Using a Manometer to Check Air Filter Condition in a Rooftop Unit

If your rooftop air handling unit has a manometer fitted, you already have a practical way to check whether the filter is starting to load up with dust. Rather than relying on guesswork, the pressure reading gives you a simple clue about how hard the system is working to pull air through the filter media.

For facility managers, mechanical contractors and industrial buyers, learning to read that gauge properly can help you spot when a filter change, inspection or deeper system check is needed.

What a manometer tells you

A manometer measures differential pressure across the filter. In plain terms, it compares the pressure on the upstream side of the filter with the pressure on the downstream side.

When a filter is clean, air passes through it more easily and the pressure drop across the filter is lower. As the filter captures more particles, resistance to airflow rises and the differential pressure increases.

That rising pressure is the maintenance clue. It does not tell you everything on its own, but it does tell you the filter is loading and the system is working harder to move air through it.

Where you will usually find the reading

On a rooftop unit, the manometer or pressure gauge is often mounted near the filter section or on the access panel. Some systems use a simple analogue gauge with a needle. Others use a manometer with two tubes or ports marked for high and low pressure.

Before opening the unit, make sure the system is operating in its normal mode if safe to do so. A reading taken when fans are off, dampers are closed, or the unit is in an unusual operating state may not be useful.

How to read the gauge

The exact scale depends on the type of manometer fitted, but the idea is the same: you are looking for the pressure difference across the filter.

On an analogue gauge, the needle may sit in a green, yellow or red range, or simply point to a value on the scale. On a fluid manometer, the difference in level between the two sides shows the pressure drop.

When reading the gauge:

• •confirm you are looking at the filter differential pressure, not another section of the unit
• •check the reading with the system running under normal operating conditions where possible
• •note the value and compare it with previous readings from the same system
• •look for a steady upward trend rather than a single isolated reading

A rising reading is the important part. A sudden jump can suggest the filter is heavily loaded, installed incorrectly, damaged, or that there is an issue elsewhere in the system.

What rising differential pressure usually means

As a filter catches more dust and debris, airflow resistance increases. That is expected. The key question is whether the pressure rise is happening in a way that matches the system’s normal operation.

A gradual increase over time usually means the filter is doing its job and is moving towards the point where it needs inspection or replacement. A sharp increase can point to a few common issues:

• •the filter is already heavily loaded with dust
• •the filter has been fitted incorrectly and air is bypassing or forcing unusual flow paths
• •upstream dust loading is higher than expected for the site
• •the wrong filter type has been installed for the application
• •the filter is damaged, wet, collapsed or blocked by debris

If a rooftop unit is serving an industrial area, dusty plant room, workshop or high-traffic entry, the pressure rise may happen faster than in a cleaner environment. Site conditions matter, so the manometer reading should always be considered alongside the application and the filter’s intended use.

How to use the reading for maintenance decisions

A manometer is most useful when it is part of routine inspection, not a one-off check. If your site has a maintenance log, record the reading each time the unit is inspected. Over time, those readings help show whether the filter is loading normally or changing more quickly than expected.

Use the gauge as one input, then inspect the filter and surrounding equipment:

• •check for visible dust build-up on the filter face
• •inspect the seals, frames and access doors for gaps or damage
• •look for signs of bypass, such as dust downstream of the filter section
• •confirm the fan and dampers are operating as intended
• •check whether the reading is consistent with the filter type fitted

If the manometer reading is high but the filter looks clean, there may be a fault with the gauge lines, the gauge itself, or the ductwork connections. If the filter looks loaded and the reading has climbed, the gauge is probably doing exactly what it should.

For guidance on sourcing the right replacement filter, you can also review our products or speak with our team about custom air filters if the rooftop unit uses a less common size or arrangement.

Common mistakes when checking a rooftop unit

A manometer is straightforward, but it is easy to misread if the system is not being checked consistently.

Watch out for these issues:

• •comparing readings from different operating conditions and assuming they mean the same thing
• •treating any increase as a failure, rather than a normal sign of filter loading
• •replacing a filter without checking whether the gauge porting is correct
• •overlooking a blocked gauge tube or poor connection
• •using the reading alone without inspecting the filter and housing

Also remember that inspection frequency depends on the system, the site conditions and the dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted.

If the unit forms part of a commercial ventilation system, the broader installation may also need to be considered against site requirements and the mechanical services engineer’s direction. For ventilation design and project context, AS 1668.2:2024 may be relevant, but site requirements should always be confirmed with the mechanical services engineer or building owner.

When to replace, clean or service the filter

The pressure reading can guide the decision, but it does not replace a physical check. If the filter is damaged, collapsed or contaminated in a way that affects performance, it should be dealt with promptly.

Some filters are designed to be washable - check the product instructions. Others are single-use and should be replaced rather than cleaned. Never assume a filter can be reused just because it looks serviceable from the outside.

If the rooftop unit is difficult to access, if the gauge reading is unclear, or if the system has a history of dust-related issues, organised filter servicing or commercial filter servicing can help keep the maintenance process consistent.

A simple field check routine

A practical rooftop check usually includes:

• •confirming the system is in a normal operating state
• •reading the manometer
• •noting any change from the last inspection
• •opening the access panel safely
• •inspecting the filter face, frame and seals
• •checking for dust bypass or damage
• •confirming the correct replacement size before ordering

If you need help measuring the existing filter before ordering a replacement, see how to measure an air filter.

Keeping rooftop units easier to manage

The best use of a manometer is consistency. When the same gauge is checked at regular inspection points, it becomes a reliable clue about filter loading and overall system condition.

That makes it easier to plan maintenance, avoid unnecessary guesswork and respond when the filter is nearing the end of its useful service in that specific application. It also helps you spot patterns, such as a site that loads filters faster than expected or a unit that needs a different filter arrangement.

For help matching the right filter to a rooftop unit, Total Air Filtration can supply or make the right filters for many applications. We are a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009. Call 07 3800 3448 or request a quote.

If you would like to discuss a specific rooftop unit or replacement requirement, contact us and we can help you work through the options.