Direct-fired heating system

Abstract

A direct gas-fired heating unit is provided which uses an axial-bladed fan for generating a main air flow through a burner unit. The burner unit has a dedicated combustion air blower supplying combustion air to the burner separate from a main air flow generated by the main fan. The burner unit includes a dedicated blower which draws a small volume of outside air as combustion air. Further, the burner unit may include bypass ports which allows for discharge of bypass air from the burner unit.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser. No. 61/216,349, filed May 15, 2009, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a heating system for heating large commercial and industrial spaces, which building spaces require heating relatively large quantities of air and distributing such air from a heating unit throughout the area. More particularly, the invention relates to a direct gas-fired heating unit which recirculates and heats interior air while minimizing the introduction of outside air.

BACKGROUND OF THE INVENTION

Two general types of heating units for large spaces are direct gas-fired and indirect gas-fired heating units. The two heating types are discussed as follows.

In an indirect gas-fired unit, such units distribute heated air to a large industrial or commercial building space. In one general construction of such a system, the heated air can be distributed through ductwork. Alternatively, the heating unit may be formed as an air-turnover or air-rotation unit which draws cold air through the bottom of the unit and pushes heated air out the top of the unit which distributes the heated air throughout the building space without the use of any additional ductwork. An example of one such system is disclosed in U.S. Pat. No. 4,480,629 (Williams).

An indirect heating unit operates on the principal where a burner is separated from the air flow such that the burner heats a chamber forming part of a heat exchanger. The walls of the heat exchanger are heated, and then air flow is generated by a blower so as to flow over the heat exchanger to pick up the heat from the heat exchanger. The heated air is then distributed or recirculated to the building space.

In this system, the heat exchanger separates the combustion chamber as well as any combustion byproducts produced therein from the air flow distributed through the building. Rather, combustion air and any byproducts are exhausted to the outside, although this venting may create a negative building pressure due to some loss of building air. Therefore, the heated air is maintained separate by drawing in return air from a building, passing the air flow over the heat exchanger thus heating the air, and then distributing such heated air to the building space.

As an alternative to indirect fired units, a direct gas fired unit uses some outside combustion air supplied to a burner but the burner is highly efficient and the heated air is mixed directly in the air flow being distributed to the building. Because the burner is able to burn clean with a relatively low amount of combustion byproducts, combustion byproducts do not need to be exhausted. Rather, a relatively large amount of outside air is drawn into the combustion chamber which mixes with the byproducts and ensures that the concentration of byproducts meets acceptable environmental standards. The heated outside air mixes with return air, i.e. interior air which has been cooled in the building space, wherein the return air is mixed with the heated outside air proximate the burner to heat same, and then recirculated by a blower back to the building space.

FIG. 1 illustrates an air-turnover configuration having a direct gas-fired heating unit 10 located in a building space 11 where there is a turnover of unheated room or return air 12 drawn into the bottom of the heating unit 10 and heated air 14 is discharged to the building space 11 from the top of the heating unit 10. The return air 12 is considered to be cooler air but is still at a temperature greater than outside environmental air located on the building exterior.

The recirculation of cooler interior return air 12 and heated air 14 is diagrammatically shown by arrows 12 and arrows 14, some of which arrows 14 terminate in ball-like shapes representing dispersion of heated air. In known units, a relatively large quantity of outside air 15 is drawn in as shown by arrows 15 as combustion air of about 15-40% of the air flow. Since this outside air 15 is drawn from the building exterior, this air is considered to be cold air that is at the temperature of the outside environment. Hence, this volume of air has the greatest amount of heating required to heat same to the level needed to maintain room temperature. Due to the volume of outside air 15, the building room pressure is elevated which tends to also cause leakage of interior air through building penetrations, such as doors 16 and windows 17, cracks and the like wherein air leakage is indicated by arrows 18.

