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I have a Grohe pressure balanced shower mixer (cartridge is model 47157000) and the check valve (model 47158000) on the cold side isn’t working.

When the check valve is installed, hardly any water comes out. When removed, the shower is great.

Knowing that the hot water is supplied by (edit) a dedicated on-demand water heater, what is the downside of removing the check valve?

My primitive notion is that the pressure on the hot side is roughly equal to the pressure on the cold side, so it seems like there shouldn’t be a problem. But I’m the first to say that I’m not a plumber, so that’s why I’m here.

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    Would do a test with your favourite SO. Have them use the shower and you turn on hot water full some place else. No screams you are good.
    – crip659
    Mar 30 at 21:51
  • Your edit takes all the fun out of it. Hot water usually is at higher pressure than cold, not much but some. One reason why there are pressure relieve valves on hot water tanks but not on cold water pipes.
    – crip659
    Mar 31 at 0:13
  • Do you have a manual for the valve? My guess, it's either defective or installed incorrectly so it's cutting off the cold water because it thinks that's the right thing to do.
    – jay613
    Mar 31 at 1:04
  • @crip659 - "Hot water usually is at higher pressure than cold" under what circumstances? Mine certainly isn't. Both come directly from the water main, but the hot gets restricted through the heater.
    – Tetsujin
    Mar 31 at 12:28
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    @Tetsujin Heating water will raise it's pressure, might just be a few pounds more than the cold water pressure. Some people place check valves on the cold water line at the tank to prevent the hot water from going into the cold water line.
    – crip659
    Mar 31 at 13:08

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Disclosure:

I do not have pressure balanced shower mixer with check valve nor the dedicated on-demand water heater.

So the risk of water pressure change is much higher then your set up. Still rarely I have to 😫🤬🤬 🤬 🤬 if someone wants to harm me by opening high water consumption cold or hot.

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