HVAC Heat Issues

[JOEMAMA]

Since the temperatures have started to drop I have been using the heat. I normally put the heat on the highest setting and put the mode selector to defrost. Then, once the car has warmed up, I turn down the heat and switch the mode to defrost/floor. However, when I do this, I get cold air from the defrost vents and warm air from the floor. Every other mode works without issue - its only the defrost/floor mode.

After a lot of research and looking the HVAC system up in the service manual, I'm inclined to think this is a design issue, but I still find that hard to accept. Does anyone else have a similar experience?

[Richard]

Mine blows warm air from the defrost vents and the floor vents when on this setting, but my a/c hasn't worked in a few years. The default is that the a/c comes on when using a defrost setting. The theory is that a/c air, which is dry, helps to defrost the windshield faster than warm "wetter" air. This is standard in all vehicles with a/c. It can be changed in the micra though by unplugging the wire to the a/c switch behind the dash bezel. This doesn't explain the difference in air temperatures though. If you haven't already turn off the a/c and see if this makes a difference.

[JOEMAMA]

I have tried turning the a/c on and off and also tried recirculate mode, but it doesn’t seem to make any difference.

If I put the vehicle into defrost only mode the temperature is fine. Same thing with floor only mode.

The temperature issue only occurs when it’s in the defrost/floor mode because the floor vent blows warm but the defrost vent is much cooler. I noticed this because lately my feet are on fire while the upper portion of my body and the cabin is cold.

The air coming out of the defrost vent and side vents is not ice cold - but much colder than the feet which more accurately reflects the current heat setting. If I have the heat on full blast there doesn’t seem to be the same discrepancy - it’s only when the heat is set to 2/3-3/4.

I presume it’s an issue with how the heater box is designed, but if others don’t experience something similar than perhaps not.

[Richard]

I double checked and I'm still about the same temp from floor and defrost at all fan speed settings. I did notice that on high about 75% of the air flow is going to the floor though, but at roughly the same temp. Only thing I can think of is that if the ducting lines aren't insulated some of the heat is being lost if the ducting from the heater core to the defrost vents are longer than those to the floor.

[JOEMAMA]

Thanks for the quick reply and for checking this on your own vehicle.

I will have to disassemble and do some troubleshooting. I don’t remember my old Micra having this issue either.

I can understand a slight temperature difference, but this is significant.

As far as air flow is concerned, you are partially correct. According to the service manual, 51% of the air flow is directed at the the floor, 35% defrost and 14% to the two side vents when in defrost/floor mode.

[JOEMAMA]

I was able to fix my issue...

6 months ago the dealer replaced the HVAC control under warranty and they botched the job. I showed the dealer pictures and they agreed to order me a replacement HVAC control unit so I could replace it myself. There were cabling issues (due to poor installation) with both the mode and air mix doors which is why I was getting cold air through the defrost vents but hot air at my feet when in defrost/floor mode.

After installing the new control unit and ensuring the cables were adjusted properly, I now have full heat and no mode issues!

[Richard]

I was able to fix my issue...

6 months ago the dealer replaced the HVAC control under warranty and they botched the job. I showed the dealer pictures and they agreed to order me a replacement HVAC control unit so I could replace it myself. There were cabling issues (due to poor installation) with both the mode and air mix doors which is why I was getting cold air through the defrost vents but hot air at my feet when in defrost/floor mode.

After installing the new control unit and ensuring the cables were adjusted properly, I now have full heat and no mode issues!

That's good to hear. Thanks for the update