this is a new sound to me and I know nothing about furnaces, whats wrong? the fan blower is on top and was feeling way warmer than usual too. After the heater turned off it started making a sound like popping or clicking from the combustion area until I turned the thermostat to the off position, then the clicking or popping stopped and the excess heat radiating from the blower stopped too.Is this dangerous? Is my gas heater going to blow up? Is this electricity or something? There was an odd smell, not gas and not burning but similar to burning rubber just not as strong.
The heat exchanger is made of aluminized steel. Over time, the heating/cooling cycle deteriorates it. What you are hearing is simply the metal expanding, then contracting when cooling off. No, your heater is not going to explode...the smell is just the aging metal and the heating of the metal.
I am guessing that this is an OLD down-flow or counter-flow furnace. Typically when you hear those kinds of noises coming from an old furnace, it is because the Heat Exchanger is cracked. I strongly advise that you immediately have that furnace carefully inspected by a reputable, qualified HVAC service company. If they know what they are doing, and they have the right tools, they should be able to look inside your heat exchanger with an inspection camera to look for cracks, holes and/or signs of fatigue in the metal of the heat exchangers. Do you have a carbon monoxide detector in your dwelling? If this is a down-flow furnace, as you seem to indicate, and it has any serious cracks or holes in the heat exchangers, many times you will see your flame turn from blue to yellow as soon as the blower turns on. When the flame is first heating up, prior to the blower starting, your flame may look mostly blue and draft upward properly through the chambers. As soon as the blower turns on and ramps up to speed, you may see that the flame begins to waver around a lot, turn yellow, and even possibly begin rolling back out of the furnace. This is a definite indication that your heat exchanger has one or more cracks or holes in it. This makes for a very dangerous situation, and it should be addressed immediately, before carbon monoxide starts building up in your home, or before any backfiring or explosions occur. If this is a very old furnace, it's time to replace it.
My guess would be that your blower motor has failed and the burner is cycling on and off, firing up and shutting down, on the high limit because it is too hot. You didn't say if the fan was running. You probably shouldn't use it until it is serviced.
