Rotation and Discharge Codes¶
Summary¶
This chapter turns the orientation conventions into the codes used on orders. It covers clockwise and counterclockwise rotation and rotation arrows, the combined rotation and discharge code, the limits on double width double inlet discharges, motor position restrictions that depend on discharge, conduit box position, plenum motor positions, and the conventions for axial and inline fans in horizontal and vertical configurations. After completing it, readers can write a correct rotation and discharge designation for a fan.
Concepts Covered¶
This chapter covers the following 14 concepts from the learning graph:
| Concept | Concept Impact Score |
|---|---|
| Plenum Motor Positions | 1 |
| Conduit Box Position | 19 |
| Clockwise Rotation | 24 |
| Counterclockwise Rotation | 24 |
| Rotation Arrow | 19 |
| Axial Rotation Convention | 2 |
| Motor Position Restrictions | 19 |
| Inline Horizontal Configuration | 2 |
| Inline Vertical Configuration | 2 |
| Rotation and Discharge Code | 21 |
| Axial Fan Configuration | 1 |
| Vertical Inline Motor Location | 1 |
| DWDI Discharge Limits | 2 |
| Bottom Angular Down Discharge | 1 |
Prerequisites¶
This chapter builds on concepts from:
- Chapter 3: Basic Fan Anatomy
- Chapter 4: Centrifugal, Axial and Mixed Flow Fans
- Chapter 5: Roof, Wall and Specialty Exhaust Fans
- Chapter 8: Centrifugal and Axial Impeller Types
- Chapter 9: Impeller Construction and Material Handling
- Chapter 12: Discharge Positions and Rotation
Welcome to Rotation and Discharge Codes
A fan that arrives with its outlet pointing the wrong way, or its motor sitting where the ductwork needs to go, costs days on a job site and a fresh order at the factory. This chapter gives you the short codes that prevent that: a rotation, a discharge, a motor position, and a conduit box location that the plant, the drafter, and the installer all read the same way. By the end you will write a complete designation for a centrifugal, plenum, or inline fan without guessing. Let's follow the airflow.
From Orientation to Order Codes¶
Chapter 12 set up the vocabulary for describing which way a fan faces: the eight discharge positions, the discharge angle, the rule that a centrifugal fan is viewed from the drive side, and the AMCA motor positions W, X, Y and Z. This chapter turns that vocabulary into the codes that go on a quote, a submittal drawing, and a factory order. Each code answers one question the factory cannot answer for you:
- Which way does the impeller turn? (rotation)
- Which way does the air leave the housing? (discharge)
- Where does the motor sit relative to the fan? (motor position)
- Which side of the motor carries the electrical connection? (conduit box position)
The sections below take these in order for housed centrifugal fans, then cover plenum fans, and finish with the different conventions used for axial and inline fans.
Clockwise and Counterclockwise Rotation¶
Every centrifugal fan turns in one of two directions, and the direction is fixed by the housing: a scroll housing is built to collect air that moves around it one way only. The direction is always judged from a stated viewpoint, because the same impeller looks clockwise from one side and counterclockwise from the other. For housed centrifugal fans, FanPedia states that viewpoint on every discharge page: rotation is viewed from the drive side (FanPedia PDF pp. 46–49). The drive side is the side of the fan where the drive is: the sheave and belts on a belt-driven fan, or the motor on a direct-drive fan.
Clockwise Rotation¶
Clockwise rotation (CW) means that an observer standing on the drive side and looking toward the fan sees the impeller turn the same way as the hands of a clock. FanPedia's CW page shows a person standing beside the bearing pedestal, looking at the drive end of the shaft, with the rotation arrow sweeping over the top of the housing from left to right (FanPedia PDF p. 46). The same page draws each centrifugal impeller type with its blades oriented for clockwise rotation, because blade shape and rotation must agree.
Worked Example: Confirming CW on an Existing Arrangement 1 Fan¶
A plant engineer wants a duplicate of an existing Arrangement 1 SWSI exhaust fan whose nameplate is painted over.
- Find the drive side. The belt guard and motor are on the east side of the fan, so the east side is the drive side.
