Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Should I flush my hot water radiant system occasionally?

HansonGHansonG Member Posts: 4
Hi- I've got a self-installed (with a bit of pro engineering in the beginning) radiant system with a Burnham RV3 cast iron boiler and 2 Grundfos pumps pushing (?pulling!) hot water through 3 radiators and 2 lower temp zone areas plus a concrete slab in one area and tubing over subfloor in another. The system has needed maintenance items- valve motors, 1 noisy pump replacement, a boiler controller over about 15 years but hasn't had a leak or plumbing issues during this time. It's only been "opened up" about 4 or 5 times with the open areas limited by isolation valves for the most part. With the advice of a couple of really great suppliers and online information, I've done all the maintenance myself. I've never had a "pro" look at this system. I clean the firebox fairly regularly but as far as I can tell, nothing else has been needed. I still have this nagging feeling I should clear that black stuff from the system and get fresh (distilled or deionized?) put back in. Anyone have a take on this question? Any help would be appreciated.


  • mattmia2mattmia2 Member Posts: 1,408
    No. The oxygen in the fresh water will make new black stuff with the metals in the system.
  • GroundUpGroundUp Member Posts: 990
    15 years is about due for a flush IMO, but if the water chemistry is still in spec it's not a necessity. The thing with radiant is the fluid velocity is typically pretty low which allows any sediment or particulate matter to settle in it and plug up the tubing. If the water is black anywhere, it's time for a flush.
  • fenkelfenkel Member Posts: 81
    I'd rise the sytem out, clean with rohmor 9100, rinse and then.use rohmar 922, i would use deminerized water, but make sure you have deminerized makeup water for autofill.. then, every year send in a sample to rohmar for a chemical evaluation..youll never have an issue after this..sentinel is a good choice as well for boiler cleaners and conditioners
    Id also have your burners inspected/ cleaned and a gas/ Combustion done every two years..
    Also inspect the exhaust venting for rust..
  • ZmanZman Member Posts: 5,870
    Does your tubing have an O2 barrier? Does the system leak? Have you checked the water PH and TDS?
    The sediment in the water is often a sign of O2 ingress. The answer to your question depends on those 3 questions.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • jumperjumper Member Posts: 1,463
    Why are sidestream filters so rare?
  • HansonGHansonG Member Posts: 4
    My comment to GroundUp is Mattmia's response: I'm sure new "black stuff" will appear soon after a flush. Black stuff was there pretty soon after initial installation, and I do recall Dan's (Holohan) comment saying "that black stuff is your friend, man" in one of his emails to me (when he graciously answered several of my "total greenhorn" questions I had in the beginning). My tubing is O2 barrier tubing (Upinor, I believe). I've not checked the PH and don't have any leaks. My ongoing habit is to check the system pressure gauge regularly which hasn't varied in years. For a gauge check: at times I've run a small amount of water out of an "extra" valve at the manifold near the boiler (valve there for that purpose) to see the gauge needle wiggle. I don't use the auto-fill valve that's hooked up to the City water except to add back the tablespoon of "test" water. I also check the Amtrol pressure at the Schrader valve. No leaks. I do check the firebox and have cleaned the "fluidized bed" propane manifold every few years. Generally just a tiny amount of dusty deposits here. I've not checked PH or TDS, but it might be good to do at this point. I do have a substantial vertical sediment collection pipe at the return side of the system before it goes back to the boiler- that might be good to unscrew and empty or replace, but I haven't yet done it. I am inspired to check for sediment, though since it can really clog things, from what I've read, and cause issues I don't want- but to replace the rest of the "black water"? I'm still on the fence for doing that. In response to Fenkel's comment: I'm concerned about adding a cleaner to the system that might inspire leaks at joints. Just draining and replacing seems safer to me if I'm going to do anything- "if it ain't broke....", maybe.
    One question I do have is: how do you pressurize your system with distilled water without the "city water" pressure coming in unless it's just through the Amtrol tank? Anyone? Thanks for all your comments so far. I thoroughly enjoy the hydronic world of heating and cooling and likely would have been happy working in the industry had I not taken up an interest in this field too close to retirement. Still a great way of bringing comfort and safety to those that appreciate it.
  • hot_rodhot_rod Member Posts: 13,481
    Most all boiler water will take on a "color" depending on metals in the system, gray, brown, even black from any corrosion of ferrous metals. In addition if the system was not flushed and a cleaner used initially there could be solder flux, oils, pipe dope, etc that can color the water.

    You can send a sample to most any of the boiler chemical companies if you want to know the water is boiler specific, Rhomar, Fernox for example.

    I am a proponent of adding hydronic conditioning additives, you can get an aerosol can to squirt it in.

    If the water test comes back questionable, I would add a cleaner to that water, circulate for a few days with all zones open, flush, add good water with a transfer pump, and then the conditioner.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • EdTheHeaterManEdTheHeaterMan Member Posts: 621
    edited June 19
    I have a Question for @HansonG. You said " I also check the Amtrol pressure at the Schrader valve. No leaks." I wonder how you "Check" that?

    More to the point, are you letting air out of the tank on each test? Also, are you testing the pressure with the tank connected to the water pressure in the boiler? If you answered yes to either or both questions, you are getting the wrong result. You don't want to measure the pressure of the boiler water that is connected to the Amtrol tank. The air pressure that is pushing against the water pressure of the system is the same. You need to lower the water pressure below the test pressure of the tank air cushion. This illustrates the point.

    The top tank illustrates a connection to the water pressure of the boiler, the bottom tank illustrates the same tank isolated from the boiler water pressure, and all the water pressure on the water side is gone. The lower pressure of 6 PSI may not be enough air pressure for expansion. When the water pressure in the boiler drops to 12 PSI when cold, there is still water in the tank. The space taken up by boiler water at cold pressure is not available for boiler water expansion at higher temperatures
  • hot_rodhot_rod Member Posts: 13,481
    Great graphic @EdTheHeaterMan , mind if I use it? Interesting too that it shows the tank installed nipple down!
    I've always heard that traps moist air above the diaphragm and shortens the life expectancy?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • EdTheHeaterManEdTheHeaterMan Member Posts: 621
    I was laying down with my head at the bottom of the bed and my feet on the pillow when I made the graphic. Sorry

    I did not even think of the upside-down connection until you mentioned it. I just made a copy of someone else's illustration and modified it in Paintbrush. Not the best but it gets the point across.

    If someone with better computer skills wants to use the idea... I have no problem with that. If it helps the next guy understand what to look for in the "mind's eye" when diagnosing and problem-solving, Then I have done my job.

Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!