Play Misty for me

[guest4219]

Whenever I turn off the air-con in my GD Jazz, I immediately  get severe misting up to all windows.
To me this indicates a build up of water in the plenum chamber (where outside air enters the ventilation system    ).
This is usually caused by blocked up drain holes in the chamber, or blockages in the drain tubes.
I cannot see anywhere obvious where such tubes exit the chamber, which would if found, at least give me a chance to make sure the drain tubes are clear.
Has anyone any first-hand experience in this area?

[guest1372]

Leaf debris in the cabin filter can also cause this.  There are a couple of rubber tubes at the base of the bulkhead underneath, but I'm not sure which are sunroof drains and which are from the a/c & heater.
--
TG

[culzean]

if the aircon is blowing cold and cools the windows in the car then both temperature and humidity will rise when aircon is turned off, which will cause condensation on cold glass.  Also if the cabin filter is blocked it can reduce airflow making misting more likely.  if you pour a little water into the grill that runs along bottom of windscreen it should drain out of bottom of intake airbox pretty quickly,  and it will show where the drains are.

[guest4219]

I should have been more clear.
If I leave the heating on and the blower fan running, but TURN OFF the air-con element of the system, so there is  just the basic heating of incoming air: that misting happens.

[culzean]

if your system is low on gas (but not low enough to stop pressure switch operating)  this upsets the way the gas expands and compresses in the system and the cooling rad can freeze up and when you turn aircon off this water will be melted and blown into cabin as water vapour.

a blocked cabin filter can reduce airflow through cooling radiator and cause freezing up.

another cause could be a leak in the heater matrix, but you would smell antifreeze in the air vents and maybe have wet carpets as well - a leaky heater matrix is an expensive job,  I have had it on other cars,  but never a Honda.

[guest6425]

Pollen filter &/Or Aircon regass

[ColinS]

Air at a certain temperature can contain a certain amount of moisture up to it’s saturation point and after that will not absorb any more.  At this point it has a Relative Humidity of 100%.  The reason it is called Relative is because it is relative to temperature.

As the temperature goes up it can absorb more moisture.  When the temperature goes down it cannot hold the moisture that it currently holds and that is when you see mist or fog in the air. (for instance, when you breath out on a cold day).

What you are seeing in your Jazz is air coming into contact with the cold windscreen and therefore reaching 100% RH, so the moisture has to come out in the form of condensation on the glass.

If you have the aircon on, then the air is cooled and reaches 100% RH before it enters the cabin, so it loses moisture (hence the puddle of water you may see under the car on occasion).  As it enters the car it will heat up and the RH will go down.  All other things considered, if the windscreen temperature is higher that the temperature inside the air conditioning unit, the air in contact with it will remain less than 100% RH so you will not get condensation.

[culzean]

If it was usual behaviour then we would all have misted up Windows in our cars

[ColinS]

Having read your second post again, I think culzean is probably correct in that something is introducing moisture into the system.  And I agree with him on the likely causes.

[guest4219]

My thanks to all you who have made helpful suggestions.
It's two or three years since I had the air-con re-gassed, but only a few months since I changed the cabin pollen filter. The pollen filter is a two minute job! I will check it though.
I will also try the pouring water into the plenum chamber trick, but I still have no idea where the drain hole/tubes are to be found. There does not seem to be any outlet visible within the engine compartment, under where the intake chamber is. Where should I look, and how the heck do I get into it to clear it if there is a blockage?
If all else fails a re-gas is probably due anyway!
Once again thanks to responders to my queries. 

[guest4219]

Check. Pollen filter - OK.
Check. Plenum chamber draining - OK.
Looks as though a re-gas is in order :-(

[MikeG1944]

I had that problem years ago with a Vauxhall and in the end turned out to be a heater rad. Would have been very expensive to replace and car was old so used Radseal and that did the trick.

[ColinS]

I had that problem years ago with a Vauxhall and in the end turned out to be a heater rad. Would have been very expensive to replace and car was old so used Radseal and that did the trick.

Thinking about it, I had exactly the same on a Morris Minor, Mike.  That was in the days before Air Con when you had to wind the window down to get cool.  Wasn't life simpler then?

[guest4219]

On a quick shopping trip today, I tried the effect of starting with air-con off, but heater full on and fan at #2 setting.
Barely any misting, which was pretty well gone  by the time I got back home.
A re-gas booked for tomorrow.
We shall see.

[guest4219]

I had that problem years ago with a Vauxhall and in the end turned out to be a heater rad. Would have been very expensive to replace and car was old so used Radseal and that did the trick.

Thinking about it, I had exactly the same on a Morris Minor, Mike.  That was in the days before Air Con when you had to wind the window down to get cool.  Wasn't life simpler then?

Yes, but a 14 year old car back then was a heap of tin-wormed rust.