[Dukka]

I purchased a house three years ago and replaced the antique steam boiler with a Weil McClain three section cast iron boiler with probe type low water cut-off and an automatic fill. One morning I smelled somethiong like burning plastic coming from the basement. When I went down to investigate it seemed to be coming from the boiler. The boiler was hot, the oil burner was off on low water I assume (the light on the low water cut out was ON). The water feeder was trying to fill the boiler but no level was showing on the sightglass. I looked into the combustion chamber and noticed a water level. I turned off the water feeder and power to the boiler. Needless to say the boiler cracked and started to fill the combustion chamber.
I replaced the boiler that same day (it was very cold that day) and when I started the new Weil McClain boiler the smell of burning plastic came back stronger than before.
Any suggestions on what caused the boiler to crack and what the smell is?