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Danfoss Thermostatic Head on 90s Block: Purmo Plate Radiators, Valve Settings & Temperature Control

m4xon 8178 15
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  • #1 16786795
    m4xon
    Level 15  
    Posts: 707
    Rate: 73
    Hi,

    I live in a block built in the 90s. Two years ago, I replaced the cast iron radiators with Purmo plate radiators, additionally Danfoss valves and thermostatic heads are installed. I have a question about the principle of operation of the heads. Namely, does program 3 mean a specific temperature? I have the impression that this head works as an open closed principle, I set 3 and nothing heats up, and how 5 goes. I don't quite understand how it works and could there be a problem with the installation?

    Thanks
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    #2 16786813
    gimak
    Level 41  
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    m4xon wrote:
    I have the impression that this head works as an open closed principle, I set 3 and nothing heats up, and how 5 goes.

    You see you have a higher temperature in the room than the "3" assigned to the head and therefore it does not open.
  • #3 16786860
    adamkowalski431
    Level 16  
    Posts: 387
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    Open the window in the room and it will start to heat you up.
  • #4 16786890
    m4xon
    Level 15  
    Posts: 707
    Rate: 73
    and do these numbers have specific temperatures assigned to them?
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    #5 16786897
    adamkowalski431
    Level 16  
    Posts: 387
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    So check the pages of Danfos, maybe you can dig up something
  • #6 16786930
    kierbedz4
    Level 36  
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    At a temperature of about 8 degrees in the room, the thermostatic valve will open automatically to prevent the radiator from freezing. Before venting the room, the valve should be closed.
  • #8 16786998
    m4xon
    Level 15  
    Posts: 707
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    Thank you very much!
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  • #9 16787775
    Zbigniew Rusek
    Level 38  
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    One more thing (quite important). Are the heads mounted horizontally? They should be horizontal, as the thermostatic head mounted vertically upwards will cause heat from the power sprig to heat the head and give a false interpretation of the room temperature.
  • #10 16788124
    BUCKS
    Level 39  
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    m4xon wrote:
    and do these numbers have specific temperatures assigned to them?

    As far as I remember, 3 is the equivalent of about 20 degrees.
    You just have to remember that the thermostatic head "measures" the temperature at the radiator.
    For example, if you have a window sill too low over the radiator, it may be too warm near the head and then the head will close or close the radiator completely, and you will have a lower temperature in the depths of the room.
    It is best to do tests with a thermometer in the room, then you will see how much the setting of 3 on the head really gives you.
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  • #11 16788955
    Zbigniew Rusek
    Level 38  
    Posts: 3610
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    BUCKS wrote:
    m4xon wrote:
    and do these numbers have specific temperatures assigned to them?

    As far as I remember, 3 is the equivalent of about 20 degrees.
    You just have to remember that the thermostatic head "measures" the temperature at the radiator.
    For example, if you have a window sill too low over the radiator, it may be too warm near the head and then the head will close or close the radiator completely, and you will have a lower temperature in the depths of the room.
    It is best to do tests with a thermometer in the room, then you will see how much the setting of 3 on the head really gives you.
    The head cannot be behind the curtain either, because the curtain causes the heat from the radiator to be "trapped" between the curtain and the glass, so it is hot there, the head senses it and shuts off the inflow of the heating medium. In general, it should not cover the radiators (curtains - yes, but to the window sill).
  • #12 16788978
    BUCKS
    Level 39  
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    Zbigniew Rusek wrote:
    The head cannot be behind the curtain either, because the curtain causes the heat from the radiator to be "trapped" between the curtain and the glass, so it is hot there, the head senses it and shuts off the inflow of the heating medium. In general, it should not cover the radiators (curtains - yes, but to the window sill).

