Unvented Cylinder Pressure Problems: Why Your Hot Water Has Lost Pressure

You turn on the hot tap and out comes a sad trickle. Cold water is flowing fine. Yesterday everything worked. Today, hot water pressure has collapsed. Sound familiar?

Pressure problems are one of the most common unvented cylinder issues I see across Orpington, Bromley, Sidcup and the wider Kent area. The good news: most are fixable, often without replacing the cylinder. The bad news: diagnosing the actual cause requires a methodical approach, because the symptoms can have many possible roots.

How an Unvented Cylinder Delivers Pressure

Understanding the system helps you understand what can go wrong. An unvented cylinder takes water from your incoming mains supply, heats it, and stores it under pressure ready for use. The pressure at your hot tap is essentially the same pressure that came in from the street, minus losses along the way.

That means hot water pressure depends on:

• Your incoming mains pressure
• The pressure-reducing valve (PRV) setting on the cylinder cold feed
• Any restrictions in the cold feed pipework
• The internal cylinder hot outlet condition
• The hot supply pipework itself
• Any thermostatic mixing valves (TMVs) on the route
• The tap or shower outlet itself

The Most Common Causes (In Order)

1. Failed or Wrongly Set Pressure-Reducing Valve (PRV)

The PRV reduces your incoming mains pressure to a safe level for the cylinder (typically 3 bar). When it fails or drifts out of calibration, it can over-restrict flow — causing exactly the "weak hot, strong cold" symptom you're seeing.

How a G3 engineer checks: Pressure gauge testing on both the inlet and outlet of the PRV, comparing readings to manufacturer specification. Replacement is usually a 30-minute job once the system is isolated.

2. Scaled Internal Heat Exchanger

In hard water areas like Orpington, Bromley and Sevenoaks, scale builds up over years on the internal heat exchanger coil. This restricts hot water flow through the cylinder. Eventually it can become severe enough to drop pressure noticeably.

How to spot it: Pressure has dropped gradually over years, often combined with longer heat-up times and audible kettling noises during heating. A fully scaled cylinder usually means replacement, but partial scaling can sometimes be addressed.

3. Thermostatic Mixing Valve (TMV) Cartridge Blocked

If the pressure problem is at one specific tap or shower (not all hot taps), a TMV is often the culprit. These valves blend hot and cold to prevent scalding, and their internal cartridges scale up and clog over time.

How to spot it: Pressure is fine at most outlets but reduced at the bath, shower, or basin with TMV. Replacement cartridges are inexpensive and quick to fit.

4. Partially Closed Service Valves

Sometimes the simplest explanation is the right one. Service valves on the cold feed to the cylinder, on the hot outlet, or at individual fixtures may have been partially closed during recent work and never fully reopened.

How to check: A G3 engineer will systematically work through every valve on the route. It's surprisingly common — and embarrassing for whoever last worked on the system.

5. Failed Expansion Vessel Affecting System Pressure

A failed expansion vessel doesn't directly cause low pressure, but it does cause erratic system pressure. You may notice pressure dropping then recovering, or pressure differing day to day. Signs include water dripping from the external discharge pipe and audible water hammer when taps close.

6. Blocked or Failed Strainers

Most unvented cylinders have a strainer in the cold feed designed to catch debris. Over years it can become almost completely blocked, dramatically reducing hot water pressure. Easy fix once diagnosed.

7. Internal Cylinder Corrosion

On older cylinders (15+ years), internal corrosion can affect the hot outlet specifically. This is rarer and usually means the cylinder is approaching end of life.

8. Incoming Mains Pressure Dropped

Sometimes the cylinder isn't the problem at all. Your incoming mains pressure may have dropped due to:

• Water company supply changes
• A leak elsewhere on your property
• A failing stop tap that's partially closed
• An issue with a property-level pressure-reducing valve

What to Try Yourself First

Before calling an engineer, three things are worth checking:

1. Run the cold tap from the kitchen sink — this is usually directly off the mains. If cold pressure is also weak there, the problem is incoming supply, not the cylinder.
2. Check tap aerators and shower heads — unscrew, soak in vinegar overnight, refit. Limescale at the outlet itself is the easiest fix possible.
3. Test all hot taps — if the problem is only at one outlet, the issue is local (TMV, partial isolation valve, pipework). If everywhere, it's likely the cylinder, PRV or main supply.

Why a Proper Diagnosis Matters

The reason I always work methodically through this list is that the symptoms can look identical regardless of cause. Replacing a PRV when the actual issue is scale build-up wastes money. Descaling a cylinder when the problem is a blocked strainer is unnecessary work. A G3 engineer with proper test equipment can identify the actual cause in under an hour.