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.
- Close the hot and cold shut-off valves to one thermostatic mixer.
- Open a cold tap elsewhere; check if warm flow stops.
- 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]
