Cistern overflows to pan when the shower is running.

[Joef]

The toilet cistern overflows into the pan when the shower on the floor below is running. Why would this be?
They say the shower seems low on pressure recently. Again why would this be and what is the connection between the problems if there is one?

 

[Chuck]

The toilet cistern overflows into the pan when the shower on the floor below is running. Why would this be?
They say the shower seems low on pressure recently. Again why would this be and what is the connection between the problems if there is one?

Interesting symptoms. Is the shower pumped? Is is a vented or unvented hot water system?

If it's just a mains-fed shower, my guess is that the toilet's float valve is borderline not sealing correctly due to low supply pressure. Running the shower reduces the pressure in the feed.

Make sure your stopcock is properly open (fully open and then close half a turn). Look for evidence of leaks (water meter turning when not water is being used). Attach a pressure gauge to a tap and check the pressure is normal for the area. Ask the neighbours if they have noticed reduced water pressure.

 

[townfanjon]

Change the inlet valve in the cistern

 

[Ben-gee]

New diaphragm needed for inlet valve, or just change it for a new valve

 

[Best]

Seen that happening with an old Fluidmaster valve that started to constantly while the basin tap was opened.
It is a reduction of pressure and new valve is what I did