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Its a funny time of the year to be talking about this but its also the best time of the year for doing jobs on the car. I've seen other threads about heaters blowing cold air and so, since I've just fixed mine, here's an explanation and a short how to.
The problem: Engine gets to operating temperature, Heater fan works, but heater blows cold air.
The Explanation - The heater works by blowing cold air over a heat exchanger (the heater core/matrix). Although inside the cabin, under the dash, the heater core is part of the engine's cooling system and it contains coolant that circulates through the engine block and head to become hot. Due to contamination from minerals the coolant picks up on its way, the heater core tends to rot out and needs replacing after around ten years. Most people dont know it is failing until their windows start to fog up and the coolant is leaking out. By then its too late. The coolant is passing through the heater core under pressure and so, when the core begins to fail, the coolant first tends to spray out as a fine mist drenching everything nearby. It can be doing this for a while before the flow becomes strong enough to be evident.
Inside the heater and airconditioning box (the big black thing screwed to the firewall) there are doorways that channel the air according to the settings you choose on the heater panel on the dash. These doorways are made of pressed steel and contain large holes (presumably to allow the air to escape if it becomes too pressurised). To prevent the air escaping through these big holes normally, the door ways are completely coated on both sides in a thin foam plastic material that also covers off the vent holes and enables the doors to make air-tight seals.
In my car this plastic material had been attacked by the escaping coolant spray and was rotted and flaking away. The heater was blowing cold air because the air could go right through the vent holes in the doorways. Cold air could not be closed off.
Repair:
To repair the problem it is necessary to remove the facia panel on the dash (including radio and switch gear). Also the center console and the covers that conceal the under dash.
When all these are removed it is possible to remove the large verical black plastic air box section that branches at the top to the under dash air hoses. It is held by two easily accessible plastic nuts. Its a bug..r to remove and replace because - especially if you have big hands - things are very tight. You might have to loosen the dash mounts too (you dont have to remove the dash but it helps if you can jiggle it a bit).
With that airbox out of the way you can see the first of the doorways immediately in front of you. Opening it carefully you can see two more doorways inside. The one on the left controls cold air, the one on the right controls hot air. (If these doorways do not want to open you might have to start the engine and operate them through the heater control). Check to see if the plastic coating is damaged on these doorways. If it is there is only one answer. It needs to be completely removed and the door surfaces needed to be cleaned and prep'd so they will bond to adhesive.
The difficulty to replace this plastic coating is that it isnt possible to replace it on both sides. Access is not good enough. Therefore it is necessary to make the repair on the facing side only. The metal door surface must be prep'd so carefully because there is only one bonding surface for the repair, not two.
When all that is done there are a few choices for coating material. I thought long and hard about sticking velcro strips to the doorways and cutting cloth covers to hook onto the velcro. In the end though I went to clarke rubber and bought a sheet of very thin adhesive backed foam. I made cardboard templates of each door and, using the templates, cut the foam to size. Because access is so difficult I found it easiest then to cut the foam section for each door into strips and to fix the material to each door one strip at a time.
This repair also improves the ability of the airconditionner to blow cold air undiluted by hot air from the heater matrix. So there's another reason for getting on to it now.
The problem: Engine gets to operating temperature, Heater fan works, but heater blows cold air.
The Explanation - The heater works by blowing cold air over a heat exchanger (the heater core/matrix). Although inside the cabin, under the dash, the heater core is part of the engine's cooling system and it contains coolant that circulates through the engine block and head to become hot. Due to contamination from minerals the coolant picks up on its way, the heater core tends to rot out and needs replacing after around ten years. Most people dont know it is failing until their windows start to fog up and the coolant is leaking out. By then its too late. The coolant is passing through the heater core under pressure and so, when the core begins to fail, the coolant first tends to spray out as a fine mist drenching everything nearby. It can be doing this for a while before the flow becomes strong enough to be evident.
Inside the heater and airconditioning box (the big black thing screwed to the firewall) there are doorways that channel the air according to the settings you choose on the heater panel on the dash. These doorways are made of pressed steel and contain large holes (presumably to allow the air to escape if it becomes too pressurised). To prevent the air escaping through these big holes normally, the door ways are completely coated on both sides in a thin foam plastic material that also covers off the vent holes and enables the doors to make air-tight seals.
In my car this plastic material had been attacked by the escaping coolant spray and was rotted and flaking away. The heater was blowing cold air because the air could go right through the vent holes in the doorways. Cold air could not be closed off.
Repair:
To repair the problem it is necessary to remove the facia panel on the dash (including radio and switch gear). Also the center console and the covers that conceal the under dash.
When all these are removed it is possible to remove the large verical black plastic air box section that branches at the top to the under dash air hoses. It is held by two easily accessible plastic nuts. Its a bug..r to remove and replace because - especially if you have big hands - things are very tight. You might have to loosen the dash mounts too (you dont have to remove the dash but it helps if you can jiggle it a bit).
With that airbox out of the way you can see the first of the doorways immediately in front of you. Opening it carefully you can see two more doorways inside. The one on the left controls cold air, the one on the right controls hot air. (If these doorways do not want to open you might have to start the engine and operate them through the heater control). Check to see if the plastic coating is damaged on these doorways. If it is there is only one answer. It needs to be completely removed and the door surfaces needed to be cleaned and prep'd so they will bond to adhesive.
The difficulty to replace this plastic coating is that it isnt possible to replace it on both sides. Access is not good enough. Therefore it is necessary to make the repair on the facing side only. The metal door surface must be prep'd so carefully because there is only one bonding surface for the repair, not two.
When all that is done there are a few choices for coating material. I thought long and hard about sticking velcro strips to the doorways and cutting cloth covers to hook onto the velcro. In the end though I went to clarke rubber and bought a sheet of very thin adhesive backed foam. I made cardboard templates of each door and, using the templates, cut the foam to size. Because access is so difficult I found it easiest then to cut the foam section for each door into strips and to fix the material to each door one strip at a time.
This repair also improves the ability of the airconditionner to blow cold air undiluted by hot air from the heater matrix. So there's another reason for getting on to it now.
Pics would be a superior addition 
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