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  1. #1
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    Default Have to keep purging air

    December of 07 we had a new boiler installed - Burnham MPO with 6 zones run by individual Taco 00R multi-speed pumps. One zone is for indirect domestic hot water and the rest for space heating. All worked more or less well - at least there were no mechanical problems. We did have problems with heat delivery so we worked on insulation and control of air infiltration. We also added a hydrocoil when a new AC system was installed in October. The hydrocoil is a separate pumped zone and supports a central humidifier as well as supplying aux heat. When we did that, we also corrected a problem involving two baseboard heaters not being on the circuit controlled by the appropriate thermostat. All the initial plumbing was " copper. The hydrocoil was plumbed with a combination of 1" copper and 1" Pex. The original install was by one contractor and modifications last fall by another.

    Now we do have mechanical problems - repeated air blocks that prevent one or more zones from circulating requiring repeated zone flushing. (At least every zone can be separately closed on each end and the boiler can also be closed off making it easy to purge, but it's still a pain to have to keep doing it --- plus we introduce oxygen that will cause deterioration of the system eventually.)

    The most recent contractor suggested the problem was an incorrectly installed air scoop. The scoop was mounted just after the boiler with the expansion tank below - that's OK. But it was at the end of a straight run of 12" and not the required 18". He suggested replacing it with a SpiroVent because he said it doesn't require a straight run and is a better system for removing air. It did help some - now only single zones quit and we have gone up to 10 days without having to purge. Better, but still bad.

    The contractor initially suggested that we should close the supply valve for make-up water a few days after the initial setup and purge. That way we would know if a leak developed in the system. Then when we had repeated problems, he suggested we keep it open. Keeping it open may have helped, but not enough to be sure it did anything.

    One more point, while we were gone for 3 days over Thanksgiving the system completely jammed so no zones were circulating despite the pumps running constantly. This seems to have happened just as we left so the pumps operated against a complete block for 3 days - one pump was dead and had to be replaced.

    The contractor says our system is "over-pumped" so the pumps suck in air. But the system worked for year - up until the recent mods.

    Any ideas???
    Last edited by k12allen; 01-04-2009 at 12:51 PM.

  2. #2
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    Smile Re: Have to keep purging air

    I've read some mighty good advice on this forum for other issues - sure hope somebody has an idea for me.

  3. #3
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    Default Re: Have to keep purging air

    At least every zone can be separately closed on each end and the boiler can also be closed off making it easy to purge, but it's still a pain to have to keep doing it

    Okay...I'll offer up an idea from afar.

    If you close off a zone before attempting to bleed off the air in the lines......you won't likely get all the air out. This because if the zone is isolated from the system.....when you open the bleeder there won't be any water getting pushed in to push all of the air out. What happens is that once enough air escapes to relieve the standing pressure....the bleeding game is over....whether or not all the air has been pushed out.

    Try bleeding those baseboards while the boiler is up and running and the target zone is calling for heat. This will insure that the circulating valve is open and that water under pressure is present to push all of the air out. (Do not close any valves to isolate the zone.)
    Last edited by goldhiller; 01-04-2009 at 09:23 PM.

  4. #4
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    Feb 2008
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    Default Re: Have to keep purging air

    It's unusual to see air in a piping system to the extent you describe.

    The piping distribution part of the system may have been poorly designed to begin with, but the three most frequent causes of chronic air in the system is 1) improper location of the expansion tank, 2) an undetectable leak somewhere in the piping (especially on a run going into a concrete floor, for example); 3) improper manifold sizing.

    Other than that, the water in all the piping acts as solid as the motor oil in a car engine that solidly lifts the hydraulic lifter rods.

    The exception to this solid aspect of the hot water, as noted, is the expansion tank, which acts as a coiled spring to relieve expansion pressure as the boiler heats the water (water expands 5% as it heats up from 65 degrees to 180 degrees).

    Many piping installers make the mistake of placing the expansion tank at the outlet location of the main supply pump (which pumps pressurized water into the flexible bladder of the ET---this causes a negative pressure at the return side of the loops & will suck in air from any vent valve or any loose/worn component that happens to be present at that location.

    Another possibility is that the diameter of the manifold piping serving the pumps is too small, which would also cause negative pressure on the return side of the loop (see the "All Choked Up" article, below).

    It should be noted that there are often primary and secondary circulator (pumps) arrangements in many of these piping arrangements, with a main pump on the boiler circuit & secondary injection pumps for the zones.

    It's only speculation at this point, as I have not seen your piping setup (diagram)---if you have any photos of the expansion tank & the manifold arrangement containing the Taco pumps please post whatever you have, also a photo of your piping diagram, if you have one.

    It will be time well spent if you take a sheet of paper & draw out exactly how your system is piped---the starting point is the boiler---the MAIN SUPPLY PIPE (usually coming out of the top of the boiler) supplies all the hot water to the manifold & the pumps---as each zone calls for heat, the hot water then flows thru the individual loops.