In some diagrams of a heating system which represent the air flows in color, the outside air 15 would be shown as blue since it is unheated, the return air 12 as orange since it has cooled but is still heated in comparison to outside air, and heated air 14 as red since it is heated to an elevated temperature necessary to maintain the room temperature as the heated air 14 is dispersed throughout the building volume.

In one known gas-fired heating unit, air is drawn into the burner unit, with the flow of heated air mixing with the building air for subsequent discharge to the building. This unit uses a backward-inclined airfoil fan downstream of the burner unit which pulls air through the burner unit, and pushes the discharge out of the unit. This single fan generates a large air flow which draws in both the return air as well as the relatively large volume of outside air. Such direct fired units essentially have 100% heat transfer to the air flow, as compared to indirect fired units which may be 80% efficient or lower due to inefficiencies in the heat exchanger configuration and exhausting of combustion gases. However, in the known direct fired heating units of the present assignee, these units still draw outside air to the combustion unit of about 15-40%.

For example, U.S. Pat. Nos. 3,630,499 (Kramer) and 4,993,944 (Potter et al.) disclose direct gas-fired heating units which use a large blower to supply combustion air to respective burners. The burners herein have inlet openings adjacent the burner flame which receives the main air flow therein as combustion air and are also believed to induce intake of a substantial amount of outside air.

In another example, U.S. Pat. No. 5,865,618 (Hibert) discloses a fan blowing a substantial amount of outside air T1 into a burner as combustion air.

Another example of a gas furnace using outside air drawn from the exterior of a building is disclosed in U.S. Pat. No. 3,591,150 (Weatherston).

One disadvantage of these known systems is the volume of outside air required to operate, which unheated air must be heated, and the flow of which increases building pressure and thus, increases the amount of leakage of building air back to the environment.

It is therefore an object of the invention to provide an improved direct gas-fired heating unit which overcomes disadvantages associated with known heating systems of this type.

The invention relates to an improved direct fired heating unit which uses an improved fan configuration and burner unit. This heating unit has a lower section, namely a filter/fan section which draws in air with an energy-efficient axial-bladed fan generating a main air flow, a middle section containing a burner unit having a dedicated combustion air blower supplying a relatively small flow of combustion air to the burner, a burner section where the burner flame and main air flow mix, and a discharge plenum which discharges and distributes the heated air to the building space.

The burner unit does not rely upon the main air flow from the main fan for combustion air, but instead relies upon its own dedicated blower which draws in a relatively small volume of outside air and feeds same directly to the burner flame. In particular, the burner unit has a gas-fed burner which generates a flame between two angled mixing plates wherein the burner flame is formed on the interior side of the mixing plates. The combustion air is supplied to the outside of the mixing plates wherein the mixing plates include perforations to allow for controlled passage of air flow into the flame region. The burner unit further includes a containment box surrounding the mixing plates and burner which isolates the combustion air flow from the surrounding main air flow, wherein the burner is supplied with a piped supply of outside combustion air by a relatively small combustion air blower. This outside air makes up only 1-5% of the heating unit air flow since it is directly piped to the burner.

This burner unit is positioned in the main air flow downstream of the main fan but the main air flow mixes at the burner flame and is not supplied as combustion air. In operation, the flame forms in the burner section, with the main air flow flowing upwardly from the filter/fan section through the burner section for heating, and then to the discharge plenum. The intake air or main air flows directly over and about the burner housing and flame for direct heating of the air flow.

The filter/fan section includes the axial bladed fan on the colder upstream side of the burner which generates the air flow but also avoids any problem with undue heating of the fan. The axial bladed fan also has greater efficiency and uses less KWH than a centrifugal fan that has been previously used on the downstream hot side of known direct fired units.

Also, the burner assembly uses the isolated supply of outside air (OA) such that less than 1-5% of the air flow is from outside air. This generates a slight positive pressure in the building which results in less leakage than known systems, and increases efficiency since most of the air flow input into the heating unit is building air that already is at a higher temperature than outside air.

