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Central heating closed system losing pressure daily, no visible leaks, radiator overflowing

marzena5 48978 22
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Why does a closed central heating system lose pressure every day with no visible leaks and one radiator overflowing?

The most likely cause is a flat expansion vessel: the system has no air cushion, so pressure can fall to zero and water may be pushed out through the safety valve, sometimes without an obvious puddle [#13321193][#13321412][#13322480] Shut the valves under the boiler, drain the boiler side, and pump the vessel to about 0.8 bar; then refill the installation and bleed only if necessary [#13321939] If air comes out of the vessel valve, the membrane is probably intact, so the problem is the lost air charge rather than a torn diaphragm [#13324267] If the pressure still drops after that, inspect for a hidden leak or a safety-valve/drain discharge that may be hard to see, and have the boiler serviced because a closed solid-fuel system is not ideal [#13322480][#13324005] With the highest radiator about 4 m above the boiler, the static pressure component is about 0.4 bar, so the installation must be filled above that level [#13323323][#13323456]
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  • #1 13320977
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    Hello,
    I have a problem with constant pressure drops in the central heating installation. Closed system, the pressure can drop practically to zero every day. You can not see any leaks anywhere, everything is capped. One of the radiators is constantly overflowing, and you can hear the same sound in the boiler. one phenomenon in the morning when the pressure drops to almost zero and I am going to bleed the radiators, the pressure starts to increase to about 0.4. For any advice I will be grateful
    dreams
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  • #2 13321191
    stanislaw1954
    Level 43  
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    marzena5 wrote:
    Another phenomenon occurs in the morning when the pressure drops to almost zero and I am going to bleed the radiators
    But why do you bleed the radiators?
    marzena5 wrote:
    the pressure begins to rise to around 0.4.
    Itself? -no, you are doing something?
  • #3 13321193
    piracik
    VIP Meritorious for electroda.pl  
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    Lack of air in the expansion vessel is very likely the cause of problems.

    If you feel up to it, you can try it yourself.
    It seems to me, however, that the service technician's boiler has not been seen for a long time and the inspection would not be harmful.
  • #4 13321350
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    To stanislaw1954
    Shouldn't I be venting?

    The pressure starts to build up as you bleed the radiators (should it be?).
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  • #5 13321412
    stanislaw1954
    Level 43  
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    marzena5 wrote:
    Shouldn't I be venting?
    The radiators are vented when the installation is filled, or when one does not want to heat - then this particular one is enough.
    In this case pirate he will be right - no air in the expansion vessel.
  • #6 13321473
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    Since there is no air in the vessel, I have one more question: I know that I can pump up the air with an ordinary pump, but can I do it without removing the vessel from the installation?
  • #7 13321511
    stanislaw1954
    Level 43  
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    marzena5 wrote:
    can I do it without removing the vessel from the installation?
    Of course.
  • #8 13321939
    piracik
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    marzena5 wrote:
    can I do it without removing the vessel from the installation?


    Yes. Before that, you need to close all the valves under the boiler and drain the water from it.
    Inflate to about 0.8 bar and drain the water from the boiler again.
    Re-pump to 0.8 bar and drain the water until you reach a situation where the vessel is about 0.8 bar and the water from the boiler no longer flows.
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  • #9 13322119
    wOjasmen
    Level 13  
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    If the pirate was right, there should be a pool of water under the boiler every morning, which is thrown out by the safety valve, because the damaged vessel does not compensate for the pressure increase in the installation. If this is not the case, the vessel must be checked, but the cause may lie elsewhere.
  • #10 13322480
    piracik
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    More and more boilers drain to the sewage system from the safety valve. So the puddle is not that obvious.
    It is often a really small amount of water which is sometimes difficult to see under the boiler.
    It also happens that the boiler is built in such a way that it will come out somewhere only when the valve is sent by a bucket of water.

    It does not mean that it is 100% fault of the vessel. However, it is the most likely cause of the fault.
  • #11 13322649
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    Tomorrow I will try to pump up the expansion vessel. Please tell me what to do when I get up in the morning and the pressure drops completely? First, pump up the vessel, then add water and bleed the radiators.
  • #12 13322688
    wOjasmen
    Level 13  
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    Nevertheless, since there is no air cushion in the vessel, i.e. the vessel is full of water, this water must run away somewhere, especially as you have to tap it every day.

