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Rear window excessive condensation.

Discussion in 'Gen 4 Prius Care, Maintenance and Troubleshooting' started by kithmo, Dec 22, 2019.

  1. kithmo

    kithmo Couch Potato

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    Hi does anyone else have excessive condensation on the inside of the rear screen ?
    My Gen 4 is parked outside on the drive in all weathers. When the humidity is high and the ambient temperature is low, I get what seems to me to be an excessive amount of condensation on the rear screen, if it's freezing out I get ice on it, which melts and dribbles water all over the cargo cover and some seeps into the plastic trim around the screen. I have never had a car with so much of it before. I have checked for leaks in the cargo area and wheel well and it's dry everywhere (except on the screen :() no leaks, no water pooling etc.
    The car is warm and dry inside when I park it up and I was wondering if this excess moisture was being drawn in through the HV battery vents, which I believe there are some that vent (in or out ?) to the outside, as the car cools down overnight. Any ideas ?
     
  2. The Professor

    The Professor Senior Member

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    It's a big window, and almost horizontal, so more of the rising warm air inside your car will contact it.

    You mentioned your car being warm when you stop. That means the air inside will be able to hold more moisture than cold. You can help a little bit by replacing the warm high humidity air inside your cabin with cooler and dryer outside air. Just wind the windows down for the last 15 seconds of your drive.

    Also don't use recirculate mode on the climate control in the colder months, as airflow from outside pushing the damp air out is the main means that moisture exits your car.

    You can also get big 1kg bags of silica gel that you can put on the parcel shelf and dash too absorb a lot of that moisture. Since using these I've not had any steamed up windows inside when I come back to my car the next morning. Get them from Amazon. Search for Apalus 1kg Silica Gel Car Dehumidifier.

    Posted via the PriusChat mobile app.
     
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  3. dubit

    dubit Senior Member

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    I drive a Prius C, but mine will act like that on the front and side windows if I'm recirculating the air. Like was mentioned above, don't recirculate that air in the cold months. Draw in the outside air instead and you'll find the condensation on the windows will be almost non-existent.
     
  4. kithmo

    kithmo Couch Potato

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    Thanks, I'll try the open windows trick, that sounds logical now you mention it.
    I don't use recirculate mode and usually have AC on all the time and I do have a silica gel car humidifier bag that I place on the dash that works well for the front screen, although it's only a 300g one. If the opening windows trick doesn't work I'll buy and try a bigger one.(y)
     
  5. The Professor

    The Professor Senior Member

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    Also remember that silica gel can't store an infinite amount of water. You have to microwave or heat the packs to evaporate the water from them periodically.

    Silica gel can absorb about 30% it's dry weight of water, so weigh your pack and if it's close to 400g you might want to dry it out.

    I use a 1kg pack in the back and front.

    SM-T813 ?
     
  6. fuzzy1

    fuzzy1 Senior Member

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    The ice that melt and dribbles all over the inside, is likely the very same water that reappears on the inside glass tomorrow. Melt, drip, evaporate, re-condense and re-freeze all over again.

    The first thing to do is to break this daily cycle by getting this old water out of the car. If liquid, wipe it up and immediately remove the towel from the car to dry somewhere else, this may need to be repeated numerous times. If the silica gel is saturated enough to be losing some water vapor, change it or dry it out somewhere else. If floormats are wet, or any wet clothing or other items are in the car, remove and dry them elsewhere before returning them to the car. If the carpets are wet, then greater effort will be needed for adequate drying.
     
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  7. kithmo

    kithmo Couch Potato

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    Thanks, but I've covered all that, mats, carpets and everything are dry, no clothing left in car and I wiped up the water off the rear cover, rear window and surround as soon as it appeared. I do dry out the dehydrator bag quite often in the microwave then on the radiator in the house.
    I tend to turn off the climate control before I shut down the car, hopefully this closes the vents. Do you think this makes any difference ?
     