- Stand on the drive side. The engineer stands east of the fan, facing west toward the shaft end and sheave.
- Watch the shaft or a blade. With the fan coasting down after a stop, the shaft turns the way clock hands turn.
- Record it. The rotation is CW. Write it down before the discharge, because the discharge code means something different for CW and CCW fans.
If the engineer had stood on the inlet side instead, the shaft would have appeared to turn counterclockwise, and an order written from that view would have produced a mirror-image housing.
Counterclockwise Rotation¶
Counterclockwise rotation (CCW) means that the observer on the drive side sees the impeller turn opposite to the hands of a clock. A CCW housing is the mirror image of a CW housing: the scroll expands in the opposite direction and every discharge comes out on the opposite side (FanPedia PDF p. 47). Neither direction is better aerodynamically; the choice comes from the duct connection, service access, and how paired fans are laid out.
Worked Example: Choosing CCW for a Mirror-Image Pair¶
Two supply fans stand side by side in a mechanical room with a service aisle between them, and both must blow north into a common plenum through Top Horizontal outlets. The west fan has its drive on its east side, facing the aisle; the east fan has its drive on its west side, also facing the aisle.
- West fan. Its observer stands in the aisle looking west. North is on that observer's right, so the top outlet blows right, which makes the top of the impeller move right: the fan is CW, ordered CW THD.
- East fan. Its observer also stands in the aisle but looks east. North is now on the observer's left, so the top outlet blows left and the top of the impeller moves left: the fan is CCW.
- Write both lines. West fan CW THD, east fan CCW THD. The discharge code is the same; only the rotation differs.
The pair looks like a reflection in a mirror placed in the aisle, which is exactly what CW and CCW housings are.
One Fan, Two Answers
Rotation is not a property you can read without a viewpoint: the same spinning wheel is clockwise from one side and counterclockwise from the other. That is why every rotation code silently carries the phrase "viewed from the drive side," and why the first question to ask about any sketch is "Check it from the drive side."
Rotation Arrow¶
A rotation arrow is the curved arrow that shows the required direction of rotation. It appears on FanPedia's discharge and impeller drawings, where it sweeps around the housing or the impeller in the direction the wheel must turn (FanPedia PDF pp. 37–49), and in common practice it is also marked on the fan housing so that whoever wires the motor can check the direction at startup.
A centrifugal impeller turning backward still moves some air, but far less than its rating, so a reversed fan often "runs but does not move much air" rather than failing outright. FanPedia is blunter about axial fans: if the impeller rotates in the wrong direction, "you will not get any airflow in the proper direction" (FanPedia PDF pp. 37–40).
Worked Example: The Startup Bump Check¶
An electrician has just wired a new belt-driven centrifugal fan with a three-phase motor, and the startup checklist calls for a bump test.
- Clear the fan. Guards are in place and nobody is inside the housing or ductwork.
- Bump the motor. Energize it just long enough for the shaft to start turning, then de-energize it.
- Compare with the arrow. While the shaft coasts, compare its direction with the rotation arrow on the housing. Read both from the same side.
- Correct if needed. On a three-phase motor, interchanging any two of the three supply leads reverses rotation. Bump again to confirm, and record the result on the startup sheet.
If the fan was ordered CW but the housing arrow shows CCW from the drive side, the housing was built to the wrong hand, and no rewiring will fix it.
Rotation and Discharge Code¶
A rotation and discharge code is the short designation that states both which way a centrifugal fan turns and which way its outlet points. FanPedia gives two equivalent ways to write it (FanPedia PDF pp. 46–49):
- Letter form: the rotation (CW or CCW) followed by a three-letter discharge abbreviation, such as CW THD.
- Angle form: the rotation followed by the discharge angle in degrees, such as CW 90, also written "Clockwise 90°".
The discharge angle steps around the housing in 45° increments. A practical way to read it from FanPedia's drawings is to start with the outlet pointing straight up (360°) and turn the outlet in the direction of rotation, as seen from the drive side: a quarter turn gives 90°, a half turn 180°, and three quarters 270°. That is why the same number means opposite outlet directions for CW and CCW fans: CW 90 blows toward the right of an observer on the drive side, and CCW 90 blows toward the left.
