Sealed-heating pressure and expansion faults

A sealed heating system is filled cold to about 1 to 1.5 bar and protected by a relief valve commonly set at 3 bar. Most pressure faults show up in how the pressure behaves between cold and hot.

How a sealed system holds its pressure

• Cold fill. A typical domestic sealed system is filled cold to about 1 to 1.5 bar. Always follow the manufacturer's stated figure where given.
• Expansion vessel. Water expands as it heats and is virtually incompressible. The cushion of gas in the vessel takes up the extra volume so the pressure stays within limits.
• Pressure relief valve. A safety device that discharges water if the pressure rises too high, commonly set to lift at 3 bar; check the appliance specification. It is not a normal pressure control, and it does not absorb expansion, vent air or top up the system.
• Filling arrangements. A removable hose forms a temporary mains connection: close its valves and disconnect it after filling. For an integrated filling link, use its specified procedure and backflow protection instead of assuming the assembly is removable. A valve left open or passing can over-pressurise the circuit or conceal a leak by continually replacing lost water.

Read the pressure cold first

Check the pressure with the system cold. That removes the effect of thermal expansion and gives a repeatable baseline to compare with the cold-fill figure; a hot reading naturally sits higher. When comparing hot and cold readings, use comparable pump conditions, for example with the pump stopped.

Confirm the indication before acting on it. A faulty gauge, sensor or connection can give a misleading reading. Use a suitable gauge within its specified pressure and temperature ranges: instrument temperature can add measurement error, separate from the real pressure change caused by water expanding.

Symptom to likely cause

• Gauge near zero and the boiler locks out. Most sealed-system boilers will not fire below their minimum pressure. Check for recent draining, filling or venting, then investigate leakage and relief discharge. Restore the specified cold pressure as the appliance instructions permit and establish the cause before returning it to use.
• Pressure falls after filling or commissioning. Trapped air can continue to leave through automatic vents, lowering the pressure without a water leak. A continuing need for top-ups still needs investigation: check for concealed leaks and relief-valve discharge, and compare cold and hot pressures.
• Pressure climbs steeply as the water heats, then the relief valve weeps. The expansion is not being absorbed: classically a vessel that has lost its charge or become waterlogged. Excessive cold filling also leaves less room for expansion.
• 2.8 bar hot with the relief discharge dripping. Lost expansion capacity or overfilling. A drip does not mean the valve is opening at that pressure: after an earlier discharge, debris on the seat can keep it passing.
• Pressure creeping up while cold. Check for water entering through a passing filling valve. On an appliance with a mains-water/primary-water heat exchanger, a breach between the circuits is another possible route; isolate and test by the model’s instructions.
• Pressure falls over days and jumps each time the system heats. The two can be one fault: with too little expansion space, the relief valve lifts when the system is hot, and the water it discharges shows up later as a low cold reading. Check the vessel and look for a wet or dripping discharge pipe, as well as searching for leaks.

Checking the vessel

Check the pre-charge with the vessel's water side isolated and depressurised, as the manufacturer specifies. Isolating it can leave pressure trapped, and a reading taken with system water pressing on it is not the pre-charge.

The maker's sealed-system rules: an example

Each model's instructions add rules of their own. Worcester's Greenstar 8000 Life instructions show the kind of detail to look for:

• Salt-softened water. Never fill the heating circuit from an ion-exchange (salt-based) softener. A permanent untreated supply, connected before the softener, fills the system and keeps its pressure up; where none can be provided, the first fill goes through the softener's bypass. An occasional top-up with softened water is tolerated.
• Second expansion vessel. More than 100 litres of system water, or more than 2.5 bar at maximum heating temperature, calls for an additional vessel in the heating return as close to the boiler as possible, pre-charged to match the built-in vessel (0.75 bar as supplied).
• Relief discharge pipe. Use pipe of at least 15 mm, in copper or another material rated for the discharge temperature and pressure, with plastic supported every 300 mm or less. It falls continuously away from the boiler to a safe place where a discharge will be seen, never above a window, an entrance door or a public path. Ending on an outside wall, it finishes in a partial bend turned back towards the wall, so hot water is directed safely down it.
• When the pipe cannot fall all the way. A remote PRV kit takes the relief valve off the boiler and relocates it higher up. Never put a trap directly in the discharge pipe without a tundish.
• Plastic primary pipework. It must have a polymeric barrier, with at least 600 mm of copper connected to the boiler. Plastic underfloor circuits need a thermostatic blending valve holding them at about 50 °C and at least 1000 mm of copper or steel at the boiler.

Reading another model's pressure figures

The Ideal Logic+ Combi2 C24/C30/C35 instructions (October 2023) illustrate why the vessel charge, filling pressure and relief setting must be kept separate:

• The integral expansion vessel holds 8 litres and starts with a 0.75 bar gas charge. Check that charge with the water side depressurised; the system gauge cannot show it.
• The filling procedure gives 1 to 1.5 bar. The commissioning check asks for the design cold pressure, 1.0 bar for a pre-pressurised system. Heating raises the pressure, but the stated working maximum is 2.5 bar. The relief valve's 3 bar setting is a safety limit, not a filling target.
• The heating design uses flow temperatures up to 80 °C, while the system components must withstand 3 bar and 110 °C. A part rated for that does not mean the system can run at it in normal use.
• In the vessel-sizing guidance, 143 litres is the maximum cold system volume without system pre-pressurisation. Adding pre-pressurisation reduces the volume the vessel can accommodate. Check the sizing table for the actual design and fit any required extra vessel in the boiler return. This table condition is not permission to run an empty or unpressurised boiler.
• For the manual's alternative of a hand-filled make-up vessel, its water level must be visible and the vessel must sit 150 mm or more above the highest system point. It feeds through a non-return valve into the radiator return, at a connection at least 150 mm below the make-up vessel. Do not confuse this water vessel with the boiler's gas-charged expansion vessel.

Common mistakes

• Topping up repeatedly. Each top-up adds fresh oxygenated water that promotes corrosion and dilutes inhibitor, while the cause of the pressure loss remains unresolved.
• Judging the system charge from a hot reading.
• Assuming every relief discharge proves a failed expansion vessel.
• Blocking, capping or unsafely redirecting the safety discharge.