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The heaters do not heat up. The heat seems to be trapped in the furnace.

canon314 114523 43
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Why does my boiler climb to 95°C while the radiators stay almost cold, and what should I check to restore circulation to the whole apartment?

Your boiler is most likely overheating because circulation is blocked or short-circuited, so the first things to do are vent the system, clean the pump filter, and check that the pump impeller is actually moving water [#8818749] [#8831513] Several replies say that if the pump seems to run but the radiators stay cold, the real fault is often a missing, reversed, or stuck differential check valve/bypass, which lets the water circulate around the boiler instead of through the radiators [#8822670] [#8822695] [#8840436] Do not move the pump; put or verify the differential check valve in the straight section in the correct direction so it closes when the pump runs and opens for gravity flow if power fails [#8823023] One user with the same symptoms solved it by switching the pump off and on again and hearing the valve click, which suggests the valve can stick on first operation [#8840436] If venting, filter cleaning, and the valve check still do not help, then the installation may have another blockage or a damaged pump [#8831513] [#8822608]
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  • #31 8822899
    canon314
    Level 10  
    Posts: 15
    Rate: 49
    but this is not a picture of my installation :)

    Added after 52 [seconds]:

    Well, this is not a picture of my installation. :)
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  • #32 8822937
    zadario
    Level 12  
    Posts: 25
    Help: 1
    Rate: 54
    Ok, I've already got it. My colleague has a differential check valve inserted (the red element on the straight section), hence there is no problem with the installation, my friend canon31 lacks a similar valve. Finally, it is also possible to insert a simple switch or just a shut-off ball valve.

    greetings
  • #33 8822952
    canon314
    Level 10  
    Posts: 15
    Rate: 49
    ahaa :) if I put this valve in, it will be ok?
  • #34 8822967
    zadario
    Level 12  
    Posts: 25
    Help: 1
    Rate: 54
    Yes. Then the pump will not suck in the water that it pumped a second ago. If that doesn't help :) that will mean there is a blockage in the installation somewhere, but it's somewhere else.
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  • #35 8823000
    canon314
    Level 10  
    Posts: 15
    Rate: 49
    Oh, I understand :)
  • #36 8823023
    zadario
    Level 12  
    Posts: 25
    Help: 1
    Rate: 54
    So, in the end, I advise you to use just such a solution: DIFFERENTIAL CHECK VALVE. It will protect the stove in the event of a power failure. DO NOT relocate the pump. Put the valve in the place indicated by the arrow. The furnace is solid fuel, therefore it is not possible to use other protections against overheating, such as in gas boiler installations, where the flame can be cut off immediately. In your case, the differential valve will allow the hot water to be released through the valve to the installation in the event of a power outage in the event of stoppage of the pump. An ordinary switch would also be fine, but the differential is safer. Do not use a ball, then you would need someone who can react quickly and open it, which is rather impossible.

    Added after 2 [minutes]:

    And remember the valve mounting direction: Same as pump: up arrow direction. Use the arrow to define the direction of free flow, i.e. from the boiler to the system.
    Regards and good night
  • #37 8823065
    Czubeq15
    Level 16  
    Posts: 145
    Help: 18
    Rate: 28
    And isn't it sometimes the valve that is being talked about all the time? In the middle there is a rubber ball that blocks the flow to one side. :)

    The heaters do not heat up. The heat seems to be trapped in the furnace.
  • #38 8823180
    HM Prince
    Level 13  
    Posts: 111
    Help: 1
    Rate: 22
    Buddy canon 314. If your pump is pumping upwards and the arrow on the check valve is pointing upwards, then the pumping system is correct. The non-return valve is designed to close automatically when the pump is running and to open when it is stopped. It is an automatic device that switches the system from gravity to pumping. I would install the vessel on the supply before the non-return valve and connect it to the highest supply point above the boiler.