The following table lists the eight discharges from Chapter 12 in both code forms.
| Discharge | Letter code | CW angle form | CCW angle form | SWSI | DWDI |
|---|---|---|---|---|---|
| Upblast | UBD | CW 360 | CCW 360 | Yes | Yes |
| Top Angular Up | TAU | CW 45 | CCW 45 | Yes | Yes |
| Top Horizontal | THD | CW 90 | CCW 90 | Yes | Yes |
| Top Angular Down | TAD | CW 135 | CCW 135 | Yes | Yes |
| Downblast | DBD | CW 180 | CCW 180 | Yes | Yes |
| Bottom Angular Down | BAD | CW 225 | CCW 225 | Yes | No |
| Bottom Horizontal | BHD | CW 270 | CCW 270 | Yes | Yes |
| Bottom Angular Up | BAU | CW 315 | CCW 315 | Yes | Yes |
Source: FanPedia PDF pp. 46–49.
Worked Example: Writing the Code from a Duct Layout¶
A contractor's sketch shows a single-width, single-inlet (SWSI) fan whose outlet connects to a duct running horizontally along the floor, leaving toward the right of an observer on the drive side. The fan turns counterclockwise from that side.
- Rotation. CCW, from the drive side.
- Discharge position. The outlet is at the bottom of the housing and blows horizontally, so it is Bottom Horizontal, BHD.
- Check the direction. For a CCW fan, start with the outlet pointing up and turn it counterclockwise: 90° points left, 180° points down, 270° points right. A right-pointing bottom outlet on a CCW fan is 270°, which agrees with BHD in the table.
- Write the code. CCW BHD, or in angle form CCW 270.
Had the contractor's fan been CW, a right-pointing outlet at the bottom would be impossible: on a CW fan the bottom horizontal outlet blows left. Rotation and discharge must be chosen together.
Write Both Forms Once
When a customer's drawing uses one form and your order entry uses the other, write both on your worksheet, "CCW BHD (CCW 270)," and check that they name the same row of the table. One alias on FanPedia's CCW pages prints Downblast as "Counterclockwise 80°"; it is a misprint, and the code is CCW 180 (FanPedia PDF pp. 47 and 49).
Bottom Angular Down Discharge¶
Bottom Angular Down discharge (BAD) is the discharge whose outlet sits at the bottom of the housing and points downward at 45° between Downblast and Bottom Horizontal. Its angle form is CW 225 or CCW 225 (FanPedia PDF pp. 46–47). On a CW fan seen from the drive side it points down and to the left; on a CCW fan it points down and to the right. FanPedia lists BAD for single-width, single-inlet fans only.
DWDI Discharge Limits¶
A double width, double inlet (DWDI) fan takes air into both sides of its impeller, which you met in Chapter 8. The DWDI discharge limits are the restriction that a DWDI centrifugal fan is offered in seven discharges, not eight: FanPedia's DWDI discharge pages list UBD, TAU, THD, TAD, DBD, BHD and BAU in both rotations, and omit Bottom Angular Down (FanPedia PDF pp. 48–49). The same pages show only three DWDI impeller types: forward curved, backward inclined and airfoil. When a layout calls for a DWDI fan blowing bottom angular down, the fix belongs in the ductwork (a Downblast or Bottom Horizontal outlet with a duct elbow) or in the fan type, and the current catalog for the product line should be checked before promising anything else.
The interactive below lets you practice writing codes, including a request that cannot be filled.
Diagram: Rotation and Discharge Code Builder¶
Run Rotation and Discharge Code Builder Fullscreen
Rotation and Discharge Code Builder
Type: microsim
sim-id: ch13-rotation-discharge-code-builder
Library: p5.js
Status: Specified
Bloom Level: Apply
Bloom Verb: apply
Learning Objective: The learner will apply the drive-side rotation and discharge-angle conventions to write the correct rotation and discharge code, in letter form and angle form, for a described centrifugal fan.
Prerequisites: drive side, clockwise rotation (CW), counterclockwise rotation (CCW), discharge position, discharge angle, SWSI, DWDI, Bottom Angular Down discharge, DWDI discharge limits (all defined in the sections above).
