The pressure of a sealed heating circuit is set cold, and it is a calculation: roughly 0.1 bar per metre of water column between the generator and the highest emitter, plus 0.3 to 0.5 bar of margin, never below the minimum stated in the manual. That check is not an end-of-visit courtesy. Annexe 1 of the French arrêté of 24 July 2020 lists the pressure check and the expansion vessel charge pressure check among the six mandatory service points on a 4 to 70 kW thermodynamic system connected to a water loop, a service whose interval cannot exceed two years under the code de l'environnement. On a boiler of 4 to 400 kW, it is the arrêté of 15 September 2009 that governs the annual service and its certificate.
What the gauge measures, and what it does not
Static pressure, working pressure, relief valve setting
The pressure gauge shows the water pressure of the sealed circuit at the point where it is tapped, almost always in the plant room, so at the low point. That reading adds the filling pressure to the expansion caused by temperature. It says nothing about flow, nothing about balancing, and nothing about the state of the vessel until cold and hot are compared. The relief valve setting is an upper protection limit, not a target for adjustment.
Why the measuring point skews the interpretation
A gauge at the low point reads the static pressure of the whole water column above it. The same circuit seen from a manifold one floor up would show a lower value. That is why a setpoint copied from one job to the next does not hold: the correct value depends on the height of the building and on where the gauge is tapped, not on habit.
Reading a boiler pressure gauge: the two needles and the zones
On a wall-hung boiler the gauge often carries two needles, and that is the most common source of error on a callout. The dark needle is the only one that measures: it gives the actual circuit pressure. The red needle is a setpoint index, moved by hand by the installer at commissioning to mark the value calculated cold. It measures nothing, it does not move on its own, and a red needle left at the factory position of 1.5 bar on a four-storey building means nothing at all.
| Cold reading, generator off | What it means | What you do |
|---|---|---|
| 0 to 0.4 bar | Circuit drained or leaking openly, low-water cut-out often tripped | Find the leak before filling anything |
| 0.5 to 0.8 bar | Below setpoint as soon as the building has more than one storey | Recalculate the setpoint against water column height, then fill |
| Calculated setpoint ± 0.2 bar | Circuit correctly filled | Record the reading, nothing else to do |
| 2.5 bar and above | Filling loop left open, or expansion vessel failed | Isolate the filling loop, check the vessel charge pressure |
| Close to the relief valve setting | The valve will lift and the circuit will lose water on every heat-up | Depressurise, then treat the cause before releasing the system |
The relief valve on a domestic wall-hung boiler is almost always set at 3 bar, a value marked on its body and not adjustable. An installation that reaches it on every heat-up is not corrected at the gauge: it is corrected at the expansion vessel.
What the gauge does not replace
Correct pressure guarantees neither flow nor balance. A network can hold a steady 1.5 bar and still leave emitters cold, because the pressure loss of the least favoured circuit exceeds the available head, or because balancing was never carried out. The gauge closes a hypothesis, it does not open a diagnosis.
Calculating the cold filling setpoint
The water column rule
The static pressure of a water column is roughly 0.1 bar per metre. The cold setpoint is that static value, measured between the gauge tapping and the highest emitter, plus a margin of 0.3 to 0.5 bar that keeps positive pressure at the high point and stops air being drawn in.
Setpoint table by building height
The values below are calculated with a 0.5 bar margin. Read them as a starting point to be set against the minimum stated in the generator's manual, which takes precedence in case of a discrepancy.
| Height, generator to high point | Static pressure | Cold setpoint |
|---|---|---|
| 3 m | 0.3 bar | 0.8 bar |
| 6 m | 0.6 bar | 1.1 bar |
| 9 m | 0.9 bar | 1.4 bar |
| 12 m | 1.2 bar | 1.7 bar |
| 15 m | 1.5 bar | 2.0 bar |
A setpoint left at the manufacturer's minimum on a three-storey building leaves the top radiator close to zero relative pressure: the classic case of a network bled every month with no leak ever found.
Matching the expansion vessel to that setpoint
The vessel's charge pressure is set with the circuit depressurised, consistently with the building's static pressure. A vessel charged too low fills with water and loses its usable volume; charged too high, it only absorbs expansion once the pressure has already risen too far. Sizing the usable volume is covered in the article on sizing the expansion vessel.
Cold-then-hot checking protocol
The cold reading
Switch off the generator and let the network return to room temperature before any reading. Record the pressure, then bleed one emitter at the high point and read again: a clear drop after bleeding signals a volume of air that was masking the real pressure. Note the value before any intervention, it is the reference for your report.
Repressurising
Open the fill valve slowly, with the gauge in sight, and raise the pressure in stages up to the calculated setpoint. Close it firmly, then remove the filling hose where the arrangement allows: a hose left connected is the single most common cause of the overpressure found at the next visit. Bleed the high point again and top up if needed.
The hot check
Fire the system back up and record the pressure once the flow temperature has settled. The gap between cold and hot is the indirect measurement of the expansion vessel: a small, consistent swing is normal, a rise approaching the relief valve setting is not. A working automatic air vent at the high point keeps residual air from blurring that check.
Diagnosing a pressure drift
The reading table
A drift is read in the direction and the speed of the change, not in the value alone.
| Gauge behaviour | What you measure next | Most likely cause |
|---|---|---|
| Slow, steady drop when cold | Record over 24 to 48 hours, generator off | Micro-leak on a connection, valve body or heat exchanger |
| Drop only after bleeding | Volume bled, second pass on the high points | Residual air, air vent not working |
| Sharp rise when hot, normal again cold | Vessel charge with circuit depressurised | Vessel undercharged or membrane punctured |
| Continuous rise even when cold | Closure of the fill valve | Filling left slightly open, check valve passing |
| Needle stuck or inconsistent | Comparison with the generator display | Clogged pressure port or failed sensor |
Do not correct before you have measured
Adding water to a leaking circuit masks the symptom, sustains oxygenation and accelerates sludge build-up. Every unjustified top-up brings in more dissolved oxygen and more dissolved salts. The correct move is to quantify the loss with a dated record before adding any water, and to log every top-up.
The six checkpoints on a water loop
For water loop distribution systems, annexe 1 of the arrêté of 24 July 2020 lists the check for sludge linked to hydrolysis, the bleeding of air bubbles where the vent is working and accessible, the pressure check, the verification that the circulators run, the inspection and cleaning of the filter on the water loop if needed, and the check of the expansion vessels' charge pressure with recharging if needed. That is the frame of your service sheet.
Traceability and the regulatory frame of the visit
What goes into the report
Record the cold pressure before intervention, the setpoint calculated with the water column height used, the pressure after refilling, the pressure reached hot, and the vessel charge pressure. Add the actions carried out and the components removed. A report with no figures in it is worth nothing if the job is later disputed. Argile attaches the report, the readings and the photos to the job file, which saves hunting for them a year later.
The service intervals to know
Servicing a 4 to 70 kW thermodynamic system falls under the arrêté of 24 July 2020, and the code de l'environnement sets an interval between two services that cannot exceed two years. For a boiler of 4 to 400 kW, the service is annual under the arrêté of 15 September 2009, which also requires an assessment of pollutant emissions and the issue of a certificate. The detail of those obligations is set out in the article on servicing a heat pump.
Safety before disassembly
Before removing anything from a hot circuit, cut the electrical supply, isolate the filling line, then depressurise through the drain and confirm the gauge has returned to zero before opening. Check that the relief valve discharge is clear and directed. A circuit the client says is cold is not a circuit you have verified is cold.
