Coils – Varitec Solutions

Designing Beyond Startup

admin — Wed, 23 Sep 2026 07:05:32 +0000

Dynamic Meta widget allowed only for Posts listing source or in Post context

6 -10 Minute Read Time

Designing Beyond StartUp

How to Preserve Your Airside Design Intent

Equipment selection establishes what an airside system can do. Controls, installation, commissioning, and maintenance determine how much of that performance actually survives.

Design intent becomes real in the space between the drawings, the installed system, and the people expected to operate it.

Correct Pieces Can Still Create the Wrong Outcome

A fan can be selected correctly and still disappoint in the field.

The scheduled airflow can be right. The static pressure can be right. The efficiency can be right. The coil can meet capacity. The damper can meet the leakage requirement.

Then all of those individually correct pieces get installed in an actual building.

A tight elbow lands too close to a fan inlet. Duct transitions don’t look quite like they did on paper. A sensor moves because the original location is inaccessible. A sequence gets interpreted differently than the engineer expected. Someone overrides a point during startup and nobody remembers to put it back.

Nothing has necessarily failed.

But the operating system can still drift away from the design.

Design Intent Can Start Drifting Before the Building Even Opens

Airside systems don’t usually lose performance because of one spectacular mistake. More often, they lose it a little at a time.

One decision affects another. A field condition creates a workaround. The workaround changes how controls need to behave. The new sequence solves one problem but creates another operating condition nobody tests because the weather isn’t cooperating on commissioning day.

By occupancy, every individual decision may have been perfectly defensible. The trouble is that buildings do not experience those decisions individually. They experience the system they create together.

Consider something as fundamental as fan performance.

A fan selection is based on more than airflow and static pressure. It also assumes the fan will be installed with inlet and outlet conditions that allow it to perform reasonably close to its tested rating.

That assumption can unravel when the physical installation is developed. The mechanical room gets tighter. Ductwork competes with structure, piping, and electrical systems. A transition is shortened during coordination. An elbow moves closer to the fan after value engineering or an equipment substitution. In the field, an installer makes the geometry fit the space available.

Each decision may be understandable on its own. Together, they can change the airflow entering or leaving the fan.

AMCA calls the resulting performance loss system effect. The fan may still be the correct model, and the scheduled values may still look correct, but the installed system no longer matches the conditions on which that selection was based.

This is why preserving design intent requires more than selecting the right fan. The design must provide enough space for the fan to perform—or explicitly account for the penalty when ideal inlet and outlet conditions cannot be maintained.

Controls cannot recover that lost geometry. A VFD can increase fan speed, but it may do so at the cost of energy, noise, and equipment stress. It cannot straighten distorted airflow or restore a transition that disappeared during construction.

Same fan selection. Different approach conditions. Different installed performance.

Controls: They Cannot Program Around Physics

A VFD can change fan speed. A static pressure reset can reduce unnecessary fan energy. A good sequence can respond intelligently to changing demand.

But controls can't make bad airflow entering a fan disappear.

The same principle applies throughout an airside system.

Fan

Poor transitions, elbows, obstructions, or uneven airflow near the fan can change what the installed system is capable of delivering compared with the conditions used to establish fan performance.

Coil

A coil isn’t only a capacity number. Its selection affects pressure drop, approach, controllability, maintenance access, and how the system behaves at part load.

Damper

A damper isn’t simply an item on a schedule. Leakage, sizing, actuator selection, authority, and installation all influence how accurately the system can control airflow.

Sensor

A sensor doesn’t become useful simply because it appears on a controls drawing. It has to measure conditions that actually represent what you’re trying to control.

This is why designing for long-term operation has to begin before controls ever arrive on the job.

Where Design Becomes Behavior

Eventually, the drawings have to become decisions.

OPEN THIS DAMPER / RESET THIS PRESSURE / RAMP THIS FAN / ENABLE THE ECONOMIZER / GENERATE AN ALARM

That’s what makes controls such an important handoff. The sequence of operation is where engineering intent becomes actual building behavior. Small ambiguities here can have surprisingly long lives.

Installing a VFD creates the ability to vary fan speed, but that doesn’t automatically mean the system will operate efficiently. The sequence still has to tell the system when and why pressure should change. The same is true of supply-air temperature reset, economizer control, demand-controlled ventilation, humidity control, and alarms.

What We See in the Field

A reset strategy exists in the design but disappears during programming.

Temperature and humidity sequences compete with each other.

An economizer technically operates, but not the way the equipment selection assumed it would.

A sensor is installed somewhere convenient rather than somewhere representative.

