hence, if all louvers 28 are closed, or at 90 degrees with the exhaust airflow, the only exhaust area left is an unobstructed area 36. Conversely, if all louvers 28 are aligned with the exhaust airflow, the exhaust area is at its maximum. In between, each pair of louvers 30, 32, 34 may be adjusted independently to suit the specific needs of an installation in a building, as will be described later.
The louvers 28 may be automatically, passively or manually adjusted. FIG. 2, now referred to, depicts an embodiment of the forced air furnace 10 where the louvers 28 are automatically adjusted. In this embodiment, one or many sensors may be used to send parameter signals to a controller 36, which in turn automatically adjusts the louvers 28. The controller 36 decides by how much the air outlet 16 must be open or closed, based on parameters such as air temperature, pressure inside the forced air furnace 10, exhaust airflow across the air outlet 16 or current drawn by the fan motor 24. The controller 36 may also use a combination of these parameters to control the louvers 28. A temperature sensor 38, a pressure sensor 40, an airflow meter 42 and a current meter 44 send their own parameter signal 46, which respectively conveys information on an air temperature, a pressure, an airflow rate and a current value, to the controller 36. The controller 36 computes the data from one or more of the parameter signals 46 received and selects the best setting of louvers opening, based on a performance table, as will be described in more details later. For convenience, this performance table may be stored in an internal memory of the controller 36. The controller then outputs a control signal 48 to one or more actuators 50. Optionally, the controller may also control the speed of the fan motor 24 through a fan speed signal 52, also based on the performance table.
Advantageously, with the forced air furnace of the present invention, it is possible to more precisely adjust the airflow rate by adjusting the air outlet 16. In a specific example where louvers 28 are used in the air outlet 16, the louvers 28, and thereby the exhaust area, are adjusted to provide an adequate amount of exhaust airflow rate. Adjusting the louvers 28 influences the backpressure in the forced air furnace which, added to the static pressure, influences the airflow rate, air temperature and amperage and speed of the fan motor 24. If it is desired to keep the exhaust airflow straight, the louvers 28 may be adjusted in corresponding pairs since both louvers of a pair are positioned equidistantly from a center of the air outlet 16.
FIG. 3, now referred to, depicts an embodiment of the forced air furnace 10 where the louvers 28 are manually adjusted. In this embodiment, the louvers 28 are mounted such that they can selectively be rotated and then locked in place by an installer. Mechanisms permitting such adjustment are multiple and may be as simple as having the louvers 28 equipped with threaded ends 54 at each end of their rotation axis, going through the enclosure 12 and locked in place with nuts 56. A person skilled in the art could easily envision different types of mechanisms capable of providing such a function and therefore, the details of such a mechanism will not be discussed in further details.
Reference is now made to FIG. 4. Another way of adjusting the outlet adjusting mechanism 26, or more precisely the louvers 28, is to provide a biasing mechanism 58 between the louvers 28 and the enclosure 12. At rest, the louvers 28 are biased in the closed position by the biasing mechanism 58. When in use, the airflow of the forced air furnace 10 presses against the louvers 28, which opens them. The biasing mechanism 58 may be one or more ordinary springs. One way of setting up the biasing mechanism 58 so that the first, second and third pairs of louvers 30, 32, 34 open gradually and somewhat sequentially is to equip each pair of louvers 30, 32, 34 with springs having a different spring constant. For example, the pair of louvers 30 may be equipped with a spring having a lower spring constant than the spring installed on the pair of louvers 34 and the pair of louvers 32 may be equipped with a spring having a spring constant in between the two others. Hence, in use, a small airflow opens, to a certain degree, mostly the pair of louvers 30 while the pair of louvers 32 and 34 are not significantly opened. A larger airflow completely opens the pair of louvers 30 and deflect to some degree the pair of louvers 32 while the pair of louvers 34 is barely opened. Similarly, a still larger airflow opens both to a large degree the pair of louvers 30 and 32 while the pair of louvers 34 are opened to some degree. Providing the right calibration of spring constants allows such behavior of the pairs of louvers 30, 32, 34 and consequently allows the forced air furnace 10 to operate within its operating parameters, as will be discussed in more details later.
Generally speaking, the more the louvers 28 close the air outlet 16, the higher is the pressure inside the forced air furnace 10, the warmer is the air temperature, the lower is the exhaust airflow rate and the higher is the fan motor speed.
The forced air furnace 10 is equipped with pre-set fan motor speeds. There may be more or less choice of fan motor speeds. For example, these could be Low, Medium-Low, Medium-High and High. These different fan motor speeds are used to adjust the airflow rate. Although the fan motor speed is adjusted to any one of these settings, the exact speed of the fan motor always slightly fluctuates as a function of the pressure inside the forced air furnace 10, which in turn is affected by the degree with which the louvers 28 are closed.
