Skimming a boiler is a process that removes the oils from the water which act like a latex glove on the water in the boiler and cause a violent and unsatisfactory boil.
The oils get into the boiler during the manufacture of the heat exchanger casting and the assembly of the piping around the boiler. This oil in the piping takes some time to get back to the boiler so skimming isn’t something that is done immediately, but after several days of running the boiler.
The skimming process is done by raising the water level and allowing the oils, slime and other floatable debris to ‘slide’ sideways off the top of the water thru a special port on the boiler known as a skim connection. It is done this way as opposed to just draining the water because, picture this in your mind, a full bathtub of water, with oil on the surface. If you were to drain the tub, the water would leave the oil stuck to the side of the tub. You’d refill the tub and presto, the oils back on the surface of the water. So a skim tapping is provided on steam boilers to slide the oils off the surface.
This process may only need to be done once, or it may need to be done several times. We have done it as many as eight times to a new boiler. That’s more the rare case. Usually it’s a couple times. This is a normal process so don’t think something is wrong. This is normal with a newly installed boiler. It may also need to be done after major piping work introduces new oil to the system.
The symptom of when a steam boiler needs to be skimmed is a violent movement of the water in the boiler gauge glass tube. Also the boiler may be going ‘on and off” rapidly. Almost as if it can’t make up its mind what to do. The heat output to the house is also poor during this time. If these symptoms appear call us to skim the boiler. Its not something that we have to stay all day for ourselves. We arrive, set the boiler up on skim, leave and go about our business, then return later to take the boiler off of skim and place the boiler back in operation. Video of skimming operation here