What is a Pressure Relief Valve and Does Your Cold Room Need One?
A missing pressure relief valve can make cold room doors impossible to open, or trap someone inside. Here's what a PRV does and whether your room needs one.
A butcher from Totnes recently found himself trapped inside his freezer room. He eventually got out, but only by beating his way through the door with a frozen black pudding.
The story comes from Marcus Stephens at Fermod, who shared it as a cautionary tale. The door handle was inspected afterwards and found to be fully functional. It wasn't faulty. The problem was something else entirely: the freezer room had no pressure relief valve.
Why Pressure Builds Up in a Cold Room
When a cold room or freezer is running, the refrigeration system constantly pulls heat out of the space. As the air inside cools, it contracts. This creates a pressure differential between the inside and outside of the room. The inside becomes slightly lower in pressure than the surrounding environment. A temperature change of just 1°C can create a pressure of 40kg per square metre, which gives some sense of the forces acting on the door.
The effect is a partial vacuum. And a partial vacuum is what makes doors notoriously difficult to open, even when the handle and hinges are in perfect working order.
When the door is opened, warm air enters the room as colder air spills out. After the door has been closed the warm air that is now trapped inside the room begins to be refrigerated and cools. As it cools it contracts and becomes denser. As it contracts it creates a partial vacuum sucking the door shut, until the pressure has balanced out. If you open a coldroom door, close it, then try to reopen it, it often won't. If door seals are worn or missing the air will slowly balance out through the gaps.
Pressure also builds during defrost cycles. If the handle and strike are not adjusted correctly, this can pop a door open unexpectedly or place additional stress on the door furniture.
This is a well-known phenomenon. Fermod and other hardware manufacturers build safety release handles specifically to address it. But even a quality safety handle struggles when there's nowhere for the pressure to equalise.
What Happens without a PRV
Without a pressure relief valve, the pressure inside the room has nowhere to go. It has to escape somewhere, and it will find the weakest point.
In the Totnes case, that weak point was the door handle. The pressure was gradually discharging through the handle mechanism, and because the handle had also been poorly fitted, ice began to build up within the internal release handle mechanism. Over time, the ice limited the handle's ability to operate as designed.
The result: a man and a black pudding, and a door that wouldn't open from the inside.
This isn't an isolated problem. Without a PRV:
- Doors become increasingly hard to open, from both inside and outside
- Pressure discharges through seals, gaskets, and hinges, accelerating their wear
- Ice builds up around door components, particularly handles and gaskets
- The entrapment risk increases significantly, especially in freezer rooms
What a Pressure Relief Valve Does
A pressure relief valve (also called a PRV or equaliser valve) is a simple device fitted to the panel wall of a cold room or freezer. When the pressure differential reaches a certain level, the valve opens briefly, allowing air to pass through and equalise the pressure on both sides.
Inside the valve are two membranes. One vents outwards and one vents inwards, allowing the valve to equalise pressure in either direction depending on which way the differential is running.
The valve does require a power supply, as it's fitted with a low amp heating coil to prevent moisture and ice build up around the membranes. Without this, the valve itself is at risk of icing over and failing at the exact moment it's needed.
The result is that your door seals stop taking the strain, your handles work as intended, and anyone inside the room can get out without needing to improvise with cured meats.
Does Every Cold Room Need One?
Cold rooms (above 0°C) can experience pressure differentials, particularly when the door is opened and closed frequently. Fitting a PRV is good practice.
Freezer rooms (below 0°C) should always have one. The pressure effects are more pronounced at lower temperatures, and the entrapment risk is more serious. If someone is trapped in a freezer room, the consequences are severe. A PRV is cheap insurance.
If you're not sure whether your existing cold room has one, look for a small circular or rectangular vent, typically around 50-100mm, fitted into one of the wall panels. Fermod PRVs are among the most widely used; you can view their range at fermod.com. Fermod also publish utility and mounting recommendations for their valve range if you want to understand correct fitting. If you're not sure which valve suits your installation, we can help.
What to Check If You're Having Door Problems
If your cold room door is hard to open, the internal release handle is not functional or gasket is missing, a blocked PRV may be the cause. Before replacing hardware, check:
- Does the room have a PRV fitted?
- Is it clear of obstruction and free to operate?
- Is the door handle correctly fitted and free of ice?
A PRV costs far less than a replacement handle, new gaskets, or an emergency callout, let alone the cost of a damage claim if someone can't get out.
Shop Pressure Relief Valves at Absolute Coldroom
We stock pressure relief valves suitable for cold rooms and freezer rooms. If you're not sure which one you need, get in touch or message us on WhatsApp on 07506 055 552 for help identifying the right part.
