Windscreen condensation demisting

[rogereld]

Climate control is usually set to auto all year round with cabin temperature dials at 20 and usually everything is fine. On a few occasions when the weather is wet recently I have noticed that the inside of the windscreen is not clearing properly. I can switch on the screen demist and the fans ramp up their speed. I am sure they used to switch back to normal auto operation automatically after clearing the screen, but this weekend the fans continued for 25 minutes until I manually switched off the screen demist button. The screen cleared within 5-10 minutes (possibly quicker) but the fan continued.
I have had a couple of occasions when driving in wet weather when the screen has started to get misty and I think the climate control should deal with this automatically.

Last year I replaced the aircon compressor and also the condenser and dryer with second hand parts from a donor volvo, because I had a condenser leak which caused aircon gas loss which seized the original compressor. I have had plenty of opportunity to test the cooling during summer 2018 and it has been working well and kept the cabin cool, but I am wondering if the change of parts has introduced a problem.

Has anyone had similar problems, and how do I fix it?

[petey]

At the risk of proffering a trivial solution, glass cleanliness makes just the biggest difference as a dirty screen offers so many more nucleation sites. Glass cleaner the inside and it'll seem like a new A/C system.
(I'm guessing you've already done this, I guess I'm more referring to other people reading this....)

[rogereld]

Yes, a good glass cleaner does help, but I have tried that already. I have owned the car for a number of years so I have got to know how it operates, and the more significant misting on the windscreen has started since I had the aircon components changed.

[OldEngineer]

There will be something damp in the car. Check all the carpets and see if you can find a wet one.

[Recognized]

Had same thing in mine, but no alterations to air con. Just put dehumidifier for an hour inside the car and problem went away. My demist function just keep going forever until I switch it off or turn the ignition off. Worth replacing the cabin filter and see if recerculation works fine.

[Brendan W]

First sign of a blocked pollen or cabin air filter is slow demisting.

[rogereld]

The pollen filter was the first thing I checked when I started to notice windscreen condensation problems in November last year, and so I replaced the filter last November. The air con work was done in September 2017 and it could have taken a few rainy damp October days last year for me to notice there was a problem, as I don't use the car every day.

We had a good summer this year with very few wet days in Southampton so I had forgotten about the screen problem until recently.

There is no evidence of damp carpets and I am not noticing problems with any other windows, just the windscreen. It does not happen all the time, and does not appear to be linked to periods when I have not used the car for a few days, which would probably be the case if I had damp carpets or water leaking in whilst parked.

When I press the screen demist button, and the fans start up, should it run for a while and then automatically reduce blower speed back to normal climate control?

[rogereld]

It is more than 18 months since I last posted on this thread, but I think I have now discovered what was causing the occasional screen demist problems that started during the winter of 2018. They were not noticeable during the summer but I was aware of dampness in the car again during winter 2019.
The car has been unused for a couple of months during lockdown and there are now fairly noticeable salty deposits on the left hand passenger footwell carpet.
I have removed the lower trim and the glovebox and the leak is from both O ring seals where the alloy pipes connect to the heater core. The clips are coated in scale.
The heater matrix joints may have been leaking for a year or two and just getting slightly worse over time but it is fairly evident now.
I have purchased two new O rings and am looking at some online videos of how to do a repair without complete dismantling of the dash. Each video seems to have different advice on how much coolant needs draining, and from where.
Can anyone who has done this repair on a UK car comment on this. Do I need to drain coolant from under the car, or can I just undo the heater matrix pipes and catch whatever coolant leaks out in the passenger footwell?

[oragex]

You can do a trick, drain the coolant reservoir (disconnect the sensor underneath when lifting it), then unplug the two wall hoses that go inside the cabin - it's tricky the first time, but it's actually simple, I have a video on my channel. Then once the hoses disconnected, put a hose on one opening and blow air with your mouth a few times - some fluid should drain from the other opening, and it should be enough not to have any leaks when disconnecting the two pipes at the heater core. Still place a piece of plastic under the core, just in case. If your recover most fluid, you can just top it up with some distilled water

[rogereld]

Originally Posted by oragex

You can do a trick, drain the coolant reservoir (disconnect the sensor underneath when lifting it), then unplug the two wall hoses that go inside the cabin

Does that require removing the PAS reservoir and removing the coolant reservoir from its position. I have changed the PAS reservoir before and it was not easy to disconnect as it had expanded slightly so the simple task of unclipping it from the coolant reservoir took ages.

I am thinking that the more hoses and pipes I need to disconnect, the more trouble I may cause that needs new pipes or clamps when I need to re fit everything.

If I just go ahead and disconnect the two pipes that connect to the heater core, where the O rings need replacing, how much coolant will I need to catch in the passenger footwell if I don't drain or dismantle any of the cooling system first?

My thinking is it could be quicker to waste a bit more coolant and just top up the coolant reservoir with fresh coolant afterwards?