Condensation in Crosstar

[ColinB]

Let’s look at the science here. If you want to change the state of a substance from solid to liquid (eg melt the ice on the screen), you have to supply an energy input, aka latent heat of fusion. So where does that heat come from? If you use a de-icer spray, some of the energy will come from the spray itself but most of it will come from the glass of the screen. In other words, you will be cooling the screen in order to melt the ice. If the interior has any moisture in it, then that moisture will condense on the cooling glass. So you are actually making the condensation worse. A similar thing will happen if you use cold water on the exterior, although the water may provide more of the latent heat (depending on the ambient and water temperatures) so the cooling of the glass should be less. Best way to clear ice from the screen without making the inside condensation worse is to remove the ice without it changing state, ie. by scraping it off. Has the side effect of warming you up on a cold morning as well!

[ColinS]

Let’s look at the science here. If you want to change the state of a substance from solid to liquid (eg melt the ice on the screen), you have to supply an energy input, aka latent heat of fusion. So where does that heat come from? If you use a de-icer spray, some of the energy will come from the spray itself but most of it will come from the glass of the screen. In other words, you will be cooling the screen in order to melt the ice. If the interior has any moisture in it, then that moisture will condense on the cooling glass. So you are actually making the condensation worse. A similar thing will happen if you use cold water on the exterior, although the water may provide more of the latent heat (depending on the ambient and water temperatures) so the cooling of the glass should be less. Best way to clear ice from the screen without making the inside condensation worse is to remove the ice without it changing state, ie. by scraping it off. Has the side effect of warming you up on a cold morning as well!

Don't follow the logic here.  At the start of the process, the screen is the same temperature as the ice as the two are in contact.  If the screen were warmer then the ice would melt naturally due to heat transfer (Delta T = T2 - T1).

Given that the temperatures are the same, how does the ice warming up take temperature from the screen.  Surely the opposite would occur.  As the water is now warmer than the ice it replaced and therefore warmer than the screen, it would start to heat the screen.

[Downsizer]

You need 80 calories to melt every gram of ice.  This energy has to come from somewhere.  That’s why it’s best to scrape it off.

[culzean]

Let’s look at the science here. If you want to change the state of a substance from solid to liquid (eg melt the ice on the screen), you have to supply an energy input, aka latent heat of fusion. So where does that heat come from? If you use a de-icer spray, some of the energy will come from the spray itself but most of it will come from the glass of the screen. In other words, you will be cooling the screen in order to melt the ice. If the interior has any moisture in it, then that moisture will condense on the cooling glass. So you are actually making the condensation worse. A similar thing will happen if you use cold water on the exterior, although the water may provide more of the latent heat (depending on the ambient and water temperatures) so the cooling of the glass should be less. Best way to clear ice from the screen without making the inside condensation worse is to remove the ice without it changing state, ie. by scraping it off. Has the side effect of warming you up on a cold morning as well!

Don't follow the logic here.  At the start of the process, the screen is the same temperature as the ice as the two are in contact.  If the screen were warmer then the ice would melt naturally due to heat transfer (Delta T = T2 - T1).

Given that the temperatures are the same, how does the ice warming up take temperature from the screen.  Surely the opposite would occur.  As the water is now warmer than the ice it replaced and therefore warmer than the screen, it would start to heat the screen.

The de-icer spray does not contain any heat ( unless it has been warmed in warm water or in front of a fire )-  ice does not warm up when you spray alcohol based de-icer onto the glass - instead the freezing point of the ice is lowered as the alcohol mixes with the ice ( basically pure water freezes at 0degC but if it has anything dissolved in it the freezing point is now going to be lower, mainly because the dissolved substance disrupts the   molecules ability to stick together and form ice ) - normal salt will lower freezing point to about -5degC.  Spraying alcohol onto most things will cool the surface because of the alcohol evaporating - it is a balance between how much heat is lost and how much the freezing point is lowered by the alcohol.  I always preferred to scrape the ice off the screen while car engine was warming up rather than use de-icer as the de-icer normally caused smears on the screen,  the best de-icer I used was AutoGlym,  it was more expensive than average but you only had to use a small amount and it did not smear. 

Using water to de-ice the screen is not a good idea, if the water is too warm it can cause thermal shock and crack the glass, and anyway the water will just find somewhere else to run off and freeze, maybe stick the wipers to the screen.

https://en.wikipedia.org/wiki/De-icing

You could try covering the screen with alcohol based de-icer and setting fire to it 

[Westy36]

It also depends on the state you leave the car in.  If it is humid and warm in the car, then that moisture has to go somewhere as the cabin cools down.  Best to let cold air into the car before you lock it up for the night.

This is what we do with our cars. Drive with the heater of for the last couple of miles, and if not raining, get the windows open for the last few hundred yards. Balances the interior temperature to the ambient temperature. Only learnt this a couple of years ago after 25+yrs of motoring, and it really does work.

The other thing I do, is when I go to the car if its cold, is pour a litre of warm/tepid water over the screen. Instantly gets the screen clear.

[Rory]

Very odd to get this before you've powered up. I know you can get windscreen misting from the dash top vents before the heater kicks in.

Are you running the aircon to help keep the inside dry ? Only asking because some turn it off in Winter thinking it 's only there to blow cold air.

It is odd to get it to a great extent the OP describes - although our Jazz's always seemed to need the a/c on.