Hundreds of alarms are created, and operators quickly learn to ignore all of them.

The equipment may still work.

That's a dangerously low bar.

Make the Sequence Explain Itself

A good control sequence shouldn’t require the next programmer, commissioning agent, or facility engineer to reverse-engineer what the designer meant.

Why is static pressure resetting?

What happens as zones become satisfied?

When does the economizer enable and disable?

What should happen if a sensor fails?

What deserves an alarm?

What should the system do next?

Those answers shouldn’t live only in the head of the person who designed the system.

ASHRAE Guideline 36 was developed around standardized high-performance HVAC sequences intended to improve efficiency, control stability, and fault detection. It is useful not because every building should be programmed identically, but because it demonstrates what happens when sequences are treated as an engineering discipline instead of an afterthought.

One of the best things a project team can do is bring controls into these conversations early enough to ask uncomfortable questions while they are still inexpensive to answer.

Can the equipment actually do what the sequence is asking it to do?

That conversation is far easier during design than after someone is standing in front of a running air handler trying to program around the answer.

Commissioning: It Runs Is Not the Same as It Works

A fan spinning in the correct direction proves that a fan can spin in the correct direction. It does not prove the system works.

START-UP

COMMISSIONING

Does the component respond?

Does the complete system behave correctly?

Command actuator → actuator moves

Test operating modes, changing demand, sensor failure, alarm response, and performance over time

One device at one moment

Interactions across different conditions

Imagine commissioning an economizer during a week when outdoor conditions never require full economizer operation. The actuator moves. The point changes on the graphic. The minimum position looks reasonable. Everyone checks a box.

Three months later, the first favorable morning arrives and the dampers finally have to travel through their full range. That is when someone discovers the linkage problem, the sensor problem, or the control loop nobody had an opportunity to watch under actual operating conditions.

A Snapshot Is Not a Year

The same thing can happen with occupancy schedules, pressure resets, supply-air temperature reset, demand-controlled ventilation, staging, humidity control, and alarms. A system can pass a snapshot test and still behave badly across a year.

Functional Performance Testing Should Demonstrate Real Modes

Test major modes of operation rather than only component response.

Use trend data to see how the system behaves over time instead of relying entirely on a momentary observation.

Verify that reset strategies actually change in response to system demand.

Test alarm logic and failure conditions.

Confirm that BAS graphics make system behavior understandable instead of merely displaying points.

Document what was changed during commissioning and why.

Berkeley Lab’s large commissioning meta-analysis evaluated hundreds of new and existing buildings and found meaningful energy and non-energy benefits from commissioning. Earlier work also found that more comprehensive commissioning produced considerably greater savings than less-thorough efforts.

Commissioning isn’t valuable because someone completed a checklist. It is valuable because somebody went looking for the gap between what the building was supposed to do and what it was actually doing.

That's the gap worth designing out.

Serviceability: The Part of Design Everyone Eventually Has to Live With

Mechanical rooms are remarkably spacious before they’re built. Then come the ducts, piping, conduit, valves, panels, and the one piece of equipment someone swears wasn’t shown there when the access door was coordinated.

A technically reachable component can still become a long-term operating problem when access requires extraordinary effort.

Five Years Later the Technician Inherits All of It

Nobody remembers how clean the installation looked during turnover. They remember the actuator buried behind piping. The filter that can’t come out without being bent. The coil that no one can get a brush into. The sensor that technically can be calibrated if you’re willing to become a contortionist.

And they definitely remember the BAS graphic that provides hundreds of pieces of information while somehow avoiding the one thing they need to know:

What is this unit trying to do right now?

These sound like inconveniences. Over twenty years, they become performance problems.

If a filter is miserable to change, maintenance is more likely to be delayed.

If coil cleaning requires an unreasonable amount of labor, it becomes easier to postpone.

If an actuator takes hours to access, the temporary workaround starts looking increasingly permanent.

If alarms are not useful, people stop paying attention to them.

If nobody understands why a sequence is doing what it is doing, overrides have a funny way of becoming operating strategies.

Performance Drift Rarely Arrives With Sirens

It accumulates: one dirty coil, one overridden point, one failed sensor, one disabled alarm, one sequence somebody stopped trusting.

The Best Optimization Opportunity May Happen Before There Is Anything to Optimize

Airside optimization is often discussed as something we do to an existing building: find the problem, tune the sequence, reset the pressure, correct the airflow, change the schedule, clean the coil, fix the damper.

Those things matter. But some of the best optimization work happens years earlier, when the project team makes it less likely that those problems will develop in the first place.

