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Hi All,

I seem to be having an issue with the centre dashboard heater vent on my car. I've read many threads where people have experienced only hot or cold air from this vent, due to air in the coolant or a valve failure, however my problem is slightly different. My blower fan functions correctly, but I only seem to get air flow from the centre vent when its independent temperature dial is set to cold. The hotter I set it, the less air flow I get, with barely any at all when the dial is set to maximum heat - 3 red dots.

I would expect the air flow from this vent to be the same, regardless of the temperature setting on its dial. Has anyone ever experienced this problem and found a solution?
 

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Mine has very slightly reduced air flow on 3 red dots vs 3 blue. But certainly not virtually zero as you describe.

If one considers the air flow paths, a cause is suggested. Three blue dots routes air from the cabin air filters in the engine compartment (or from the cabin interior when in recirculate mode) to the blower, through the A/C evaporator and then the dash vents. Three red dots directs air off the evaporator through the cabin heater core before going to the dash vents. So a slightly reduced flow when the blower is held at constant speed is to be expected since the heater core will have a small restriction.

Heavy accumulation of some sort on the heater core fins could be blocking air flow. Has the car been run without cabin air filters in place? If these symptoms have been present since you owned the car it might be that the previous owner replaced the heater core with a defective part, ducting was damaged or a rag left in the duct.

Based on diagrams of air flow control I don't think it could be a fault in the red/blue diverter flaps. But I've never had one apart for inspection so can't say for a fact. Perhaps someone else can comment.

If the heater core is clogged, I'd expect you would get very little air flow to the footwells or windshield. Those air paths are always over the heater core - not ever across A/C evaporator only. Is that what you see? Also, is the air flow equal on both sides of the dash vents? If not, I'm wrong and there is some other problem.

There are methods to flush the A/C evaporator & heater core - usually to remove mold that grows in the moisture from A/C operation and generates a moldy socks smell. Search the best links thread in this forum for methods. Alternatively, one would have to remove the heater core for inspection & cleaning, which is an awkward job.
 
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