  8. fuzzy1

    fuzzy1 Senior Member

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    I always leave mine open, on the theory that when in use, the absolute (not necessarily relative) humidity inside the car should always be greater than outside. Human occupants always emit water vapor through respiration, a necessity of life, and this increases interior moisture content above ambient, even when the cabin warmth reduces relative (%) humidity and makes it seem dry.

    Once the humans park the car and exit, and the car cools to ambient, then relative (not absolute) humidity rises as the temperature cools. If the increased water vapor inside remains trapped there, then it should begin condensing on the inside surfaces before there is any similar condensation on the outside. So I'm inclined to leave vents open to let that trapped moisture gradually escape, allowing the interior humidity to equilibrate with the exterior.

    Rapidly changing exterior conditions can make a mess of all this.
     
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  9. kithmo

    kithmo Couch Potato

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    Update: bought a 1kg silica gel car dehydrator bag off Ebay £12 and it has completely cured it. I guess the little 300g bag I have was not up to the job, I just leave this on on the dash now. The new bag has a blue indicator on it that apparently turns pink when it needs recharging (6-12 minues in a 650W microwave) and came with a non-slip tacky pad so I stuck it on the aluminium bar of the extendable rear hatch cover, behind the middle head rest, so it can't move whilst driving. Thanks for the tip Professor, no more misted rear window. (y)
    Thanks you other guys for your suggestions too.
     
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  10. ice9

    ice9 Active Member

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    Use defrost to dehumidify your vehicle - front windshield defrost. I believe the rear defrost only turns on the hatchback window heater. The front defrost runs heated ac which removes moisture in the cabin more quickly.
     
  11. kithmo

    kithmo Couch Potato

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    I always do, didn't make any difference. Windows were not misting up during driving, just overnight.
     
  12. ice9

    ice9 Active Member

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    I didn't have a condensation problem with my gen 2 Prius. However, I just replaced it with a gen 4. Perhaps the AC used in the gen 4 is less effective at removing moisture - though it has worked fine whenever I have used it.

    When I first read you comments, I thought perhaps you had a circulation problem. Did locating the desiccant packs at the defrost vents fix the problem, or was it fixed only after relocating them to the cargo area?
     
  13. kithmo

    kithmo Couch Potato

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    I had always placed the small (300g) pack on the dash when I parked up and removed it to the console tray when driving, still doing that as it prevents the front screen misting up but it didn't prevent the rear screen misting. I have the large (1kg) pack I just bought at the back on the parcel shelf rail, which I leave in place (stuck to a tacky pad) and that has cured the rear screen misting completely.
     
  14. ice9

    ice9 Active Member

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    I don't know. If you had your AC operating in re-circulate, you might not be getting enough air-flow to the rear of the car. It sounds like you just have a serious moisture problem.

    When you say you get "ice" on inside of the hatch-back window. Is it really "ice" or did you really mean "frost"?
     
  15. kithmo

    kithmo Couch Potato

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    I hardly ever use re-circulate, I'm always on fresh (outside) air mode, with the AC always on.
    Ice on back window was thick frost, on the inside as well as out.
     
  16. ice9

    ice9 Active Member

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    One other thing. If you are operating the AC in ECO mode, I believe that this minimizes fan operation. If you are like most people, I'm guessing you use the defrost only as required then revert to ECO AC. So, your defrost might be effectively drying out the air with both the compressor and maximum air flow. However, once you go back to ECO AC, your blowers probably drop back down to between 2 and 4 bars which would reduce airflow in the cabin. My guess is that this would allow warm, moist air from exhalation to collect near the roof of the car and eventually condense out on the hatchback window before it can exit via the rear vents.
     
  17. kithmo

    kithmo Couch Potato

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    Nope, I always have mine in NORMAL mode unless the car is frosted up then it's front and rear demisters on then when the front is clear I change to FAST mode (which I believe is not an option on US models).
    I think I used ECO only a couple of times on my commute to work, stood in traffic, when I first got the car and was obsessed with trying to achieve high mpgs. Now I'm retired and not really bothered about the mpg as I only fill up every 5 or 6 weeks due to my low mileage.
    Thanks for the suggestions though. (y)
     
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