Before the Drawings Are Issued Ask Five Questions

01 ABOUT THE FAN

Does the installation give the fan a reasonable chance of performing the way it was selected to perform?

02 ABOUT THE COIL

Did we consider what happens at part load, not simply whether it meets peak capacity?

03 ABOUT THE CONTROLS

Does the sequence clearly explain how the system should respond as conditions change?

04 ABOUT COMMISSIONING

Can we demonstrate those operating modes before turnover, or at least trend and verify them when conditions allow?

05 ABOUT SERVICE

Can someone maintain the thing without fighting everything installed around it?

These aren’t separate conversations. They’re the same conversation at different points in the building’s life.

A Better Test for Design Intent

Before a project leaves design, imagine the documents landing on three desks.

ENGINEER

Will this meet the performance requirements?

CONTROLS CONTRACTOR

Can I make the system behave this way?

TECHNICIAN

Can I keep the system behaving that way?

The first question gets a lot of attention.

Long-term performance depends on all three.

Long Term Performance Depends on All Three

The first question gets a lot of attention. Long-term performance depends on all three.

Design intent isn’t preserved by the specification sitting in a project folder. It is preserved through thousands of small decisions made by the people who inherit that specification afterward.

Good airside design gives everyone a fighting chance.

Twenty years from now, that may matter considerably more than how good the mechanical room looked on turnover day. So, if you do one thing, talk to the facilities team if you can. Find out what matters to them or what struggles they’ve run into that they’d like to avoid in the future.

If there isn’t a team in place, take the extra moment to get feedback from a mechanical contractor or service provider that you trust. Chances are that they will have insights that will not just benefit the owner but will bolster you and your career for years to come.

Want to Go Further

Airside performance doesn’t stop with controls, commissioning, or serviceability. Fan selection and system effect, coil performance, leakage, reset strategies, sensor location, controls, and maintenance all interact. Sometimes the smallest decision in one part of the system creates a surprisingly large consequence somewhere else.

WATCH THE AIRSIDE OPTIMIZATION WEBINAR

Explore practical ways to improve airside performance from design through operation.

What the Schedule Doesn’t Tell You About Fan and Coil Performance

admin — Thu, 27 Aug 2026 19:26:34 +0000

Dynamic Meta widget allowed only for Posts listing source or in Post context

Coils

6 -10 Minute Read Time

The easiest thing to blame when an air handler comes up short on airflow is the fan.

It has a curve, a scheduled operating point and a motor with enough horsepower. So when TAB measures less air than the schedule calls for, the fan is usually where the finger lands first.

Two air handlers can satisfy the same airflow and cooling-capacity requirements on paper and still behave very differently once they’re operating. — One may spend much of its life comfortably at part load. Another may require more fan energy to overcome pressure losses. One coil may give the designer plenty of room for dehumidification and control. Another may meet capacity while leaving less margin when conditions change.

Both selections can be correct.

They are not necessarily equivalent.

A Fan Selection Is More Than a Design Point

Most fan selections begin with required airflow and static pressure.

Those two conditions establish an operating point on the fan curve — but the fan doesn’t operate independently of the system connected to it.

Loads rise and fall. VAV boxes modulate. Outdoor-air quantities change. Filters load. Economizers open and close. Spaces get used differently than anticipated. As airflow requirements and system resistance change, the fan’s operating point can change with them.

That makes understanding the fan curve important beyond the initial selection.

Air leakage, pressure drop, component selection and installation conditions all influence how the airside system ultimately performs.

There is another consideration behind that curve.

Fans are rated under standardized test conditions in controlled test labs. The installed conditions aren’t always as cooperative.

An elbow close to the inlet, an abrupt transition, limited straight duct or an unfavorable discharge arrangement can affect performance. Even fan rotation relative to the first duct turn can matter.

These installation-related losses are commonly referred to as system effect.

The fan curve isn’t wrong. The fan simply may not be operating under the same conditions used to establish its published performance.

Image Credit: https://www.greenheck.com/resources/blog/understanding-fan-system-effects

Published fan performance assumes standardized test conditions. Installed inlet and discharge conditions can affect actual performance.

The typical field response is understandable, but that is why an airflow problem shouldn’t automatically lead to one response:

Increase the speed.

Before Increasing Speed, Follow the Law – Fan Law

Sometimes increasing fan speed is exactly what needs to happen. It just shouldn’t be automatic.

The fan laws show why:

Airflow ∝ Speed

Pressure ∝ Speed²

Power ∝ Speed³

A 10% increase in fan speed theoretically increases airflow by about 10%, pressure by about 21% and power by about 33%.

