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[Solved] No Hot Water from 35L Boiler After 1-Month Shutdown: Reasons & Solutions for 1.5-Year-Old System

Tomek.93 69930 31
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Why is there no hot water at the tap after shutting down and draining a 35 L boiler for a month, even though the boiler heats up normally?

The most likely cause is trapped air or a vacuum in the boiler/hot-water line, not a failed heater, so you need to bleed the system and let the hot side refill [#17149618] Turn on the cold-water supply to the boiler, open the nearest hot tap or disconnect the hot outlet hose, and wait several minutes until the air is forced out and water starts flowing from the hot side [#17149618][#17150052] One reply also pointed to a clogged safety valve/negative pressure as an earlier symptom, and cleaning the valve restored flow on the cold side, but the hot side still needed air removal from the boiler/outlet section [#17148517][#17148592][#17149370]
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  • #1 17148471
    Tomek.93
    Level 7  
    Hello, I have a 35l boiler. It was constantly in use, there were no problems. Leaving the house for a month, I turned it off and turned off all the valves. After returning, I turned on everything as it was before, but I have no hot water in the tap, and the boiler is on. What could be the reason? I will add that the water installation is about 1.5 years old.
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  • #2 17148501
    arturdip
    Level 31  
    Maybe one of the valves has broken and does not work?
  • Helpful post
    #3 17148517
    Witold5
    Level 32  
    Check the safety valve to see if it is clogged.
  • #4 17148568
    Tomek.93
    Level 7  
    When I open the safety valve, the water runs for a moment, a few seconds, and then stops.
    No valve broke, it was disconnected from the power supply for a month, nothing else was done with it
  • #5 17148592
    Witold5
    Level 32  
    So you already have a clogged safety valve in front of you. Or there was a vacuum.
  • #6 17148703
    Tomek.93
    Level 7  
    The valve is on cold water. The cold water hose works fine, I checked it. The boiler took cold water normally and warmed up.
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    • No Hot Water from 35L Boiler After 1-Month Shutdown: Reasons & Solutions for 1.5-Year-Old System IMG_20180404_152143.jpg (3.55 MB) You must be logged in to download this attachment.
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  • #7 17148713
    markutek87
    Level 10  
    Maybe the boiler is not completely full of water? Turn on the cold water supply, open the nearest hot water tap and see if after some time air has escaped and water starts to flow.
  • #8 17148721
    Witold5
    Level 32  
    I wrote about negative pressure.
  • #9 17148736
    markutek87
    Level 10  
    @ Witold5 Yes, of course, but you only stated the fact that my colleague, the author of the topic, wanted to know what to do with it, so I just explained it to him. No offense.
  • #10 17148742
    Witold5
    Level 32  
    I thought that having a boiler at home, the owner knew how to operate it. So I just hinted at the possibility. And these are just the laws of physics.
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  • #11 17148760
    Tomek.93
    Level 7  
    So what am I supposed to do? Because I'm completely green in this topic .. I've already done what Markutek recommended, but nothing happened
  • #12 17148783
    Witold5
    Level 32  
    Do what we advise.
  • #13 17148792
    michalek1988
    Level 26  
    markutek87 wrote:
    Turn on the cold water supply, open the nearest hot water tap and see if after some time air has escaped and water starts to flow.

