Sealed Heating System Pressure: Gauges, Filling Loops, and How to Repressurise a Boiler

Sealed Heating System Pressure: Gauges, Filling Loops, and How to Repressurise a Boiler

Low system pressure is one of the most common reasons a combi boiler or sealed system boiler locks out and stops producing heat. Understanding what the pressure gauge shows, why pressure drops, how to repressurise safely, and when a pressure loss indicates a more serious problem is essential knowledge for any heating engineer — and useful for the homeowners you advise.

How a Sealed Heating System Works

A sealed (pressurised) central heating system is a closed loop of water — unlike an older open-vented system, there is no feed and expansion cistern in the loft. The system operates at a positive pressure above atmospheric, maintained by a pre-charged expansion vessel (usually fitted at or near the boiler). The expansion vessel absorbs the increase in water volume as the water heats up, preventing the pressure from rising to the relief valve setting during normal operation.

The system pressure gauge (usually a Bourdon tube dial gauge or a digital gauge on modern boilers) indicates the static pressure of the water in the system. This is the pressure measured when the system is cold — at operating temperature, pressure will rise by approximately 0.3–0.5 bar due to thermal expansion.

Correct Operating Pressures

• Cold fill pressure: 1.0–1.5 bar is the correct cold fill pressure for most domestic sealed heating systems. Many boiler manufacturers specify 1.0–1.2 bar cold as their standard commissioning pressure. Check the boiler installation manual for the specific recommendation.
• Hot operating pressure: With the system at full operating temperature (75–80°C flow), pressure should be 1.5–2.0 bar. If pressure at operating temperature rises above 2.5 bar, either the expansion vessel is undersized, has lost its pre-charge, or is incorrectly sized for the system volume.
• Pressure relief valve (PRV) setting: The boiler's safety relief valve is typically set at 3 bar. If the PRV lifts (discharges water through the relief valve outlet), the system pressure has reached 3 bar — this is a safety function and indicates a fault (failed expansion vessel, overfilled system, blocked or closed circuit) that must be investigated before the system is re-operated.

Why Pressure Drops

In a truly sealed, leak-free system, the cold fill pressure should remain stable indefinitely. A system that repeatedly loses pressure indicates one of the following:

1. A water leak: Any loss of water from the system reduces the total volume and therefore the static pressure. Leaks can be at joints, fittings, radiator valve packing, pump gland seals, or the boiler heat exchanger itself. Even a small drip at a compression fitting will cause gradual pressure loss over days or weeks. Look for wet patches, rust staining on pipes, or water marks on ceilings below radiators.
2. A failed expansion vessel: The expansion vessel contains an internal diaphragm separating a nitrogen pre-charge chamber from the system water. If the diaphragm fails, the pre-charge escapes and the vessel becomes waterlogged — it can no longer absorb thermal expansion. A waterlogged expansion vessel causes the pressure to shoot up to the PRV setting (3 bar) when the system heats up, causing the PRV to discharge. After the PRV discharges, the pressure drops back, appearing as a cycle of high pressure when hot and low pressure when cold. This is the most common expansion vessel failure symptom.
3. Air in the system: Air pockets in a sealed system compress when the system heats up and expand when cooling, mimicking pressure loss. Bleed all radiators and air vents if the system has been recently refilled.
4. A faulty filling loop that drips back: If the filling loop check valves are not sealing properly, water can bleed back from the heating circuit to the cold mains when the system is under pressure, causing gradual pressure loss.

Pressure Gauges: Types

Two types of pressure gauge are used on sealed heating systems:

• Dial (Bourdon tube) gauge: The traditional round dial gauge with a needle pointer. Reads 0–4 bar or 0–6 bar. The gauge face typically has colour-coded zones — red for too low (below 0.5 bar), green for normal (1–2.5 bar), red for too high (above 3 bar). Gauges are available in 1/4" BSP and 1/2" BSP back or bottom entry connections. Dial gauges require periodic replacement if the Bourdon tube or pointer mechanism fails — a gauge that does not return to zero when the system is depressurised indicates a faulty gauge.
• Digital pressure gauge / display: Modern combi boilers have a built-in digital pressure display (often on the LCD screen rather than a separate gauge). These read from a pressure transducer in the system and are generally more accurate than Bourdon tube gauges.

