The reason for burner flames to be burning yellow is because they are deprived of oxygen. With incomplete combustion there will be less CO2 Carbon Dioxide and more CO Carbon Monoxide.
High efficient furnace are two pipe system that have combustion air going into the furnace
and exhaust air coming out. They both should be piped to an outside wall with the least amount of horizontal lines as possible . The reason for this is the furnace will produce water with vent temperature running warm and condensation will be produced. The water will be piped to a floor drain or pump.
The first thing you should check is outside intake pipe make sure it is not plugged with snow or frost blocking or restricting air flow. Exhaust air is warm and moist and the intake will suck this air in while mixing with cold air from outside and the moisture will frost up on the intake pipe. exhaust pipe should be extended 8 inches past the intake pipe to minimize this problem. As well be careful of fences or walls where the exhaust could bounce back to the intake pipe.
Next thing to check is the intake and exhaust pipe for places where water could collect due to wrong slope or sages in the pipe. If the pipe does not have enough hangers then it will sag and collect water in the valleys .
high efficient furnaces with only one exhaust pipe to the outside and combustion air inside building will bring air into the building similar to a mid efficient furnace . Both will bring in combustion air in through a combustion air pot usually 4 or 5 inch insulated pipe that comes from outside. It must be located as per code level to one foot above burners and within two feet horizontal. Outside will have a mesh screen that is designed to keep small animals and birds out, the screen could be plugged and needs to be cleaned, lots of times the backdraft safety switch will trip if there is not enough combustion air.
If cold air is noticed coming out of combustion air pot when furnace is not running this means the basement is in a neg pressure . The cold air can be stopped by a Hoyme combustion air damper that is connected to the furnace heat cycle and only opens on a call for heat.
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