While all of the blower air flow can be supplied at a rate which supplies the necessary flow of combustion air, the blower air also preferably has a portion of such blower air flow diverted back into the main air flow. In this regard, the blower flow path can be provided with exit ports that allow for a selected portion of blower flow to divert to the main air flow as bypass air flow, while a second portion of blower air flow continues to the burner as the combustion air flow. By generating the bypass air flow, the concentration of combustion byproducts can be maintained at a low level that satisfies environmental regulations. Further, the diversion ports may have controllable openings which control the amount of diversion air flow, for example, to adapt the same burner unit to different sized heating units.

Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating a building with a direct gas-fired heating unit therein.

FIG. 2 is a front elevational view of a heating unit of the invention.

FIG. 3 is a top view thereof.

FIG. 4 is a perspective view of the heating unit of the invention.

FIG. 5 is an enlarged view of a burner unit of the invention.

FIG. 6 is a perspective view of the burner unit.

FIG. 7 is a side elevational view of the burner unit.

FIG. 8 is a bottom view of the burner unit.

FIG. 9 is a perspective view illustrating an alternate embodiment of the burner unit.

FIG. 10 is an end view thereof.

FIG. 11 illustrates a second alternate embodiment of the invention.

Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.

DETAILED DESCRIPTION

Referring to FIGS. 1-3, the invention relates to a direct-fired heating unit 20 which preferably uses a combustible gas such as natural gas to heat a building space 11 (FIG. 1). The heating unit 20 of the invention is usable in a similar manner to that described above relative to the heating unit 10 illustrated in FIG. 1. Hence, a detailed discussion as to operation of the heating unit 20 within a building space 11 is not required based upon the foregoing description of such environment and the operation of direct-fired heating units 10 in such building spaces 11.

The inventive heating unit 20 uses an improved fan configuration and burner arrangement which has improved energy efficiency and minimizes the introduction of outside air used in the burner configuration so as to increase the overall efficiency of the heating unit 20.

More particularly as to FIGS. 2-4, the heating unit 20 generally has a box-like, upstanding construction defined by an exterior housing 22 formed in a box-like shape with a hollow interior 23 which is hollow along the vertical length of the housing 22 to allow for entry of the return air flow 24 at the bottom thereof and discharge of the supply air flow 25 at the top thereof (FIG. 4). The heating unit 20 is formed with a lower section 27 which is formed with rectangular openings on a plurality of sides thereof, in which openings air filters 28 are mounted. Rectangular air filters 28 are of a generally rectangular flat construction typical of filters of this type. Thus, upon the generation of an air flow within the heating unit 20, the return air 24 is pulled into the filter/fan section 27 through the filters 28 as generally illustrated in FIG. 4.

The upper portion of the filter/fan section 27 includes an internal support frame 29 which supports an axial-bladed fan 30 therein. The fan 30 includes a fan motor 31 (FIG. 2) which drives the fan blade 32 (FIG. 4) in a conventional manner. Operation of the fan 30 is controlled by a control unit 32 which controls the fan 30 in accord with thermostat temperature requirements of the building space 11.

The axial-bladed fan 30 has reduced kilowatt hour requirements and is more energy efficient than other fans used in heating units of this type. The fan 30 has a diameter which is closely proximate the width of the housing 22 so as to generate a substantial air flow through the housing interior 23. The amount of air flow generated by the fan 30 can vary depending upon the overall heating capacity of the heating unit 20 being constructed. It is understood that the heating unit 20 may have different capacities as to total air flow and BTUs of heat generated for distribution within the building space 11. Hence, the invention is not limited by the specific capacity of the heating unit 20, although it is understood that the heating unit 20 is of the large capacity type as used in industrial and commercial building spaces 11.

In one construction, the fan 30 can generate 40,000 cfm to generate a sufficient amount of air flow including air speed and distance of travel of the supply flow 25 so as to sufficiently disperse heat through the building space 11.

