Warmer Weather, Hidden Risk

Christmas is behind us, and we’re now entering the time of year when many of us start looking ahead to spring and summer - longer days, warmer temperatures, and (hopefully) a few well-earned breaks.

For building services, the shift into warmer weather can feel like a relief. Heating demand reduces, boilers run less, and energy consumption falls - all positive outcomes for both sustainability and operating costs.

However, for steam heating systems, the move from winter full-load conditions to spring and summer part-load operation can introduce a less obvious challenge: stall.

And in critical environments such as hospitals, a stall-related failure can be expensive, disruptive, and entirely avoidable.

Why steam coils are most at risk in spring

Many steam-using building services applications - particularly hospitals and healthcare estates - rely on steam coils in air handling units (AHUs) for frost protection and heating.

In winter conditions, when the ambient temperature is low, the AHU requires significant heat. That means:

Under these conditions, the steam pressure is usually high enough to push condensate out of the coil and across the trap, even if there is some backpressure downstream.

What changes when the weather warms up?

As ambient temperatures rise, less heat is required, steam demand falls and the control valve begins to close.

This is where the risk begins.

With reduced steam demand, the pressure at the coil can drop significantly. If there is also backpressure in the condensate return line - for example due to:

then the steam pressure may no longer be sufficient to push condensate through the trap.

The result is condensate backing up into the coil.

This condition is known as stall.

What is stall (and why does it matter)?

Stall occurs when a steam coil is operating at low load, causing the inlet steam pressure to drop, therefore the steam trap does not have enough differential pressure to discharge condensate.

It’s important to be clear on this point:

Stall is not a steam trap failure.
It is a differential pressure problem.

Even a perfectly selected and fully operational steam trap cannot discharge condensate if the available differential pressure is too low.

The consequences of stall

If condensate cannot drain, it accumulates inside the coil. This can lead to:

For a hospital or large building services site, a failed frost coil or damaged AHU can be hugely inconvenient and costly particularly if it affects ventilation performance, infection control requirements, or downtime across critical areas.

A common factor: control valve leakage

Stall risk can be made worse when a control valve is not performing correctly.

One common issue is seat erosion, often caused by:

When the seat erodes, the valve may no longer shut tightly. This can allow low-pressure steam to “weep” continuously into the coil even when there is minimal heating demand.

That creates two major problems:

In the worst cases, this combination can accelerate damage significantly.

Prevention and root-cause checks

The good news is that stall can be predicted, and the risks can be reduced with a combination of good design, correct selection, and practical system checks.

1) Control valve selection and protection

A correctly sized control valve, protected by the right upstream components, can significantly reduce the risk of erosion and leakage.

Key considerations include:

If steam quality is exceptionally poor, upgrading to a leak-tight control valve may be worth considering — such as a polymer-seated design.

2) Steam trap performance and suitability

Even though stall is not a trapping problem, the trap must still be correctly selected and fully operational.

Ask:

3) Differential pressure on low-load conditions

This is the critical question:

Does the trap have sufficient differential pressure to evacuate condensate when the coil is at low load?

If not, condensate will back up — regardless of trap condition.

4) Backpressure and condensate lift

Backpressure is often the hidden culprit, especially in building services systems.

Check:

Even a relatively small lift can create enough backpressure to prevent condensate discharge when the coil pressure drops.

5) Consider an Automatic Pump Trap (APT)

If differential pressure is too low, or backpressure is unavoidable, replacing the trap with an Automatic Pump Trap (APT) can provide a reliable solution.

An APT is designed specifically to:

Stall is predictable and preventable

Stall can result in significant damage to heat exchangers, coils and associated equipment, and it prevents the system from doing the job it was designed to do.

But the key takeaway is this:

Stall is a differential pressure issue not a trap issue and it can be predicted with a simple calculation.

Speak to Spirax Sarco

If you operate steam coils in AHUs particularly for frost protection spring is the ideal time to review stall risk before the next seasonal change catches you out.

Speak to your Spirax Sarco Area Engineer today to discuss a stall risk analysis and identify practical solutions for your site.