Do I need to flush a steam heat boiler system? If so, how often?

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Topic: Do I need to flush a steam heat boiler system? If so, how often?

Heating November 26, 2012 at 11:07 am

Do I need to flush a steam heat boiler system? If so, how often?

Steam people, I need advice.
I recall being told once that the boiler should be flushed every so often- like opening the valve to empty the rusty water into a bucket in the boiler room.
Is this really necessary?  And if so, how often should it be done?

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Yes, the sludge builds up.  I did mine once a month.

We used to flush into the bucket but then someone told me that flushing introduces oxygen into the system, which just creates more rust, and so on. So we stopped doing flushing monthly, get it serviced once a year, and so far nothing bad has happened.

Yes, you need to flush the system periodically, but not as often as you’ve probably been told, or for the reasons you may think.
Every system is different. Once a month is often fine if you’ve got a probe-type low water cut off. Mechanical, or float-type low water cut offs require flushing more often. Like weekly or so. Post a pic of your boiler and I’ll tell you which one you’ve got.

Draining of the boiler periodically is necessary because steam contiliously escapes trough steam valves and fresh water is added to boiler systematically. Water impurities such as minerals, salts and so on accumulate in boiler water and lead to thicker steam bubble shell when water is boilng. Then leftovers of steam bubble shells can be carried over to steam system and pushed to the system. It can lead to steam hammer. Also water after coniniously boiling can start to break up and develop acidity and will start eating trough the boiler sections and piping. It can lead to leaks and boiler failure. 


I often enjoy your posts, but I disagree with many of the points you cover.

Firstly, steam should NOT be escaping through the AIR elimination valves, that should close (plink) as soon as the air escapes and a s soon as steam hits them. The auto filler is there to replenish the water should it drop too low, but certainly not regularly.

You want the old water to stay in the system, because it has precipitated out its impurities already. Not that draining or flushing a system is never called for,just rarely.

Heating water cannot make it acidic, nor cause it to “break up”. H2O has (3) physical states, and atomically contains Hydrogen and Oxygen. There is no there there.

The OP is probably confusing flushing the low water sensor with flushingthe system.

Steam always escapes system. Air vents must see steam to close, and they release some steam in process of closing but it is not visible,  also there always some microopenings in the system. As per acidity as heat is applied to water containing bicarbonate ion and carbonate is formed, carbon dioxide gas is also released and is carried out with the steam. When the steam condenses the carbon dioxide dissolves in water to form carbonic acid, which lowers the pH of the returning condensate and can corrode the steam condensate piping system. This carbonic acid needs to be neutralized to prevent costly failures due to corrosion and leaks in the condensate piping system.

When i say boiler water, i do not mean clean discilled h2o pure, There are lot of things in the tap water.besides h2o. Of cource acid can be neutralized, but nobody pays attention to water quality in residential heating. People who know are not in this field.


I read through your recommended article. In industrial applications (they are talking refineries), there is significant new water being introduced.

This is the opposite of a brownstone steam system, unless something is drastically wrong. The volume of steam that escapes a Gorton valve is miniscule, and condensed to liquid form is drops, at most.

As for micro openings, the head pressure should be low (4 lbs max) and you never say never, but if the system is pressure checked multiple times a day, it should be pretty air/steam tight. Once the air is eliminated, steam pressure should trip the burner circuit, not leaving much time or pressure to force any steam out a micro-leak.

I understand what are you saying, but still, i find acidity in the boiler water when i test it, and i set all steam boilers i service to 8-16 oz of pressure with vaporstats depending on system size and configuration. in industrial applications such as power plants system is closed , unlike residential, and same water recirculates trough the system, some water is lost trough deaerators, some trough blowdowns and so on, but still rate of fresh water introduced into the system percentage vise is smaller than in brownstone steam system.

I think it depends on your boiler.  We had an ancient one from the first part of the last century (think room sized) in our 4-unit coop. and we used to get lots of dirty sludge out of it, without fail, draining off water weekly.

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