Or, check what actually happens when "input" on full, "output" shut all night (can you trust them not to fiddle??) & see how hot in morning. If it isn't very hot maybe it's bust so get it repaired or replaced..
I have such a situation myself currently & I have already offered tenant new, replacement, storage heater (only a few hundred ££££) .
Things go wrong & break: But you knew that ??
I am legally unqualified: If you need to rely on advice check it with a suitable authority - eg a solicitor specialising in landlord/tenant law...