Unlikely Sources Of Trouble For Windshields: Car Defrosters And Heaters

If your car is starting to get up there in age, you know how all the systems in the car can start to act up. However, your car's defroster and heater tend to stay rather silent when they start to have trouble. Problem is, when a heater or defroster starts to fail, they can have detrimental effects on your windshield. Some of these are secondary—they don't hurt the glass but lead you to do things that could. But the problems could also damage the glass directly. You have to make sure your heater and defroster continue to work properly to avoid a host of problems.

Thermal Stress

A heater or defroster that's starting to break down can be brushed off as just a temporary thing—maybe the day was just extra cold, so it felt like the heater wasn't working as well, right? Or maybe the heat seemed to ebb and flow, occasionally making your car too hot—again, that's just because the sun came out and warmed up your car, right?

Not quite. If your car's heater—and thus the defroster, which is normally part of the same system—starts to act in unpredictable ways, that could cause your windshield to crack. The trouble lies in the way the defroster can concentrate hot air onto specific spots on the windshield. That heat, coupled with too-high temperatures, can make the glass expand and contract too drastically—and it can crack.

This is why you're not supposed to pour hot water on your windshield if it's icy, or why a glass can shatter if it comes in contact with water that's too hot. Car windshields are tough and can deal with heat in general -- that's why you can drive through the desert Southwest in summer and still have a windshield—but if super-hot air or water contacts the glass, especially in spots, those spots can become overwhelmed by thermal stress.

Damage and Delays

If your defroster and heater are unpredictable and occasionally sending out air that's hotter than you want, it could crack the glass. But if they're also not sending out as much heat as you want, three things can happen. One is that you turn up the heater—and if the heater suddenly starts working again and sends out a lot of heat, the glass could crack. Or, the heater doesn't put out more heat, the glass stays fogged up, and you can't drive because you can't see. Or, if your windshield is icy, you try something like scraping it with a scraper, which can leave chips in the glass if you're not careful.

When you get your car serviced, have the heater and defroster checked out on a regular basis. Get them fixed if you notice any strange behavior. Doing so will save your windshield from thermal stress and save you from a more expensive repair bill down the line. Consult with professionals in your area who service auto glass for any further questions you might have.