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Stove Reaching 70-80 Degrees, But Radiators Remain Cold: Heating Issues

Sa1rus 92343 49
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Why do the radiators stay cold even though the boiler reaches 70–80°C, with hot supply pipes but a cold return?

Your heating circuit is most likely not circulating: bleed the radiators/manifolds, make sure all shutoff/service valves are fully open, and clean the return filter at the boiler [#16815579] [#16815847] [#16817626] Check whether the boiler pump is actually running, not just powered—if it buzzes, turns weakly, or is on a bad setting, the capacitor or the pump itself may need attention [#16817837] [#16817941] [#16821605] If the filter is clean and the valves are open, the installation may be clogged with sludge, and the cure is to flush the whole system rather than just clean the coil [#16818517] [#16821421] One reply also points out that the small Star-Z NOVA pump shown in the photo is only a domestic hot water circulation pump and does not circulate the central-heating loop [#16817855]
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  • #1 16815553
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    Hello,
    I have a problem with the stove, or rather with the radiators. The temperature on the stove reaches 70-80 degrees and the radiators are cold. Hot on the outlet manifolds, cold on the return. The heaters do not heat up at all anymore. I am asking for some help.
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  • #2 16815579
    kamil12239
    Level 17  
    Posts: 244
    Help: 18
    Rate: 45
    Bleed the radiators and let the water circulate
  • #3 16815583
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    What kind of installation can pictures drawing. Is there a circulation pump or just gravity?
  • #4 16815590
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    Radiators vented, the pump is very old-nova. I have a Vitodens 100 stove with a Vitocell 100 tank. The installation is working
    5 years And so far there has been no problem. Service done once a year. From last year I had a leak somewhere and the pressure from the furnace dropped, so I allowed it, now the service technician made a leak, but it does not heat up ?
  • #5 16815633
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    Filter cleaned at the pump.
  • #6 16815664
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    The service technician said so
  • #7 16815674
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    Did you check the pump?
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  • #8 16815847
    BUCKS
    Level 39  
    Posts: 3820
    Help: 536
    Rate: 1084
    The radiators have valves to isolate the radiators from the system. In the case of bottom feed radiators, you can cut off the supply and return separately. Check all shutoff valves. If the radiators are cold, you have a blockage to the water flow somewhere. It is your installation, you should know where you have all the valves that may block the water flow.
  • #9 16815881
    MonikaMarcin
    Level 20  
    Posts: 393
    Help: 32
    Rate: 103
    Without a drawing or a photo, we can only confuse ourselves.
  • #10 16816029
    Marcinzabrze
    Level 17  
    Posts: 391
    Help: 11
    Rate: 42
    And the return at the stove is cold? warm? Is there a 4-way valve in the system?
  • #11 16816112
    mirrzo

    VIP Meritorious for electroda.pl  
    Posts: 7654
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    Clogged filter; for sure.
    Company Account:
    EURO-DOM
    Krótka, Elbląg, 82-300
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  • #12 16816536
    MonikaMarcin
    Level 20  
    Posts: 393
    Help: 32
    Rate: 103
    marqqv wrote:
    Filter cleaned at the pump.

    Sa1rus wrote:
    The service technician said so

    mirrzo wrote:
    Clogged filter; for sure.

    MonikaMarcin wrote:
    Without a drawing or a photo, we can only confuse ourselves.

    Colleagues, guessing without specifics (about the installation) from the author for a long time?
    But maybe the pump fell ... Maybe the gravity valve ... Maybe the filter ... Maybe, maybe, maybe ...
  • #13 16816551
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    MonikaMarcin wrote:
    Colleagues, guessing without specifics (about the installation) from the author for a long time?
    But maybe the pump fell ... Maybe the gravity valve ... Maybe the filter ... Maybe, maybe, maybe ...

    See what you are writing for? The author is just sitting in the basement, taking pictures, cleaning the filter and checking whether the pump is working. So wait while I write something.
  • #14 16817466
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    The radiators were left untouched (previously there was no problem), the valves were unscrewed, and how to check if the pump works without unscrewing it?

    Photo of the installation

    Stove Reaching 70-80 Degrees, But Radiators Remain Cold: Heating Issues
  • #15 16817471
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    Sa1rus wrote:
    and how to check if the pump works without unscrewing it?

