I wonder if you have asked the landlord to install cooling/heating. Maybe he would be interested in doing so for a slight increase in your rent.
We once installed aircon for a long term tenant who actually asked if they could install a window box and take it with them but fix the window up. We trusted them implicitly to do as they said, but we decided to install a split system to make it easier to rent next time. We didn't put their rent up because they were good enough to allow us to build a deck on the back while they lived there and they were great tenants who were looking to buy, so we knew we would be putting the house up for rent sooner rather than later and waited until then to up the rent.
In other cases, we have waited until a tenancy is about to finish, and tenants about to leave and then installed air-con or a dishwasher. Each of these things gets more rent, or makes renting easier, so they pay for themselves very quickly via higher rent.
Witzl, if you have asked for heating/cooling and the landlord has refused, then they are probably biting off their noses to spite their face, because the problem will not go away.