Differential Pressure Switch (DPS) in AHU – A Practical Engineer’s View

A Differential Pressure Switch (DPS) is one of the simplest devices in an AHU, yet one of the most critical. It does not control airflow, optimize energy, or make systems intelligent. Its job is far more basic—and far more important.

It doesn’t calculate, optimize, or modulate anything. It simply answers one critical question:

It tells you whether the system is actually doing what you think it is doing.

What a DPS Really Is (and What It Is Not)

A DPS is a simple mechanical device that senses the pressure difference between two points and changes its contact when a preset value is crossed.

or we can say that A DPS senses the pressure difference between two points and changes its contact when a preset limit is crossed.

As per experience if there No signals, no PID, no fine control.

• It does not control airflow
• It does not regulate pressure
• It exists purely for proof, safety, and protection through alarms
• Output is binary: ON or OFF
• It is not meant for control or modulation

Think of it as the system’s truth detector.

Where DPS Is Used in an AHU (Real Applications)

1. Filter Choking Detection

This is the most common use and also the most frequently misunderstood one that i have seen on site.

The DPS is connected:

• One side before the filter
• The other side after the filter

As the filter loads with dust, pressure drop increases. When it crosses the set value, the DPS trips and tells the BMS:

Filters are dirty. let’s clean or replace them, that’s the common practice i have seen on site.

In most systems:

• This is an alarm only
• The AHU continues to run
• Maintenance decides the replacement schedule

Simple, effective, and energy-saving if used correctly.

2. Fan Airflow Proof – The Most Critical Role

A running fan does not guarantee airflow.

Best site practice:

• High-pressure port connected to fan discharge plenum
• Low-pressure port left to atmosphere (inside AHU or return section)

Why this works better than suction-to-discharge:

• It proves the fan is actually pushing air into the duct
• It avoids false proof when a downstream damper or fire damper is closed

This DPS answers a life-saving question:

“Is air really moving, or is the fan just spinning?”

3. Heater and Coil Protection

For heaters, airflow proof is not optional.

If air stops and the heater stays ON:

• Coils burn
• Fire risk increases
• Equipment damage is guaranteed

That’s why, in many projects:

• The heater airflow DPS is hard-wired
• It physically cuts heater power
• BMS only monitors the status

Software logic is good.
Hard-wired safety is better.

Why DPS Matters

From site experience, DPS prevents:

• Heater failures
• Coil freezing
• Fans running blindly against closed systems
• Energy waste due to choked filters
• Compliance failures in pharma and hospitals

Most importantly, it prevents assumed operation, which is where most HVAC failures start.

How DPS Actually Works

Inside the DPS:

• A diaphragm senses pressure difference
• A spring sets the trip point
• A micro-switch changes contact.

When Delta-P exceeds the set value, the contact flips.

Because it’s mechanical, DPS devices are:

• Reliable
• Easy to test
• Predictable during failures

DPS vs DP Transmitter – Clear Line You Should Never Cross

Here’s the rule that avoids 90% of confusion on site:

• DPS = proof and safety
• DP Transmitter = control and optimization

If you need:

• ON / OFF confirmation → DPS
• Continuous value for control → DP transmitter

Trying to control pressure using a DPS is simply wrong.

Parameter	DPS (Differential Pressure Switch)	DPT (Differential Pressure Transmitter)
Basic Function	Provides proof or alarm	Measures and reports pressure continuously
Output Type	Binary (ON / OFF)	Analog (4–20 mA or 0–10 V)
Nature of Signal	Discrete status	Continuous value
Accuracy	Low (threshold-based)	High (measured value)
Typical Use	Safety, interlocks, alarms	Control, monitoring, optimization
Control Capability	Cannot control	Used for PID control
Use with VFD	Not suitable	Fully suitable
BMS Input Type	Digital Input (DI)	Analog Input (AI)
Typical Applications	Fan airflow proof, filter dirty alarm, heater safety	Room pressure control, duct static control, filter trend analysis
Response Expectation	“Is condition met?”	“How much pressure exists?”
Installation Sensitivity	High (tubing, moisture, range)	Moderate (needs calibration)
Maintenance Needs	Low, mechanical	Periodic calibration required
Cost	Low	Higher
Failure Impact	Missed alarms or nuisance trips	Control instability or wrong modulation
Best Role	Truth check and protection	Control and performance tuning

Delta-P Settings – Where Most Mistakes Happen

Filter DPS Settings (Typical)

• Pre-filter: around 100–150 Pa
• Fine filter: around 250–300 Pa
• HEPA: as per OEM, often 400–600 Pa

Golden rule:
Never set DPS at clean filter pressure.
Set it around 1.5 to 2 times the clean value.

Fan Airflow Proof Settings

These are low values, because the goal is proof, not measurement.

• Supply fan: roughly 30–80 Pa
• Return or exhaust: slightly lower

If airflow exists, DPS should trip. That’s all it needs to do.

Practical Field Problems

• DPS range too high → diaphragm never reacts
• Tubing connected backwards → false healthy status
• Long or sagging tubes → delayed response
• Moisture trapped in tubing → random alarms

A very common issue:
Condensation forms in sagging tubes and blocks pressure transmission.

Solution

• Keep tubing short
• Maintain continuous slope
• Avoid U-loops
• Provide drain points if required

Open Tubes vs Static Pressure Probes

• For filters: open-ended tubes are fine
• For fan airflow proof: static pressure probes are better

High air velocity blowing directly into a tube can create false readings. Static probes avoid that.

This small detail makes a big difference in high-speed systems.

DPS and BMS – How They Work Together

From a BMS point of view:

• DPS is wired to a digital input
• Usually as a dry contact

Typical fan proof logic:

1. Fan command ON
2. Delay timer (10–20 seconds)
3. Check DPS status
4. If not proven → trip and alarm

For filters:

• Continuous monitoring
• Alarm only
• No shutdown unless process demands it

Limitations of DPS

A DPS:

• Cannot modulate or control airflow
• Does not show actual pressure values
• Is sensitive to poor installation (tubing, moisture, vibration)
• Can drift over time if abused

It is reliable—but only when installed and used correctly.

Advancements Around DPS

Modern systems improve DPS usage by:

• Pairing it with DP transmitters (safety + control)
• Using smarter BMS logic with delays and confirmations
• Applying predictive maintenance using pressure trends
• Improving accessories like probes and moisture-resistant tubing

The DPS itself is simple—but the system around it has become smarter.

Final Takeaway

A Differential Pressure Switch will not improve your efficiency numbers on paper.
What it does is far more important — it tells you the truth about what’s really happening inside the system.

It makes it clear:

• Whether air is actually moving or the fan is just running
• Whether filters are genuinely loaded or just assumed to be fine
• Whether it is safe to keep the equipment running or time to stop

When applied and set correctly, a DPS quietly protects equipment, people, and regulatory compliance.
When installed casually or ignored, it turns into just another alarm that no one trusts.