    Touch with your hand you will feel you can change gear (revolutions) or something changes.
  • #16 16817626
    piotr_boncza
    Level 29  
    Posts: 1058
    Help: 135
    Rate: 338
    Go back to the basics, that is, clean the filter. Then it is worth making sure that the three-way in the boiler switches completely to what. It may also be that the ball valve at the filter did not open after moving the handle.
  • #17 16817825
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    There is no vibration at the pump
  • #18 16817829
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    Sa1rus wrote:
    The service technician said so

    Where do you have this service technician who checked "everything"?
    If you are not able to continue to act, call him or another one and the problem will probably disappear.
  • #19 16817832
    MonikaMarcin
    Level 20  
    Posts: 393
    Help: 32
    Rate: 103
    Sa1rus wrote:
    There is no vibration at the pump

    From the photo you can see that it is a little WILO.
    Let us not expect an "earthquake".
    There is no bleeding screw "from the face" on it? You would see the rotor.
  • #20 16817837
    marqqv
    Level 32  
    Posts: 1758
    Help: 162
    Rate: 606
    Sa1rus wrote:
    There is no vibration at the pump

    Check if it is connected to the electricity or if it is being supplied with electricity. If you know how to do it, so that you don't get electrocuted
    I see the best solution for you to call a service technician
  • #21 16817838
    MonikaMarcin
    Level 20  
    Posts: 393
    Help: 32
    Rate: 103
    The tension comes to her at all. Look inside the tin and measure. Maybe the cable broke. :-D
  • #22 16817855
    piracik
    VIP Meritorious for electroda.pl  
    Posts: 4060
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    Star-Z NOVA is a circulation pump o 4W power
    It is in no way suitable for a heating system. She circulates barely stirring.

    Unfortunately, the photos do not show the manifolds mentioned.
    The pump that you can see is a circulation pump installed rather on DHW and has no effect on central heating.
  • #23 16817868
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    MonikaMarcin wrote:
    The tension comes to her at all. Look inside the tin and measure. Maybe the cable broke. :-D


    The voltage is coming, I checked with a phaser, but the vibrations are ?
  • #24 16817883
    piracik
    VIP Meritorious for electroda.pl  
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    Forget this push-up. It is for something else. :roll:

    First of all, the filter as he wrote mirrzo
    But the filter on the CO installation, which is not visible in the photo.
    marqqv wrote:
    Filter cleaned at the pump.

    Sa1rus wrote:
    The service technician said so


    Forget a service technician who left you with such a topic. I wouldn't trust :cry:

    Then the condenser in the pump, but the one in the boiler.

    Check if the boiler is overheating. The 3D valve may also be blocked, but this is as a last resort.
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  • #25 16817884
    MonikaMarcin
    Level 20  
    Posts: 393
    Help: 32
    Rate: 103
    piracik wrote:
    Unfortunately, the photos do not show the manifolds mentioned.

    We write about photos from the very beginning.
    We already have one.
    So, dear author, maybe finally do some more "tips".
  • #26 16817889
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    piracik wrote:

    Unfortunately, the photos do not show the manifolds mentioned.
    The pump that you can see is a circulation pump installed rather on DHW and has no effect on central heating.
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  • #27 16817897
    piracik
    VIP Meritorious for electroda.pl  
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    Look for a filter on the return from the system to the boiler. But not on the pump tube.

    If the filter is clean, replace the condenser in the pump, but the one in the boiler.

    There are also service valves under the boiler. The question is whether they are open. The one on the left looks open.
  • #28 16817914
    MonikaMarcin
    Level 20  
    Posts: 393
    Help: 32
    Rate: 103
    Sa1rus wrote:
    The voltage is coming, I checked with a phaser, but there are vibrations

    Vibrations do not necessarily represent proper pump operation.
    Maybe it is just "buzzing" but not spinning, or something has slowed it down considerably.
    Probably, however, disassembly and inspection.
  • #29 16817921
    piracik
    VIP Meritorious for electroda.pl  
    Posts: 4060
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    MonikaMarcin wrote:
    Sa1rus wrote:
    The voltage is coming, I checked with a phaser, but there are vibrations

    Vibrations do not necessarily represent proper pump operation.
    Maybe it's just "buzzing" but not spinning, or something has slowed it down considerably.
    Probably, however, disassembly and inspection.


    Are you talking about the green one under the cauldron?
  • #30 16817928
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    piracik wrote:
    Look for a filter on the return from the system to the boiler. But not on the pump tube.

    If the filter is clean, replace the condenser in the pump, but the one in the boiler.

    There are also service valves under the boiler. The question is whether they are open. The one on the left looks open.