That creates two questions:

Where did the additional pressure requirement come from?

And:

Can the motor handle the additional brake horsepower?

A motor selected closely around the original operating point may not have enough capacity for the additional load.

Before adding speed, it’s worth understanding what changed in the system — and whether the fan and motor can support the new operating point.

And one of the first places worth looking is sitting upstream or downstream of the fan.

A Coil Selection Is Also an Airside Decision

Cooling coils tend to be judged first by capacity.

Can the coil meet the sensible and latent load at the scheduled conditions?

It has to.

But the first coil selection that meets capacity isn’t necessarily the right selection for the unit.

During AHU selection, each internal component contributes to the unit’s total pressure requirement. Filters, dampers, coils and other components are configured before the final fan selection is validated.

That means changing the coil can change the fan selection.

A coil may meet capacity but carry more airside pressure drop than another option. A larger coil face may reduce face velocity and pressure drop, but require a larger casing and more first cost. Additional rows may provide more heat-transfer surface while also affecting pressure drop and access for cleaning.

This is where “meets capacity” stops being enough.

TECHNICAL NOTE | Why Face Velocity Matters

Face Velocity = Airflow ÷ Coil Face Area
For the same CFM, a larger coil face means lower face velocity. That can reduce airside pressure drop and help reduce the potential for moisture carryover from a wet cooling coil — but it can also increase AHU size and first cost.

Then there’s the coil five years from now.

Can it be accessed? Can it be cleaned? What happens to pressure drop when the coil and filters no longer look the way they did on startup day?

Those aren’t just maintenance questions.

They’re performance questions.

Capacity tells us whether the coil can meet the load. Face velocity, geometry, pressure drop, controllability and serviceability help determine how well the system performs while doing it.

Design Insight

The best-performing fan or coil isn’t necessarily the one with the highest efficiency. It’s the one that best supports the way the building will actually operate.

There is rarely one “correct” selection.

A hospital may place a premium on humidity control, redundancy and reliability. An office building may prioritize energy performance. Another project may be constrained by footprint, acoustics or first cost.

The goal isn’t to maximize every variable.

It’s to understand which compromises the project is making — and why.

Energy, humidity control, acoustics, redundancy, first cost, serviceability, controls and lifecycle performance all influence what better means for a particular application.

That is what turns equipment selection into engineering.

The Controls Eventually Inherit Those Decisions

Once the AHU starts, the controls system works with the machine that was selected and installed.

It can’t remove pressure drop from a coil. It can’t correct poor fan inlet or discharge conditions. It can’t manufacture fan capacity that isn’t there.

But controls can have an enormous influence on how the available capacity is used.

Consider duct static pressure in a VAV system.

A fixed static-pressure setpoint can ask the supply fan to maintain pressure even when downstream demand has fallen. The VFD makes the pressure. The VAV dampers throttle it away.

Static-pressure reset allows the target to respond to actual system demand.

The question becomes:

How much pressure does the building actually need right now?

The same principle applies to coil control. A coil that satisfies a peak-load calculation still has to be controllable as airflow, water conditions and load change.

Selection establishes the operating envelope. Controls determine how intelligently the system moves within it.

When the Fan Really Is the Problem

None of this means the fan gets a free pass.

If an AHU is short on air, actual fan speed should be compared with commanded speed. Rotation, VFD limits, fan-array operation, motor capability, wheel condition and the measured operating point all deserve verification.

The difference is that by then, we know more.

We know the pressure drop across the filters and coil. We know something about system resistance. We know whether the fan is seeing reasonable inlet and outlet conditions. We know what static pressure the controls are asking it to maintain.

Now, if the fan cannot make the duty, we can say so for a reason.

The Schedule Is the Beginning, Not the Finish Line

Schedules give us a common basis for selection.

Buildings are less cooperative.

Filters load. Dampers actuate. Coils get wet and dirty. Loads change. Components need maintenance. Controls respond to what is happening throughout the system.

That is why the best-performing fan or coil isn’t necessarily the one with the most impressive number on a selection report.

It’s the one that best supports the way the building needs to operate.

Sometimes the consequences of those choices don’t become obvious until TAB is standing in the mechanical room looking for 2,000 missing cfm.

By then, the question is no longer whether the schedule was right.

It’s whether the entire airside system had what it needed to succeed from the start.

And that’s a much easier question to answer during selection than during TAB.

Next month, we’ll bring the pieces together and look at what happens after startup — and why controls, commissioning and serviceability are essential to achieving the performance the design intended.