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ᐅ The Vaillant VCW 196EC is experiencing intermittent operation issues.

Created on: 28 Nov 2025 10:46
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Curiosius
Original post
Good morning,

a Vaillant Vcw 196EC heating system frequently stops working intermittently. It runs normally, and then, without any changes to the settings on the unit or the radiator thermostat, it displays the error E12. Sometimes also F40 or F23. Pressing reset stops the issue for hours. A new expansion tank and several other parts have been continuously replaced. It still needs to last a while... There is enough water in the system. What could this be?

Curiosius
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Curiosius29 Nov 2025 11:01
#13
You mean to slightly loosen the mentioned valve at the bottom of a heating circuit first?
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Nauer29 Nov 2025 12:45
#14
If by “loosen” you really mean unscrewing, then I would strongly advise against it—that kind of weekend adventure is something no one wishes for. The idea is only to apply a minimal turning force, more like checking whether the fitting can move at all...
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Curiosius29 Nov 2025 14:40
#15
Of course, carefully. I have special training in the renovation sector, no matter what. Always with thoughtfulness and calm.

What’s still not quite clear to me is whether you mean the side of the thermostat or the opposite side. Naturally, below in each case?
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Nauer1 Dec 2025 23:00
#16
Yes, the return valve is located below, opposite the thermostat. It is usually just a small Allen key screw, so it’s easy to overlook. Give the valve insert only a very slight turn, no more. If a radiator remains noticeably colder at the return side, you probably have a flow rate loss exactly there, which is disrupting your central heating system.
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Curiosius1 Dec 2025 23:05
#17
Many thanks!
I noticed that for older models, there is a special key... Is it necessary because of the precise fit?
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Nauer1 Dec 2025 23:13
#18
Yes, the special key ensures precise movement of the valve cartridge without damaging it. Just a small turn is enough; only check, do not fully open!