Where a gauge is being replaced on an existing installation, match the connection size and confirm the pressure range (0–4 bar for domestic systems). Some boilers use proprietary gauge connections — always check the boiler manufacturer's approved parts list before fitting a third-party gauge.

Filling Loops: Types and Operation

The filling loop is the connection used to top up the sealed system with mains water when the pressure drops. It connects the cold mains supply to the heating circuit, with at least one (and ideally two) double-check valves to prevent contamination of the potable water supply by heating circuit water (a cross-connection risk under Water Regulations).

Three common filling loop configurations:

• External braided filling loop: A short braided flexible hose (typically ¾" BSP both ends, 300–450mm long) with double-check valve fittings, connected between isolation valves on the cold supply and the heating circuit. The loop must be disconnected from the cold supply after filling — a permanently connected loop with failed check valves creates a cross-connection between the potable water supply and the heating circuit, which is a Water Regulations breach and a potential contamination risk. Some engineers use a keylock isolation valve on the filling loop supply to prevent accidental connection.
• Internal (built-in) filling loop: Many modern combi boilers have an internal filling loop with a lever or key-operated valve accessible through the boiler casing. No external hose is required — the connection between cold supply and heating circuit is made internally. The same rules apply regarding disconnection from the potable circuit after filling — some internal loops have a keyed mechanism that returns to the disconnected position as the design default.
• Pressurisation unit (commercial): For commercial systems or where frequent repressurisation is expected, an automatic pressurisation unit maintains system pressure without manual intervention. The unit monitors pressure via a transducer and automatically tops up when pressure falls below the set point. Requires connection to mains cold supply and to the heating circuit, with appropriate backflow protection (a Reduced Pressure Zone valve, not just check valves, where required by water undertaker).

How to Repressurise a Sealed System Boiler

Step-by-step procedure for an external braided filling loop:

1. Ensure the boiler is switched off and the system has cooled to below 40°C — do not fill a hot system as the pressure reading will be artificially high due to thermal expansion.
2. Connect the filling loop between the isolation valves on the cold supply and the heating circuit return (or the designated filling loop ports on the boiler).
3. Open both isolation valves slowly — the sound of water entering the system will be audible. Watch the pressure gauge rise.
4. When the gauge reaches 1.0–1.2 bar (or the boiler manufacturer's specified cold fill pressure), close both isolation valves.
5. Disconnect the filling loop from the cold supply isolation valve immediately. Do not leave the loop permanently connected.
6. Switch the boiler on, allow the system to heat up, and verify that pressure does not exceed 2.0–2.5 bar at operating temperature. If pressure rises above 2.5 bar, the expansion vessel pre-charge should be checked.
7. Re-bleed all radiators if air was introduced during the filling process.

If the system pressure drops again within days of refilling, the underlying cause (leak or expansion vessel failure) must be identified and rectified rather than simply topping up repeatedly. Repeated topping up introduces fresh oxygenated mains water to the system, accelerating corrosion.

Expansion Vessel Pre-Charge Check

The expansion vessel nitrogen pre-charge should be checked at every boiler service. Pre-charge is typically 0.5–1.0 bar (check the boiler installation manual — it varies by boiler and system height). To check:

1. Isolate the boiler and relieve system pressure to zero by opening a drain valve or releasing from a radiator bleed point.
2. Remove the dust cap from the Schrader valve on the expansion vessel (the same type as a tyre valve).
3. Check the pre-charge with a tyre pressure gauge. If the gauge reads zero (no air pressure) with the system pressure at zero, the diaphragm has failed and the vessel must be replaced.
4. If the pre-charge is low but not zero (below specification), top up with a pump (a standard bicycle pump or tyre inflator works) and recheck.
5. If the vessel is waterlogged (discharges water when the Schrader valve is depressed with system pressure at zero), the diaphragm has failed — vessel replacement is required.

Pressure Gauges and Filling Loop Components at APM Plumbing & Electrical

APM stocks dial pressure gauges (0–4 bar, 1/4" and 1/2" BSP), braided filling loop hoses (300mm and 450mm, ¾" BSP double-check valve type), filling loop isolation valves, and expansion vessels (replacements for domestic combi and system boilers). All components are suitable for sealed heating systems to 3 bar working pressure.

Browse our heating controls and system accessories or contact the Acton trade counter. Trade orders placed before 2pm ship same day.

Products Available from APM Plumbing & Electrical

Key filling loops and pressure components available for trade collection or delivery:

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.