HVAC A/C effect on fuel consumption and demisting the windscreen

Discussion in '1st Generation (2004-2011)' started by ec5779, Sunday 6th Oct, 2019.

  1. ec5779 Junior Member ☆ ☆ ☆ ☆ ☆

    United Kingdom ec5779 London
    19
    7
    Hello

    Apologies if this is a rather novice question but when its raining outside, the windscreen tends to get steamed up. Turning on the blower seems to fix it, but in the winter I tend to turn off the air conditioning (i.e. I press the A/C button) because I figure its not necessary. But as soon as I do that the windscreen starts to get steamed up again. I've tried blowing hot and cold air and it doesn't seem to make much difference (surprisingly!) unless I turn the A/C back on again.
    Not sure if this is related but we've noticed a slightly musty smell when the air conditioning is turned on that tends to dissipate after it's been running for a few minutes.

    This leaves me with a few questions, if anyone would be kind enough to chip in with their thoughts please:
    1. Is it normal that you have to have the air conditioning on to demist the windscreen (i.e. blower alone isn't enough)? My previous car didn't have air conditioning and I'm sure hot air was enough?
    2. What impact does using the air conditioning have on fuel consumption. When the car is idling you can hear when the air conditioning system kicks in and out, so I presume it must use some extra power/fuel?
    Thanks in advance for any advice!
     
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  2. MattBeardless Service Team H-Tune Staff

    United Kingdom Matt Derby
    283
    94
    In answer to your questions...

    1. My 8th Generation Accord automatically puts the A/C on to demist the windscreen - though I have to turn it off and just wait for the car to warm up as the A/C is broken.

    2. Usually, there's an approximate difference of 10% to Instant MPG from what I observed in a few cars, it saps a bit of power too on smaller engine vehicles such as my Dad's 1.4 K12 Micra.

    As for the musty smell, next time you take your car for a service ask for an 'air-con bomb' treatment - the smell is usually due to bacteria in the system, not nice stuff - then again, neither is the A/C bomb.
     
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  3. Sandy52 Valued Contributor ★ ★ ★ ☆ ☆

    United Kingdom Sandy NORWICH
    304
    129
    I run my A/C ALL the time on AUTO. I just set the temperature at whatever I prefer.
    It does a great job in winter and I never have misted up windows. The effect on MPG is negligible, I have tried to measure it, on different cars, but cannot see any difference in normal use. Maybe if you are a Hypermiler you can see it but I cannot.
    It might make a very slight difference to MPG but I prefer to see out of my car windows all the time.
    Another reason NOT to turn off the A/C is if it is not used the internal seals can dry up and fail and it is expensive to have it repaired.
     
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  4. MattBeardless Service Team H-Tune Staff

    United Kingdom Matt Derby
    283
    94
    I haven't actually had that issue after 10 years-the technology in question has gotten a lot better recently. The leak on mine is actually one of the hardlines from the condenser where it's rubbed through on the chassis mount.
     
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  5. Mr Honda Club Member ★ ☆ ☆ ☆ ☆

    Ireland Dublin
    89
    37
    Honda tends to do things differently so yes, my Accord will fog up similarly, but now I run the air con all the time with the recirculate button on. Change your pollen filter, spray a little perfume or fabreze onto it before you install it, should help with the mould/musty smell. Fuel loss is negligible as said.
     
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  6. Heckler Club Moderator Club Staff

    The screen mists because of the difference in temp between the outside and inside of the car, along with the level of moisture. Cars will be several degrees warmer inside that out and this gap gets bigger the more extreme the temp gets.

    Condensation can occur on the inside of windows due to this, so it's a good idea to keep a dehumidifier bag in the car to combat this. I have a couple and one is in the car and one in the house. I swap them over each week to allow them to dry out... Especially in the winter when this problem is worse.

    As for fuel consumption, tests that I've seen done can be anything up to 10-12% difference in fuel use when using the AC. But effects will be different for everyone based on driving style, temp settings and so forth.

    Make sure to use the AC at least once or twice a week to avoid any issues with seals and so forth.
     
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  7. ec5779 Junior Member ☆ ☆ ☆ ☆ ☆

    United Kingdom ec5779 London
    19
    7
    Thanks everyone for your thoughts.
    Interesting these questions seem rather controversial as I've had opposing answers to both questions...

    My mechanic said he put a 'disinfectant' through the system (whatever that is) and he changed the pollen filter so lets see how it goes.
     
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  8. 155695 Premium Member Club Supporter

    England Keith Essex
    734
    418
    The A/C (or more accurately comfort cooling - I am unaware of any car that has full A/C as used in buildings) works on a reverse cycle heat pump system. This means that once the required temperature is set by the user, the system maintains that temperature by either providing heating or cooling as necessary until the required temperature, measured from the extracted air, is attained and it then maintains it. It also provides fresh air make-up, thereby ensuring that the inside of the car remains free from condensation and other extraneous smells, etc,

    If you do not have any fresh air entering the vehicle, i.e. if the A/C is turned off and windows closed, inevitably condensation will occur in winter as well as the interior becoming stuffy and unpleasant.

    Modern car A/C systems do not have a significant impact on fuel consumption and it is best for the reasons set out above and for the system itself, to be run every time the car is driven.

    Keith
     
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  9. Heckler Club Moderator Club Staff

    When you turn on the A/C it engages a clutch that puts extra load on the aux belt and the system... it's why the revs increase at idle when you turn it on.

    This in turn has a direct impact on the entire system, which has a direct impact on the fuel economy. There are so many studies on this and the systems installed on cars haven't exactly changed in the last 30yrs or more.

    The amount of an impact this will have varies greatly and is dependent on the outside temperature as well as the interior temp settings. The size of the cars engine also plays a part. Making sure your car is well maintained, has regular oil changes and clean filters are all a factor as is making sure the system has the correct level of refrigerant in it.

    In very high temps such as above 30º with the AC set to it's lowest setting (around 16º) that can be as much as 20-25% on a large petrol engine. Lower outside temps and ac temp on a smaller engine reduces that considerably... which is why some people mistakenly think it has no impact at all.
     
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