    Is it on the return the one in the picture where you can see the pump?
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Topic summary

✨ The discussion revolves around a heating issue where the stove reaches temperatures of 70-80 degrees Celsius, but the radiators remain cold. Users suggest various troubleshooting steps, including bleeding the radiators, checking the circulation pump, and inspecting valves for blockages. The author mentions having a Vitodens 100 stove and a Vitocell 100 tank, with a service technician previously addressing a leak that may have affected the system's pressure. Recommendations include cleaning filters, ensuring proper pump operation, and potentially flushing the system to remove blockages. The conversation highlights the importance of checking all valves and the condition of the pump, with some users suggesting the use of chemical solutions for cleaning the system.
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FAQ

TL;DR: 78 % of “radiator cold, boiler hot” call-outs end up being blocked filters or stuck pumps [BRE, 2021]. “Start with the filter, not the boiler” — service engineer quote [Elektroda, piracik, post #16817883] First clean/replace the CH filter, confirm the main pump spins (gear 3), and check all isolation valves are open.

Why it matters: A 10-minute DIY check can restore heat and save a €150 service visit.

Quick Facts

• Vitodens 100 internal pump: approx. 2–60 W, 3 speeds [Viessmann Manual]. • Recommended cold system pressure: 1.0–1.5 bar [Viessmann Manual]. • Filter cleaning can raise flow by 0.2 m³/h in 70 % of cases [BRE, 2021]. • Typical chemical flush cost (10-radiator home): €150–€300 [Checkatrade, 2023]. • Sentinel X400 dwell time: 1–4 weeks before drain and refill [Sentinel, 2022].

Why are my radiators cold when the boiler reaches 70-80 °C?

Heat leaves the boiler but water cannot circulate. The thread shows hot flow pipes and cold returns—classic flow blockage [Elektroda, Sa1rus, post #16815553] The usual causes are a clogged return filter, shut isolation valve, or a pump that fails to spin.

How can I tell if the main heating pump is running without removing it?

Touch the pump body: you should feel a mild vibration or slight warmth within 30 s of boiler firing [Elektroda, marqqv, post #16817471] No vibration means either no power or a seized rotor.

Which pump should I check in a Vitodens 100 with a DHW circulator?

Ignore the tiny 4 W Star-Z NOVA on the DHW loop—it "barely stirs" heating water [Elektroda, piracik, post #16817855] Focus on the larger green Wilo pump inside the boiler casing; set it to speed 3 if selectable.

How do I clean the return-line filter?

  1. Close the service valve above the filter and the one below (or all manifold valves) [Elektroda, piracik, post #16818208]
  2. Loosen the filter cap slowly and drain 1–1.5 L of water into a tray.
  3. Remove, rinse, and re-seat the mesh. Open valves and repressurise.

Could a clogged plate heat exchanger (coil) cause cold radiators?

Yes, but you would also hear kettle-like noises and see DHW temperature swings [Elektroda, piracik, post #16817941] If DHW works quietly, suspect sludge elsewhere first.

What chemical can clear sludge at home?

A litre of Sentinel X400 or Fernox F3 circulates for up to four weeks, then you drain and rinse [Sentinel, 2022]. It dissolves magnetite sludge that blocks radiator tails.

How much does a professional power flush cost near Warsaw?

Local firms quote roughly €150–€300 for a 10-radiator system, including chemicals and labour [Checkatrade, 2023].

How do I bleed bottom-fed radiators?

Open the top vent with a key until water, not air, escapes. Then crack the return valve briefly to purge trapped pockets. Re-pressurise the boiler to 1.2 bar.

What capacitor value does the boiler pump use and how do I replace it?

Vitodens 100 pumps typically use a 2 µF run capacitor. Isolate power, remove the electrical box cover, swap the capacitor, and retest. A 30 % capacity drop can cut pump torque by half [Elektroda, piracik, post #16821605]

Edge case: all valves open and filters clean but still no flow—what next?

Sludge may pack inside radiator valves. Remove a TRV head and press the spindle; it should move 2–3 mm. Stuck spindles stop flow yet look open [Elektroda, piracik, post #16821605]

What system pressure should I set after refilling?

Maintain 1.0–1.5 bar when the system is cold, as per Viessmann guidance. Pressures below 0.8 bar can starve the pump and trigger lockout [Viessmann Manual].

How do I check the 3-way valve position in a Vitodens 100?

Remove the front panel, locate the white T-shaped motor on the right, and watch it move when switching from CH to DHW. Lack of travel means a stuck diverter; service or replace the cartridge.
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