3 Bar Expansion Vessel Fault Traced on a Baxi Duo-Tec with a Rothenberger Gauge

Cold pressure reading 3 bar, then a swing down to 0.5 the second the pump and burner kick in. That pattern on a Baxi Duo-Tec points at the expansion vessel before it points at anything else. A membrane that has lost its air charge cannot absorb the volume change when the water heats, so pressure spikes hard on heat-up and collapses on cool-down. The Rothenberger tyre-style gauge, the one with the bleed button on the side, reads the vessel precharge straight off the Schrader valve once the system is drained down to zero.

The first move is to isolate and drop system pressure to nil. Close the flow and return service valves under the boiler, open a drain point or a low radiator bleed, and watch the built-in gauge fall to zero. Only then does the Schrader reading mean anything. With water still pressing on the far side of the diaphragm you get a false high number every time.

Reading the precharge with the Rothenberger

The Schrader valve on a Duo-Tec vessel usually sits at the back or underside, sometimes behind the removable case panel. Pop the dust cap, press the gauge on square, and take the number. A healthy sealed-system vessel on a combi wants roughly 0.75 bar to 1 bar precharge for a typical two-storey house, matched to the cold fill pressure of around 1 to 1.2 bar.

When the gauge reads 0.1 bar or shows nothing at all, the vessel has lost its charge. That is the fault. If you press the valve core and water sprays out instead of air, the diaphragm has split and the vessel needs replacing, not repumping. Water on the air side is the end of that component.

A slow hiss of air with a reading near zero is the recoverable case. Recharge with a foot pump or a small compressor, checking with the Rothenberger between pumps so you do not overshoot. Bring it to match the intended cold fill, not above it, or the vessel loses usable capacity and the same symptom returns in a different disguise.

Why the built-in vessel gives up

The internal expansion vessel on a Baxi Duo-Tec is small, often around 8 litres, and it works hard on a system with a lot of water in it. Every heat cycle flexes the rubber diaphragm. Air migrates slowly through the membrane over years, and the precharge bleeds away until there is nothing left to cushion the expansion. That is a wear item, not a defect.

Systems running hot flow temperatures accelerate it. If the boiler has been set to 80 degrees flow for years with no weather compensation trimming it back, the water expands more per cycle and the vessel takes more punishment. Dropping the flow target and letting weather compensation controls modulate the temperature against outdoor conditions eases the thermal load on the whole sealed circuit, vessel included.

There is a second failure that mimics a dead vessel. Scale and sludge in an uninhibited system can partly block the vessel connection, so the diaphragm cannot move freely even with good precharge. A dose of Sentinel X100 inhibitor after a proper flush keeps the internal waterways clear and slows corrosion that would otherwise clog the narrow vessel neck. On a system that has never seen inhibitor, expect the vessel and the automatic air vent to be the first casualties.

Where the built-in vessel keeps failing every couple of years, the honest fix is an external vessel plumbed into the return, sized properly to the system volume. A 12 or 18 litre external vessel takes the strain off the tiny internal one and stops the annual repump ritual. It costs more in parts and a bit of pipework, but the callback rate drops to near nothing.

The 3 bar spike itself

Three bar cold is high before you even fire the boiler. That number alone says the system was overfilled, probably by someone topping up through the filling loop chasing a low-pressure fault that was really the dead vessel. Overfilling a system with no working vessel just moves the problem, the pressure relief valve then weeps at 3 bar and drips outside, and the pressure falls again overnight.

Confirming it is not a genuine leak

Before condemning the vessel, rule out water loss. Refill to 1.2 bar cold, mark the gauge position, and leave the heating off for an hour. If cold pressure holds steady with the burner idle, there is no significant leak on the fabric of the system. Pressure that only misbehaves under heat is thermal, which lands you back at the vessel or the PRV.

Check the pressure relief valve discharge pipe outside. A pipe that is wet or dripping after a heat cycle confirms the PRV is lifting at 3 bar because there is no vessel capacity to absorb expansion. Sometimes the PRV seat is also damaged by the repeated lifting and passes even after the vessel is fixed, so it gets replaced in the same visit.

The filling loop is the other quiet culprit. A flexible loop left connected with a passing valve slowly raises system pressure over days. Disconnect one end after filling. If pressure still climbs with the loop off and the boiler cold, the leak is genuine and hides somewhere in the pipework or a radiator valve gland.

Radiator sizing and the load the vessel carries

Vessel size follows system water volume, and water volume follows how much radiator and pipe the house holds. A radiator sizing calculation done properly, room heat loss against emitter output at the design flow temperature, tells you the total litres in the circuit. More radiators, bigger volume, more expansion per heat cycle, larger vessel needed.

As a rough working figure, expansion is around 4 percent of total system volume across a full cold-to-hot swing. A system holding 100 litres expands about 4 litres. An 8 litre vessel with correct precharge has an acceptance capacity well under its physical size, often only 3 to 4 usable litres, which is why a marginal system tips the internal vessel over the edge and lifts the PRV. Fabric upgrades change this too. Better insulation means lower heat loss, which allows smaller radiators and a smaller water volume, so the vessel sees an easier life on a well-insulated house than on a draughty one.

What the Rothenberger cannot tell you

The gauge reads precharge and nothing else. It will not tell you whether the diaphragm is intact until you press the valve core and see air or water come back. So the sequence stays fixed: drain to zero, read the Schrader, press the core to check for water, then decide between repump and replace. Skip the drain-down and every reading you take is a guess dressed up as a measurement.

A vessel that holds its recharge for a week and then sags again has a slow diaphragm leak that a single gauge reading cannot catch. That is the case that sends engineers back three times before someone fits an external vessel and ends it. The question worth sitting with is how many repumps a customer should pay for on an 8 litre internal vessel before the external one stops being an upsell and starts being the actual repair.