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boiler short cycles/ water level drops

Everything was fine until 2 weeks ago, then I noticed the boiler short cycling. I went to check the water level and it was fine. Here's the problem when it first fires up it runs fine but after a while the water in the sight glass starts to bounce, slight at first then it drops a couple of inches to the point where the low water level switch kicks in and shuts it down. The water trickles back in from the top of the sight glass until the glass is full then it starts again repeating the cycle. We unhooked the auto fill unit [round black ball] and flushed water through the top and bottom lines; separately; and we got good flow from both. The service man put a rust inhibitor in the system when he cleaned and serviced the boiler could this have something to do with it??? Any help is greatly appreciated. 


  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,376
    Bouncing Water Level

    A bouncing water level usually indicates surface contamination--oily residue floating on top of the boiler water. The cure is skimming.

    Because the contaminants are floating on the surface, you can't remove them by draining and refilling the boiler. When you drain the boiler, the contaminants just stick to the walls of the boiler. When you refill the boiler and fire it up, everything seems great for a while, but the contaminants gradually work loose from the walls and float back to the top and you're right back where you started.

    Skimming slowly removes water from the top, so the material floating on the surface comes right along with it.

    This article explains the procedure:

    The location of the skim tapping should be included in the Installation and Operation Manual that came with your boiler. If you can't find it, post the make and model of your boiler. A few pictures wouldn't hurt either.

    If you know what the "rust inhibitor" was we can tell you if it's likely to have caused the problem. In general it's not a good idea to add anything to boiler water unless it's required to correct a serious water chemistry issue. Nothing can stop rust from occurring, but since boiling removes oxygen from the water, and the water becomes saturated with iron, it doesn't happen at a significant rate. The best way to minimize rust is to avoid changing water, add water only immediately prior to running the boiler, and making sure the pH, alkalinity and hardness of the water are moderately high.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • ChrisJ
    ChrisJ Member Posts: 11,966

    I agree with Hap to an extent.

    While I swear by water treatment I will agree that none is better than too much.  I agree that skimming is in order either way as it will both remove oils and the rust inhibitor.  If you wish to add water treatment afterwards you can do so and keep track of how much is added as well as observe if it causes problems.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Something must have changed for the worse

    You could drain the boiler several times to get the additive out, and see if things return to normal. Only pure water should be used in these boilers as the additives are not necessary, unless the local water contains certain chemicals, which must be removed. I am sure that your tech does not realize that, and yet he has been causing problems including wasted fuel without knowing it.--NBC
This discussion has been closed.