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Boiler sight glass turned black

I could use some help diagnosing the cause of this issue.

This is my first winter with oil heat and I’ve encountered a few issues (and this forum has been a big help) but this one confuses me.

We had our boiler inspected when we moved in 2 months ago (all good) and skimmed about a month ago now (tech mentioned there was some oil l, but not a ton). 
Before we had it skimmed, it was producing rust flakes in the sight glass and I drained it twice where red-brown, rusty water pretty much came out the whole time (about 4 gallons each time). A boiler technician suggested we have it skimmed and we did. Since it was skimmed, the sight glass has gotten dirtier as the days have gone on. It did not do this before we skimmed it. Initially you could still see the water moving in the column, and it appeared to be cloudy, but not opaque. Now the tube appears to be completely coated in something that prevents seeing the water in the tube.

What is the cause of this staining? Do I need to do something besides cleaning the inside of the sight glass?

Photos to follow in just a moment here.

Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 11,082
    Remove the sight glass and clean it. It's good to have a spare glass and washers just in case. Skim the boiler again. Drain and refill and flush the vinegar probably losend up some stuff
    LastFreeMason