Reference is now made to FIG. 5, which depicts a typical installation of the forced air furnace 10 in a building 58. In such installation, the forced air furnace 10 is connected to a ductwork 60. At least one ductwork outlet 62 is located in each room 64. Not shown in FIG. 5, for reasons of clarity, is a return air ductwork, which routes back air from each room 64 to the forced air furnace 10 so that the air in the rooms is renewed periodically. The ductwork 60 creates a resistance on the forced air furnace 10, which increases the pressure both at the air outlet and inside the forced air furnace 10. Since this resistance, or static pressure, is proportional to the length, size and configuration of the ductwork 60, the forced air furnace 10 often necessitates adjustment on the site since not all ductwork 60 create the same static pressure. Indeed, the static pressure depends on many parameters such as the ductwork 60 length, its cross-section, bends in the ductwork 60 and other restriction, such as filters, adding to the static pressure. This adjustment is part of the forced air furnace installation method, which will now be described. A forced air furnace having manual adjustments is considered first in the following example.
FIG. 6, which presents an excerpt of a performance table, and FIG. 7, which schematically represents the adjusting method, are now concurrently referred to. Although, initially, it does not absolutely have to, it may be more convenient to first connect the forced air furnace 10 to a ductwork 56 to ease the measurement of the static pressure of the ductwork 56. Based on the static pressure measurements and in accordance with the manufacturer's performance table, the fan motor speed is selected to provide an adequate airflow rate for the building. At this point, the forced air furnace 10 needs to be connected to the ductwork 56. Then, the heating element 20 is turned on so that the following elements may be measured: airflow rate, heating element amperage, electric voltage of the forced air furnace, exhaust air temperature, and incoming air temperature. If all measures are satisfactory, then no further adjustment is necessary and the forced air furnace 10 would be ready for use. However, in real life, this is rarely so. Reference to the performance table of FIG. 6 is then made to select the right operating parameters. For example, assuming that the heating element 20 of the forced air furnace 10 has a power of 15 kW and that the static pressure is 0.5 wc (inches of water column), required airflow rate of 925 cfm (cubic foot per minute) and a required temperature difference (ΔT) between the exhaust and an incoming airflow of 45° F. to 60° F., adequate settings of the forced air furnace must be selected. Different solutions exist: first, the fan speed may be set at Med-Hi and the first pair of louvers 30 may be set at 30° from the direction of the exhaust airflow while the others are closed. Adjusting the opening of the louvers 28 adjust the exhaust area. This provides an exhaust airflow of 917 cfm and a ΔT of 49° F. Another solution is to set the fan speed at Med-Low and set the first pair of louvers at 0° from the direction of the exhaust airflow. This setting yields an airflow rate of 929 cfm and a ΔT of 48° F. This setting would even work better since it will be quieter and operate at a lower fan motor amperage. Airflow rate, heating element amperage, electric voltage of the forced air furnace, exhaust air temperature, and incoming air temperature may be measured again to confirm that the adjustments to the forced air furnace 10 worked as supposed to. If not, the forced air furnace 10 may need further adjustment.
If the forced air furnace 10 is provided with a controller 36 and one or more temperature sensors, then the measurement step may be done completely or partly (if not all sensors are present) automatically. As previously described, the controller 36 then sends the control signal 48 and fan speed signal 52 so that the forced air furnace is properly calibrated and ready for use.
The ΔT is very subjective as it is a question of comfort which not only varies from one building occupant to another, but also depends on ambient temperature, humidity level, airflow speed, etc. ASHRAE recommends a method to evaluate comfort so that a ΔT may be more easily determined.
It is possible to see, in the performance table of FIG. 6, that some settings do not have an electric current value. This means that at that particular setting, the fan motor 24 would exceed its maximum rated amperage and damage could occur. Hence, these values are not provided in the performance table so that the installer cannot select them. The temperature values that would fall within the allowable working range of the forced air furnace are located within the bold lines. It may be seen that in some circumstances, more than one setting is possible. In these cases, it may be preferable to select the setting where the amperage of the fan motor is lower.
In the case where the forced air furnace 10 is automatically adjusted, the controller 36 receives parameter signals 46 from one or many sensors such as the temperature sensor 38, the pressure sensor 40, the airflow meter 42, the current meter 44 or any combination thereof. Upon installation, the controller 36 verifies which parameter signals 46 are available, checks whether it has sufficient information to adjust the forced air furnace 10 and if information is missing, requests the user to input the missing information so as to be able to complete its task of adjusting the forced air furnace 10. Once all information about the required parameters is available, the controller 36 consults the performance table and selects the appropriate louvers and fan motor speed adjustments. This selection is basically made in the same way the installer would manually select the adjustment based on the same performance table. If more than one choice is available to the controller 36, the controller 36 may select the one that is more energy efficient, or the one that provides more comfort (i.e. quieter, meaning a lower fan speed) to the occupant of the building. Similarly to the manual installation, the controller 36 would not select adjustments that would exceed the maximum amperage of the fan motor 24.
The present invention has been described with regard to preferred embodiments. The description as much as the drawings were intended to help the understanding of the invention, rather than to limit its scope. It will be apparent to one skilled in the art that various modifications may be made to the invention without departing from the scope of the invention as described herein, and such modifications are intended to be covered by the present description.

