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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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  • #31 16817935
    MonikaMarcin
    Level 20  
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    I read that:
    piracik wrote:
    Star-Z NOVA is a 4W circulation pump
    It is in no way suitable for a heating system. She circulates barely stirring.

    But I don't see another one at the moment.
    And I don't know what the author checked by touch. ;-)
    I have generally written about "external pump control".
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  • #32 16817938
    Sa1rus
    Level 7  
    Posts: 24
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    Is it possible that the coil in the furnace is stony and because of it?
  • #33 16817941
    piracik
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    MonikaMarcin wrote:
    I read that:
    piracik wrote:
    Star-Z NOVA is a 4W circulation pump
    It is in no way suitable for a heating system. She circulates barely stirring.

    But I don't see another one at the moment.
    And I don't know what the author checked by touch. ;-)


    Re-read the topic, analyze it and don't be confused.

    Sa1rus wrote:
    Is it possible that the coil in the furnace is stony and because of it?


    Yes. But the boiler would also overheat on hot water heating. You would hear characteristic noises. Something like an electric kettle, like water that heats up quickly.

    First you have to eliminate the simplest things.

    Filter, valves, capacitor.

    Remove the casing from the boiler (slightly upwards and towards you). The screws are unscrewed from the bottom.

    At the bottom right is the boiler pump. Check if it is set to 3rd gear.

    Before you open the filter, you must close the valves. This service (a flat wrench or a wide screwdriver is required) and below the filter must be the second standard one with a handle.
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  • #34 16817980
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    If it is the one behind the panel, I do not see any regulation anywhere
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  • #35 16817991
    piracik
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    It may not be.

    The switch looks like in the photo below. That black toggle on the electrical box.
    Stove Reaching 70-80 Degrees, But Radiators Remain Cold: Heating Issues
  • #36 16818024
    Sa1rus
    Level 7  
    Posts: 24
    Rate: 23
    Well, there is no regulation, when it comes to this coil, when there was a service technician, he drained the water into a bucket And there was some black muck in the coil, earlier there was a problem with these noises from the stove (but the stove was heating) as you wrote, but how to overflow ukal, so far it is quiet, but it does not heat, maybe he forgot to unscrew the valves, he did not unscrew one, maybe and the service technicians also not, how can they look? Are these these?
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    • Stove Reaching 70-80 Degrees, But Radiators Remain Cold: Heating Issues IMG_20171112_142018.jpg (2.14 MB) You must be logged in to download this attachment.
  • #37 16818040
    piracik
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    This one is open. The other side is the same. The notch on the valve is to be along the pipe. Check this square from below with a notch.

    Look for the second valve somewhere under the filter and check if it's open.

    I wouldn't trust a service technician to clean this filter.

    Personally, I very often come across a dry condenser in the pump. As a rule, if it is below 70% of its capacity, problems with the flow begin.
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  • #38 16818118
    Sa1rus
    Level 7  
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    Both are open, or to clean this filter it is enough to close only these 2 valves?
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  • #39 16818121
    piracik
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    Not. One above the filter and one below the filter. Otherwise you will flood the bathroom.
  • #40 16818169
    Sa1rus
    Level 7  
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    Well, I only have a service valve in front of the filter, and then only in the manifolds, do you need to drain the water somewhere else?
  • #41 16818208
    piracik
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    May be. Close the valves on the manifolds and the service one above the filter.
    You will only need to drain a little more water.
    It should not be more than about 1 - 1.5 liters
    Loosen the stopper slightly and catch the water until it stops flowing.
    Don't wring it out all at once.
  • #42 16818313
    Sa1rus
    Level 7  
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    The filter is clean
  • #43 16818517
    mychaj
    Level 35  
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    The same situation with my in-laws last year, we thought that the installation was frozen - except that some of the pipes were warm and the radiators were cold. It turned out that during the stop, every now and then the muck accumulated in one place blocked the installation and only flushing the installation helped.
  • #44 16818647
    Sa1rus
    Level 7  
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    Only at the entrance to the building I have 2 filters, then another 2, so there is no muck to get in, and how did you flush the installations?
  • #45 16818732
    mychaj
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    We dumped water in turn from all the radiators on the return valves, I allowed and watched what was pouring out of the main pipe, which - a lot of muck fell out, and there are filters on the central one, there was such a fine black sludge, something like dissolved copper patina?
  • #46 16821127
    Sa1rus
    Level 7  
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    Maybe someone has an idea what else can be done, so far filters have been cleaned, radiators deaerated, a checked three-way valve, a proven pump in the furnace (it turns, but it's hard for me to judge). Hot pipes at the entrance, already cold by the heaters at the entrance, any idea? The service technician wants to clean my coils, but is she not also responsible for the warm water in the tap?
  • #47 16821421
    mirrzo

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    Just cleaning the coil for nothing. Your system is clogged with silt, so you need to "unplug" it. Use a measure such as Sentinel X400
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  • #48 16821588
    Sa1rus
    Level 7  
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    Rate: 23
    When draining the water it is slightly yellowish, but you can barely see it, the installation is 5 years old, so rather not too old. Does anyone know someone from the southern suburbs of Warsaw worth recommending? What may be the cost of flushing the system?
  • #49 16821605
    piracik
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    A day ago, the reason they were looking for a circulation pump.
    Flushing the entire installation today.
    I would be afraid of what tomorrow will bring :)

    Bleed the manifolds, radiators, check if the spindles in the valves under the heads are working properly, replace the capacitor in the pump (even on trial. The cost of one in an electrical store is about PLN 5)

    I have been working in the service for 10 years. I replaced hundreds of capacitors, unblocked dozens of blocked valves on the heads, vented thousands of radiators and flushed one installation that had been air-conditioned for several years.

    Good luck
  • #50 16821607
    mirrzo

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    You will do it yourself. Buy "chemistry", add to the system, and then follow the manufacturer's instructions. It will be the cheapest and you will become the 'hero of the house' :)
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    Krótka, Elbląg, 82-300
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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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