Hvac Notes by Ocean

Check the RCP (Reflected Ceiling Plan)

Never decide a wall opening just by looking at duct lines. Start by checking the elevation of that wall section and the ceiling height. Then calculate how many inches of actual space you have to work with.

I usually start my measurement about 4 inches above the finished ceiling. For example, in a T-bar ceiling, the grid and wires are supporting the ceiling system. If you install your duct or hanger too close to the finished ceiling, Otherwise, you’ll find yourself standing with the GC and the framer, pointing fingers at each other about why the ceiling doesn’t fit anymore.

Always measure from the ceiling, not the floor. The floor height can change later when flooring finishes are added, and technicians working from lifts measure more accurately from above anyway.

Identify the Largest Duct

The first question before making any wall opening is: “What’s passing through here?”
Even if multiple ducts cross the same section, always locate the largest one (by height) that will determine the opening size and elevation.

If you don’t base the opening on the largest duct, you’ll regret it later when insulation or sealant makes the smaller ones suddenly not fit.

If There’s a Unit, Check the Submittal

This connects directly to the previous step. Sometimes, it’s not just ducts, there’s a unit attached to the line, often an exhaust fan that discharges through the exterior wall to a louver.

Before marking your opening, always review the submittal (spec sheet) of that unit.
Don’t rely solely on the schedule or plan, if the submittal later changes the unit’s dimensions, you’ll be redoing everything. The centerline of the largest duct or unit should align with the center of your wall opening.

How Much Clearance Should You Leave?

There’s no such thing as a “perfect cut.” No one - not you, not me - can make an opening exactly match the drawing. Even small deviations can cause grille misalignment or rework later.

That’s why I usually add 1-2 inches of clearance beyond the required opening size.

There’s also another kind of clearance worth noting: Try to position your duct or unit in the vertical center of the available space. It looks cleaner, helps with alignment, and prevents other trades from asking you to “just move it a little bit” later. (show them who is dominant)

Check the Exterior Finish

Exterior walls never end with bare concrete, there’s always a finish layer: stucco, EIFS, metal panel, or brick veneer, for example. So make sure to review the Architectural Plan (A-sheets) along with your mechanical drawings. It’s like checking with your next-door neighbor before drilling through their wall.

Avoid cutting openings near joints or seams, which can complicate waterproofing and detailing later. And if it’s a panel-type facade, once it’s installed, cutting through it is nearly impossible , so coordinate the opening early with the GC during the shop drawing stage.

ICC M1504.3 – Exhaust Openings

• Must be at least 3 ft (914 mm) from any property line, operable window, or door.
• Must be at least 10 ft (3048 mm) from any fresh-air intake opening.
• Exception: if the exhaust outlet is 3 ft higher than the intake opening, it may be closer.
  🔗 ICC Code Reference

Quick Reference Checklist

The first stage of any mechanical inspection is the Pre Wrap Inspection. It’s the check that must be done before ducts are insulated or enclosed. Once the ductwork is wrapped, you can’t see what’s underneath; so if you skip this inspection, you’d have to tear off the insulation later. That’s why the duct must be fully connected and approved before insulation begins.

A Rough Start

I went in half excited, half nervous but things didn’t go smoothly at first. I fumbled a bit during the initial inspection, and the inspector asked me to get better prepared and reschedule.

Then, we submitted two more inspection requests, but both were canceled by the GC. Their reason was that the installation “wasn’t perfect yet.” Technically, though, Pre Wrap isn’t a final inspection it’s more of a mid stage check, before the ducts are covered. A few minor imperfections are fine.

Still, the GC insisted that everything must look perfect before inspection. Looking back, I think they just wanted to avoid any red flags, since the overall schedule was tight. After about two weeks of delay, they suddenly rebooked it on their own, and ironically, the inspection passed easily almost anticlimactically.

Preparation

The first thing the inspector said when he arrived was:

“Do you have the approved plan set?”

I didn’t realize what that meant at first, but the GC handed over a stamped, approved plan set and that’s what the inspector wanted. He flipped through each sheet, verifying that the installed units matched the approved mechanical schedule exactly.

