Steaming Up With The Heater...

[matt-c]

Something I've noticed over the past week or so now the weather has got a bit colder, is I've increased the temp on the dial for the interior heater (over the summer it was set on 18 and I only used it with the aircon to cool the cabin) to 21/22 and run it on the LO (or LO +1) setting, and it actually fogs up the glass inside the car. Not a lot, but a noticeable amount. However, if I press the aircon button to on, it demists very quickly. Turn it off, and a few mins later I get mild fogging again inside.

I've checked all round the interior, and nothing is wet or damp, so I don't think it's a leak or anything like that, but maybe there is too much moisture coming out the vents when the interior heater is on without the AC - I know AC air is dryer, so that would explain why it demists when I turn that on.

Why would the air coming out the heater vents have too rich a moisture content to fog up the glass inside?

[The Transporter]

There will be residual moisture in the box that contains the heater matrix and on the air conditioning eveporator... Soon as you turn on the heater blower then this moist air will be blown onto your windscreen.. Also worth while checking that your cabin filter is clean and able to pass fresh air through it

[matt-c]

I was wondering if a filter might be to blame. I realise there will be residual moisture, but I'd have thought it'd be "gone" after initial use (after the heater has not been used for months etc), and not do it everytime it's used - it's not something I've found on all the other cars I've owned over the years (granted this is the first I've had with aircon, although my works Transit does have aircon, and I've not had any misting up due to using the heater blower without the AC on at all in that over the last winter, or currently)

[clossie]

I've had he same problem now for 3 years (ever since I bought the car) I've tried running the heater at full pelt and 28 degrees for half an hou, tried changing cabin filters. Tried everything. Just gave up and leave aircon on all the time now. It seems the aircon system just helps remove ambient moisture quicker than on heat alone. No leaks in my car either.

[is200 Newbie]

It sounds like you have a leak somewhere in the cabin or like the transporter said above about the cabin filter in that needs replacing as it wont let air in/out of the car freely. When ever heat is applied to water or in a damp area it will create moisture which will settle on a cold surface, see it in your house when drying clothes on a radiator. Check the carpets and in and arount the heater vents for wet patches. Is the headliner damp, sunroof misting up, check around the seal in that area.

[CoopDog]

Sounds to me that the heater matrix has a leak, that will be what is causing the fogging

[The Transporter]

Because you are turning the aircon on and off, you are generating condensation on the evaporator coils inside the heater box (It is a function of the air conditioner, condensation forms on a cold surface) It is the same moisture that can cause mouldy odours from the aircon.... I would suggest that this is where the moisture is coming from... My van and my Jeep do the same... Not sure on your year of car, but whenever you set the demister on the air con runs too to help demist the car so whenever you switch to demist, then the coils will be getting wet and will set up the conditions you describe next time you start the car.