In another smaller heating unit, the air flow generated by the fan 32 can be about 25,000 cfm.

This fan 30 thereby generates a main air flow 45 (FIGS. 2 and 5) which preferably flows vertically from bottom to top through the housing 22.

The heating unit 20 further includes a middle section 35 which contains a burner assembly 36 therein which is located within the main air flow. The housing 22 in particular includes support frame members 37 which support the burner assembly 36 thereon. The burner assembly 36 comprises the burner unit 38 that is mounted to the support frame 37 and further includes a dedicated combustion air blower 39 supplying a relatively small flow of combustion air to the burner unit 38. This burner unit 38 generates a flame 40 to generate the heat necessary for heating the main air flow 45 in a burner section 41 that is located above the burner assembly 36. Preferably, the burner section 41 is enclosed on four sides and is that location wherein the burner flame 40 and main air flow 45 mixes to essentially heat the main air flow 45 prior to its passage to the discharge plenum 42 which thereby discharges the supply flow 25 to the building space 11 near the ceiling thereof. The discharge plenum 42 includes discharge ports 42 having adjustable grills 43 therein which control the discharge of the main air flow therethrough.

The positioning of the fan 30 on the cold or upstream side of the burner assembly 36 generates the air flow but also avoids problems with undue heating of the fan 30.

The burner unit 38 generally is positioned in the main air flow 45 downstream of the main fan 30, but the main air flow 45 mixes at the burner flame 40 although the main air flow is not supplied as combustion air. The flame 40 forms in the burner unit 38 with the main air flow 45 flowing upwardly from the filter/fan section 27 through the burner section 41 for heating, and then to the discharge plenum 42.

The heating unit 20 of the invention is particularly improved as to the arrangement of the burner assembly 36 and its positioning within the main air flow 45. This self-contained burner assembly 36 includes the aforementioned blower 39 which provides the combustion air supply to the burner unit 38. Referring more particularly to FIGS. 5-8, the burner unit 38 does not rely upon the main air flow 45 from the main fan 32 for combustion air which passes exteriorly of the burner unit 38, but instead relies upon its own dedicated blower 39 which draws in a relatively small volume of outside air 46 and feeds same directly to the burner flame 40. In particular, the burner unit 38 has a gas-fed burner 47 which receives gas 48A from a gas supply pipe 49 and discharges a gas flow 48B to generate a flame 40 between two angled or divergent mixing plates 50 wherein the burner flame 40 is formed on the interior side of the mixing plates 50 and distributed along a gas manifold 51. The combustion air 46 is supplied to an air mixing box 52 through the bottom thereof gas flow 53 and then to the outside of the mixing plates 50 as indicated by arrows 54 wherein the mixing plates 50 include progressively-increasing perforations 56 to allow for restricted passage of air flow 57 into the flame region. The burner unit 38 further includes a containment box 59 defined by box walls 60 surrounding the mixing plates 50 and burner 47 which isolates the combustion air flow 54 from surrounding air flow 45, wherein the burner 47 is supplied with a piped supply 53 of outside combustion air by the air blower 39. The outside air 46 makes up only 1-5% of the heating unit air flow since it is directly piped to the burner 51.

This burner unit 38 is positioned in the main air flow 45 downstream of the main fan 30 but the main air flow 45 mixes at the burner flame 40 as indicated by arrows 62 and is not supplied as combustion air. In operation, the flame 40 forms in the burner section 41, with the main air flow 45 flowing upwardly from the filter/fan section 27 through the burner section 41 for heating, and then to the discharge plenum 42. The intake air 24/45 flows directly over and about the burner housing 60 and flame 40 for direct heating of the air flow.

