Gas Leak/ furnace problem in Rented house

Have you filed a claim with your own insurer?

Normally, if you have a standard Belgian contract (Main Residence - 9 years duration) you are required to support urgent structural reparations even if those would take more than 40 days. (look in your contract).

If your own home insurance doesn't offer you to take a hotel room during the reparation works, or if you are not able to negotiate a temporary rent reduction with your landord, the only one possibility is to trigger a court case and to submit your written arguments to the "Juge de Paix".

(S)He is the only one who can decide if you may get a rent deduction and for how long.

You can also post your question on a specialized website like:
http://www.pim.be/

You will get very fast lots of knowledgeable and legal answers for free.