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Brazing Flux

Last week I was changing out a compressor in a Hydron Module water to air heat pump. This unit is a 2005 vintage unit and the factory brazing in the unit did not look very good. So when brazing in the suction line to the compressor I could not for the life of me get this to seal up. Pipe and compressor fitting were perfectly clean, no oil or residue. I tried for a few hours and gave up. I'm using Sil Fos 15 so copper to copper joints are good to go with no topical flux.

The next day I tried again and put a very small amount of brazing flux on the joint on my next try, sealed perfectly.

I typically do not use any brazing flux but now I'm thinking about using it on second hand joints such as this one. Also when making connections to brass service valves. I know we want to keep the interior piping spotless but a very small amount really could make life easier.

Anyone use flux on a regular basis with copper to brass or second hand joints in a refer system? I'm guessing its not all that popular with how hard it was to find. 1 supplier had it in stock and the container was 5 years old.


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,538
    I have never had a problem getting 15% silver on copper to copper unless very dirty. Did you have pressure in the system from residual refrigerant or nitrogen ?
  • bnjmnbnjmn Member Posts: 45
    Not under pressure. It wasn’t like pressure blowing a hole in the molten alloy. I couldn’t get any to suck in like it was being repelled. It was a dirty joint to begin with even after being cleaned and prepped then after multiple attemps I’m sure that didn’t help. Once a very small amount of flux was on there it sucked in the chicken turd and sealed nicely.

    The poor design of the Hydron unit really made the replacement a pain in the butt, nightmare actually.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,538
    I have had some success heating a crappy looking joint really hot and then wiping it with a clean wet rag soaked in water "steam cleaning" then using a fitting brush and mesh sand cloth on it, can usually get it to take. Using flux like you did is helpful as well
  • DZoroDZoro Member Posts: 1,037
    I have had a bad batch of sil fos 15 about a year or so ago. Couldn't get it to draw, thought I was loosing my mind. Changed to a new box and walla, it was like I learned how to braze once again.....:)
  • pecmsgpecmsg Member Posts: 1,573
    Check with a magnet some compressor fittings are copper coated steel. Discharge lines always get flux and 35 or 55%.

    Dissimilar metals require flux and higher silver content.
  • bnjmnbnjmn Member Posts: 45
    Thanks for input. I don't mean to beat the brazing topic to death. I thought the flux in the application was helpful.

    I do question the quality of the copper fitting on the compressor.
  • pecmsgpecmsg Member Posts: 1,573
    Copper to Copper flux is not needed but doesn't hurt.

    Again Compressor Discharge lines I use a higher silver content and flux is then required!
  • ChrisJChrisJ Member Posts: 11,337
    edited April 2019
    Silvphos uses phosphorus as it's flux for copper to copper joints.

    Personally, I've only used Harris Stay-Silv 15 and it works fantastic.

    If adding flux to the joint helped, either the joint isn't copper to copper or your rod is junk in my opinion.

    Any chance the compressor housing was sucking up too much heat?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • bnjmnbnjmn Member Posts: 45
    I did try another batch of rod, different manufacturer with the same result. The fitting was plenty hot. The original brazing was poor, this was probably the first prototype unit the Amish built.

    After the multiple attempts of re-brazing the joint I did inspect the inside of pipe and compressor inlet when applying flux to the joint. Everything was clean, no carbon or puddles of rod.

    The dirty original joint was most likely my problem. Candle light brazing I guess.
  • LeonardLeonard Member Posts: 903
    Not knowing any better maybe clean rod with steel wool???
    Maybe being old it developed a surface oxide??
  • mikeg2015mikeg2015 Member Posts: 1,179
    We never use flux when bracing. But use emery cloth to get bare copper and clean up the brass is it’s not new. Never have a problem. A guy with 40 years experience that started in Florida taught me. He’s got terrible arthritis, shaky hands, but his brazed joints never leaks and look clean.

    Key is getting the torch adjusted right. You using MAP or oxy acetylene torch? We only use a torch only for brazing. Too easy to get cold joints with MAP, especially outdoors with any breeze. With the later, just have ot not overheat it. But it goes fast. I’m still learning myself. We save the MAP for sweating copper on water heaters and low temp boilers.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,538

    True about compressors with steel stubs. I haven't seen one in so long that I forgot about that. Could be the problem.

    I think filter driers are copper coated steel. Have to check
  • ratioratio Member Posts: 2,610
    I keep a tube of 55% in the truck for the really hard jobs. Wet the joint with that, then it'll take 15%. I also have some flux (I forget the name), but that drys out pretty quickly. I read that you can add water to make it pasty again, but I haven't tried that yet.
  • pecmsgpecmsg Member Posts: 1,573
    I add a drop or 2 of water to the dry flux. No problem
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,538
    Yes the flux dries out and you can add water to it.
  • mikeg2015mikeg2015 Member Posts: 1,179
    Clarification, We are using self fluxing brazing rod, so we save the step. We of course are purging continuously with nitrogen. I’m a newbie, still learning. Pretty cool trade. Doing 24 units on a re-roof on. SO lots of practice including cutting out filter dryers, and replacing the liesets we removed, so plenty of bending and swaging.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,538
    Steel stubs on compressors and filter driers you have to be careful nor to over clean them. The steel stubs are copper coated and if you sand off some of the copper then your brazing to steel and need flux.

  • mikeg2015mikeg2015 Member Posts: 1,179

    Steel stubs on compressors and filter driers you have to be careful nor to over clean them. The steel stubs are copper coated and if you sand off some of the copper then your brazing to steel and need flux.

    Good point. We don’t clean new tubing or the filters.

  • pecmsgpecmsg Member Posts: 1,573
    mikeg2015 said:

    Steel stubs on compressors and filter driers you have to be careful nor to over clean them. The steel stubs are copper coated and if you sand off some of the copper then your brazing to steel and need flux.

    Good point. We don’t clean new tubing or the filters.

    The Harris Site has a lot of information:

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