We tried showing him our tablet version of the drawings, but he refused, saying he only reviews the official stamped plans.

• Lesson learned: Always have a printed copy of your Approved Plan Set ready on site.

The Inspection

The inspector started by saying,

“Let’s go through each unit according to the drawings.”

He asked for the tonnage of each unit, then had his assistant take photos of the labels to verify against the schedule. Then came the big question,

“Where’s your seismic bracing?”

This project didn’t include any seismic bracing details in the design. Our boss had casually said, “It’ll be fine; we’ll deal with it later.” Well… we learned the hard way that “later” usually means “too late.” 😂

Even without specific bracing details, if the plan states SDC-D (Seismic Design Category D), you’re still required to follow CMC Section 605 and ASCE 7 for seismic restraint.

Fortunately, since this was just the Pre Wrap stage, the inspector agreed to defer seismic bracing to the next inspection (after insulation was complete).

What the Code Actually Says

1. Joints and Seams (Based on CMC Section 603.9)

Each duct joint should be tightly fixed with screws so that there’s no movement or air leak and sealed airtight using approved materials such as UL 181A or UL 181B tapes, mastics, or gaskets.

We used DP-1010 (UL 181 approved) sealant, and all joints were S & Drive connections, fully sealed after fastening.

At the grille connections, the ducts were wrapped with a blue self adhesive wrap, then taped over once more to prevent any air gaps or dust infiltration around the collar/boot.

2. Supports and Hangers (Based on CMC Section 604)

Duct supports must follow SMACNA standards or the manufacturer’s installation requirements. Additional supports are required near changes in direction or at transitions. Rod nuts must be double nutted or locked to prevent loosening.

We kept the support spacing within 8 feet, used 26 gauge galvanized steel ducts, and installed trapeze hangers at every elbow section. Any dents or visible damage on the duct surface can be grounds for rejection so visual inspection before wrapping is crucial.

3. Bracing (Based on CMC Section 605)

All duct systems must be braced and restrained to prevent excessive movement or lateral/horizontal swing due to vibration or earthquakes. Braces are usually set around 45°, and all anchors must be ICC-ESR approved.

We didn’t have the bracing installed yet, so this will be the focus of our next Seismic Bracing Inspection.

Helpful Resources

• Inspection Checklist (Unofficial Reference)
  It’s an old version, but still a good baseline for inspection prep.
  buildingincalifornia.com/checklists
  
  
• California Mechanical Code (Free Online Viewer)
  The official public version, searchable and always handy on site.
  epubs.iapmo.org/2022/CMC/index.html
  
  

Wrapping Up

The night before the inspection, I studied CMC Section 603 - Section 605 like I was preparing for a presentation…
and then, of course, the inspector didn’t ask a single code question 😅. After about 10 minutes of walking through the units and drawings, he signed off the plan, took a photo for records, and left. And just like that  Pre Wrap Inspection was done.

Next up: Seismic Bracing Inspection. Let’s hope I can nail that one too.

While reviewing the project’s mechanical submittals, I noticed something unusual. All units were supposed to be R-410A, but a few had been changed to A2L refrigerant.

At first, I thought it was just a simple model revision. Then I followed the list and saw where those units were going. One of them was assigned to the MDF/MPOE room. That’s when it clicked. A2L... mildly flammable... things get different when that goes into an electrical room.

I remembered what I’d read before. A2L isn’t the same as R-410A.

According to ASHRAE 34, R-410A falls into the A1 safety group “A” for low toxicity and “1” for non flammable under normal operating conditions. In short, it’s the reason most commercial HVAC systems have safely used R-410A for years without additional protective systems. But R32, “Mildly flammable” literally means what it says. Once that refrigerant enters a space filled with electrical equipment, extra safety measures aren’t optional anymore, they’re mandatory.

What Exactly Is an MDF/MPOE Room?

The MDF (Main Distribution Frame) and MPOE (Minimum Point of Entry) might sound like simple telecom spaces, but in reality, they’re the neural center of a building where electricity and data intersect. The MDF is the main hub where external communication lines like internet, phone, and data, first enter the building and get distributed to each floor.