    In exceptional situations, if there is no option to discover the thermostatic head, you can use a head with a capillary, i.e. a wire 1.5-2 m long at the end of which there is a temperature sensor. Then the head can be covered, because the temperature on the external sensor is taken into account, i.e. the sensor must be mounted in an open place at a suitable height for the radiator to properly heat the room.
    There is always a way out of a given situation, but the capillary head is more expensive than the usual one.
  • #13 16789281
    Zbigniew Rusek
    Level 38  
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    BUCKS wrote:
    Zbigniew Rusek wrote:
    The head cannot be behind the curtain either, because the curtain causes the heat from the radiator to be "trapped" between the curtain and the glass, so it is hot there, the head senses it and shuts off the inflow of the heating medium. In general, it should not cover the radiators (curtains - yes, but to the window sill).

    In exceptional situations, if there is no option to discover the thermostatic head, you can use a head with a capillary, i.e. a wire 1.5-2 m long at the end of which there is a temperature sensor. Then the head can be covered, because the temperature on the external sensor is taken into account, i.e. the sensor must be mounted in an open place at a suitable height for the radiator to properly heat the room.
    There is always a way out of a given situation, but the capillary head is more expensive than the usual one.
    And it is not always commercially available.
  • #14 16852967
    leso
    Level 10  
    Posts: 10
    I understand that you shouldn't cover the thermostatic head during the day when I don't want the heat to be wasted, but am I not logically saying that it might make sense at night? If I cover the window with a curtain at night, then the head will automatically let less heat through the valve into the radiator and I am walking at night. This way I don't have to "screw" the head on before the night. If I am wrong, please pay attention to me.
  • #15 16853299
    BUCKS
    Level 39  
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    leso wrote:
    If I cover the window with a curtain at night, then the head will automatically let less heat through the valve into the radiator and I am walking at night. This way I don't have to "screw" the head on before the night. If I am wrong, please pay attention to me.

    But you have to cover or expose the head on purpose, so what's the difference to potentially screwing the head on.
    This is probably why you bought a thermostatic head to set the temperature relatively precisely, and now having this head you do not want to use it properly.
    If you don't want to shoot, buy yourself electronic heads and program temperatures for specific hours, the head will automatically change the settings.
    For now, I use a constant temperature of 20 degrees for 24 hours on the room regulator and 2 heaters are without heads, and in the remaining ones I set 21, 21.5 degrees on the head and I do not turn anything.
    The temperature is between 20.1-20.8 degrees depending on the room and I'm not shooting anything.
    My regulator does not allow me to precisely set a higher temperature, so I am not trying to do so, because the constant temperature seems optimal to me.
  • #16 16853388
    m4xon
    Level 15  
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    thanks gentlemen!
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Topic summary

✨ The discussion revolves around the operation of Danfoss thermostatic heads installed on Purmo plate radiators in a 90s block building. The main inquiry concerns the temperature settings associated with the head's numerical scale, particularly whether setting "3" corresponds to a specific temperature. Responses indicate that the thermostatic head operates based on the room temperature relative to the radiator, with "3" approximately equating to 20 degrees Celsius. If the room temperature exceeds this setting, the head will not open, leading to confusion about heating performance. Users suggest checking the installation orientation of the heads, ensuring they are mounted horizontally to avoid false temperature readings. Additionally, it is advised to keep the heads unobstructed by curtains or furniture to maintain accurate temperature control. For those seeking more precise control, electronic thermostatic heads are recommended.
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FAQ

TL;DR: On Danfoss-style heads, setting 3 ≈ 20°C; “3 is the equivalent of about 20 degrees.” Use a thermometer to verify and avoid covering or mis‑mounting that skews sensing. [Elektroda, BUCKS, post #16788124]

Why it matters: This FAQ helps 90s‑block apartment owners with Purmo radiators and Danfoss valves get stable room temperatures and avoid waste.

Quick Facts

What do the numbers on my Danfoss thermostatic head actually mean?

They are comfort setpoints. Typical mapping: “3” ≈ 20°C. The head throttles flow to hold that room temperature. Use a separate room thermometer to confirm your exact result, because the sensor is near the radiator where air is warmer. Adjust a half‑step if needed. “5” means a higher setpoint, not full‑open forever. “The head ‘measures’ temperature at the radiator.” [Elektroda, BUCKS, post #16788124]

Why is the radiator cold on 3 but hot on 5?