More particularly as to the combustion air blower 39, this blower 39 has a substantially lower capacity than the fan 30 in that this blower supplies an air flow which is less than 1-5% of the total air flow, which combustion air flow 46 is piped in as outside air. In particular, the blower assembly 39 includes an inlet pipe 66 (FIG. 5) which connects to and feeds the blower 39. This pipe 66 projects through the housing 22 and then extends to a source of outside air. In particular, the supply pipe 66 connects to a vertical pipe riser 67 which in one exemplary embodiment projects through the roof 68 and terminates at an inlet pipe 69 that receives outside air OA as indicated by inlet air flow 70 in FIG. 4. The intake pipe 66 is also shown in FIGS. 2 and 3 as extending horizontally through the housing 22. In this manner, the intake air 46 is isolated from the main air flow 45 and is solely supplied with outside air 70 which avoids mixing of the combustion air 54 with the main air flow 45. Referring to FIG. 8, the mixing box 52 includes an inlet opening 71 that opens vertically into the chamber defined by mixing box 52 and is in communication with air passages 72 (FIG. 6) which open vertically into the space defined between the containment box side wall 60 and the mixing plate 50. The combustion air flow 54 (FIG. 5) flows vertically through these air passages 72.

In the embodiment shown in FIGS. 5-8, the combustion air supplied to the burner unit 38 is completely isolated from the main air flow 45. Where the burner unit 38 generates a flame 40 that produces combustion byproducts that are at a sufficiently low level so as to satisfy environmental standards, the combustion air flow 46 can be kept at as low a rate as possible as dictated by the amount of combustion air 46/70 necessary to supply the flame 40. Where low amounts of combustion air are required to generate low amounts of combustion byproducts, the percent of combustion air 46 relative to the main air flow may be approximately 1% and possibly less. Preferably, the combustion byproducts produced as a result of the flame 40 are at a sufficiently low level so as to satisfy applicable environmental standards including NCANSI Z83.18b-2008 which governs recirculated air.

Where such standards are satisfied, it is preferable that all of or only a minimal amount of the combustion air is provided by outside air which typically is required to satisfy the environmental standards, and that this outside air be isolated from the main air flow so as to minimize the amount of intake of outside air necessary to operate the heating unit 20. This provides for substantially increased efficiency due to the reduction in total volume of outside air and the amount of outside air which requires additional heating by the system. Further, by minimizing the amount of outside air provided as combustion air, this minimizes the amount of building pressure, although some small amount of pressure is still generated since the outside air is still being mechanically pulled into the building space 11 by the blower 39. The substantially low volume of outside air percentage minimizes the building pressure so as to minimize the air leakage through building penetrations as described above.

Referring to FIGS. 9 and 10, an alternate embodiment of the invention is illustrated which uses an alternate burner unit 38-1. The burner unit 38-1 in most respects is formed the same as burner unit 38 described above. However, the air mixing box 52 is provided preferably with a plurality of bypass ports 75 which open sidewardly to generate a discharge of bypass air flow 76 therefrom.

More particularly, the burner unit 38-1 is formed substantially the same as burner unit 38 described above such that a detailed description of the component parts thereof is not required. This burner unit 38-1 as seen in FIG. 10 is disposed within the main air flow 45 to generate the flame 40 and mix the combustion air flow that is indicated by arrows 53, 54 and 57 with the gas flow 47 for generation of the flame 40 between the mixing plates 50.

However, it may be found that the combustion byproducts generated by the flame 40 when supplied with the completely isolated air flow 46 may still exceed environmental standards depending upon the burner unit 38 being employed. Hence, the provision of the bypass port 75 in the side walls of the mixing box 52 allows for a flow of bypass air 76 to be generated, which flow 76 exits through each port 75 for subsequent mixing with the main air flow 45. These discharge ports 75 are also diagrammatically illustrated in FIG. 7. As seen in FIG. 10, intake air 53 supplied by the blower 39 essentially enters the air mixing box 52 with a first portion of such air flow 77 flowing upwardly through the passages 72 (FIG. 6) to define the combustion air flow 54. However, a second portion of such combustion air flow 53 is then diverted as interior flow 78 so as to pass through the discharge ports 75 and define the bypass flow 76 described above. Preferably, the blower 39 is operated so as to generate an adequate air flow to feed the burner flame 40 which is the first component or portion of the intake flow 53 received from flow 46. The interior additional component of the inlet air flow 53, however, diverts as flow 78 so that the blower 39 essentially is supplying two components of outside air flow, the first component being a combustion air flow 54 and the second component being the bypass air flow 76 (FIG. 10).

