Pressure gauge: checking heating circuit pressure

Understanding the pressure gauge and heating circuit pressure

What is the pressure gauge for on a boiler or a heat pump?

The pressure gauge shows the water pressure in a closed heating circuit, on a boiler as well as a heat pump. It's your dashboard. Too low, and water circulates poorly and the unit may go into safety mode. Too high, and you risk the safety valve triggering and leaks.

Reading pressure in bar: green and red zones and useful benchmarks

Pressure is read in bar. The green zone corresponds to the normal operating range, often around 1 to 2 bar on a domestic system. In the low red zone, water is missing. In the high red zone, pressure is excessive. The right benchmark also depends on the height between the unit and the highest emitter.

Cold / hot pressure: what you should compare on the circuit

Always compare when cold, with the system stopped and cooled down, then when hot and running. A moderate rise is normal. If pressure climbs sharply when hot or keeps dropping when cold, think leak, air, or an expansion vessel that needs checking. Top up the water gently, then bleed and recheck.

Knowing what pressure to aim for based on your system

Common pressure ranges for a domestic heating circuit

On a standard system, you often aim for 1 to 1.5 bar when cold, then a slight rise while heating. The right benchmark is read on the boiler's pressure gauge. If you're approaching 2.5 to 3 bar, that's generally a sign of an expansion-vessel problem or an overflow.

Influence of water height (floors) on circuit pressure

Pressure also depends on the water column height. Count roughly 0.1 bar per meter between the boiler and the highest point. In a multi-floor house, you therefore set the pressure higher than in a single-story home, to avoid air in the top-floor radiators.

Radiators, underfloor heating: points to watch depending on the circuit

With radiators, watch for air ingress and pressure drops after bleeding. With underfloor heating, the pressure stays similar, but the sealing and the expansion vessel are even more sensitive. If you're topping up frequently, look for the leak rather than just adding water.

Checking and correcting pressure with method, without damaging the circuit

Step-by-step check: bleeding, refilling, and verifying the gauge

Turn off the boiler or the heat pump, then let the network cool down. Do a quick bleed of a radiator at the top of the home. Close it as soon as air stops escaping. Then read the pressure on the gauge. If it drops below 1 bar, a refill is often needed.

Repressurizing via the fill valve: the right moves

Locate the fill valve under the unit. Open it slowly and watch the gauge continuously. Increase in small steps up to the value recommended in the manual, often around 1.2 to 1.5 bar when cold. Close it firmly, then replace the cap or disconnect the hose if the setup allows it.

After adjusting: stabilization, leak check, and additional bleeding

Wait 10 minutes, then check the pressure again. Do another light bleed if air remains, and top up if needed. Inspect the connections, radiators, and the safety valve. A repeated drop or overpressure when hot points to a leak or an expansion vessel that needs checking.

Diagnosing pressure anomalies: the most common faults

Pressure dropping: micro-leaks, air vent, safety valve, radiators

If pressure keeps dropping steadily, first look for a micro-leak at the connections, the safety valve, the automatic air vent, or a weeping radiator. A bit of air in the circuit can also throw off the gauge reading after a bleed. Look for water traces and monitor the trend over 24 hours.

Pressure rising: expansion vessel, filling, scaled-up heat exchanger

Pressure that climbs, especially when hot, often points to a deflated or punctured expansion vessel. Also check the fill valve, which is sometimes left slightly open. A scaled-up heat exchanger can cause local overheating and pressure swings. Beyond the green zone, put the system into safety mode and have it checked.

Inconsistent gauge readings: stuck needle, faulty sensor, clogged pressure port

If the needle doesn't move, jumps abruptly, or doesn't match the symptoms, the gauge may be stuck, the sensor may be faulty, or the pressure port may be clogged. Compare with the pressure shown on the boiler (if available) and avoid correcting things "at random" by adding water.

Good job-site practices and useful obligations in 2026

Checks to build into your maintenance routine: quick pressure-gauge checkpoints

Quick check of the pressure gauge when cold. Verify consistency with the building's water column height and the manual (often around 1 to 1.5 bar in a house). If pressure drops after bleeding, look for water being topped up too often, a leak, or a worn-out expansion vessel.

Traceability and reports: recording circuit pressure before/after a visit

Record the pressure before and after, along with the actions carried out (bleeding, top-up, adjustment). Provide a dated report. Useful proof of maintenance for the client, especially when the equipment is covered by a maintenance obligation.

Safety: safety valve, expansion vessel, and depressurizing before disassembly

Before disassembly, cut the power, isolate the fill line, then depressurize via the drain. Check the safety valve (free discharge) and the expansion vessel (proper inflation). Zero rushing on a hot circuit.