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Proper way to skim a boiler? With boiler on, or off?

ChrisF Member Posts: 46
I've been skimming my boiler by first turning off the main switch, after it goes through a heating cycle, and the water inside is hot. I open up the skim valve, and turn the water flow valve on just a bit to add more water so it can start the skimming process. I let the water flow out, (about 2 and a half buckets), until the water becomes warm, and clearer. Then I close the water flow valve, as well as the skim port, and turn the main power back on, and then it starts up as normal. Recently though, I've read that you should have the boiler on while skimming. If that is the suggested way to do it, I'll start doing it that way.


  • ethicalpaul
    ethicalpaul Member Posts: 5,742
    No, I think you're doing it fine. You could let the boiler fire up if you wanted to do more skimming and the water was getting cold, but I don't think you need to have it running while you are skimming. Skimming takes a long time and the boiler would start making steam.

    Don't forget to drain your boiler to the correct level after skimming. At least on mine, the skim level is quite higher than the normal water line.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • nicholas bonham-carter
    At the first fire up of our Peerless, we followed their instructions for adding Arm&Hammer washing soda, and simmering for an hour or so. Then I began to skim, with the boiler off, until the water out of the skimming port was cold. The theory was that the hot water with washing soda would float on top of the cold makeup water, and all be washed out.
    Subsequent skimmings were just as you describe, and luckily there is a floor drain nearby to pipe over to temporarily.—NBC
  • Fred
    Fred Member Posts: 8,542
    Two and a half buckets probably isn't enough to get all the oils out. Be sure the water supply as you skim is very, very slow. You don't want the water boiling at all during the skim process. The goal is for the water in the boiler to be very still so that all the oils float to the top. I have skimmed both hot and cold, both ways are effective as long as it is very slow and long enough. Oil will float, hot or cold.
    EBEBRATT-Ed Member Posts: 15,635
    In Lost art of steam @DanHolohan describes every type of skimming as recommended by various boiler mfgs. I use Dans method add TSP or whatever your using get the boiler hot shut burner off and skim the longer and slower the better
  • clammy
    clammy Member Posts: 3,113
    I would agree w Ed if it’s really dirty use a wand and flush it out thru a tee installed in the return tapping of the boiler if it’s a side tap boiler install a tee and wand it out from top to bottom letting it all flush out refill and bring the boiler to a steam and start skimming . I use a bucket w a pump and a switch I hit on and off w my foot . I have found that if my skim and return piping are on the same side it may take multi visit and skims to finally be clean so what I have started to do is install another valve in the opposite return and hose feed my make up for skimming this way it flush opposite the skim port I ve found it usefully on dirty system . I also have started to pre flush nipples and pipe usually w dawn contentract usingna coil foamier and then a rinse w fresh water . I ve had excellent results doing this and cut skims down to 2 times one on start up w a good wand Andy flush and then a few weeks later On some the intinal skimming cleaning was enough I steam them skim them till cool flush steam skimand usually done but it all depends as w everything if life peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating