Pressure imbalance is one of those spray foam problems that can send a crew down the wrong troubleshooting path fast.

One gauge is running higher than the other. The spray pattern changes. Output drops. The foam doesn't look right.

The first reaction may be to blame the proportioner.

But an A-side/B-side pressure imbalance doesn't necessarily mean the proportioner itself has failed. A restriction at the gun, clogged screen, material-supply problem, transfer pump issue, hose restriction or temperature difference can all affect what you're seeing at the gauges.

The key is understanding what the pressure readings are telling you.

What Does a Spray Foam Pressure Imbalance Mean?

In a properly operating plural-component spray foam system, the A-side (ISO) and B-side (resin) need to be delivered at the correct ratio and under appropriate operating conditions.

When one pressure begins running significantly higher or lower than the other, something in the system may be restricting material or preventing adequate material supply.

One of the easiest ways to begin troubleshooting is to ask two questions:

Which material appears to be lacking at the gun?

And:

Is that material's pressure higher or lower than the other side?

Those two observations can help point you toward the part of the system that deserves attention first.

High Pressure Can Point to a Downstream Restriction

Here's one of the most useful troubleshooting principles for spray foam contractors:

If a material is lacking at the gun but that same side shows HIGHER pressure, start looking for a restriction downstream of the pressure gauge.

Think about putting your thumb over the end of a garden hose.

The water has a harder time escaping, so pressure builds behind the restriction.

Graco uses essentially this same analogy in its Reactor pressure-imbalance troubleshooting documentation.

For example, if A-side material appears to be lacking at the gun while the A-side pressure is higher than B-side pressure, possible A-side restrictions can include:

• Mixing chamber
• Gun screen
• Whip hose
• Hose temperature sensor area
• Main fluid line
• Fluid manifold outlet

Cold material or a heater problem can also increase viscosity and contribute to increased resistance.

So don't automatically assume:

High A pressure = too much A material.

The opposite may actually be occurring at the gun.

The high reading may be telling you that the A-side material is encountering resistance somewhere between the gauge and the mixing chamber. Graco specifically identifies these downstream restrictions in its E24 pressure-imbalance troubleshooting procedure.

Low Pressure Can Point Toward Material Starvation

Now reverse the situation.

If a material appears to be lacking at the gun and that side's pressure is LOWER, the problem may be upstream of the pressure gauge.

In other words, the system may not be receiving enough material on that side.

Possible causes include:

• Transfer/feed pump problems
• Restricted feed hose
• Clogged filter or strainer
• Improperly vented drum
• Clogged desiccant breather
• Restricted pump inlet
• Debris or a stuck check ball
• Material being diverted back toward the drum

These are among the supply-side conditions Graco identifies when diagnosing pressure imbalance.

That gives contractors a useful starting rule:

HIGH pressure → investigate a restriction downstream.

LOW pressure → investigate material supply upstream.

It's not a complete diagnosis by itself, but it can dramatically narrow down where to start looking.

A-Side High and B-Side Low: What Should You Check?

Suppose the A-side pressure is considerably higher than the B-side.

There are two broad possibilities worth investigating.

There may be an A-side restriction downstream, preventing A material from flowing normally through the hose, gun screen or mixing chamber.

Or the B side may be experiencing a supply problem upstream, causing B pressure to fall.

That's why pressure readings should always be considered alongside what is actually happening at the gun.

Don't diagnose the problem from one gauge alone.

B-Side High and A-Side Low: Reverse the Troubleshooting

The same principle works in the opposite direction.

If B-side pressure is unusually high, investigate possible restrictions on the B side downstream.

If A-side pressure is unusually low, investigate whether the A side is being starved upstream.

The important thing is to troubleshoot the system logically rather than immediately replacing expensive components.

Check the Spray Gun Before Assuming You Have a Major Equipment Failure

The gun is one of the first places worth inspecting because small restrictions can create surprisingly large problems.

Check the:

• Mixing chamber
• Gun screens
• Side seals
• O-rings
• Material passages
• Gun block
• Fittings

A partially clogged screen or mixing chamber can reduce flow without completely stopping the gun.

Spray Foam Gear's guide on when to replace your spray foam gun parts covers several warning signs contractors should watch for.

Replacement gun parts, O-rings, filters and other components are available in our spray foam tools, parts and equipment collection.

Our existing gun-parts guide also identifies clogged filters and screens as possible contributors to reduced flow, pressure imbalance and off-ratio conditions.

Don't Overlook Filters and Screens

Filters are inexpensive compared with the equipment they're protecting, but they can cause major headaches when neglected.

A filter doesn't have to be completely blocked to affect material flow.

Contamination and buildup can gradually restrict flow, changing the way the system behaves.

That's why filter and screen inspection should be part of routine equipment maintenance—not something the crew checks only after the rig stops spraying.

We've also covered this in our guide to the five most common spray foam equipment failures. Dirty filters can reduce flow and contribute to pressure and spray-performance problems.

Inspect the Heated Hose and Whip Hose

If the gun looks good, work backward.

Inspect the whip hose and main heated hose for anything that could interfere with normal material delivery.

Look for:

• Kinks
• Crushed sections
• Damaged fittings
• Leaks
• Abnormal wear
• Temperature problems
• Material restrictions

Heated hoses deal with pressure, heat, movement and abuse on every job. Small problems can eventually affect material delivery and equipment performance.

Our 10-minute morning equipment check includes hose, gun and proportioner inspections designed to catch problems before spraying begins.

You can also shop spray foam hoses and related equipment.

Check the Transfer Pumps and Material Supply

If the pressure indicates that one side may be starving, move toward the drum.

Verify that the transfer pump is operating correctly and that material can move freely from the drum toward the proportioner.

Inspect:

• Drum material level
• Transfer pump operation
• Air supply to the pump
• Feed hoses
• Strainers
• Drum venting
• Pump inlet
• Connections

A feed-side problem can make it look like something much more complicated is wrong with the machine.

Graco specifically includes transfer-pump malfunction, restricted feed hoses or strainers, improper drum venting and inlet/check-ball problems among potential causes of low-pressure material starvation.

Temperature Matters More Than Many Contractors Realize

Pressure isn't determined only by pumps and restrictions.

Material viscosity matters too.

Cold material is thicker and harder to move through the system. That increased resistance can affect pressure.

So if the pressure readings suddenly change, don't ignore temperature.

Verify that the material and equipment are operating within the chemical manufacturer's specified conditions and make sure the heating system is functioning correctly.

A pressure difference isn't always evidence of a failed mechanical component.

Moisture and Contamination Can Create Problems Too

Keeping material and equipment protected from moisture and contamination is another important part of reliable operation.

Moisture can contribute to buildup, restrictions and equipment problems—particularly on the ISO side.

Keep connections clean when they're opened, maintain the air system properly and monitor desiccant condition.

For a deeper explanation, read why moisture is the silent killer of spray foam equipment.

Use the Pressure Gauges as Diagnostic Tools

The gauges aren't just there to tell you how much pressure the machine is producing.

They're giving you clues about what's happening inside the material-delivery system.

When you see an imbalance, think logically:

High pressure?

Look for something restricting material after the gauge.

Low pressure?

Look for something preventing enough material from reaching the pumping system.

Then verify your diagnosis by checking what is actually happening at the gun.

That approach can help you avoid replacing parts unnecessarily and get the crew spraying again faster.

Prevent Pressure Problems Before They Stop the Job

The best time to discover a clogged screen, worn O-ring, damaged hose or struggling transfer pump is before the crew starts spraying.

Build a short equipment inspection into the beginning of every workday.

Check the gun.

Check the filters.

Inspect the hose.

Verify the transfer pumps.

Look at your pressure readings.

Check temperatures.

And make sure the truck carries the common replacement parts needed to handle predictable failures.

A ten-minute inspection can be a lot cheaper than losing several hours of production.

For more troubleshooting and maintenance information, visit the Spray Foam Gear Field Guide.

And when you need spray foam gun parts, hoses, O-rings, filters, PPE, tools and contractor supplies, visit Spray Foam Gear.

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