Evidence of Mastery: For each of 8 job cards, the learner selects a rotation (CW or CCW) and one of the 8 discharge letter codes, or selects "Not offered", and submits. A submission is correct when it matches the Answer column in Content. Mastery is 7 of 8 correct on the first attempt. Rotating the free-practice housing is exploration, not evidence.
Misconceptions: (1) The same angle number points the same way for CW and CCW fans. (2) Rotation can be judged from the inlet side. (3) Every discharge is available on every centrifugal fan, including DWDI.
Instructional Rationale: Apply-level skill needs repeated use of the rule on varied cases with a check after each one. Each card describes the outlet direction in words as seen from the drive side, so the learner must convert direction plus rotation into a code rather than match a picture to a label.
Content:
Job cards, in this fixed order. Directions are as seen by an observer on the drive side.
| # | Fan | Rotation stated on card | Outlet description on card | Answer | Why (shown as feedback) |
|---|---|---|---|---|---|
| 1 | SWSI | CW | Outlet blows straight up | CW UBD (CW 360) | Straight up is Upblast, 360°, for either rotation. |
| 2 | SWSI | CCW | Outlet at the top blows horizontally to the left | CCW THD (CCW 90) | For CCW, a quarter turn counterclockwise from straight up points left: 90°, Top Horizontal. |
| 3 | SWSI | CW | Outlet blows straight down | CW DBD (CW 180) | A half turn from straight up is 180°, Downblast, for either rotation. |
| 4 | SWSI | CCW | Outlet at the bottom blows horizontally to the right | CCW BHD (CCW 270) | For CCW, three quarter turns counterclockwise from up points right: 270°, Bottom Horizontal. |
| 5 | SWSI | CW | Outlet at the bottom points down and to the left at 45° | CW BAD (CW 225) | 225° clockwise from up is down-left, Bottom Angular Down, which SWSI fans offer. |
| 6 | DWDI | CW | Outlet at the bottom points down and to the left at 45° | Not offered | DWDI fans are offered in seven discharges; Bottom Angular Down is not one of them. |
| 7 | SWSI | CCW | Outlet at the top points up and to the left at 45° | CCW TAU (CCW 45) | 45° counterclockwise from up is up-left, Top Angular Up. |
| 8 | DWDI | CCW | Outlet at the bottom points up and to the right at 45° | CCW BAU (CCW 315) | 315° counterclockwise from up is up-right, Bottom Angular Up, which DWDI fans offer. |
Discharge choices shown to the learner: UBD Upblast, TAU Top Angular Up, THD Top Horizontal, TAD Top Angular Down, DBD Downblast, BAD Bottom Angular Down, BHD Bottom Horizontal, BAU Bottom Angular Up, and Not offered.
After each submission the sim shows a drive-side outline of the housing with its outlet drawn in the answer's direction and a rotation arrow in the answer's rotation, labeled with both code forms.
Provenance: Discharge names, letter codes, angle forms and the DWDI list are from FanPedia PDF pp. 46–49 and this chapter's section "Rotation and Discharge Code." The outlet direction for each angle follows the chapter's reading rule. Job cards are illustrative scenarios and are labeled "illustrative".
Rules: Outlet direction, seen from the drive side, is the direction "straight up" turned through the discharge angle in the direction of rotation: clockwise for CW, counterclockwise for CCW. Angles: UBD 360, TAU 45, THD 90, TAD 135, DBD 180, BAD 225, BHD 270, BAU 315. A DWDI card whose only matching discharge is BAD has the answer "Not offered". In free practice, the rotation is CW or CCW (default CW) and the discharge angle runs from 45 to 360 in steps of 45 (default 360, unit degrees); the sim shows the matching letter code, and when the fan type is DWDI and the angle is 225 it shows "Not offered on DWDI".
Learner Activity:
- The learner reads job card 1 and predicts the code by choosing a rotation and a discharge, then submits.
- The sim checks the answer. A correct answer shows the feedback and draws the housing outline with the correct outlet and rotation arrow. A wrong answer shows a hint only, with no code or drawing; the learner may retry once (not scored) or press Show answer, and after that final attempt the "Why" text, the correct code and the housing drawing appear.