    From there, start and write what effect.
  • #14 17148809
    Tomek.93
    Level 7  
    There is no effect whatsoever. It was supposed to be here, excuse me, the dictionary on the phone ... The post has been improved, as you can see
  • #15 17148823
    Witold5
    Level 32  
    That is, you should read what you wrote.
  • #16 17148838
    icooz
    Level 16  
    And write whether warm or cold water is flowing from the warm tap, or only the air itself hisses? Wait, from the boiler, if you don't have pressure, you haven't unscrewed the valve or the diaphragm that gives the pressure is broken. And how long does it take cold water to heat up? Can it fly from the safety valve when water is being tapped?
    I have no knowledge, but when I see a diagram of a typical boiler, we will see if it is the fault of the hot water outlet being blocked or the diaphragm rupture.
    EDIT is this effect cold water or air?
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  • #17 17148851
    Witold5
    Level 32  
    The author of the topic writes that the installation is 1.5 years old, so I would rather not suspect mechanical damage.
    How do you know there is hot water in the boiler? If you are not sure that the water in the boiler is there, do not turn it on.
    Before you turn on the boiler, you must first turn on the water.
  • #18 17148883
    rafbid
    Level 33  
    Do you have one hot water outlet? The battery may be clogged.
  • #19 17148892
    markutek87
    Level 10  
    Maybe a stupid question: did you open all the valves supplying cold water to the boiler? Main for an apartment / house, separate for a bathroom? The "new" water flows into the boiler, or is it just "old"?
  • #20 17148961
    Tomek.93
    Level 7  
    So yes: there was water in the boiler, because I drained it all, and then I refilled it. The boiler is 35l, filled up quickly, less than 5 minutes. I turned it on, the water heats up because I drained it a bit again to check it and it was flying warm. I unscrewed all the valves, I even tried to unscrew the faucet in the kitchen, the hot water pipes are empty. The water is standing in the boiler
  • Helpful post
    #21 17149013
    Witold5
    Level 32  
    How did you come down, which way? Did you unscrew the pipes, hoses?
    The battery is probably not, because there is no more water at the safety valve and you have to look for it there.
  • #22 17149016
    markutek87
    Level 10  
    Wait - where did you drain the water from the boiler? First it was cold to fill it completely, and then it was warm? Safety valve? Show a picture of these hoses under the boiler.
  • #23 17149057
    michalek1988
    Level 26  
    Since you physically unscrewed the battery and you did not have hot water, maybe one of the valves did not open physically, even though you turned the tap or moved the lever. A clogged pipe cannot be ruled out, but in such a new installation, if you have not made a revolution in the walls now (which could damage the pipes), disconnect the hot water hose coming from the boiler (maybe just loosen it), the point is that it would be known that this warm water is physically in it, and that no sediment has accumulated that is blocking the drain. Do this with the cold water supply closed and the boiler off. Be careful as the water can get really hot. PS The hot water hose has a different temperature than the cold one?
  • #24 17149342
    Witold5
    Level 32  
    The author of the topic does not speak, so the topic has been solved.
  • #25 17149367
    rafbid
    Level 33  
    Witold5 wrote:
    The author of the topic does not speak
    Sometimes you have to wait 3 days.
  • #26 17149370
    Tomek.93
    Level 7  
    I figured it out a little more, unscrewed the safety valve, cleaned it, screwed it on and now the water is running normally, but the hot one is still not there
  • #27 17149451
    Witold5
    Level 32  
    So now the section between the safety valve and the battery.
  • #28 17149527
    Tomek.93
    Level 7  
    Witold5 wrote:
    So now the section between the safety valve and the battery.

    As I wrote, I think everything is fine with the safety valve, because it works as it should, the battery is also functional, I also checked it, i.e. I screwed in a different one and it did not change.
    I unscrewed the hot water hose from the boiler, a lot of air escaped at first, and nothing else. Not a drop of warm water. Rather, there is a problem, but I don't know how to solve it. There is one more point. The apartment was not heated in my absence and the temperature was quite low. Could this have had any effect? I will note that the water at the main riser was turned off, and I drained the water from the pipes
  • Helpful post
    #29 17149618
    markutek87
    Level 10  
    Air in the boiler for me. There is still air in the boiler, possibly in the pipes too. Turn on the cold water supply to the boiler, unscrew the hose from the hot water outlet and wait. Not only 5-10 seconds, but only 5-10 minutes. Do not connect the heater.
    Cold water will flow into the boiler, it will force the air out, and water will flow from the hose on the hot side. Cold water flows from the tap normally? How do you turn on the tap, it does not shoot / spray?
  • #30 17149900
    Tomek.93
    Level 7  
    markutek87 wrote:
    Air in the boiler for me. There is still air in the boiler, possibly in the pipes too. Turn on the cold water supply to the boiler, unscrew the hose from the hot water outlet and wait. Not only 5-10 seconds, but only 5-10 minutes. Do not connect the heater.
    Cold water will flow into the boiler, it will force the air out, and water will flow from the hose on the hot side. Cold water flows from the tap normally? How do you turn on the tap, it does not shoot / spray?

    Cold tap water runs normally, and if there is water in the boiler, should I drain it and then refill it to push the air out?
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Topic summary

✨ A user reported no hot water from a 35L boiler after a month of shutdown. Initial troubleshooting suggested checking valves and the safety valve for clogs. The user confirmed the cold water supply was functioning, and the boiler was filled with water. Further suggestions included ensuring the hot water outlet was not blocked and checking for air trapped in the system. The user eventually resolved the issue by allowing cold water to flow into the boiler, which expelled trapped air, restoring hot water flow.
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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?

  1. Switch off power to the heater.
  2. Fully open the cold-water inlet and leave the hot-outlet hose loose over a bucket.
  3. 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].
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