The MPOE, on the other hand, is the entry point that connects the building directly to the service provider’s network.
Both are usually housed in the same room, where power panels, UPS units, switching hubs, and network racks are installed together.

In other words, it’s not just a communication room. It’s a hybrid electrical space packed with high density electronic equipment, power distribution panels, and tangled network cabling.

Physical Characteristics of This Space

The environmental conditions inside this room are completely different from an ordinary office space.

High Heat Load.
Network racks, servers, UPS units, and switching hubs operate 24/7. Each generates continuous heat, which is why these rooms require constant cooling, not comfort cooling, but operational cooling.

High Sealing and Limited Ventilation.
For security and dust control reasons, there are no windows, and air exchange with the outside is minimal.
In many cases, even the door gaps are sealed effectively making it a confined or sealed space.

Uneven Air Circulation.
Hot air accumulates near the ceiling while cooler air stays near the floor. This stratified condition means that any leaked gas could pool or stagnate in certain areas instead of dispersing evenly.

Code Classification: “Critical Electrical Space”

According to CMC, and ASHRAE standards, spaces like these are treated as critical electrical environments when flammable refrigerants are present.

CMC 2022

According to CMC 2022 Section 320.4, MDF/MPOE rooms (telephone and technology equipment centers) require cooling but not mechanical exhaust unless specified by the equipment manufacturer.

https://epubs.iapmo.org/2022/CMC/index.html

ASHRAE 34

ASHRAE 34 Section 6.1 classifies R-32 as an A2L refrigerant "low toxicity and mildly flammable"

ASHRAE 15
Section 7.3 defines the Maximum Allowable Limit (MAL) as 0.44 × LFL, which sets the maximum charge a space can safely contain.
Section 7.4 specifies that when the refrigerant charge exceeds this limit, the space must be classified as a machinery room and equipped with required safety controls.
Sections 8.11 and 8.12 further require leak detection and automatic emergency ventilation when A2L refrigerants are used in confined or machinery spaces.

https://www.ashrae.org/technical-resources/standards-and-guidelines

The Core Issue, R-32 Is an A2L Refrigerant

Once an A2L refrigerant is introduced, several safeguards automatically come into play:

• Leak Detector
  Detects when the refrigerant concentration in the room exceeds a certain threshold. Because A2L refrigerants are heavier than air and tend to settle near the floor, sensors must be installed low and close to the equipment rather than on the ceiling.
  (Referenced in ASHRAE 15 2019 Section 8.11.6 and Section  8.11.8 “Refrigerant Detection and Response.”)
• Emergency Exhaust Fan
  When a leak is detected, the exhaust fan must automatically start at high speed to dilute the refrigerant and prevent the room from reaching flammable levels.
  (Referenced in ASHRAE 15 2019  Section  8.11.11 Section  8.11.14  “Mechanical Ventilation and Level 2 Ventilation.”)
• BMS Integration
  The leak detector must be tied into the building management system:
  Detector → BMS → Exhaust Fan Override + Unit Power Shutdown.
  Only when this logic chain is complete does the system qualify as a proper “automatic mitigation response.”
  Detection alone isn’t enough, ventilation and shutdown must follow immediately.
  (Referenced in ASHRAE 15 2019  Section  8.11.6.2 and Section  8.11.7 “Automatic De-energization and Remote Controls.”)
• A2L Warning Signage
  Every person entering the space must be aware of the refrigerant class. Labels such as “Contains A2L Refrigerant Mildly Flammable” must be clearly posted, as required by UL 60335-2-40 and the appendices of ASHRAE 15-2019 Section 9.17  “Marking and Labeling.”
  https://www.hvacinformed.com/news/understanding-ul-60335-2-40-standard-co-1668171588-ga.1715243565.html

The Supervisor’s Comment “No Exhaust Fan Needed If It’s Under 6.6 lb”

When I brought this issue up to the team, my supervisor immediately responded:

“If the refrigerant charge is under 6.6 pounds, you don’t need an exhaust fan”

At first, I wasn’t sure.
Could that possibly be written anywhere in the code?
I dug out and according to CMC 2022 Section 1106.1.1, a machinery room is required only when the refrigerant charge exceeds the limits defined in Table 1102.3. Section 1106.2.5 further specifies that leak detection and automatic emergency ventilation apply exclusively to such classified machinery rooms, not to standard equipment rooms with smaller refrigerant quantities.