- The learner moves to the next card; previous cards cannot be re-answered for score.
- After card 8, free practice unlocks: the learner chooses SWSI or DWDI, a rotation, and an angle, and the outline and code update at once. The learner should notice that switching rotation at a fixed angle mirrors the outlet left to right.
Feedback: 8 cards, fixed order, one scored attempt each, then a second unscored attempt is allowed. Correct: "Correct:
Starting State: Card 1 is shown with no rotation or discharge selected and the question "What code goes on the order?"
Chapter Anchors: Eight discharges for SWSI fans; seven for DWDI, without BAD. CW 90 blows to the right and CCW 90 to the left of an observer on the drive side. The worked example's answer is CCW BHD (CCW 270).
Motor Position Restrictions¶
For SWSI Arrangements 1 and 3, FanPedia states that the motor can be mounted in any of the four AMCA motor positions W, X, Y and Z from Chapter 12 (FanPedia PDF pp. 162–163). Motor position restrictions are the combinations of rotation, discharge and motor position that cannot be built, or that need a check, because the motor would block the outlet or would not fit under it.
FanPedia lists the same restrictions for SWSI Arrangements 1, 3 and 3F (FanPedia PDF pp. 162–163):
- BHD discharge, because the motor interferes with the outlet opening: CW rotation, position Z is not allowed; CCW rotation, position W is not allowed.
- TAD discharge, for the same reason: CW rotation, position W is not allowed; CCW rotation, position Z is not allowed.
- THD discharge, a height restriction, because the motor may not fit below the discharge: CW rotation, position W; CCW rotation, position Z. On an Arrangement 1 fan, the motor will fit if the pedestal is lengthened to accommodate it.
As Chapter 12 showed, W is the right-hand position and Z the left-hand position beside the housing, seen from the drive side. Notice the mirror pattern: every CW restriction on W becomes a CCW restriction on Z, and the reverse, because a CCW housing is the mirror image of a CW housing.
Other arrangements carry their own rules. Swingout fans (Arrangements 9SS and 9ST) open the housing for impeller access, and a K mount motor, also called piggyback or scroll mounted, sits directly on the housing scroll on an adjustable motor base, so the outlet decides where there is room for it (FanPedia PDF pp. 53–54). The table summarizes these rules.
| Arrangement | Motor position rule | Source |
|---|---|---|
| 9 and 9F (SWSI) | Standard: CW fans use (L), CCW fans use (R) | FanPedia PDF p. 52 |
| 9SS swingout | Fixed: CW with (L) only (right swing); CCW with (R) only (left swing) | FanPedia PDF pp. 53, 166 |
| 9ST swingout | Standard (R) on right swing, (L) on left swing; motor on slide base above the pedestal | FanPedia PDF pp. 53, 166 |
| 10 (SWSI) | Motor position unique to the pedestal design, no options | FanPedia PDF p. 52 |
| 3F (DWDI) | W and Z standard; consult factory for X and Y | FanPedia PDF p. 167 |
| K mount, SWSI and DWDI Arrangement 3 | Clock positions depend on discharge: UBD 9 and 3 o'clock; THD 9, 10:30 and 12; DBD 9, 10:30, 12, 1:30 and 3; BHD 12, 1:30 and 3 | FanPedia PDF p. 54 |
Worked Example: Rejecting a Motor Position Before It Ships¶
A specifying engineer sends a schedule line: "SWSI Arrangement 3 fan, CW, BHD, motor position Z, belt drive."
- Look up the discharge. BHD is one of the three discharges with a restriction.
- Apply the rotation. For CW rotation with BHD, position Z is not allowed: the motor would interfere with the outlet opening.
- Offer the alternatives. Positions W, X and Y carry no restriction for CW BHD, and the engineer picks W.
- Write the corrected line: "Arrangement 3, CW BHD, motor position W."
Rotation Changes Which Letter Is Forbidden
A common trap is to remember "BHD means no Z" and forget that it only holds for CW fans; for CCW BHD the forbidden position is W. Always look up the restriction with the rotation and the discharge together, and recheck it whenever either one changes late in the job.