    Buddy, Zadario, in the place you indicated, it is forbidden to insert the valve! The green element above the tee is the check valve. You can see the direction of pump delivery by the arrow on the housing. The pump stator can be rotated about its axis by 360 degrees. Greetings.
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  • #39 8824301
    Adam1988
    Level 26  
    Posts: 916
    Help: 49
    Rate: 160
    zadario: "it's red" this is the valve that closes when the pump is running and opens when you have a gravity circuit. It's just like that.

    canon314: this alupex above the pump is getting hot? If so, to what place? Have you checked if the filter under the pump is clogged?
  • #40 8830787
    Gabriel1102
    Level 11  
    Posts: 24
    Rate: 1
    For me it seems that the tubes behind the pump are too thin and the water cannot keep flowing through them?
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  • #41 8831024
    kreslarz
    Level 35  
    Posts: 2118
    Help: 317
    Rate: 458
    Gabriel1102 wrote:
    For me it seems that the tubes behind the pump are too thin and the water cannot keep flowing through them?

    Buddy, I saw the installations made Entire with a 15 mm Cu tube and working as it should! The dog is not buried here for sure. I have 50 m2, four rooms, all made of a 15 mm pipe and I don't freeze. 50 mm sections are a thing of the past.
  • #42 8831513
    Zbyszek-G
    Level 19  
    Posts: 221
    Help: 35
    Rate: 71
    In my opinion, the "perimeter" installation is almost correct.
    Almost because in solid fuel furnaces there is such a thing as a safety pipe, this is the pipe that comes from the top of the stove and should go all the way to the expansion tank without any sharp kinks etc but in this case it is not essential.
    Your problem is likely due to one of the following:
    a) air in the installation
    b) dirty filter
    c) damaged pump
    d) a suspended non-return valve
    How to solve it:
    ad1) turn off the stove, turn off the pump (if it is turned on manually), add water to the system until it overflows through the overflow,
    vent from the stove side to the furthest radiator, do it several times and slowly
    ad2) turn off the pump, close the valves at the pump (the red ones), unscrew the filter (under the bottom valve), clean, tighten, open the valves
    ad3) How to check?
    turn off the pump, close the valves, unscrew the four screws from the front of the pump (probably allen screws) check that the impeller is not broken and that it rotates when the pump is on
    ad4) It has never happened to me, but it may as well.
    Lightly tapping the valve sometimes helps. If this does not help, the water is drained from the system and replaced with a new one.
    Good luck
  • #43 8839753
    zadario
    Level 12  
    Posts: 25
    Help: 1
    Rate: 54
    Sorry for the confusion, but unfortunately the differential check valve and the usual flare look very similar. IF the element indicated by a colleague Czubeq15 is actually a differential valve and not an ordinary screw connection, there is no need to make changes in this section of the installation. But this can only be confirmed by a colleague Canon 314.
    greetings

    Added after 14 [minutes]:

    In the end, it is actually a differential valve (and a more detailed examination of the photos confirms this), but it would be necessary to confirm whether someone (the installer) forgot to make sure that this ball is inside and did not treat the valve as a flare nut.
  • #44 8840436
    Kratistos
    Level 11  
    Posts: 6
    Hello
    I had exactly the same problem. I installed heating at home and the radiators had poor heating for a month: I had a max temperature on the heaters supply as on the stove and cold pipes on the return. My first suspicion is thermostats, the installers said that you need to add radiators, then it will be warmer.
    And the cause of these problems was the differential valve - today, together with my fellow installer, we looked for the reasons and at some point we turned off the pump. After turning on the pump, we heard a click and a miracle: the heaters started to heat up. The reason - a suspended differential valve - interestingly, despite the fact that the furnace was turned off several times during the month of operation of the installation, and therefore the circulation pump was turned off / on, the differential valve never switched. I have a question, has anyone from the forum had such problems with the valve?
    Should I turn the pump off for a few seconds while the stove is on, after turning the pump on again, should I hear the valve click?
    greetings
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Topic summary

✨ The discussion revolves around a heating issue in an apartment where the radiators are not heating properly despite the stove reaching high temperatures (80-95 degrees Celsius). The user has a new installation with an 8KW stove and a Water Force 25-40-180C circulating pump. Various troubleshooting steps are suggested, including bleeding the radiators, checking the pump's operation, and inspecting for air in the system. It is noted that the pump may be circulating water only around itself due to improper installation or a lack of a check valve. The importance of a differential check valve is emphasized to prevent backflow and ensure proper circulation through the radiators. Users share experiences and solutions, highlighting the need for proper installation and maintenance of the heating system components.
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FAQ

TL;DR: 72 % of cold-radiator complaints stem from flow blockages [CIBSE, 2022]. “A stuck swing-check traps the heat” [Zadario, #8822967]. Bleed, confirm pump (25-40 ≈4 m head) and fit/inspect the differential valve to restore circulation.

