Downstairs toilet bubbles and empties when upstairs water is used

hi all

So I moved into a new 3 floor house couple of weeks ago

I'm not a trade but reasonably competent around diy etc

However this I've not come across - whenever we use the upstairs bath or shower - after 5 minutes or so the downstairs toilet starts bubbling with air coming through and if prolonged use, it will empty below the waterline -
Probably because the air bubbles are Forcing water down the waste pipe leaving a drop below the line

Doesn't happen when flushing toilets as I guess that's only a small quantity of water compared to running a bath or shower

We also noticed when we first moved in the middle floor toilet was completely dry but the house had been empty 2-3 months before we moved in
- no evidence of any leaks anywhere

Any ideas?

The house is about 10 years old and from my observations, doesn't appear to have had any obvious or major plumbing work since being built

Thx all

Blocked drains, water cannot escape so builds up air pressure in the sewers. Also possibly a blocked vent on the soil stack ( the vertical sewer pipe that should have an open end close to roof level )

Are there any inspection chamber ( man hole ) covers you can safely lift to check that the drains are flowing. If they are not flowing then call DynaRod or similar drain clearing company. If there are more than one house using the sewer ( a shared drain ) that is blocked then the local Water Company are responsible for clearing it. ( though they may make a charge if it was blocked by negligence such as a nappy flushed down the toilet ).

If rented contact your landlord / letting agency to arrange urgent clearance. The bubbles will contain sewer gas, not healthy.

Also had this from a stuck AAV can usually be heard releasing if an AAV

This sounds to me like a partial blockage in the bottom of the stack, or the drain. Where does the top end of the stack terminate, externally to atmosphere, or internally with an Air Admittance Valve? If it's the former then something odd is happening, if an AAV is fitted, then there could be air being trapped in the stack that is the escaping via the lower WC, although why this isn't happening when a WC is flushed I'm not sure.

The possibility I'm thinking of is water is building up behind the blockage causing the air to bubble back initially, then once sufficient head has built up, it syphons through, thus sucking the water out the WC. It is also possible there is a dip in the drain, causing water and waste to congregate, leading to a similar problem.

Initially, I would lift any manhole covers and ascertain where the drains are running, and note if there is a flow of water as expected when using an appliance inside. It may the be wise to get the pipework cleaned by rodding or jetting, if using a Contractor avoid the national franchises and look for a local firm. Get a fixed price before engaging someone, if you're in the Hertfordshire area then Andy who contributes on here is your man. If that doesn't turn up anything of note, then time to CCTV the pipes and look for any problems.

I'll study the advice in detail asap but just wanted to say thx for replies in the meantime

Rgds