Could be it the a/c was stuggling to clear it as the temp was too low - a/c supposed to stop working at 3-4C although I have to say in our Jazz's it was never obvious that it wasn't working.   On cars in general I used to try and remember to turn the a/c off before arriving home in order to dry out the evaporator so it's not left wet overnight only to dump its moisture into the car on startup - however I found with the Jazz's that the car would just mist up anyway, and on my Mercedes (and, I suspect, wife's VW) it apparently never goes fully off anyway, they don't have a clutch on the a/c like the Jazz does.

I

[ColinB]

Given that the temperatures are the same, how does the ice warming up take temperature from the screen.  Surely the opposite would occur.  As the water is now warmer than the ice it replaced and therefore warmer than the screen, it would start to heat the screen.

Think of it this way. If you start off with a quantity of ice at 0 deg C, then apply just enough heat to melt it, you’ll end up with the same quantity of water at 0 deg C. But you’ve had to add energy to do that, so think about where that energy has come from.

Culzean is correct in pointing out that adding the de-icer will lower the freezing point, but no matter what the freezing point  is you still can’t have a change of state without providing the latent heat of fusion. And that will cool down the glass and promote condensation.

[culzean]

Very odd to get this before you've powered up. I know you can get windscreen misting from the dash top vents before the heater kicks in.

Are you running the aircon to help keep the inside dry ? Only asking because some turn it off in Winter thinking it 's only there to blow cold air.

however I found with the Jazz's that the car would just mist up anyway, and on my Mercedes (and, I suspect, wife's VW) it apparently never goes fully off anyway, they don't have a clutch on the a/c like the Jazz does.

The Jazz uses a fixed displacement scroll type compressor - so to stop it compressing the gas it needs to be completely disconnected from the drive ( which is what the solenoid operated magnetic clutch does ). Even variable displacement compressors can be prevented from compressing gas if the compressor is bypassed or piston stroke is prevented  - the displacement control valve replaces the clutch used on a fixed displacement type.

https://axleaddict.com/auto-repair/Variable-Displacement-Compressors

[Downsizer]

Internal condensation releases 540 calories of latent heat from every gram of water condensing, which is enough heat to melt over 6 grams of ice.  Some of this heat must go into the glass.  So perhaps when parking on a frosty night, it would be sensible to keep plenty of warm moist air in the car to reduce ice formation.  Is it easier to mop up condensation inside than to scrape ice off outside?

[culzean]

Internal condensation releases 540 calories of latent heat from every gram of water condensing, which is enough heat to melt over 6 grams of ice.  Some of this heat must go into the glass.  So perhaps when parking on a frosty night, it would be sensible to keep plenty of warm moist air in the car to reduce ice formation.  Is it easier to mop up condensation inside than to scrape ice off outside?

Don't fancy scraping ice off the inside of car windows....

[Jocko]

I've had to do that on several occasions. Even worse, My FIAT 126 had a single skin roof with a felt stick-on pad in the centre and one winter we had 10 days of freezing fog. The car was so frosted inside I couldn't use the heater. My mate was even worse. He had the fabric roof version, and every time he went over a bump, he had a blizzard inside the car as the thick frost was dislodged.

[richardfrost]

I've had to do that on several occasions. Even worse, My FIAT 126 had a single skin roof with a felt stick-on pad in the centre and one winter we had 10 days of freezing fog. The car was so frosted inside I couldn't use the heater. My mate was even worse. He had the fabric roof version, and every time he went over a bump, he had a blizzard inside the car as the thick frost was dislodged.

My mini van was basically just a tin can, with sliding windows. No chance of that ever keeping warm in winter. At least the windows were small and easy to reach for scraping!

[madasafish]

I used to drive to Scotland in mid winter  late at night using country roads to get quicker to Kirkcaldy - in 1999 before the motorway was extended .

I was in a company Rover 820 and at -22C on the car external thermometer, the windscreen misted up despite heater on flat out directed at the windscreen. I had to slow down to 20mph to enable the heater to clear the screen.

(in all other aspects, the car was rubbish - not just the heating system)

[Expatman]

Big pity you can’t specify a heated windscreen, cars with heated windscreens clear any condensation immediately and defrost is so much easier than having to scrape, scrape, scrape with a scraper!! Wonder why it’s not even an option?

[jazzaro]

In my opinion, Honda made a mistake in not designing the electrically-powered air conditioning system to run in reverse as a heat pump and they also seem to have missed the next best solution which would be a heating element under the windscreen vents so there is instant warm air. I thought that Japanese winters were cold but maybe they are a dry cold, with minimal condensation problems, rather than the damp cold that we get here.

A dual mode heating pump would be a complicated and expensive solution for a hybrid car with a heat source always avaiable as the thermal engine. A Jazz is not a plugin hybrid or a battery electric vehicle so it does not have to run many kilometers with the petrol engine off, so the lack of heat is limited to the first 3-4 minutes after a cold startup.
In a hybrid car like Jazz or Yaris or Corolla, you only have to wait for the engine warmup (both for  and there are specific strategies and protocols for fast warmup; keep in mind that the engine must be warm because the kat must be warm to limit pollution  And if you cannot wait (or you want immediately warm), a simple and cheap 1000watt PTC resistor would be enough for the first 3-4 minutes after a cold start, as in some diesel engines.
The other issue, talking about condensation, could be the aircon: we have two ways to demist our glasses, one is blowing hot air, the other is blowing dry air. We can obtain dry air switching on the aircon, but this will work bad with  low temperatures due to ice growing in the internal heat exchanger. So we should set manually the climate, setting aircon on, full hot air to the windshield and recirculation on if external temperature is lower than 3°C, this to prevent ice on the evaporator.