The checker below drills exactly that habit across the three restricted discharges and a few arrangements with their own rules.
Diagram: Motor Position Restriction Checker¶
Run Motor Position Restriction Checker Fullscreen
Motor Position Restriction Checker
Type: diagram
sim-id: ch13-motor-position-restriction-checker
Library: html
Status: Specified
Bloom Level: Evaluate
Bloom Verb: judge
Learning Objective: The learner will judge whether each proposed combination of arrangement, rotation, discharge and motor position can be ordered as written, using the FanPedia restriction rules.
Prerequisites: arrangement, CW and CCW rotation, discharge letter codes, motor positions W, X, Y and Z, motor positions (L) and (R), swingout arrangement 9SS, motor position restrictions (all defined in the chapter above this block).
Evidence of Mastery: For each of 10 order lines, the learner selects one verdict, "Allowed", "Not allowed" or "Height check needed", and submits. A verdict is correct when it matches the Verdict column in Content. Mastery is 9 of 10 correct on the first attempt. Opening the reference rule card is exploration, not evidence.
Misconceptions: (1) A restriction on a position holds for both rotations. (2) The THD restriction forbids the position outright instead of calling for a height check. (3) Arrangement 9SS lets the buyer choose either side for the motor.
Instructional Rationale: Evaluate-level judgment requires the learner to weigh a proposal against stated criteria and commit to a verdict before seeing the answer. Mixing allowed lines with restricted ones forces a real check on every line instead of pattern guessing.
Content:
Order lines, in this fixed order:
| # | Order line shown | Verdict | Why (shown as feedback) |
|---|---|---|---|
| 1 | SWSI Arrangement 3, CW, BHD, motor position Z | Not allowed | For CW BHD the motor in Z interferes with the outlet opening. |
| 2 | SWSI Arrangement 3, CW, BHD, motor position W | Allowed | For CW BHD only Z is restricted; W is available. |
| 3 | SWSI Arrangement 1, CCW, BHD, motor position W | Not allowed | The mirror rule: for CCW BHD the restricted position is W. |
| 4 | SWSI Arrangement 1, CW, TAD, motor position W | Not allowed | For CW TAD the motor in W interferes with the outlet opening. |
| 5 | SWSI Arrangement 3F, CCW, TAD, motor position Z | Not allowed | For CCW TAD the restricted position is Z, on Arrangement 3F as on 1 and 3. |
| 6 | SWSI Arrangement 3, CCW, TAD, motor position W | Allowed | For CCW TAD only Z is restricted. |
| 7 | SWSI Arrangement 1, CW, THD, motor position W | Height check needed | The motor may not fit below a CW THD outlet in W; on Arrangement 1 a lengthened pedestal makes it fit. |
| 8 | SWSI Arrangement 3, CCW, THD, motor position Z | Height check needed | For CCW THD the height restriction applies to Z. |
| 9 | SWSI Arrangement 3, CW, UBD, motor position Y | Allowed | FanPedia lists no motor position restriction for UBD. |
| 10 | Arrangement 9SS swingout, CCW, UBD, motor position (L) | Not allowed | Arrangement 9SS is built CW with (L) or CCW with (R) only. |
Reference rule card the learner can open at any time:
| Discharge | CW restriction | CCW restriction | Reason |
|---|---|---|---|
| BHD | Z not allowed | W not allowed | Motor interferes with outlet opening |
| TAD | W not allowed | Z not allowed | Motor interferes with outlet opening |
| THD | W, height check | Z, height check | Motor may not fit below the discharge; Arrangement 1 pedestal can be lengthened |
| Arrangement 9SS (any discharge) | (L) only | (R) only | Fixed swingout pairings: CW with (L) on a right swing, CCW with (R) on a left swing |
Provenance: Restrictions for Arrangements 1, 3 and 3F from FanPedia PDF pp. 162–163; Arrangement 9SS rule from FanPedia PDF pp. 53 and 166. Order lines are illustrative and are labeled "illustrative".
