---
id: 2026-06-04T10:05:03-0400
title: 2026-06-04 10:05:03
tags: []
daily: "[[2026-06-04]]"
---
# 2026-06-04 10:05:03

## Mechanical Rules of Thumb

### Cooling towers

Avoid single cell tower

Avoid tower without a bleed-off

Avoid tower without heat trace and insulation on the make-up water

Avoid line-to-line bypass (in lieu of dumping into the sump)

### Chillers/chilled water


Avoid one chiller system (exception: dual circuit air-cooled)

Avoid series chillers without bypasses

Avoid a coil with more than 14 fins/inch

### Equipment/misc.

Avoid simplex air compressor

Avoid simplex sump pump

Avoid dx built-up systems (vav)

Avoid coil face velocity above 550 fpm (could send condensate down ductwork)

Avoid horizontal cold water lines or horizontal roof drain lines above ceilings without insulating (dripping/sweating)

Isolation valves on both sides of all equipment - good practice

Think about future service: don't locate where dangerous to service

Avoid discharging roof drains onto a driveway or where pedestrian traffic

Avoid floor mounted water closets in commercial buildings - cleaning issue

Minimum 4" housekeeping pads underneath all floor mounted equipment

Ashrae recommends 400 fpm face velocity maximum at intake louvers

Avoid standard filters at more than 500 fpm (see mfgr recommendations)

Trap primers on all floor drains

### Ashrae 62 (ventilation)

* 20 cfm/person oa in office buildings.
    exceptions: large assembly areas

* 15 cfm/person oa in schools.

note: this ventilation might exceed the local codes,
but ashrae recommendations should be met.

### Smacna pressure class

* 1" wg - low pressure
* 3" wg - medium pressure
* Under 3" wg - smacna does not recommend duct testing

If higher static pressures are specified,
try to get lower pressure classes approved.
The savings in duct costs are significant.

### Ductwork sizing

| Case         | Max Friction Per 100ft |
| ------------ | ---------------------- |
| Low pressure | 0.10"                  |
| Exhaust duct | 0.15"                  |


| Case                  | Velocity FPM                    |
| --------------------- | ------------------------------- |
| Medium pressure riser | Max 3000                        |
| Medium pressure       | Max 2500                        |
| Low pressure          | Max 1800-2000                   |
| Return air shaft      | Max 1000                        |
| Outside air shaft     | Max 1000                        |
| Grease duct           | Target 1800, Min 1500, Max 2200 |

Due to excessive noise,
Avoid fan-powered or VAV boxes larger than 2000 CFM.

Install a balance damper at each low-pressure runout to diffusers.

No more than 90° turn with flex duct, straight duct run into boxes.

Where space permits,
consider converting rectangular ductwork to round ductwork.

Energy code requires all duct to be sealed.

### Pumps

Try to locate condenser water pumps as low as possible
as compared to the cooling tower basin - NPSH - gravity flow.

Select pumps for "non-overloading".

Select impeller no larger than 90% of maximum impeller size.

### Air units

| Tons          | CFM/Ton |
| ------------- | ------- |
| under 20      | ~400    |
| 25 and larger | ~350    |


Avoid VFD bypass on VAV fan systems
because it can blow apart duct in manual mode.
Consider a spare VFD, or dump damper/blow-out doors.

Avoid larger economizer systems without powered exhaust.

### Useful formulas (standard conditions)

Quantity (CFM) = Velocity (FPM) * Area (SF)

Sensible load (BTUH) = 1.08 * CFM * (Delta-T)

Latent load (BTUH) = 0.68 * CFM * (Delta-GR)

Total load (BTUH) = 4.45 * CFM * (Delta-H)

* Formula for water:
    * Load (BTUH) = 1.08 * CFM * (Delta-T)

$$
\rm{GPM} = \frac{\text{Load (BTUH)}}{500 \times \text{Delta-T}}
$$

* A/C equipment efficiencies:

$$

$$

### Conversion factors

* 1 PSI = 2.307 ft of water
* 1 ton A/C = 12,000 BTUH
* 1 hp = 2545.1 BTUH = 745.7 watts
* 1 kW = 341 5 BTUH
* 1 cubic foot = 7.48 gallons
* 1 gal. of water = 8.345 lbs

### pipe sizing

* 8 to 10 FPS max. Velocity
* 6 FPS max. Velocity (public areas - noise)
* 10 to 15 FPS velocity in central plant headers or on roof
* 3" ano smaller: max head loss 6 ft/100 ft
* Schedule 40 blk. Stl. Pipe sizing: (* std wt)

### Hvac piping

* Chilled water: 2 GPM/ton (12°F Delta-t)
* Condenser water: 3 GPM/ton

#### Thermal Expansion

(steel pipe): 1" expansion/100 ft of pipe/100°F rise

### A/C condensate/waste sizing

| Tons     | Pipe Size |
| -------- | --------- |
| Up to 20 | 3/4"      |
| 21--40   | 1"        |
| 41--90   | 1-1/4"    |
| 91--125  | 1-1/2"    |
| 126--250 | 2"        |

### Air device sizing

| CFM       | Neck Size |
| --------- | --------- |
| Up to 150 | 6"        |
| 151--250  | 8"        |
| 251--450  | 10"       |
| 451--650  | 12"       |
| 651--800  | 14"       |

### FPB/VAV sizing

| Inlet Size | Max CFM |
| ---------- | ------- |
| 6"         | 450     |
| 8"         | 700     |
| 10"        | 1200    |
| 12"        | 1700    |
| 14"        | 2300    |

Note: runout distance is very important.
The inlet size shown is not necessarily the correct runout size to the box.
(Medium pressure runs 0.3 ft/100 ft max)

### Cooling loads

| Building Type      | SF/Ton   |
| ------------------ | -------- |
| Office buildings   | 300--325 |
| Tempered warehouse | 500      |
| Call center        | 275--300 |

* Typical air distribution for office bldgs:
    * north 1.5 CFM/SF
        (assumes 12 ft zone east 2.0 CFM/SF and 25°F Delta-t)
    * south 2.5 CFM/SF
    * west 2.5 CFM/SF
    * interior 0.8 CFM/SF
    * total should avg 0.8 to 1.0 CFM/SF gross

* Smoke evacuation: 1 CFM/SF

* Stairwell pressurization: 2000 CFM/floor

### Plumbing Fixture Runout Minimum Size Schedule

| Fixture                | Waste  | Vent   | Cold | Hot  |
| ---------------------- | ------ | ------ | ---- | ---- |
| Water Closets          | 4"     | 2"     | 1"   | -    |
| Lavatories             | 1-1/2" | 1-1/2" | 1/2" | 1/2" |
| Urinals                | 2"     | 1-1/2" | 3/4" | -    |
| Sinks                  | 2"     | 1-1/2" | 1/2" | 1/2" |
| Service Sinks          | 3"     | 2"     | 3/4" | 3/4" |
| Hose Bibbs             | -      | -      | 3/4" | -    |
| Box Hydrants           | -      | -      | 3/4" | -    |
| Electric Water Coolers | 2"     | 1-1/2" | 1/2" | -    |
| Showers                | 2"     | 2"     | 1/2" | 1/2" |
