My question involves landlord-tenant law in the State of: California

In December of 2007 my landlord was informed of inoperative furnace. The gas company red tagged the furnace in January 2008 as dangerous possible loss of life and turn the gas off to the furnace. I was given a small portable heater for a 3 bedroom 2 story home (did nothing but raise my electric bill). In June of 2008 the landlord hired a company to replace the furnace. Our city requires a mechanical permit and inspection on furnace change outs. No permit is on file with the city and no inspection of the furnace was done. The pipes leading to the new furnace are cracked and rigged with the wrong connectors causing gas to leak into the apartment. SoCal gas has red tagged this unit 4 times as dangerous possible loss of life , and turned off the gas to the furnace. The landlord has been given all the orders for repair from Socal gas as well as written request by me . He refuses to get the proper permit and inspection by the city. Our furnace is leaking gas again after he sent up another repair man. The repair man taped the cracked gas lines.
In short I have been without a working heater since December of 2007. I would like to request rent abatement from January of 2008 to December 2008.
What is the procedure for rent abatement?