Rules: A line is "Not allowed" when its rotation, discharge and position appear as NOT ALLOWED in the rule card, or when it breaks the Arrangement 9SS pairing (CW with L, CCW with R). A line is "Height check needed" when it is CW THD with W or CCW THD with Z. Every other line is "Allowed".
Learner Activity:
- The learner reads order line 1, predicts a verdict, and submits it.
- The sim marks the verdict, shows the "Why" text, and highlights the matching row of the rule card.
- The learner continues through all 10 lines in order.
- At the end the sim shows each line with the learner's verdict and the correct verdict side by side, and the learner should notice that every restriction flips between W and Z when the rotation flips.
Feedback: 10 lines, fixed order, one scored attempt each. Correct: "Correct:
Starting State: Order line 1 is shown with three verdict choices and the question "Can this line be ordered as written?" The rule card is closed.
Chapter Anchors: Restrictions apply to BHD, TAD and THD on SWSI Arrangements 1, 3 and 3F. The worked example's line, Arrangement 3, CW, BHD, Z, is not allowed and is corrected to W.
Conduit Box Position¶
The conduit box is the motor's power connection point, the box where the supply wiring enters and joins the motor leads (FanPedia PDF p. 75). Its conduit box position is the side of the motor it is mounted on, coded F1, F2 or F3 (F3 is on top; FanPedia PDF p. 69) and always viewed from the drive side of the motor, looking at the motor shaft (FanPedia PDF pp. 52–57). The motor position pages use only F1 and F2, and FanPedia marks F1 as the standard position on its motor position pages. Its motor modification page adds that F1 or F3 is standard, and that F2 is standard on IEC frames but a special modification for NEMA motors (FanPedia PDF p. 75).
The conduit box position matters because the box can end up facing a wall, the fan housing, or the belt guard, where an electrician cannot reach it. FanPedia gives two rules tied to motor position:
- For Arrangement 9 and for plenum Arrangement 3SM, motor position (L) is normally ordered with an F1 conduit box; select F2 if there are clearance issues (FanPedia PDF pp. 52 and 55).
- For Arrangement 9SS in the left swing, CCW rotation and (R) motor position, the fan must have an F2 conduit box (FanPedia PDF p. 53).
Worked Example: Choosing F1 or F2 for an Arrangement 9 Fan¶
A distributor orders an SWSI Arrangement 9 fan, CW rotation, for a tight equipment room. The fan's left side, seen from the drive side, sits 18 inches from a block wall (an illustrative site dimension).
- Motor position. Arrangement 9 standard is (L) for CW, so the motor hangs on the left side of the pedestal, on the wall side.
- Default conduit box. Position (L) is normally ordered with F1.
- Check clearance. Ask which side of the motor F1 puts toward the wall. If the electrician cannot reach or route conduit to the box in that space, FanPedia's note applies: select F2.
- Write the line: "Arrangement 9, CW, motor position (L), conduit box F2 (clearance)."
Had the fan been an Arrangement 9SS in left swing, there would be no choice to make: CCW and (R) require F2.
Plenum Motor Positions¶
A plenum fan has no scroll housing, so its motor positions are named differently from a housed fan's. Plenum motor positions are the motor locations FanPedia defines for belt-driven plenum fan arrangements (FanPedia PDF p. 55):
- Arrangement 3 (horizontal): motor on the floor or fan base, in AMCA positions W, X, Y or Z.
- Arrangement 3HS (horizontal, top-mounted motor on a slide base): Standard (S), also known as "R", or Optional (O), also known as "L".
- Arrangement 3HA (horizontal, top-mounted motor on an adjustable motor base): Top Center (TC).
- Arrangement 3SM (horizontal, side-mounted motor on a slide base): (L) or (R), with (R) standard.
- Arrangement 3VA (vertical, adjustable motor base): Top Center (TC).
- Arrangement 3VS (vertical, slide base): Standard (S), also known as "R", or Optional (O), also known as "L".
The arrangement code tells you which set of positions applies: H or V for horizontal or vertical, then S, A or M for slide base, adjustable base or side mounting.
Axial and Inline Conventions¶
Axial, mixed flow and inline fans have no scroll, so the eight centrifugal discharge codes do not apply to them. They use their own rotation rule and a configuration code instead.