According to ASHRAE 15 Table 7.3,
R-32 has an LFL (Lower Flammability Limit) of 0.307 kg/m³ (0.019 lb/ft³).
By definition, the Maximum Allowable Limit (MAL) = LFL × 0.44,
which equals 0.135 lb/ft³.

That value defines how much refrigerant a space can safely contain before it crosses into “Machinery Room” territory.

For our MDF/MPOE room (13 ft × 16 ft × 9 ft = 1,872 ft³):

0.135 × 1,872 = 252 lb

The system’s charge of 6.6 lb was just a fraction of that, about 2.6% of the threshold, meaning the safety margin was over 40 times larger than required by code.

And that “6.6 pound” figure wasn’t arbitrary. Most mini split systems ship with around 3 lb of factory charged refrigerant. Even with an extended lineset, the total charge rarely exceeds 6 lb. So from a practical standpoint, it’s nearly impossible for such a system to approach the MAL threshold. In other words, even under the most conservative assumptions, an MDF/MPOE room equipped with a single 18,000 BTU mini split system would still remain well below Machinery Room classification, no leak detector, no exhaust fan, and no override logic required.

That night, I slept like a baby.

When a new project begins and I open the drawings for the first time, I can still remember that feeling clearly. Hundreds of plan sheets stacked in front of me, sections overlapping, details scattered everywhere. It wasn’t excitement that I felt at that moment. It was overwhelm. As I flipped through each sheet, I couldn’t help thinking:

“Can I really go through all of this without missing anything?”

Right now, I work on commercial projects in an HVAC QA/Assurance position. My job is to study drawings of spaces that don’t exist yet and spot potential issues before they become real ones to shape the work so it flows smoother when construction begins. That’s the essence of what I do.

I believe even the most complicated job becomes manageable when you break it down step by step. So every time a new project starts, I ask myself: “What’s the best order to dissect these drawings this time?”

Of course, reading through every single page would be ideal but in reality, nobody has time to go through hundreds of sheets. And honestly, most of them won’t matter.

That’s why this post isn’t a manual or a checklist from a textbook. It’s my personal note on drawing review priorities, shaped by what I’ve actually seen, missed, and learned on site. Whenever a new project starts, these are the parts I reach for first. In the end, my job isn’t just to fix problems after they happen, it’s to sense them before they appear. If something looks suspicious in the drawings, that’s my cue to start digging.

So here’s what I look for first when I open a set of drawings at the beginning of a project.

1. General Notes

The very first sheet, usually G0.01 is where I always start. The “G” stands for General, and this page carries the language of the entire project. Code references, mechanical standards, material specs, construction notes, and most importantly, the Seismic Design Category (SDC), you can tell what kind of project you’re dealing with just by reading this one sheet.

What is SDC?

SDC (Seismic Design Category) defines how much seismic load a structure must be designed to withstand.
It ranges from A through F:

• A → very low seismic risk
• E or F → essential facilities, like hospitals, that must not collapse under any circumstance

Somewhere in between categories B, C, and D are where most commercial buildings fall. By code, the SDC level is determined per ASCE 7-16, Table 11.6-1. But in practice, almost nobody calculates it from scratch in the field. If you’re working in Los Angeles, assume SDC D or higher it’s the safest bet. (There’s a whole truckload of stories behind how I once figured that out, but that’s for another post.)

In short: SDC D or above means seismic bracing is not optional. It’s required.

“Not shown” doesn’t mean “not required.”

This is the part many teams overlook. Even if the drawings don’t include detailed seismic bracing,
that doesn’t mean you can skip it. It means the QA or construction team is expected to handle it.The plans had zero duct or pipe bracing details  but the requirement still applied. Under SMACNA Seismic Restraint Manual, any SDC-D or higher project must provide both:

• Lateral bracing (side to side)
• Longitudinal bracing (along the run) for ducts, pipes, and equipment alike.

