Recently I have been experiencing a strange occurence in the NGX's Window heat system due to the high temperatures of Summer. Departing for instance KLAS during the middle of the day TAT's reach about 35-40 degrees Celsius. On the ground the Window Heat lights cycle on and off in order to keep the windshields set temperature constant. I understand that and it has been documented in many posts in the forums and in the FCOM's. However where I'm receiving trouble is once I'm airbone it seems the window heat never turns back on, even if I'm at FL350 with temperatures below zero the switches which remained in the on postions the entire time still result in the green "on" light to remain extinguished. Toggling the window heat switches off and back on again initially results in the green light to illuminate only to disappear seconds later. I'm completely perplexed and haven't been able to find any information in the FCOM's about operating procedures for a situation like this. I have attached a photo of my overhead panel documenting the issue. Any assistance would be greatly appreciated. Thank you.

Try turning them off, doing the window heat test, and turning them back on. Also, the procedure calls for either of the engine generators to be selected on the electrical panel after pushback (and you left it on the APU generator)

I copy by 737NGXFCOM:

 

Window heats on light :

 

Illuminated green: window heat is applied to selected windows

 

Extinguished : switch is off or, an overheat is detected, or a system failures occurred, or system is at correct temperature

 

If you have an overheat condition or a system failure you can read it in the annunciator panel; from your screen it seems you have system at correct temperature

Do the overheat yellow lights come on? If yes, open QRH at page 3.6.

If not, don't use shift+6 combination in VC. I've got this problem when i used this combinations. http://support.precisionmanuals.com/KB/a101/the-main-2d-panel-is-corrupted-when-i-open-it-with.aspx

It is a normal condition when heat is not required (window is warm).

If you want to verify if it's working move the switch to PWR TEST, this will power all the windows controller forcing them to full power, the lights MUST come on, if the windows are near to overheat, forcing them to PWR test can cause OVERHEAT to appear, in this case, cycle the switch off, then ON again waiting some seconds.

Thanks everyone for the information, I have tested the system on the ground using the power test and verified that it's operating correctly, so I took off and climbed to cruise as usual, however at FL360 the TAT is -7 and the "On" lights have not re-illuminated. Does anybody know how long it will take for the windshields to "cool down" in order for the system to automatically engage?

See my response in the other thread-