According to the environmental standards, a particular threshold amount or concentration of the byproducts of combustion may be permitted with a flow of 1% of outside air, but the total permissible amount then increases if the percentage of outside air is also increased. Since the burner assembly 38 typically does not need the outside air beyond the amount necessary for combustion, the bypass air 76 can then be diverted into the main air flow 45 which therefore supplements the air flow and allows for an increased threshold value that is satisfactory to meet environmental standards. For example, if 2-3% of outside air is provided by the blower 39, only 0.5-1% may go through the burner unit 38 as combustion air while the remainder enters the recirculation air as the bypass air flow 76. Hence, a controlled amount of minimal bypass air is provided as outside air so that operation of the burner unit 38-1 may be specifically defined so as to satisfy environmental standards, particularly with respect to different size units which may have different requirements due to the different flow capacities and sizes.

In the one embodiment, the discharge or bypass ports 75 preferably are formed as three circular holes through the side walls of the mixing box 52 which holes 75 are provided on one or both of the opposite sides of such box 52. As such, the bypass flow 76 has a fixed amount defined by the total open area of the ports 75.

Referring further to the embodiment of FIGS. 9 and 10, one preferred embodiment preferably allows for controlled adjustment of the bypass flow 76. This is accomplished by the provision of a slide plate 81 on one or both sides of the mixing box 52. These slide plates 81 serve as gate dampers and have respective bypass holes 82 therein which are sized substantially the same as the ports 75. Each slide plate 81 is movable linearly by linear actuator 83 which drives the slide plate 81 sidewardly in a reciprocating manner to vary the overall alignment of the holes 82 with the bypass ports 75 which thereby varies the size or open area of the opening defined by the overlapped holes. Hence, the total amount of bypass flow 76 can be varied by selective positioning of the slide plate 81.

It is possible to fix these plates 81 prior to assembly so as to define a fixed opening size for the overlapping holes 75 and 82.

Also, the relative amount of overlap and hence the total passage size may also be controlled by a pressure-sensing arrangement. This arrangement includes tubing 85 that passes through a pressure transducer 86 that senses the pressure within the box 52 to ensure adequate pressure being supplied to the burner. The pressure transducer further is connected by wiring 87 back to the linear actuator 83 to selectively control the position of the slide plate and control the opening or closing of the holes 75 and 82. In effect, the slide plate 81 acts as a damper to govern the bypass flow 76.

Referring to FIG. 11, a further alternate heating unit 20-2 is illustrated which uses the burner 38 described above but has an alternate piping assembly 90 connected thereto. In particular, the blower 39 discharges to a piping T 91 that has a first leg passing to the inlet pipe 92 that supplies the combustion air flow 53 to the burner 38 and thereby generate the flame 40. The second leg of the T 91 connects to a discharge pipe 93 that allows for the passage of blower air therefrom to form the bypass flow 94. This bypass flow 94 is substantially the same as the bypass flow 76 described above as to the purpose and function thereof. Preferably, the outlet pipe 93 includes a damper 95 at the end which is rotatable to vary the discharge passage and the amount of bypass flow 94. This damper 95 may be manually set in a conventional manner, or may be electronically controlled in a manner similar to that described above by the pressure transducer 86 so as to automatically or electronically adjust the amount of such bypass flow 94.