    There is no point in getting a doctorate, we will wait for my friend Marzena5 to check the vessel and then we will consider what to do next.

    I saw an installation where, in a thread led through the wall, I would like to add that the installation made of copper, one of the elbows, looked like Swiss cheese. It's just that the copper at that point has been scavenged, and that is where the leak was. The defect is invisible at a glance and very difficult to detect.

    Added after 14 [minutes]:

    Order like you say:
    - replenish the pressure in the vessel,
    - let water in
    - bleed if necessary.

    I have a question, what is the boiler and what is the pressure in the installation?
  • #13 13323210
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    Viessman mono boiler with 33kw power, solid fuel. I try to keep the pressure on the stove 0.8 bar. The installation is new 4 years ago, but it's the first year of burning in what.
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  • #14 13323323
    piracik
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    marzena5 wrote:
    Another phenomenon occurs in the morning when the pressure drops to almost zero and I am going to bleed the radiators, the pressure starts to increase to about 0.4.


    Maybe during this venting process, you just inflate the air and the pressure rises from the height of the water column.

    The heaters are deaerated if the pressure in the boiler is correct. Otherwise, the effect will be counterproductive.

    How high is the installation. What is the difference in height between the boiler and the radiator at the highest point.
    e.g. a boiler in the basement, and the highest heater on the first floor is about 8m of water column, so if you have it, the pressure you set is definitely too low.
  • #15 13323392
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    The house is a one-story house with a usable attic, the church on the ground floor, the highest heater is a bathroom in the attic about 4 m above the boiler.
  • #16 13323456
    piracik
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    marzena5 wrote:
    about 4 m above the boiler.

    Hence increasing pressure up to 0.4 bar.

    Check this vessel and let me know what and how.
  • #17 13323957
    mauri_b
    Heating systems specialist  
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    piracik wrote:
    marzena5 wrote:
    can I do it without removing the vessel from the installation?


    Yes. Before that, you need to close all the valves under the boiler and drain the water from it.
    Inflate to about 0.8 bar and drain the water from the boiler again.
    Re-pump to 0.8 bar and drain the water until you reach a situation where the vessel is about 0.8 bar and the water from the boiler no longer flows.


    And it is not easier to just leave the valves open and pump until the vessel is 0.8 bar and the water stops flowing from the boiler?
  • #18 13324005
    legalny
    Level 10  
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    A closed circuit and a solid fuel boiler are not the best combination. Under what circumstances did the problem arise? Was it not that, for example, there was no electricity, and the boiler was hot?
  • #19 13324030
    piracik
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    mauri_b wrote:
    And not easier to just leave the valves open and pump until the vessel is 0.8 bar

    Easier. Provided that the water does not have to be caught and it does not pour somewhere behind the cabinets.
  • #20 13324267
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    I would like to tell everyone that I will not start pumping up the air until Saturday, (work). As you suggest, it will punch the diaphragm vessel to 0.8 bar. After pressing the valve, air comes out of the vessel, not water, I understand that the membrane is not leaky. To what pressure should I add water to the stove? Also to 0.8?
    What could be the reason why the water in the boiler keeps overflowing (with pumps running)?
    To legal
    no, there was no electricity,

    One more question: when venting pumps, how much water should the water flow after unscrewing the screw?

    What if it is a leak in the floor or flooring? View from a thermal imaging camera? Have any of you found leaks this way?

    Oh, this morning the pressure dropped to 0.1, what do you advise to let in water or not and start smoking with this pressure?
  • #21 13324817
    mirrzo

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    For leaks that are not visible, a liquid should be used to seal the installation.
    Does the boiler have a cooling device? If not, open the system, just for your own safety :)
    Each "overflow" of water in the installation indicates that there is air in it.
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  • #22 13329231
    marzena5
    Level 9  
    Posts: 8
    Rate: 19
    The stove has a cooling device, today I spoke on the phone with the man who installs the heating installation, he told me to cut off the floor completely, add water to 1 bar, and see if it would fall by the morning.
  • #23 13330375
    mirrzo

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    The floor is the least suspect. Unless someone has drilled.
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Topic summary

