When we use 2 sources of water at the same time (say a shower and a toilet) this will cause the downstairs toilet to flush on its own.(we can use the sink and flush a toilet downstairs and the upstairs toilet will flush, we can use different combinations, although the same combindation causes the same toilet to flush). We have a combi boiler in case this is the cause. We have lived here 5 years and this only just seems to have happened in the last few months (which coincided with a new washing machine that only uses cold water) any ideas?

I'm being haunted by the word "flush". When you say the toilets flush do you ne3an the tank opens up and flushes the bowl or do you mean the water level in the toilet bowls gets sucked out of the bowl? My bet's on the second explanation. How old is your house? Do the traps under the sink and lavatory go into the wall or do they go down to the floor? Let me know, Tom

Thanks for the response.
What happens is a little water starts to trickle into the bowl then because of the 'escaping' water the cistern starts to replace the water which then pushes more water into the bowl as if the toilet has been flushed (makes the normal flushing sound :) .
Most of the traps go into the floor but 1 goes into the wall.
We can use the washing machine and outdoor tap and this still causes the toilet issue.
The house is 6 years old.
Thanks
M

What area do you live in? This will take a little time. Let's check the adjustments in your toilets to start.
Check the tank water level, it should be at the mark in the tank or 3/8 to 1/2" below the top of the over flow tube when the tank's filled.. Now look at the flapper, If it has wavy edges on the flat surface, replace it. The linkage between the flapper and the flush lever should have 1/4" play when the tank is filled. Also, check the small tube coming out of the top of the ballcock and make sure it's connect to and draining into the white overflow tube and not shoved way down inside it.
Check those and get back to me. Good luck, Tom

Thanks for the advice.
The issue seems to be someone turned the water tap to the house almost off. All the toilets were therefore losing pressure when another water source had been used and this was causing the toilets to "flush".
Thanks
M

Thanks for letting us know. Tom

I don't think we are going to get anywhere until the pressure is measured in the house. Try to get one of these: Water Pressure Gauge For Residential Water Supply - Mfg# WTG254LS300

I'm thinking low water pressure. Both too high and too low of a water pressure may cause valves to leak.

The second option is that maybe there is something wrong with the toilet valve itself.