    Try to draw a piping schematic that includes everything all the way back to the MAIN RETURN PIPE (usually at the bottom or side of the boiler).

    Very simple piping diagrams are listed below in the heating help.com site to give you an idea of basic piping diagrams---click onto "series piping" and "diverter tee piping".

    More elaborate piping arrangements are included with the Siegenthaler articles.

    A number of following articles by Siegenthaler at the Pmmag.com site address the air-bound piping issue & should be entered into the "search box" to access the article:

    "All choked up", "Righting Some Wrongs", "Beyond closely Spaced Tees", "visualizing pressure differences in hydronic design".

    You should also keep after the designer & installer of your system---they broke it, they should fix it.

    Another possibility once you have the diagram drawn, is to contact toll-free the "TECHNICAL DEPT." of Taco, Burnham & other mfgrs of boiler components---they will often offer expert help free of charge (and believe me, they've seen this problem before many times)---their toll-free number is available at the web sites.

    Sometimes they will only talk to heating technicians, but many will offer help if you fax or e-mail your diagram.


    http://www.heatinghelp.com/heating_qa.cfm
    http://www.pmmag.com
    http://www.taco-hvac.com
    http://www.burnhamboilers.com
    Last edited by NashuaTech; 01-05-2009 at 01:57 AM.

  5. #5
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    Thumbs up Re: Have to keep purging air

    Thanks, ****hiller and thanks, NashuaTech. I will follow all the suggestions, go to the reference sites and get back in a few days but here are a couple of points right now.

    The expansion tank is mounted under the SpiroVent and both are between the boiler outlet and the manifold that supplies the 7 Taco pumps. One possibility that my contractor suggested was that the system was "over-pumped" and sucking air in through the SpiroVent. Didn't think the vent would allow that and I have never seen the boiler pressure under 12 lbs - but better check that again when all the pumps are running.

    A leak in the system would explain a lot, but I don't think so if the water make-up valve is open as it is now.

    Will do the diagram and send photos of the system but can't do that until later this week after a pressing work deadline is met.

    Advice is much appreciated.

    Allen

  6. #6
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    Jan 2009
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    7

    Default Re: Have to keep purging air

    I posted a diagram of the system and pictures.
    Original Install 07.jpg (middle) shows the initial configuration that didn't give a problem for 12 months. The air scoop after a 12" run can be seen there as well as lack of valves to isolate boiler.


    Boiler & Supply Manifold 08.jpg (left) shows the additional circulator, SpiroVent and new valves on boiler supply/return. The new zone is piped with 1" while othes are 3/4".

    Return manifold 08.jpg (right)shows the return manifold which is inside heated space while the rest in in the garage.

    I don't understand ****hiller's advice: "If you close off a zone before attempting to bleed off the air in the lines......you won't likely get all the air out. This because if the zone is isolated from the system.....when you open the bleeder there won't be any water getting pushed in to push all of the air out. What happens is that once enough air escapes to relieve the standing pressure....the bleeding game is over....whether or not all the air has been pushed out." Perhaps I didn't explain how I have been bleeding. I close off the boiler on both sides, close off all but one of the valves below the circulator pumps, open the supply regulator, attach a hose to the drain cock and open the cock. What happens is that water flows into the one zone, all through that zone, back to the return manifold and out through the drain cock. If I do that for each zone, I have max flow through the zone and best chance of getting trapped air out of the system. Or so I have been thinking.

    Don't know if this matters, but the boiler is ~2 feet above lower level floor, lower level piping goes through a crawl space (3 zones plus new zone for hydrocoil) while upper level goes up (2 zones). I do have air cans on some of the baseboard heaters on the upper level.

    What's the magic that will purge these demons?
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    Last edited by k12allen; 01-08-2009 at 12:39 PM. Reason: fixing pics made problems with text

  7. #7
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    Feb 2008
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    Default Re: Have to keep purging air

    Allen,

    Thanks for the photos and the diagram---the schematic is excellent.

    I know you will eventually find the source of the problem---although there are a lot of components to the system, from the photos it looks like it was correctly assembled.

    There are a number of expert sources of help on the internet & elsewhere that should eventually help resolve the problem.

    Be aware that any new water that enters the piping system contains entrained air, which it liberates into the piping once it heat up, to complicate matters.

    Troubleshooting a problem of this type often involves isolating and operating one loop at a time to see if you can detect a defect in one of the loops as the source of the problem.

    I've also seen chronic air problems in systems that have insufficient pressure (psi) for the height of the upper floor convectors---the fact that you have a lower loop that is lower than the boiler may be an indicator.

    Thus, please check your upstair baseboard & heat exchanger bleed valves---you should get some water out of the bleeder when you open it---if you get nothing, or only a weak air sound, check the psi on the boiler gauge.