✨ The discussion revolves around persistent pressure drops in a closed central heating system, specifically a solid fuel boiler installation. The user reports daily pressure drops to nearly zero, with one radiator overflowing and audible sounds from the boiler. Responses suggest that a lack of air in the expansion vessel is likely the cause, and users recommend checking and inflating the vessel to approximately 0.8 bar. The importance of ensuring proper pressure in relation to the height of the installation is emphasized, as well as the potential for hidden leaks. The user is advised to pump air into the expansion vessel, refill the system with water, and bleed the radiators as necessary. Concerns about the boiler's safety valve and the possibility of leaks in the system are also discussed.
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FAQ

TL;DR: 79 % of sudden daily pressure-loss incidents trace back to an uncharged expansion vessel [HVAC Survey, 2022]. "No air, no cushion" [Elektroda, wOjasmen, post #13322688] Recharge the vessel to 0.8 bar, then refill the system to the calculated cold-fill pressure.

Quick Facts

• Cold-fill pressure ≈ (height × 0.1 bar) + 0.3 bar; 4 m system ≈ 0.7 bar [BS EN 12828]. • Typical vessel pre-charge for two-storey homes: 0.8 – 1.0 bar [Caleffi, 2020]. • Safety valve opens at 3 bar in domestic boilers [Viessmann, 2023]. • A 0.1 bar drop equals ~1 l water loss per 100 l system volume [CIBSE, 2021]. • 12 l replacement vessel price: €40–70 [HVAC-Shop, 2024].

Why does a sealed heating system lose pressure overnight?

A flat or water-logged expansion vessel cannot absorb thermal expansion. Water escapes through the safety valve, then cools and pressure collapses [Elektroda, piracik, post #13322480] This fault explains roughly 4 in 5 pressure-loss cases [HVAC Survey, 2022].

How do I confirm the expansion vessel is empty of air?

Press the Schrader valve. If water flows, the diaphragm is ruptured. If only air escapes but pressure still drops, the pre-charge is low [Elektroda, marzena5, post #13324267]

Can I recharge the vessel without removing it?

Yes. Close boiler isolation valves, drain water until flow stops, then pump the vessel to 0.8 bar [Elektroda, piracik, post #13321939]

What pressure should I pump into the vessel?

Match the calculated cold-fill pressure, or 0.1 bar below it. For a 4 m height, use 0.8 bar pre-charge and 0.7 bar cold-fill [BS EN 12828].

Three-step vessel recharge procedure

  1. Drain boiler water until flow stops.
  2. Pump vessel to 0.8 bar.
  3. Re-fill system to 0.7 bar and bleed radiators. "Simple, clean, done" [Elektroda, piracik, post #13321939]

Why does system pressure rise while I bleed radiators?

Air replaces water in the gauge path; the water column shortens, so indicated pressure climbs by ≈0.4 bar for every 4 m vented [Elektroda, piracik, post #13323456]

Could the safety valve dump water unseen?

Yes. Many modern boilers route the valve outlet to a drain, hiding leaks [Elektroda, piracik, post #13322480] Check the tundish or condensate pipe for moisture traces.

How much water should appear when venting a pump?

Expect a steady trickle for 2–5 seconds. Continuous flow signals residual pressure; stop and refill after vessel recharge [HVAC Guide, 2021].

Is under-floor piping a likely culprit?

Rarely. Only 3 % of leaks originate in embedded plastic circuits unless drilled [CIBSE, 2021]. Use thermal imaging or pressure-decay tests before opening floors.

What if recharging fails to stop pressure loss?

Look for edge cases: 1) ruptured diaphragm (water at Schrader valve), 2) corroded copper elbows forming pinholes [Elektroda, wOjasmen, post #13322688], 3) micro-leaks at auto-air vents. Replace faulty components.

Is it safe to run a solid-fuel boiler in a closed loop?

Only with a certified cooling coil and 3 bar safety valve. Overheat plus power cut can spike pressure to 6 bar in 15 minutes [Feinmesser, 2019].

Can leak-sealing liquid help invisible leaks?

Sealing additives work on small weeps (<1 l/day) and withstand 3 bar max. They are temporary; plan permanent repairs within six months [Elektroda, mirrzo, post #13324817]
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