Boiler Pressure Too Low: How to Refill to 1.5 Bar
Independent guide — no official affiliation with the platforms mentioned.
Short answer
When boiler pressure drops below 1.0 bar, slowly open the filling loop underneath until the gauge reads 1.5 bar, then close it. It takes 2–3 minutes. Frequent drops indicate a leak or faulty expansion vessel — call a technician.
01 — Step-by-step fix
Step-by-step fix
Let the boiler cool down
Refilling a hot system gives false readings due to expansion. Switch off and wait 15–20 minutes.
Find the filling loop
It's a small tap or flexible connector under the boiler, labelled 'filling loop' in the manual.
Raise pressure to 1.5 bar
Open the valve slowly and watch the gauge. Close it fully at 1.5 bar.
Above 2.5 bar the safety valve discharges water. Never fill beyond 2 bar.
Bleed radiators if needed
Open radiator bleed valves until water appears, then re-check the pressure.
Log frequent drops
If pressure drops more than once a week, suspect a leak or expansion-vessel membrane — call service.
Tips
- Forgetting the filling loop open is the most common mistake — re-check after an hour.
- Photograph the gauge to track changes over time.
02 — Frequently asked questions
Frequently asked questions
What should boiler pressure be?
1.0–1.5 bar when cold is ideal. Rising to 2.0 bar during heating is normal; 3 bar is the safety limit.
Pressure keeps dropping — what should I do?
Check visible leaks first (valves, filling loop). If none, the expansion vessel membrane may be faulty — needs a technician.
How do I lower excessive pressure?
Bleed a radiator slightly to release water until it returns to 1.5 bar. Don't use the boiler's drain valve.
Links in this guide point to official platform pages. Steps may change with platform updates; check the page date.
03 — Related guides