The Number One Unvented Hot Water Issue (And How It's Easily Preventable)

If you own an unvented hot water cylinder, there's one fault that causes more call-outs than anything else: a failed expansion vessel. It's the single most common problem we see, and the good news is that it's almost entirely preventable with a simple annual service.

Here's what's actually going on, why it matters, and how to stop it happening to your cylinder.

What Is the Expansion Vessel and Why Does It Fail?

An unvented hot water cylinder heats water in a sealed system, so as the water heats up, it expands. That expansion has to go somewhere — that's the job of the expansion vessel. Inside it sits a rubber diaphragm, with air on one side and system water on the other. As water expands into the vessel, it compresses the air pocket, absorbing that extra volume and keeping pressure in the cylinder stable.

Over time, that internal diaphragm loses its air charge. It's a gradual process — a small amount of pressure loss year on year — until eventually the vessel can no longer absorb the expansion it's designed for. When that happens, the cylinder has nowhere to send the extra volume of expanding water.

How This Leads to the Pressure Relief Valve Passing

Every unvented cylinder has a pressure relief valve as a safety back-up, set to open and discharge water if the system pressure gets too high. When the expansion vessel stops doing its job, that excess pressure has to escape somewhere — and it's the pressure relief valve that ends up dealing with it.

You'll usually notice this as a slow, intermittent drip from the discharge pipe, often outside the property or into a tundish. Homeowners frequently describe it as the valve "weeping" or "passing" — a small trickle rather than a dramatic leak. It's easy to dismiss, but it's a direct symptom of the expansion vessel behind it having failed, not a fault with the valve itself.

Left unaddressed, this generally goes one of two ways:

• The valve continues to pass intermittently, wasting water and (if it's mains-fed) adding to your water bill.

• Pressure inside the cylinder continues to build during heating cycles, putting unnecessary strain on the vessel, valve, and cylinder itself.

Why This Is So Easily Preventable

This is the part that frustrates us most, because a failed expansion vessel is one of the easiest things to catch early — if the system is checked regularly.

During an unvented hot water cylinder service, we check the pre-charge pressure of the expansion vessel against the system's cold fill pressure. If the vessel has lost its charge, it gets re-pressurised or replaced there and then, before it ever gets to the point of the pressure relief valve passing. It's a five-minute check that prevents a call-out.

We also check:

• The cold fill (pressure reducing) valve

• The temperature and pressure relief valve itself

• The tundish and discharge pipework for correct operation

• The expansion vessel pre-charge pressure

• General condition of the cylinder and associated safety devices

Manufacturers and building regulations (G3) recommend an annual service for unvented cylinders for exactly this reason — not just for safety compliance, but because the parts most likely to fail (the expansion vessel especially) are quick and inexpensive to check compared with dealing with the consequences of ignoring them.

The Bottom Line

If your pressure relief valve is dripping or passing, don't just watch it and hope it stops — it won't. It's very likely telling you the expansion vessel behind it has already failed. And if it hasn't happened yet, an annual service is the simplest way to make sure it never does.

This is the single biggest reason to keep your unvented cylinder serviced. It's a small, predictable check that avoids the most common fault we see on unvented systems, keeps your cylinder running safely, and stops a minor issue turning into a bigger one.

If you're not sure when your cylinder was last serviced, or you've noticed the discharge pipe dripping, get in touch and we'll get it checked and sorted.