If you miss this check on the General Notes sheet, an inspector later saying “Where’s your bracing?” can easily turn into a full scale rework.

2. Structural & Slab Type

Before anything gets built upward, the very first thing to check is “What type of slab does this project have?” That’s not just a structural engineer’s concern, for QA, this one line of information can cut the project risk in half. Post tension, metal deck, precast, or cast in place. Whichever type it is determines the anchoring method we can use. If it’s a post tension slab, then drill in anchors are basically off-limits. You need to embed the anchor channels before the concrete is poured. 

From experience, post tension slabs demand extreme caution. Once the concrete is poured, you can’t just drill through ceilings or floors. On one project, the sprinkler trade said they’d start field work after the pour. The GC immediately pushed back:  “If you drill, you’ll need a 3D scan to locate the cables, and you’ll sign off that any damage repairs are 100% your responsibility.”  The reason? Post tension cables are under high stress. Hit one, and you’re looking at several thousand dollars in repair costs, minimum. And if it triggers a structural inspection, your schedule stops on the spot. 

For QA, a post tension slab is full of “do not touch zones.” It’s far safer to install anchors in the deck before the pour. When possible, I also use walls for support especially if they’re plain concrete walls without PT cables. But even then, you have to check the architectural and electrical plans, or ask other trades if they’re already claiming that wall space. It’s cheaper, safer, and saves everyone headaches later.As I like to put it:

“Embed early, sleep better.”
Trying to set anchors after the pour is almost a suicide mission.

For reference, while the 2022 California Mechanical Code doesn’t explicitly mention “limitations on post installed anchors by slab type,” the intent is strongly reflected in the California Building Code, Chapter 17, and related standards such as ACI 318. In short, verifying slab type before installation remains a mandatory QA step.

🔗 2022 California Building Code, Chapter 17 – Special Inspections and Tests (Section 1705.3)

3. Anchor Channel & Insert Details

For any SDC D or higher project, it’s mandatory to use ICC-ES-approved anchor channels. This isn’t about brand preference. It’s about using hardware structurally certified to resist seismic loads. According to SMACNA and ASCE 7-16, a seismic bracing system must be designed to withstand at least 1.4 times the design load.
So from a QA standpoint, the first question is always:

“Is this anchor system code approved?”

While reviewing drawings, you’ll often see the note “embed by structural.” That phrase means:

“This anchor channel must be embedded in the concrete before the pour by the structural team.”

In other words, it’s not something you drill in later. It must be installed into the slab ahead of time. This distinction becomes critical during layout. If you miss an embed before the pour, you’ll soon hear the dreaded line:

“There’s nowhere left to install anchors.”

At that point, your only option is a drill-in anchor, which is prohibited in post tension slabs. One overlooked embed can throw off the entire anchorage plan.

The next step is to create a dedicated anchor layout sheet, basically an anchor map. Using the drawings, I mark every anchor point and make sure the spacing between supports does not exceed 8 ft (≈ 2.4 m). This limit follows ASCE 7-16 Chapter 13 – Seismic Design Requirements for Nonstructural Components, which references the SMACNA Seismic Restraint Manual for maximum brace spacing.

🔗 ASCE 7-16 Chapter 13

During installation, elbows need the most attention. They’re vibration hot spots, so installing 45-degree braces at each elbow is ideal. That angle offers the best stability and keeps the duct steady during seismic movement.

For heavy equipment like HRUs, AHUs, or condensers always anchor at two opposite points. Uneven load distribution can pull a single anchor loose under stress.

At transition sections (the short ducts connecting units), adding one extra anchor is always worth it.That zone carries concentrated vibration and torque whenever the unit starts up. One additional anchor there can prevent the kind of mysterious ceiling rattle everyone hears months later.

We once learned this the hard way. During one project, we closed the ceiling without installing the seismic bracing anchors. At the time, we assumed that zone didn’t require bracing  but the inspector later pointed out,

“This is an SDC-D area; you still need a lateral brace here.”

