The best idea would be to carefully look over the rental agreement and see exactly what the landlord is liable for and what you are paying for. Also investigate the history regarding checks etc. That have been carried out on the faulty equipment, if there is a statutory obligation the Landlord is liable for and this has not been done it will make your case stronger. Once you have done that and collected written evidence of their lack of action seek advice from an independent party (another rental agency might be good, pretend you are looking to rent and pose this as a hypothetical question as it happened to the friend of a friend). I know things are a little different in the UK but in my case I was without heating for 9 months (including winter), I wrote several times and kept copies of all the mail and replies, eventually I reduced the rental payments by £200.00 Per Month to cover the cost of buying and running electrical heaters and extra bedding/clothing. The landlord made noises about recovering the outstanding rent once he had replaced the boiler and even threatened court action. I invited him to go ahead with the court action but he has still not after 5 years as I also discovered that the boiler had been condemned and put out of service before I moved in, so any time in that period I could have been seriously injured.