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how to descale nti combi heat exchanger (both sides)

Hi guys.
I have a NTI TX-150 combi unit. It was a replacement to the original, and it looks to me like they used bad groundwater to fill the system initially 20 years ago. The replacement was just put in place, and the scale in the hydronic side has caused a few valve failures. One really odd failure was the spirovent has a little plug in the center tube of the core. That little plug slid down in the tube then got scaled in place isolating the pressure tank from the rest of the system...
Anyway, after a system flush with city water, I would like to make sure that the internal water to water heat exchanger (this is a combi unit) is clean.
I am currently having to reduce the flow rate of the city water to the houshold demand hot water, and I expect it is due to a partially plugged heat exchanger on the boiler side.
I did also do a vineagar flush of the demand hot water side, and did get some scale out on that side with the system off. The User manuals give NO instructions on how to flush that heat exchanger (or any of the system for that matter).
I did the pump in a pail with a bunch of vinegar for the demand hot water but I expect that would go thru the heat exchanger on one side.
I did a similar thing on the boiler side using taps in the external piping, but I have no idea if that would go thru this heat exchanger or not.

Taking it out looks a little messy, but I am up for it. Maybe there is a repair manual around somewhere for it, "he said hopefully".


  • Leon82
    Leon82 Member Posts: 684
    I would follow the piping and see if you can make a loop by closing a combination of valves. It sounds like you tried that.

    Pulling it off, you can do a vinegar soak and will be able to visually inspect it
  • hot_rod
    hot_rod Member Posts: 19,287
    Get a tankless descaler kit It’s a bucket with pump and acid, you’ll get better and quicker results compared to vinegar

    For the boiler look a cleaner kit aerosol cleaner from Rhomar or Fernox
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ottawa_craig
    ottawa_craig Member Posts: 7
    Pulling it off looks a little harder than I would like. The Installer did not use unions on the outlet pipes to the client side plumbing (hot and cold demand hot water). I have a pump, and can hook into the external piping, but not sure if that is all that is needed on this unit. Plus the ONLY thing the customer manual says is to use fernox 40 to descale. No proceedure. I can try just running an acid thru the boiler side (did it once for only about 10 mins)
    but I have no way of telling if it ever makes it to the internal heat exchanger (the water to water one).
  • hot_rod
    hot_rod Member Posts: 19,287
    edited June 2019
    If the boiler was filled just one time, and has not taken on addition fill water for any reason, the boiler side of the HX is probably ok.

    The "B" side which see continue fresh water tends to be the problem side. Flush it with one of these kits. I use this kit to clean 3 way thermostatic valves, plate heat exchangers, tankless water heaters, even old tarnished copper fittings. I'd start with just a DHW side flush.

    The boiler side generally does not require an acid cleaner, the products I mentioned above are really a strong detergent to clean out the usual boiler gunk. I'd run the entire heating system with the cleaner on the boiler side. you need some valves to inject it, then to flush it clean.

    Boiler fill water should be a low TDS, and some conditioner added to good quality fill water.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ottawa_craig
    ottawa_craig Member Posts: 7
    Thanks Hot Rod.
    Like I said, I think the boiler side was initially filled with ground water. I flushed it all out, (replaced some pipe, and valves in the system, they were scaled). Ran some acid thru, and things seem better but I was nervous around running acid (even week) for any real amount of time thru the boiler.
    What PH level do you use for an acid flush, and how long, considering that it will be going thru the boiler.
  • hot_rod
    hot_rod Member Posts: 19,287
    If you fill the system with good quality water, really no need to ever flush and clean it again, on the boiler side. Watch hardness, and TDS, add a good hydronic conditioner and it should be fluid troublefree for many years.

    Unless the boiler is severely scaled from hard fill water you should not need an acid based cleaner. The hydronic specific cleaners are blended to be friendly to multi-metal systems, and less aggressive then acid cleaners. They will handle small layers of hard water scale inside the boiler, according to the manufacturers.

    I've used acid cleaners when I know a system has taken on large amounts of hard fill water for extended periods. Usually a radiant slab system with a pinhole leak, loose connection, etc Those boilers will generally make it know that they are scaled with various noises, lack of heat transfer.

    You might even consider a fill tank instead of a fill valve to assure the system does not take on water, bad water, in the future.

    On the domestic side, the descaling frequency will be based on water hardness, how much water passes thru it, operating temperature, quality of the heat exchanger, etc.

    There are flat plate style heat exchangers, high pressure drop styles, that provide higher velocity in the channels for less fouling.
    Every home has different DHW flows and usage, plan on descaling it in the future
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Steve_Wheels
    Steve_Wheels Member Posts: 25
    As Hot Rod said, if water that was used to fill the boiler side meets the specs in the I&O manual then a product like Fernox 1 injected into the system at the right amounts will help protect the system.

    Of course if you ave that much scale problem I would turn off your fill valve and see how long the system holds pressure. IMHO it sounds like you have a leak somewhere.

    Is the system copper, steel, cast iron?
  • JoeyG_2
    JoeyG_2 Member Posts: 14
    Use Fernox DS40 System Cleaner and pump it backwards through the Heat Exchanger to clean it. That's what NTI wants you to do.

    Joey G