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overpreaurizzation of hot water heating systems

Doug_7 Member Posts: 210
Jack - I think you are 100% correct on problem cause and solution.

For perspective, we have a three story 45,000 sq ft building and the heating system is a 1,000,000 Btu / hour boiler with 3 inch supply and return piping. The expansion tank size is 80 US gallons. It sits quietly on the floor in the corner and does a great job.

We run the pressure at 20 psig in the boiler room to make sure there is always a positive pressure on the third floor. If the building was one story we would run the system pressure lower.

Our expansion tank is probably oversize because I see no real pressure change between hot and cold. It is always about 20 psig. I like it that way. The system make-up water is always left valved OFF and the system does not require any water addition unless we change a side-stream filter. It is important to always leave the make-up water valved OFF to minimize the consequence of a catastrophic water leak. I speak from experience on this.

There are proper methods for sizing expansion tanks. This is not a matter of opinion - it is physics. Sizing depends on total system volume, max - min operating temperature, and how much max-min pressure change you want to have.

See manufacturers information.


Ask the "powers that be", to show you the expansion tank size calculations. Then when they can't, have a proper expansion tank size calculation done for your system and have the proper size tank properly installed.



  • JackEnnisMartin
    JackEnnisMartin Member Posts: 70
    over pressurization of heating systems

    Good Morning
    I work for the largest school division in Manitoba Canada and I am concerned with some of the system designs. It seems I am running into jobs that are running at 45 to 50 psig on single story buildings and are blowing the relief valves constantly. Why would a design engineer want a single floor school to run at fifty psig? the solutions I am getting from the powers to be here are ,if the relief valve is weeping it is defective and should be replaced? Gentlemen, I am getting very sick of this scenario to say the least. Please ,if I am missing something could you tell me what. My experience tells me, fifteen pounds gauge would be more than enough. On one job ,the expansion tank for a fifty thousand square foot building is 12 gallons.The installer also never introduced more air pressure into the extrol tank ;so, it started out at 12 psig and now the rubber bladder has burst. Another job has very large tanks at least 20 gallons: but, once again ,they have never been cahgrged correctly and the fifty pound relief is blowing -- sooooo -- it is defective and I am supposed to change it again Is there in your experince ,any reason to run a single story building at these pressures? My initial thoughts are they went very small on pipe size and this is compensaton for friction lose. This does not hold up to logic ,the mains are huge and the pumps are very large indeed. I cannot convince this bunch that the systems are not right. They hold the engineer card up saying ;this is the design -- it does not work. I have allot of respect for good engineering; but, this is to my mind ,someone who has missed the boat so to speak. Any thoughts on the matter would be appreciated. If I am wrong tell me ,and please explan how I go about having the systems run correctly, without blowing a fifty pound relief valve all the time.

    Thanks and hug your kids they are the future we are the past. Jack Ennis Martin

  • Sounds like the Expansion tanks may be the issue as you said. If the rubber diaphragm has burst and there is no air in the tank...

    Take Dans advice and add " TL. DMS " after your name on your business card,.. maybe then the Engineers may give you a little more respect.

    Jack Ennis Martin TL, DMS

    (These Letters , Don't Mean Squat)

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Dave_12
    Dave_12 Member Posts: 77
    Some things to look at


    Blowing relief valves can be due to various reasons. One is a bad relief valve, which is rare. A second reason is an undersized or bad expansion tank. A third is due to a bad, or leaking, PRV which is letting city water pressure get through to the system. And lastly, poor design with regard to placement of pumps versus the point of no pressure change (expansion tank tie in) and point of tie in of makeup water.

    First, it is important to pump away from the expansion tank and the tie in of makeup water. It is also important in most commercial sytems to pump away from the boiler so that system pressure loss is not added to the pressure at the boiler, thus creating a problem for the relief valve.

    It is also important to "engineer" properly, emphasis on properly, the sizing of the expansion tank. One needs to consider the total water volume of the system, the relief valve setting on the boiler, the static charge of the system, the cold water temp, and the average heated water temp. The challange is to determine the expansion tank size that will 1.) have adequate acceptance volume to accomodate the expanded water, and 2.) have enough total volume to accept the expanded water and not have the pressure rise above a point that is, say 10% or better 20% below the setting of the relief valve.

    Once you have all of the above in place, it is also critical to charge the expansion tank to the at rest system pressure, and do this with the tank isolated from the system and vented to atmosphere. Many people do not do this.

    So it boils down to looking at the sizing of the expansion tank, charge it correctly, make sure the relief valve is good, make sure that the PRV is holding back city water pressure, and pumping away from the boiler and the point of no pressure change.

    Lastly, buy and read Dan's book "Pumping Away" and the above will become more clear.

    Good Luck,

  • JackEnnisMartin
    JackEnnisMartin Member Posts: 70
    over pressurization of heating systems

    Thanks for the advice ,I am glad to have my opinions verified by other people in the trade. I have purchased Dans books and they are excellent. I have also amongst others ;Hydronic Heating for Residental and Light Commercial and I find it to be excellent as well. The Taco program for sizing expansion tanks does an excellent job for me when I am rubbing the tiny amount of hair I have left off my shiny pate.

    I would suppose ,like other people, you start to doubt what your education and experience is telling you ;when all around you, seem to have cinderblocks for heads. I have said it before and I will keep saying it; refusing to entertain other ideas is self defeating. I have done the calculations for both buildings and they tell me not adding in the 5 psig for relief valve sizing the pressure should be approixmately 18 to 20 psig as you indicated. I would think a thirty pound relief is more than high enough.

    I was greatly amused by the letters ,very droll -- unfortunately ,my experience with the engineering community is ( some ) have quit reading anything too many years ago. However ,I also have to say ,I have met some very knowledgable people over the years as well.

    Thanks again, tell your kids they are the most important thing in the world --- because they are.

    Jack Ennis Martin
  • Ragu_4
    Ragu_4 Member Posts: 44

    I would say that 90% of the expansion tanks that I have installed either have either a low or no charge in the bladder; this is brand new out of the box. As per Mark Eatherton I charge and write with magic marker the pressure and date right on the tank.

    An old teacher told me that there is no such thing as too much expansion capability. I like that.

    In theory I agree with shutting off the boiler feed valve, but I always get a little scared about depending that much on the low water cutoff to do its job. In the case of a school district, maybe it would be a good idea to have the "Authority having jurisdiction" make a written decision. Safety first.
    EBEBRATT-Ed Member Posts: 10,371

    I havent got much to add to the above replies-they pretty much got it nailed.

    The only thing I would add is go to the higest point in the system and find a vent or a drain valve (maybe on some radiation) and install apressure gage. Shut the pumps down and see what the pressure is. 2-3 psi is sufficient.

    Although with a one story building this is unnecessary-12 psi is plenty.

    Also, running with the starting pressure too high reduces the size of the expansion tank (you would need a bigger tank)--in other words the higher the initial pressure-the less differential you have between the initial pressure and the relief valve setting requires a larger tank. Maybe you SHOULD measure the pressure and lower it as much as possible If you suspect the expansion tank is too small.

    I know what I mean but I can't even explain it to myself!!!!!!!!!

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