Hence, the bypass flow is controllable and supplied solely by a relatively small amount of outside air. This outside air in this inventive heating unit is able to be maintained in the range of 1%-5%. For example, the fan 30 may supply 40,000 cfm and the blower supplies approximately 800 cfm or 2% of the air flow. Of this 800 cfm, 220 cfm may be provided as combustion air to the burner unit 38 while the remaining 580 cfm may pass to the main air flow as the bypass flow.

Therefore as to the foregoing, a highly efficient heating unit is provided which minimizes the amount of outside air while providing overall improved efficiency to this system.

Although particular preferred embodiments of the invention have been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.

Claims

1. A direct fired heating unit comprising;

a filter/fan section which draws in air with an energy-efficient axial-bladed fan generating a main air flow;

a middle section containing a burner unit having a burner generating a burner flame for heating said main air flow and a dedicated combustion air blower supplying a flow of combustion air to the burner which said combustion air is isolated from said main air flow;

a burner section where said burner flame and main air flow mix; and

a discharge plenum which discharges and distributes the heated air to the building space.

2. The heating unit according to claim 1, wherein said burner unit solely uses said combustion air for flame generation and does not rely upon the main air flow from the main fan for combustion air.

3. The heating unit according to claim 3, wherein said blower relies solely upon said dedicated blower which draws in a relatively small volume of outside air and feeds same directly to the burner flame.

4. The heating unit according to claim 1, wherein said burner unit has a gas-fed burner which generates said flame between two angled mixing plates wherein the burner flame is formed on the interior side of the mixing plates, and said combustion air is supplied to the outside of the mixing plates wherein the mixing plates include perforations to allow for controlled passage of air flow into the flame region.

5. The heating unit according to claim 4, wherein said burner unit further includes a containment box surrounding the mixing plates and said burner which isolates the combustion air flow from the surrounding main air flow.

6. The heating unit according to claim 1, wherein said burner is supplied with a piped supply of outside combustion air by said air blower.

7. The heating unit according to claim 6, wherein said outside air makes up only 1-5% of the heating unit air flow since said outside air is directly piped to the burner.

8. The heating unit according to claim 1, wherein said burner unit is positioned in the main air flow downstream of the main fan wherein the flame forms in the burner section, with the main air flow flowing upwardly from the filter/fan section through the burner section for heating, and then to the discharge plenum, said main air flowing directly over and about the burner housing and flame for direct heating of the air flow.

9. The heating unit according to claim 1, wherein said filter/fan section includes said axial bladed fan on a colder upstream side of said burner which generates said main air flow.

10. The heating unit according to claim 1, wherein all of said blower air flow is supplied at a rate which supplies the necessary flow of said combustion air.

11. A direct fired heating unit comprising;

a filter/fan section which draws in air with an energy-efficient axial-bladed fan generating a main air flow;

a middle section containing a burner unit having a burner generating a burner flame for heating said main air flow and a dedicated air blower supplying a flow of blower air along a blower air flow path to the burner which said blower air is isolated from said main air flow upstream of said burner unit;

a burner section where said burner flame and main air flow mix;

a discharge plenum which discharges and distributes the heated air to the building space; and

a first portion of said blower air comprising combustion air supplied to said burner and a second portion of said blower air being diverted back into said main air flow as a bypass air flow.

12. The heating unit according to claim 11, wherein the flow path of said blower air is provided with one or more exit ports that allow for said second portion of said blower air flow to divert to said main air flow as said bypass air flow, while said second portion of said blower air flow continues to the burner as the combustion air flow.

13. The heating unit according to claim 12, wherein said burner unit includes a chamber which includes first passages supplying the combustion air to said burner and second passages defining said exit ports exiting from said chamber to discharge said bypass air flow to said main air flow.

14. The heating unit according to claim 13, wherein said bypass air flow allows the concentration of combustion byproducts to be maintained at a low level that satisfies environmental regulations.

15. The heating unit according to claim 13, wherein said exit ports have controllable openings which control the amount of said bypass air flow.