Claims

1. A forced air furnace comprising:

an enclosure having an air inlet and an air outlet;

an adjusting mechanism for adjusting said air outlet between an open position and a partially closed position;

an interface proximate said air outlet for connecting said air outlet to a duct;

an electric heating element inside said enclosure; and

a fan powered by an electric motor, said fan being inside said enclosure.

2. The forced air furnace of claim 1 further comprising a controller for controlling said adjusting mechanism.

3. The forced air furnace of claim 2 wherein said controller is further operative to control a speed of said fan.

4. The forced air furnace of claim 3 further comprising a temperature sensor for measuring a temperature inside said enclosure and outputting a temperature signal to said controller, said controller being operative to adjust said adjusting mechanism and said fan speed based on said temperature signal.

5. The forced air furnace of claim 4 further comprising an airflow meter for measuring an airflow and outputting an airflow signal to said controller, said controller being operative to adjust said adjusting mechanism and said fan speed based on said airflow signal.

6. The forced air furnace of claim 5 wherein said airflow meter is located proximate said air outlet.

7. The forced air furnace of claim 4 further comprising a pressure sensor for measuring an air pressure inside said enclosure and outputting a pressure signal to said controller, said controller adjusting said adjusting mechanism based on said pressure signal.

8. The forced air furnace of claim 4 further comprising a current sensor for reading a current value of said electric motor and outputting a current signal to said controller, said controller being operative to adjust said adjusting mechanism and said fan speed based on said current signal.

9. The forced air furnace of claim 4 wherein said adjusting mechanism comprises louvers.

10. The forced air furnace of claim 1 wherein said adjusting mechanism comprises louvers, said louvers being manually adjustable in pairs.

11. The forced air furnace of claim 1 wherein said adjusting mechanism comprises louvers and a biasing mechanism attached to said louvers, said biasing mechanism being operative to bias said louvers in a closed position.

12. A method of installing a forced air furnace comprising the step of adjusting a size of an exhaust area of an air outlet of said forced air furnace to produce a desired exhaust airflow at a selected fan speed through said exhaust area.

13. The method of claim 12 wherein said step of adjusting is performed based on a temperature inside said forced air furnace.

14. The method of claim 13 wherein said step of adjusting is performed based on an airflow through said air outlet.

15. The method of claim 14 wherein said step of adjusting is performed based on a pressure inside said forced air furnace.

16. The method of claim 15 wherein said step of adjusting is performed based on a value of a current drawn by said electric fan motor inside said forced air furnace.

17. The method of claim 16 wherein said step of adjusting comprises adjusting louvers.

18. The method of claim 12 wherein said step of selecting a fan speed is performed based on a pressure inside said forced air furnace.

19. The method of claim 18 wherein said fan speed is selected based on an airflow through said air outlet.

20. The method of claim 12 wherein said step of adjusting is performed once said forced air furnace is connected to a ductwork.