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Why is it that when you turn on the cold water, hot and sometimes even hot water flows?

Nick05 20676 11
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  • #1 19654859
    Nick05
    Level 2  
    Posts: 4
    Rate: 1
    I live in a brand new 7-unit building.
    I have two Hansgrohe taps with thermostats in the bathroom and ordinary mixer taps in the rest of the flat.
    Recently, however, I've had the problem that when I turn on the cold water, the water usually runs warm/hot for the first few seconds. This happens with both thermostatic and mixer taps. Sometimes hot water even comes from the toilet cistern.
    The connections from the boiler room are connected correctly (the hot water connection is hot when touched by hand and the cold water connection is cold).
    Do you have any suggestions?
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    #2 19654867
    stanislaw1954
    Level 43  
    Posts: 15002
    Help: 1981
    Rate: 4560
    brzozowskikamil wrote:
    Do you have any suggestions?
    As someone rummaged through the installation?
  • #3 19654894
    Nick05
    Level 2  
    Posts: 4
    Rate: 1
    No changes have been made to the installation.
    I am wondering if it is possible that, for example, the thermostat in the tap has broken due to its age. And now, even though the tap is turned off, hot water flows through the thermostat and into the cold water connection?
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    #4 19654919
    Robewit
    Level 26  
    Posts: 679
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    I am betting on damage to one of the thermostat taps. If you have the opportunity, cut off the cold water supply to one then the other and check the effect.
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    #5 19654921
    misiek1111
    Level 37  
    Posts: 3970
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    Turn off the shut-off valves in front of that thermostat and you'll find out.
  • #6 19655015
    Nick05
    Level 2  
    Posts: 4
    Rate: 1
    Thank you for your answers. However, with these thermostatic taps I do not have shut-off valves, they are wall taps. In that case, instead of the thermostatic taps, I will install ordinary taps with valves and check the effect.
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    #7 19655245
    stanislaw1954
    Level 43  
    Posts: 15002
    Help: 1981
    Rate: 4560
    Nick05 wrote:
    In this situation, instead of thermostatic taps, I will install ordinary taps with taps and check the effect.
    Install one ordinary tap as a replacement and test. This will tell you which thermostatic mixer has failed.
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    #8 19655317
    Aleksander_01
    Level 43  
    Posts: 12626
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    There is definitely something wrong with the thermostats, but is it definitely a fault? In my opinion, it could still be a situation where the hot water has more pressure than the cold water, when opening another tap or bardash, the pressure of the cold water drops dramatically and the hot water 'overflows' from the thermostat.
    One thing is for sure, something is wrong with the thermostats.
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    #9 19655446
    misiek1111
    Level 37  
    Posts: 3970
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    Rate: 1047
    As a quick aside, you can still set the thermostat temperature knob to minimum and see if the situation changes.
  • #10 19656190
    Nick05
    Level 2  
    Posts: 4
    Rate: 1
    Ok thank you for your replies, I will follow the suggestions given. I will give an answer from the tests carried out.
  • #11 19666171
    Zbychul
    Level 22  
    Posts: 442
    Help: 31
    Rate: 189
    I had the same thing at my place - a flat in a block of flats , a hansgrohe thermostatic mixer and instead of hot water in the taps - lukewarm or cold water . Due to the difference in pressure, cold water entered the hot water pipes through the mixer. In general, the faucet is to blame and if it is under warranty, it should be reported. Even such a company starts to make mistakes. I temporarily installed one-way valves behind the water meters ( on the pecks ) without any problem and it helped for the time being.
  • #12 19667253
    piracik
    VIP Meritorious for electroda.pl  
    Posts: 4060
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    If you are close to the boiler room then the cause could be a lack of air in the diaphragm vessel, or a faulty non-return valve at the DHW cylinder.
    The heated water expands, increases in pressure and backs up into the cold water system.
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Topic summary

✨ The discussion revolves around a plumbing issue in a new 7-unit building where cold water taps initially deliver warm or hot water. The user has Hansgrohe thermostatic taps in the bathroom and standard mixer taps elsewhere. Suggestions include checking for damage to the thermostatic taps, as a malfunction could allow hot water to flow into the cold water line. Other potential causes discussed include pressure differences between hot and cold water, leading to backflow, and issues with the boiler room's diaphragm vessel or non-return valve. Recommendations include replacing one thermostatic tap with a standard tap to diagnose the problem and adjusting the thermostat settings.
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FAQ

TL;DR: Troubleshooting warm flow on cold lines in a 7‑unit building? "Sometimes hot water even comes from the toilet cistern." Most cases are crossflow from a faulty thermostatic mixer or pressure imbalance; isolate mixers, verify check valves, and balance pressures. [Elektroda, Nick05, post #19654859]

Why it matters: This FAQ helps apartment residents, landlords, and installers quickly diagnose and stop hot‑into‑cold cross‑connections that waste energy and risk scalding.