Had we installed it, the plan was to invert the channel and attach a 45-degree adjustable brace for controlled movement the ideal setup for that condition. That experience changed how I look at anchor details forever. Now, every time I review drawings, I don’t just look for where anchors go and I ask whether the bracing geometry actually works in real space. Because one misplaced anchor can shift the entire seismic load path.

4. Equipment Schedule

When reviewing the Equipment Schedule, I don’t see it as just a list of models I see it as a map of installation conditions and potential risks. Every line on that table hints at how the project will behave once construction starts.

Mounting Type (ceiling / wall / pad)

The anchoring method changes completely depending on how the unit is mounted. Ceiling-mounted equipment requires hanger rods and trapeze frames, while pad-mounted equipment is fixed on a housekeeping pad using anchor bolts or snubbers. As a QA, I always cross-check whether the structural slab type matches the anchorage detail in the drawings. If those two don’t align, it’s a coordination issue waiting to happen.

Refrigerant Type (R-410A / R-454B)

R-454B is classified as an A2L refrigerant, meaning mildly flammable. Under CMC 2022, any system using A2L refrigerants requires additional safety measures like leak detectors and exhaust interlocks. So whenever I see “R-454B” on the schedule, my first QA question is:

“Does the sensor plan reflect this?”

If it doesn’t, that’s a red flag, the code compliance gap starts right there.

Condensate Drain (gravity vs. pump)

For gravity drains, confirm a 1/100 slope can be maintained and that the pipe won’t clash with structural beams. For pump drains, make sure the float switch interlock is properly tied into the BAS (Building Automation System). Condensate drains seem small, but in my experience, they’re one of the most common post–ceiling closeout issues. That’s why I always trace every drain line directly on the plan before anything gets covered.

Grille Locations

Grille locations on walls are just as critical. If the wall is concrete, the openings must be cored early and always double check that the opening sizes on the mechanical plan match those on the detail sheet.

QA Note: Submittal Sheet Management

Once drawing review is done, the next step is organizing the Submittal Sheets. It’s how we connect design intent to real materials. At the start of every project, I sort submittals by equipment type and inside each folder, I keep the submittal sheet, cut sheet, and ICC-ES report together. Then I cross check them against the Equipment Schedule:

• Does the model number match the drawing?
• Has the refrigerant type changed?
• Are the weight, anchorage method, and drain type consistent with the plans?

If the refrigerant is A2L (R-454B), I flag that page in the submittal  it usually includes the leak detector and ventilation requirements. When the inspector later asks,

“Where’s your sensor plan?”,
I can pull up that page immediately. I keep all of this organized in a file listing the equipment name, submittal number, refrigerant type, mounting type, voltage, and any special notes. If someone asks for anchor torque specs or clearance dimensions, I can find them in seconds.

5. Detail Notes & Opening Sizes

Even when the plan says

“Dryer Vent Opening: 8x32,”
the Detail Sheet might quietly add:

“Opening to be 4 inches larger in both directions.”

I learned that the hard way once. The main plan showed 8x32, so the GC cored it to that exact size. Later, I noticed a small note on Detail Sheet the same one above. By then, the grille was installed, but the sleeve didn’t cover the wall. We had to re-core, replace the sleeve, patch, repaint, and delay ceiling closeout. One line of small text turned into a full rework cycle.

That’s why QA can’t rely on the plan alone. Plan sheets show intent; detail sheets show reality. They’re often developed at different stages  plans early, details after shop drawings  and that timing gap causes dimensional drift (1–6 inches isn’t unusual). When “Opening,” “Sleeve,” and “Frame” are revised separately, a mismatch is almost guaranteed.

Here’s how I check every opening before it becomes a field issue:

1. Check the Mechanical Plan – confirm the base size (e.g., “Opening 8x32”).
2. Cross-check the Detail or Section Sheet – look for notes like “opening 4” larger,” “sleeve by structural,” “flange clearance.”
3. Check the Architectural Wall Legend – if it’s a fire-rated wall, reference UL 1479; the core usually needs to be larger.
4. Mark it in the field – add a simple note:

“Dryer Vent Opening @ East Wall – Verify with Detail 5/M5.01.”

That one step prevents most rework loops.