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What happens to heating circuit's water during off season ?

Roohollah Member Posts: 135
Dear Members ,

I have came across a technical problem of a heating system which composes of a non condensing wall mounted boiler ,and poly propylene pipe net work and Aluminum radiators . I examined the system which is as follows :

_ City water as a make - up - cold pressure between 1.5 and 1.6 bar - working pressure at normal operation just for providing domestic hot water for bath and kitchen is 2 bar . PH of heating circuit 8.5 and TDS 70 ppm . Clear water inside of the radiators .

When I got the system checked I found that the heating circuit ( supply and return lines ) did not have shot off valves and the heating system just turned off by its switch . The working pressure was 3.0 bar and there were some drops of water beneath of the wall mounted boiler . As I noticed that there were not valves at heating circuit lines , so I checked the radiators first . When I was trying to pen the bleeding valve of the radiator I heard a noise inside of the radiator which was look like a surge or may hammering .To my surprise , the radiator was full of air and I let the air went out until water appeared out of the bleeding valve . I saw that the rest of radiators were full of air ,and after I took the air out of the whole radiators , the manometer of the boiler showed 2 bar . i checked the make up valve which was closed and it did not let water added into the system . Thus, there are two questions , how the heating circuit water turned into air during off season ,and whether there is chemical phenomenon between aluminum and copper coil of boiler which must be balanced ?

Thank you in advance for taking the time to help me and sharing your invaluable experience with us ,

I look forward to hearing at your convenience time ,

Yours Sincerely,



  • bob_46
    bob_46 Member Posts: 813
    Pressures are way to high. Check out X-tank, relief valve, and fill valve. Where is the drip coming from ?
  • hot_rod
    hot_rod Member Posts: 21,820
    Is that the normal city water 8.5 ph and 70 ppm of TDS? That TDS seems low, like it is filtered or DI water? Ph seems high. Over 8.5 ph in aluminum systems is a concern, usually low ph is friendlier to most metals. How does that city water taste, low TDS is usually flat, or bland tasting water.

    Low, low ph, 7 being neutral, is aggressive to ferrous, so you have a narrow ph range for those multimetal systems.

    Often ph like that indicates something has been added to the water, like conditioners. Rhomar ProTech conditioner typically leaves neutral ph water with an 8- 8.5 ph.

    Was the air completely bleed out when the system was started the first time? It helps to heat the water to drive air out of solution and systems may need to be bleed several times after start. radiators are a good hiding place for air, wide spaces, low flow velocity typically.

    Does the system lose pressure from a possible leak?

    I am a big fan of conditioners especially in a mixed metal system. Aluminum is fairly fluid sensitive, and low TDS can make the water aggressive, wanting to pull elements from the mixed metals. If you ever use conditioners or glycols they must be aluminum specific blends.

    Hydronic conditoners will balance PH, scavenge some O2, and also provide a very thin layer of film to protect bare metals. Every 20° temperature increase about doubles the corrosion potential. So from 60°F to 180° corrosion potential skyrockets. higher temperatures cause hardness minerals to precipitate also, so a double wammy.

    Of course assure a leak free system before adding chemicals, disconnect the fill valve and use a fill tank instead so fluids cannot become diluted..
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Roohollah
    Roohollah Member Posts: 135
    Dear Friends ,

    I profusely thank you for prompt replies . Indeed, there are many points in your valued replies and I must peruse them .So, I do appreciate such a friendship and support.

    With regard to the questions , I would like to say that the PH and TDS of city water in Tehran especially in Tehranpars is 7.6 to 7.7 ,and 139 ppm ,respectively. But, the PH of heating water circuit was 8.5 which I checked by Digital pH meter .Also, the TDS of the heating circuit water decreases to 70 ppm . In terms of the taste I strongly say that it is excellent and there is not sign of insipidness or bitterness in city water .

    No. I mean that the pressure of system has increases since the heating mode turned off slightly .On the day of inspection , I saw the relief valve outlet was dropping out and pressure was 3.0 bar . I checked all its boiler's inner parts and did not see any leakage .

    As regards conditioner , we just once used it for our recent boiler room and its was great and we tested this boiler room's water regularly and as you stated it created a thin layer to protect the piping net work .Here, using conditioner is not prevalent amongest heating guys . Recently , I have seen that EMMETI ( an Italian brand ) provides heating installers with conditioner .

    The system during the heating season was bleed out and the inhabitants of the unit never noticed cold radiators . It operated well .

    I talked too much , I am sorry for such long post , Thanks for your patience and considering my situation . As always, I feel such a honorable hospitality and kindness from you all .

    I am at your disposal if there are unclear subjects about the problems,

    Yours sincerely,

  • Zman
    Zman Member Posts: 7,542
    The extreme changes in pressure you are seeing are likely contributing to both the air issues and perhaps the water chemistry problems.
    When the pressure goes too high it will cause the pressure relief valve to discharge. The system will then replenish itself with fresh, oxygenated water.
    This would explain the air in the radiators. The oxygen will also contribute to the corrosion. The change in the chemistry may be a byproduct of that corrosion.
    I would begin by looking at what the correct pressure for the system should be. The most likely cause would be an issue with the expansion tank followed by a bad prv or a cross connect from a domestic coil.

    Generally the correct pressure would be 1 bar for every 10 meters of building height with a little safety factor (0.2Bar?)added to prevent air intrusion.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Roohollah
    Roohollah Member Posts: 135
    Thank you for your technical explanation of fluctuation of the pressure in the heating system. I will check them and follow the pressure that you stated above .

    All the best ,