    Most residential boiler systems are set for the standard 12 psi---each psi of boiler pressure raises the water in the pipes 2.31', thus 2.31' X 12 psi = 28'---thus, the highest baseboard upstairs should be no more than 28' above the lowest piping loop in your system---if your highest baseboard is HIGHER than that, you probably won't get any water coming out of the upper floor bleed valves------the hot water never gets to the top convectors & they often suck air into the system thru any pinhole leak, as well.

    Post back if you don't get water from the upper bleeders---it's a simple matter to slightly raise the boiler gauge psi.

    As the heating season winds down in March, you will be able to try some isolation techniques to run only one or 2 zones at a time to check to see if there's any change in the air trapped.

    I've included a PM with additional resources.
    Last edited by NashuaTech; 01-08-2009 at 09:42 PM.

  8. #8
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    Jan 2009
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    Default Re: Have to keep purging air

    Quote Originally Posted by NashuaTech View Post
    Allen,

    Thanks for the photos and the diagram---the schematic is excellent.



    I know you will eventually find the source of the problem---although there are a lot of components to the system, from the photos it looks like it was correctly assembled.

    There are a number of expert sources of help on the internet & elsewhere that should eventually help resolve the problem.

    Be aware that any new water that enters the piping system contains entrained air, which it liberates into the piping once it heat up, to complicate matters.

    Know that and am concerned about a hidden leak - or one I haven't taken the time to find. Fortunately, we have no pipes in concrete so nothing is totally hidden.

    Troubleshooting a problem of this type often involves isolating and operating one loop at a time to see if you can detect a defect in one of the loops as the source of the problem.

    If I waited for warmer weather and do that without the boiler running, that would be more convenient - but I wouldn't get the outgassing from heating - so, not a good idea. Will wait for less frigid weather so we don't have to freeze too bad in unheated areas.

    I've also seen chronic air problems in systems that have insufficient pressure (psi) for the height of the upper floor convectors---the fact that you have a lower loop that is lower than the boiler may be an indicator.

    Thus, please check your upstair baseboard & heat exchanger bleed valves---you should get some water out of the bleeder when you open it---if you get nothing, or only a weak air sound, check the psi on the boiler gauge.

    Will check those bleeds, but since we didn't have problems until the recent work that's less likely. But worth checking anyway.

    Most residential boiler systems are set for the standard 12 psi---each psi of boiler pressure raises the water in the pipes 2.31', thus 2.31' X 12 psi = 28'---thus, the highest baseboard upstairs should be no more than 28' above the lowest piping loop in your system---if your highest baseboard is HIGHER than that, you probably won't get any water coming out of the upper floor bleed valves------the hot water never gets to the top convectors & they often suck air into the system thru any pinhole leak, as well.

    Right on the 12 psi cold setting. As for height, the upper radiators are about 10' above the top of the boiler so the pressure should be OK.

    Post back if you don't get water from the upper bleeders---it's a simple matter to slightly raise the boiler gauge psi.

    As the heating season winds down in March, you will be able to try some isolation techniques to run only one or 2 zones at a time to check to see if there's any change in the air trapped.

    Guess I should have read your whole response before answering

    I've included a PM with additional resources.
    Thanks,

    Allen

  9. #9
    Join Date
    Aug 2007
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    1,131

    Default Re: Have to keep purging air

    allen,

    Just a few thoughts to add.

    Since the problem didn't appear before installing the hydro-coil, that procedure/install becomes most suspect.....not?

    There is a possibility that one of the PEX connectors is allowing air to enter your system.

    Since you seem to have an abundance of isolation valves int he system, I'll suspect you also have valves to isolate the hydro-coil. If so, I guess I'd suggest you try shutting off the flow to that zone entirely and then bleed out all the baseboards via their integral bleeder valves without isolating those zones whatsoever. This *should* rid you of any and all air in those zones and won't introduce any new entrained air from the introduction of new water into the system.

    You mentioned that one guy had you turn off the make-up supply to the boiler to see if that would help. I would make sure this make-up water is turned on/available to the boiler when you bleed off the baseboards because you will lose some/a tad of water when you perform the bleeding.

    Once the hydro-coil is out of the equation and the bleeding of everything else is complete.....see how things perform for a couple/few days. If no new air-locks develop, then you could reintroduce the hydro zone and see what happens. This procedure should tell you if the problem is with the HC or has a different/additional source.

    Good luck.

  10. #10
    Join Date
    Sep 2008
    Location
    Bismarck, North Dakota
    Posts
    9

    Default Re: Have to keep purging air

    Maybe corrosion is happening. A few years ago I lived in a rented house with circulating hot water heating and had to vent radiators frequently. The cause was corrosion of piping or radiators. I proved that by opening the vent valve on a radiator and holding a match next to the venting air, and the air began to burn. I think it must have been hydrogen gas that had been produced in the corrosion.

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