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How to properly flush a hydronic system? Mike Holzworth

What is the correct way to flush a new hydronic boiler system (boiler, pumps, piping & terminal equipment. What chemicals should be used (I heard some are actually worse that the stuff your trying to get out of the piping)? How long do you circulate it in the system. Do you need to flush it with clean water after this cleaning?
Another question related to the 1st:
How do you flush a domestic, potable water system?


  • Boilerpro
    Boilerpro Member Posts: 410
    Cleaning hot water systems

    From B& summary. Add 1 lb Trisodium phosphate per 50 gallons system water. Fill, vent and circulate system a few hours, drain and refill system. A little bit of cleaner will stay in system and should provide a slightly alkaline solution....shoot for a Ph of 7 to 8. Just got this info, and plan on starting to use it.

  • hr
    hr Member Posts: 6,106
    My current procedure

    First I power flush the system with tap water at 20- 25 lbs. Just below relief valve pop pressure. This generally gets dirt, wood chips, solder balls, teflon tape, etc flushed out. A Y strainer in the system is cheap insurance, be sure to check it after system cleaning however.

    Next I use a hydronic system cleaner Rhomar 9300 corrosion and scale remover. A liquid product with a strong cleaning ability. Potassium hydroxide based I believe. It also has other ingrediants (inhibitors) that start providing a film layer on the steel and cast components. I try to run it up to temperature once,and run the pump overnights.

    The EPA has put the hammer down on TSP, unless you find a commercial chemical supplier willing to sell it, the hardware shelf stuff is pretty weak. Most is labeled "phospate free" these days. Still, I suppose any cleaning is better than none at all.

    I then fill the system with good quality water and either Rhomar 921 hydronic conditioner or Dowfrost HD and DI water. If the water quality on the job is poor, new wells especially, I buy demineralized water in 5 gallon containers for about 8 cents a gallon. Fill and label the system as to the contents. It takes a few hours more, but really eliminates component failure.

    Test the ph when all is said and done 8-10 is a good range, according to chemical and boiler manufactures specs. TSP has a ph of 11 and systems cleaned with that end up in the 10 ph range generally. Dowfrost HD runs 9-10.7 out of the container, below 7 ph it needs to be flushed and changed.

    A good cleaning and ph adjustment is a very important step. Failure to do so will start to haunt you and the owners about 3 to 5 years down the road. The new radiant systems are much different from the old copper tube baseboard/ cast iron boilers of years ago, for some reason? Maybe that sneaky O2 molecule :)

    hot rod

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