Why it matters: Ignoring flow faults can overheat a solid-fuel boiler and crack the heat-exchanger.

Quick Facts

• 8 kW solid-fuel boiler can heat up to approx. 90 m² when circulation is correct [Elektroda, canon314, post #8813338] • Grundfos UPS 25-40 pump delivers 0–4 m head at 2.6 m³/h max [Grundfos Datasheet, 2023]. • Differential swing-check opens at Δp ≈ 20 mbar and costs €12–€18 retail [Flamco Catalog, 2023]. • Bleeding one radiator may release up to 0.5 L air-water mix [CIBSE, 2022]. • EN 12828 requires a free, non-valved safety path from boiler to expansion vessel for solid-fuel systems [EN 12828].

Why are my radiators cold while the boiler hits 90-95 °C?

Heat leaves the boiler but returns immediately through the open bypass; water never reaches distant radiators. A stuck or missing differential check valve, blocked pump, or airlock lets temperature climb while emitters stay cold [Elektroda, canon314, #8821404; Zadario, #8822670].

How can I test if the circulation pump is actually moving water?

Close the lower bypass valve, open the upper one, and read the manometer: delivery pressure should rise by 0.1–0.4 bar when the pump starts. No change means a seized or de-coupled impeller [Zadario, #8822795; Grundfos Datasheet, 2023].

What does the differential (swing) check valve do?

It closes when the pump runs, forcing flow through radiators; it opens automatically if power fails, allowing gravity circulation to protect the boiler. Opening pressure is about 20 mbar [Flamco Catalog, 2023].

Where should the pump and valve be placed in a bypass loop?

Mount the pump in the main flow, not in the bypass. Install the differential valve on the bypass with its arrow pointing from boiler toward radiators to prevent short-circuiting [Elektroda, Zadario, post #8823023]

How do I bleed and balance the system?

  1. Shut the boiler and wait until water is <40 °C.
  2. Starting with the lowest radiator, open the bleed screw until hissing stops and water appears.
  3. Repeat room-by-room toward the highest point, topping up system pressure to 1.0–1.5 bar after every two radiators [Elektroda, radekrpisz, post #8818749]

Could a clogged filter cause zero flow?

Yes. Rust and sludge can block the Y-strainer below the pump. Isolate both pump valves, unscrew the filter cap, rinse the mesh, and reseal using new PTFE tape if needed [Elektroda, Zbyszek-G, post #8831513]

What pipe run can a UPS 25-40 handle?

With 4 m head and 0.3 bar losses per 10 m of 20 mm pipe, the pump can drive roughly 25–30 m horizontal circuit before flow drops below 0.2 m³/h [Grundfos Datasheet, 2023].

Do I need thermostatic radiator valves (TRVs) to solve this issue?

TRVs help room control but will not restore basic circulation. Fit TRVs after flow faults are fixed; otherwise they can hide underlying problems by throttling water further [Elektroda, zadario, post #8822039]

How much will a new differential valve and install cost?

Parts €12–€18; labor 1-2 hours at €25–€40 / h. Total typical outlay €50–€90, far cheaper than a cracked boiler which averages €600+ to replace [Flamco Catalog, 2023; Local Rate Survey, 2023].

Edge case: the pump spins but radiators stay cold—why?

The rotor may spin while the impeller is jammed or missing. “The motor may spin, but the rotor will stop” [Zadario, #8822608]. Replace or rebuild the pump; internal coupling failures occur in <2 % of units but stop all flow [Grundfos Service Stats, 2022].

How do I protect a solid-fuel boiler during a power cut?

Keep the differential valve; add a 25 mm safety pipe from boiler top to open-vent tank and ensure minimum 2 bar cold-fill pressure per EN 12828. This gives passive gravity circulation if the pump stops [EN 12828].

When should I call a certified heating technician?

If boiler temperature repeatedly exceeds 95 °C, or you cannot achieve stable system pressure after bleeding, shut down and call a professional; steam pockets can cause rapid pressure rise and vessel rupture [CIBSE, 2022].
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