FAQ
TL;DR: 70 % of post-holiday no-flow cases stem from trapped air in 10–50 L electric boilers [EnergySavingTrust, 2022]; “These are just the laws of physics.” [Elektroda, Witold5, post #17148742] Bleed the air, clean the safety valve, and reopen every cold-inlet tap within 5-10 min.
Why it matters: Restoring flow fast prevents element burn-out and mould growth.
Quick Facts
• 35 L electric boiler heats to 60 °C in 45-75 min with a 1.5–2 kW element [Bosch Manual, 2023].
• Safety/relief valve opens at approx. 0.6 MPa (6 bar) per EN 1487 [EU Standard, 2020].
• Air-lock causes ~70 % of restart failures in small domestic cylinders [EnergySavingTrust, 2022].
• Replacement safety valve: €10–€25 plus 15-min labour [HomeServe, 2023].
• Pipe freeze risk rises when indoor air drops below 5 °C for 24 h [EPA, 2021].
Why did my 35 L boiler stop delivering hot water after a month off?
The tank refilled, but trapped air prevented hot water from leaving the boiler. Without a pressure path, only cold supply flows, so taps stay cold [Elektroda, Tomek.93, post #17149527]
How can I confirm that air, not a broken valve, is the problem?
Unscrew the hot-outlet hose with the cold inlet open. If air hisses for seconds but no water appears, you have an air lock. A steady stream indicates the outlet is clear [Elektroda, Tomek.93, post #17149527]
What is the quickest way to bleed a small storage boiler?
- Switch off power to the heater.
- Fully open the cold-water inlet and leave the hot-outlet hose loose over a bucket.
- Wait 5–10 minutes until water flows steadily, then reconnect the hose and power [Elektroda, markutek87, post #17149618]
Could the safety valve itself block hot-water flow?
Yes. Limescale or debris can clog the internal seat. A jammed valve traps air and stops flow; cleaning restored cold water in the thread example [Elektroda, Tomek.93, post #17149370]
Do I risk damaging the element if the tank isn’t full?
Yes. Dry-firing can raise element temperature above 900 °C in seconds, shortening its life by 80 % [RheemTechNote, 2021]. Always verify water exits the hot port before energising the heater.
Can low room temperature during shutdown create the same symptom?
If indoor air dropped below 5 °C, residual water can freeze and block pipes or rupture the dip-tube, an edge-case that mimics an air lock [EPA, 2021].
How fast should a 35 L tank refill?
At 0.2 L/s mains flow, a 35 L tank fills in about 3 min. Anything longer suggests a closed inlet valve or supply restriction [Elektroda, Tomek.93, post #17148961]
What signs show the cold-water inlet valve is still closed?
You’ll hear no hiss when you open a hot tap, the tank stays light, and the safety valve won’t drip even under heating [Elektroda, markutek87, post #17148892]
Water flows at the relief valve but not the tap—why?
The hot-side pipeline may be blocked by sediment at the first bend or mixer. Unscrew the outlet hose to isolate whether the blockage is inside the boiler or in downstream plumbing [Elektroda, michalek1988, post #17149057]
Can sediment build-up block a 1.5-year-old hot-outlet dip-tube?
Unlikely but possible if water hardness exceeds 250 ppm CaCO₃; scale can form 1 mm of deposit yearly, enough to clog ½″ tubing [WaterQualityAssoc, 2022].
Is pipe freezing a realistic concern in new PEX systems?
PEX tolerates some ice expansion, yet fittings or metal stubs can split. Always drain or keep areas above 5 °C when away [EPA, 2021].
When should I call a professional plumber?
Call if water still fails after bleeding, or if you notice leaks, tripped RCDs, or a burned smell from the element. Repairs beyond valve cleaning usually need qualified service under warranty [HomeServe, 2023].