Axial Rotation Convention¶
The axial rotation convention is the rule that an axial fan's rotation is determined by viewing the impeller from the discharge side of the fan, not from the drive side (FanPedia PDF pp. 37–42). Axial rotation is written as left hand (LH) or right hand (RH), and it must match the impeller's hand and the airflow direction (PM, air through the impeller then over the motor, or MP, air over the motor then through the impeller) that Chapter 8 covered. Do not convert an axial LH or RH into CW or CCW for an order; each family keeps its own viewpoint.
Two Families, Two Viewpoints
Notice the pattern: a centrifugal fan is judged from where the drive is, and an axial fan from where the air leaves. Before you read any rotation on a drawing, ask which family the fan belongs to, and the viewpoint follows from that one answer.
Axial Fan Configuration¶
An axial fan configuration is the code that states how an inline, mixed flow or axial fan is oriented and supported: horizontal or vertical, which way the air goes, and what brackets, cap or hood it carries (FanPedia PDF pp. 50–51). It replaces the centrifugal discharge code, because a duct-shaped fan always discharges along its own axis.
Inline Horizontal Configuration¶
An inline horizontal configuration is a configuration in which the fan's axis is horizontal. FanPedia defines three codes: HOR, horizontal with no brackets; HCH, horizontal ceiling hung; and HBM, horizontal base mount (FanPedia PDF p. 50). For a belt-driven Arrangement 9 horizontal fan, the motor positions are lettered A through H around the casing as seen from the discharge, with A at the top. A floor-mounted fan shows positions A, B, C, G and H, the five not blocked by its base; a duct or ceiling mount shows all eight (FanPedia PDF p. 56).
Inline Vertical Configuration¶
An inline vertical configuration is a configuration in which the fan's axis is vertical and the air moves up or down. FanPedia defines ten codes (FanPedia PDF p. 51):
| Code | Airflow | Support or top |
|---|---|---|
| VUN | Vertical up | No brackets |
| VUI | Vertical up | Floor mount brackets on inlet |
| VUO | Vertical up | Ceiling hung brackets on outlet |
| VUS | Vertical up | Stack cap |
| VUH | Vertical up | Hood |
| VDN | Vertical down | No brackets |
| VDI | Vertical down | Ceiling hung brackets on inlet |
| VDO | Vertical down | Floor mount brackets on outlet |
| VDH | Vertical down | Hood |
| VRH | Vertical reversible | Hood |
The letters read left to right: V for vertical, then U, D or R for up, down or reversible, then the support or top.
Vertical Inline Motor Location¶
The vertical inline motor location is the place the motor occupies on a belt-driven Arrangement 9 vertical inline fan. FanPedia states that there is no specified motor position for these configurations (FanPedia PDF p. 57): on a roof-mounted fan the motor is centered on the curb cap, and on floor, ceiling and duct mounted fans it is centered between the support brackets. The order needs the configuration code and the conduit box position, but no motor position letter.
Designations You Can Defend
You can now turn a layout sketch into a rotation and discharge code, check the motor position against the discharge, pick the conduit box side, and switch to the axial and inline conventions when the fan has no scroll. That is the full orientation line on a centrifugal fan order, written so the factory, the drafter and the installer all build the same fan.
Key Takeaways¶
- Centrifugal rotation, CW or CCW, is viewed from the drive side; a CCW housing is the mirror image of a CW housing, and the housing's rotation arrow is the startup check.
- A rotation and discharge code is written in letter form (CW THD) or angle form (CW 90); the angle turns from straight up in the direction of rotation.
- DWDI fans are offered in seven discharges; Bottom Angular Down (BAD, 225°) is SWSI only.
- On SWSI Arrangements 1, 3 and 3F, BHD and TAD each forbid one motor position and THD needs a height check; the letter flips between W and Z with rotation.
- Conduit box position is F1, F2 or F3 (top), viewed from the drive side of the motor; the motor position pages use F1 and F2; F2 is the clearance fix and is required on 9SS left swing.
- Plenum, axial and inline fans use their own codes: plenum position letters, LH or RH from the discharge side, and configuration codes such as HOR or VUH.