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Thread: Heater off, and it heats the car!

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    Heater off, and it heats the car!

    Sorry, I should have posted this in summer!
    I drive mostly long distances, and after a half hour or so, I regularly begin feeling hot in this car. It has always been hotter than inside others. Especially so after about half an hour's drive or longer, uncomfortably hot. Initially I thought it was the sun heating the black plastic dashboard, but I wanted to get to the bottom of this and try to do something about it, if anyhow possible.
    The findings were astonishing, and may affect you too! Here is what happened and what was done to correct it:


    I grabbed a digital thermometer with an external sensor. I put the sensor out of the window while driving on the highway, and measured 31C outside temperature. With the heat all the way off, and the air blower at full blast, I put the sensor into the air stream blowing into the inside. It measured 3.5C (6F?) hotter than the outside air!
    I had to recheck. The outside temperature was as before. This time I put the sensor into a different outlet blowing into the car, and it too was 3.5C hotter than the outside air.


    My next step was to check if the hot air flap of the heater assembly was closed all the way.


    This is what a fully closed heater flap (heater all the way off) looks like: Observe the rearmost lever standing vertically up.

    Name:  heat off.jpg
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    This is how the heat control flap looks like with the heat fully on: Observe the rearmost lever being horizontal.

    Name:  heat on.jpg
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    If the hot air flap does not close all the way, the cord that pulls the flap can be easily adjusted to make the rearmost lever stand upwards and firmly close the flap. To adjust it, you could unclip the cord out of its holder and clip it back in correctly. See picture:

    Name:  adjustment heater cord.jpg
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    In my case the heater flap is correctly closed all the way, meaning the heater is turned off fully, but after time the car gets hotter and hotter because of warmer air blowing to the inside!
    The reason for this can only be that the now hot heater assembly heats the "cool" fresh air going through it, albeit with the heat correctly turned off! It is simply a primitive construction!

    What can be done about it?
    Since I don't want to simmer and dehydrate in this uncomfortable car, I built a coolant bypass for the heater core. It is in place in summer, and gets taken back out in winter. I did not want to pinch off the heater hose in an effort to stop the coolant flow, as that flow has other important purposes and must not be interrupted.

    What difference does it make?
    With the bypass in place and under the otherwise same conditions as before, the air blowing inside was now practically the same temperature as outside.
    It was now much better inside the car, the difference was almost as much as turning the airconditioning on! It is like having one airconditioning unit secretly and constantly on, without needing to turn on the real one!

    I would be interested in knowing if other folks can measure the same temperature differences between the outside air and the air blowing to the inside, and that with both the heat and the airconditioning turned off. To measure the difference, the warmed-up car has to be driven at least a half an hour to get the heater assembly itself hot enough to heat the car without the heater turned on.

    The following picture is my coolant bypass for the heater core. It is made of simple copper pipe, with an outside diameter of 18mm (medieval and non-standard: .71 inches). A local plumber could easily make it at little cost.

    Name:  Bypass tube.jpg
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    The bypass tube goes onto the rubber hoses connecting the heater core.
    The following picture is without the tube connected, because the cold season has begun in this part of the world.

    Name:  heater core connection.jpg
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    Last edited by foama; 09-14-2018 at 06:29 AM.

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    Sounds like a possible busted flapper door to me. One of the guys who got his heater core replaced due to leakage said they revised the door design early on. Since you have a 2014, its possible you have the old design. I'll be checking for this when I get my heater core removed as well.
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    If I turn my heat off but still have the vent turned on, I would expect the temperature of the air coming out of the vents to be similar to the outside air temp...which is what you are saying. Am I misunderstanding something?

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    What he said^
    Do you have the controls on recirculate or fresh air while doing this?

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    1) @ Daox: thanks for checking the flap, a photo would be great! I'm curious too if it may be a better design on newer models.

    2) @ Top_Fuel: Yep, the heat is turned all the way off, the flap is closed all the way, and yet the air inside leaves the vents hotter than it is outside.

    3) @ Fummins: I have tried both with recirculate and with fresh air, because it could have been the bottom of the air canal being heated by hotter air in the engine compartment. There was practically no difference. I also measured the air in the recirculation flap, and it was always cooler than out of the vents, regardless which vent and on which side.
    Last edited by foama; 09-13-2018 at 04:44 PM.

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    Quote Originally Posted by foama View Post
    ...With the heat all the way off, and the air blower at full blast, I put the sensor into the air stream blowing into the inside. It measured 3.5C (6F?) hotter than the outside air!
    I'm still lost. I would expect the outside temp to be about the same as the air coming out of your vents.

    You're only talking about a 3.5C difference. I wouldn't even notice that. My car could be doing that right now. I drive around with my vent on 99% of the time.

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    Me too, when it's +30C here I just turn the ac on. But it's snowing damnit.......

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    Quote Originally Posted by Top_Fuel View Post
    I'm still lost. I would expect the outside temp to be about the same as the air coming out of your vents.

    You're only talking about a 3.5C difference. I wouldn't even notice that. My car could be doing that right now. I drive around with my vent on 99% of the time.

    Sorry, I should have posted this in summer.
    I'm saying the air coming inside is blowing at 3.5C = 6F hotter into the cabin than it is outside!


    A practical example:

    It is +35C (95F) outside, and therefore 38.5C (102F) inside the cabin, with heat and AC turned off and air blower at full blast.

    If the airconditioning now gets turned on, it will eventually settle to about 33C (91C) inside, but this is only 2C or around 4F lower than outside, because the heater is adding 3.5C / 6F to the cabin temperature.
    (And yes, my airconditioning unit was checked out by the dealer and by a specialist aircon company.)

    At 35C (95F) outside I would rather have it be 29.5C or 85F inside, instead of 33C or 91F and that it would be if the heater would really be off when turned off.



    I would be interested to know if other folks measure the same temperature difference between the outside air and the air blowing to the inside, and that with both the heat and the airconditioning turned off. To measure the difference, the warmed-up car has to be driven at least a half an hour to get the entire heater assembly itself hot enough to heat the car without the heater turned on.
    Last edited by foama; 09-14-2018 at 06:45 AM.

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    @foama: Is it air, which is flowing through your bypass, or some fluid?

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    Quote Originally Posted by foama View Post
    Name:  heater core connection.jpg
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    Hey foama, what is that black plastic box mounted on your firewall just above and beyond the heater hoses in your picture?


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