New bathroom fitted but vent pipe replaced with valve.
Also macerater fitted to attic for bowl and loo on floor above.
The smell is overpowering when anything is used.
If the attic toilet is flushed the smell in the bathroom below is particularly awful.Can anyone help solve this problem?

Take valve off vent pipe or open valve. You are not getting flow so it is sucked back in your house.

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vent pipe replaced with valve.

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By "valve" do you mean AAV,( air admittance valve)?
If so it sounds like it failed in the open position. Replace it!

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Also macerater fitted to attic for bowl and loo on floor above.

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You have a ejector system installed in the attic? What's that pumping up to? The, loo located on the floor above the attic? Sorry, but that's how it reads, Can you explain?

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[The smell is overpowering when anything is used.

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If the AAV has failed you have a direct open connection between your flat and the sewer. Good luck, Tom