The room temperature rises on my honeywell 8500 thermostat when the backlight(feature 280) is on. The dim light works, the bright light lasts for 8 seconds. If I turn off the feature and the stat is dark the temperature is correct. Turn on the dim light and temp will rise 3 or 4 degrees in less than an hour. Is this the wiring, the current or voltage or what? This is the second thermostat i have used, then first one was ruined, it is a new Trane 13 heat pump and my installers are stumped. Need some help please.