On “3,” the room is already at or above that setpoint, so the head stays closed. When you turn to “5,” you request a warmer room, and the valve opens to deliver more heat. This is normal proportional control, not a fault. [Elektroda, gimak, post #16786813]

Is there built‑in frost protection and when does it activate?

Yes. The frost setting opens the valve at about 8°C to protect the radiator and piping from freezing. If you plan to air a room in winter, close the valve first to avoid over‑cooling triggering long reheating afterwards. [Elektroda, kierbedz4, post #16786930]

Should the thermostatic head be mounted horizontally or vertically?

Mount it horizontally. A vertical head can be warmed by the supply pipe or riser, causing the sensor to think the room is warm. That leads to premature closing and underheating. Re‑orient the valve body or use an angled adapter if needed. [Elektroda, Zbigniew Rusek, post #16787775]

My sill/curtain covers the head and the room is cold—why?

Coverings trap hot air near the sensor. The head then senses high local temperature and shuts the valve, while the room core stays cool. Keep the head exposed, and hang curtains only to the sill so convection to the room remains clear. [Elektroda, Zbigniew Rusek, post #16788955]

Is it okay to open a window to ‘force’ heating?

If you open a window near the head, the sensor cools and the valve opens, making the radiator heat. This proves the head responds to local air; it doesn’t fix placement problems. Air rooms briefly with the valve closed to limit waste. [Elektroda, adamkowalski431, post #16786860]

Can I deliberately cover the head at night to lower temperature?

Avoid that. You bought a head to set precise temperatures. Use the dial or install electronic heads to schedule night setbacks automatically. One user reports a stable 20.1–20.8°C band using constant setpoints and minimal fiddling. [Elektroda, BUCKS, post #16853299]

What is a capillary (remote‑sensor) thermostatic head?

It’s a head with a 1.5–2 m capillary tube to a remote bulb that senses room air away from the radiator. Use it when the valve must be covered or recessed. It costs more but restores accurate control in difficult locations. [Elektroda, BUCKS, post #16788978]

How do I test and set my Danfoss head correctly?

Use this quick method: 1) Place a room thermometer away from the radiator. 2) Set the head to “3” and wait several hours. 3) Nudge half‑steps until the thermometer matches your target. Remember the head senses near the radiator. [Elektroda, BUCKS, post #16788124]

Why does vertical warmth from the pipe fool the sensor?

Heat rising from the supply warms a vertically mounted head. The sensor sees a micro‑climate hotter than the room, closes early, and you feel cold. Horizontal mounting keeps the sensor in ambient room air for accuracy. [Elektroda, Zbigniew Rusek, post #16787775]

Do low window sills affect thermostatic heads?

Yes. Low sills trap convective heat over the radiator, making the head read higher than the room. Result: shutdown and a colder interior. Raise the sill clearance, redirect airflow, or relocate the sensor using a capillary head. [Elektroda, BUCKS, post #16788124]

What should I do before airing rooms in winter?

Close the thermostatic valve before opening windows. This prevents the sensor from over‑reacting to the cold draft and running the system unnecessarily hard afterward. Reopen to your normal setting after you finish. [Elektroda, kierbedz4, post #16786930]

Does setting 5 equal maximum heat output?

No. Setting 5 requests a higher room temperature. If the room never reaches it, the valve can stay open longer, but output still depends on system temperatures and radiator size. Verify with a room thermometer, not pipe warmth. [Elektroda, BUCKS, post #16788124]

What’s a simple control strategy to avoid tinkering every day?

Pick a comfortable constant setpoint and leave it. For automation, use electronic heads to schedule setbacks. Reported results show tight control within about 0.7°C without daily adjustments. “Constant temperature seems optimal to me.” [Elektroda, BUCKS, post #16853299]

Edge case: the head isn’t sold locally—what then?

Some capillary heads may be harder to source. If unavailable, improve exposure, change valve orientation, or try a different brand compatible with your Danfoss valve body. Ask suppliers about lead times or alternatives. [Elektroda, Zbigniew Rusek, post #16789281]
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