Quick Facts

  • Typical operating pressure for thermostatic mixers is 1–5 bar; balanced hot and cold supplies improve stability. [Hansgrohe, 2021]
  • Backflow prevention is required; install non‑return valves per EN 1717 to stop hot‑to‑cold cross‑connection. [EN 1717, 2000]
  • Symptom pattern: cold tap runs warm for the first few seconds; even cisterns can fill warm when crossflow exists. [Elektroda, Nick05, post #19654859]
  • Plant‑side faults near the boiler (loss of expansion‑vessel air or failed DHW check valve) can push hot into cold. [Elektroda, piracik, post #19667253]

Why does hot water come out when I open the cold tap?

Hot water can backfeed through a mixer that links hot and cold internally. A worn thermostatic cartridge may leak across ports. Pressure imbalance can make hot push into the cold line when other taps open. This is classic crossflow through a mixer. [Elektroda, Aleksander_01, post #19655317]

How do I test which mixer is causing crossflow?

Use shut-offs to isolate mixers one at a time.
  1. Close the hot and cold shut-off valves to one thermostatic mixer.
  2. Open a cold tap elsewhere; check if warm flow stops.
  3. Reopen and repeat for other mixers to find the culprit. This isolates crossflow to a single mixer. [Elektroda, misiek1111, post #19654921]

Can a thermostatic cartridge leak between hot and cold when the tap is off?

Yes. A failed cartridge or its internal checks can pass hot to cold even when closed. That creates warm flow on cold openings. “There is definitely something wrong with the thermostats.” Replace or rebuild the faulty mixer. [Elektroda, Aleksander_01, post #19655317]

I don’t have shut‑off valves at the mixer. How can I test?

Temporarily swap one thermostatic mixer for a simple valve or basic mixer with shut‑offs. Test the system; if the symptom disappears, that mixer was the source. Replace mixers one by one to identify the faulty unit. [Elektroda, stanislaw1954, post #19655245]

Why is my toilet cistern filling with warm water?

Warm water can backfeed from a mixer into the cold network and reach the WC branch. The cistern then fills with warm water for a short time. This indicates a cross‑connection, often via a faulty or unbalanced mixer. [Elektroda, Nick05, post #19654859]

Could building equipment near the boiler cause warm water on cold lines?

Yes. A flat or waterlogged DHW expansion vessel increases system pressure on the hot side. A failed non‑return valve at the cylinder lets hot migrate into the cold circuit. These plant faults can mimic mixer crossflow. [Elektroda, piracik, post #19667253]

Where should I install check valves to stop crossflow?

Install non‑return valves on the cold and hot branches feeding mixers, or directly behind the water meters. This blocks reverse flow paths and stabilizes temperature at outlets. Users report that adding one‑way valves eliminated the issue. [Elektroda, Zbychul, post #19666171]

My hot pressure is higher than cold; will that trigger crossflow?

Yes. Thermostatic mixers are designed for balanced dynamic pressures. A pressure‑reducing valve on the higher side helps equalize supplies. Balanced pressures stabilize outlet temperature and prevent hot backfeeding. [EN 1111, 2017]

Does turning the thermostat knob to minimum help diagnose?

Yes. Set the mixer’s temperature knob to the minimum and open a cold tap. If warm water still appears on the cold line, suspect internal crossflow. This quick check guides further isolation steps. [Elektroda, misiek1111, post #19655446]

What are safe operating pressures and temperatures for thermostatic mixers?

Typical operating pressure is 1–5 bar. Many mixers ship with a 38°C safety stop to reduce scald risk. Follow manufacturer limits, and ensure check valves are installed. Balanced supplies improve accuracy and safety. [Hansgrohe, 2021]

If I replace a mixer, do I need to replace both?

No. Replace or temporarily swap only one mixer first and retest. This approach identifies the faulty unit without unnecessary expense. Move to the next mixer only if symptoms persist. [Elektroda, stanislaw1954, post #19655245]

Is this issue worth a warranty claim?

Yes. Report it if the mixer is under warranty. Manufacturers may repair or replace leaking cartridges. Users have resolved crossflow after warranty action or by adding check valves. [Elektroda, Zbychul, post #19666171]
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