Boiler & Appliances IT

Boiler Pressure Too Low: How to Refill to 1.5 Bar

~5 min 5 steps Verified:

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

Step-by-step fix

  1. Let the boiler cool down

    Refilling a hot system gives false readings due to expansion. Switch off and wait 15–20 minutes.

  2. Find the filling loop

    It's a small tap or flexible connector under the boiler, labelled 'filling loop' in the manual.

  3. Raise pressure to 1.5 bar

    Open the valve slowly and watch the gauge. Close it fully at 1.5 bar.

  4. Bleed radiators if needed

    Open radiator bleed valves until water appears, then re-check the pressure.

  5. Log frequent drops

    If pressure drops more than once a week, suspect a leak or expansion-vessel membrane — call service.

02 — FAQ

Frequently asked questions

01What 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.
02Pressure 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.
03How 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.