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Stabilizing Pressure in storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by arngnick, Feb 24, 2013.

  1. arngnick

    arngnick Member

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    I am just getting my system up and running I have about 700gal in my system I have 105gal of expansion tanks. When allowing my storage to swing from 130 to 200 degrees I am seeing a large pressure fluctuation.

    What am I missing? I thought the point of the expansion tanks are to relieve pressure changes. The tanks come precharged at 12psi and I would like to limit my system pressure at about 20 psi as my boiler is only rated at 23psi.

    What pressures should my tanks and incoming water supply be set at to provide the best stability?

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  2. Jeff S

    Jeff S Feeling the Heat

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    Expansion tanks don't eliminate pressure changes they only make room for expansion.The more you increase the temperature of your system the more it expands which in turn fills your expansion tanks which in turn will increase the pressure.If you want to limit your system to a desired maximum pressure you can either add more expansion or limit system temperature.
  3. arngnick

    arngnick Member

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    Thanks, I am running my system with the recomended 13-15% expansion capacity what is an acceptable minimum pressure for my system so I can coincide that with my minimum system temperature?
  4. Jeff S

    Jeff S Feeling the Heat

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    You all stated that you don't want to run the system over 20 psi and since your expansion tanks are pre-charged at 12 psi you are really limiting yourself.The best way for you to figure out what the max temp should be is to monitor your system until you reach your desired 20psi.
  5. arngnick

    arngnick Member

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    I want to get the maximum temperature swing with my system so I need to start out with the lowest possible pressure which I am willing to change. Then go all the way up to 20 psi. Should I consider lowering my expansion tanks to 5psi?
  6. Tennman

    Tennman Minister of Fire

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    FWIW there's a nice expansion tank calculator in the tidbits sticky that computes all the lines, tank, storage, etc. Assuming you have your total system volume correct 15% is the rule of thumb you read here. I start the boiler with about10 psi ambient and it goes to 20 psi at 185 boiler setting. I'll be adding more expansion to get that to a more comfortable 15 psi. So what was the ambient starting pressure? You may just need to vent some water.
  7. Jeff S

    Jeff S Feeling the Heat

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    Do you have a bladder type expansion tank?It may be possible that you over filled your system initially and have expanded your bladder before you even started the heating cycle.This of course would leave less room for expansion and prematurely raise system pressure.
  8. arngnick

    arngnick Member

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    After talking with Jeff I believe that I may have satuarted my system...My ambient pressure was about 12psi but the pressure was on the verge of the 23psi max so I let some water out.

    My plan going forward is to take the pressure off of the tanks and lower the starting pressure in the expansion tanks to about 5psi then refill the system at 5 psi and see how hot I can go before I max out at 20psi.
  9. arngnick

    arngnick Member

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    Yes the tanks are bladder tanks
  10. arngnick

    arngnick Member

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    Here is a pic of my tanks

    Attached Files:

  11. Bob Rohr

    Bob Rohr Minister of Fire

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    The fill pressure is determined by how high the uppermost point in your heating system is. Is 5 psi enough to fill the system to the highest point? Generally we suggest that you have a 5 psi at the highest point, so add 3-5 psi to the pressure needed to lift the water to your highest point.

    lowering the fill to 5 psi could cause some major air problems. Water at 60°F would exert 4.33 ponds pressure on a 1 square inch column of water, in other words you need 4.33 psi to lift water 10 feet, or .433 per foot of lift. Most round that to .5 or 1/2 psi to make the math easier and give you the extra positive pressure. 20 feet to the highest point X .5 would require 10 psi. So calculate from the boiler location to the uppermost point. Ideally add 3-5 psi to that number. that is why 12psi is the common fill pre-charge pressure on tanks and the preset on fill valves. it will cover most buildings up to two stories.

    What is limiting the boiler to a 23psi operating pressure?
  12. arngnick

    arngnick Member

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    The Varmebaronen UB 50 is only rated at 1.5 bar so that is the limiting factor. My highest point in the system is just under 9 feet. (highest point is the first floor radiant zone)
  13. ewdudley

    ewdudley Minister of Fire

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    So putting it all together with the information BR provided you, if high point in system is 9 ft above the bottom of the boiler, the it takes 3.9 psisg to raise the water to that point. Then as recommended you need 3-5 on top of that for the minimum cold system pressure for a bare minimum of 6.9 psisg.

    Maximum system pressure at bottom of boiler (or top of boiler? Does anyone know?) is 21.7 psisg (1.5 bar, not 23 psisg) so your maximum pressure swing is 14.8 psi at the the boiler. If your expansion tanks is at the same level as the boiler then the expansion tank would be set to 6.9 psisg minimum pressure and would rise to 21.7 psisg maximum. However if the expansion tank was in a closet on the first floor, then minimum expansion tank pressure could be 3.0 psisg with a maximum of 17.8 psisg. In this case the pressure rise is the same, but you can accept more water going from 3.0 to 17.8 psisg than you can going from 6.9 to 21.7 psisg. The highest you can place the tank is 6.9 feet above the highest point in the system, which would result in a minimum pressure of 0.0 psisg at the expansion tank and the same rise of 14.8 psi.

    It looks like your tanks are elevated a couple feet above the boiler, so you can subtract 0.4335 psi from the boiler level minimum of 6.9 psisg per foot elevation when setting minimum tank pressure, but your maximum pressure at the tank needs to be lowered by the same amount.
    Floydian likes this.
  14. maple1

    maple1 Minister of Fire

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    Is that 105 gallons the acceptance volume?

    I might consider putting more expansion in - although you should be pretty close to OK.

    I am using a 110 gallon propane tank as non-bladder expansion, for 660 gallons of storage plus whatever else is in the piping & boiler. It is sitting upright on the same floor the storage is on. I have around 8psi in the system when it cools down to the re-lighting point (maybe around 120°f), and it tops out around 15psi when everything is hot (190° at top of storage). I have two stories above my boiler, so the highest point in my system is maybe 14' above my boiler outlet. I have no problems with air or keeping the flows going.

    I'd try reducing your cool pressures to 8psi. Isolate your expansion, drain the water out of it, reduce the air side to 8psi, drain some water out of your system to reduce that to 8psi, open your system to expansion again, and heat your system again to see what happens. I've also noticed you'll see more psi rise on the higher end of the temp rise as opposed to the first part.

    Just re-read the posts again before hitting submit - sounds like you're planning on doing that anyway but to 5psi (missed that the first time). You might be good with that to.

    Keep us posted!
  15. arngnick

    arngnick Member

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    So I think I found my problem...after I isolated the tanks and drained all water away I checked the air pressure and found that the tanks were actually set to 20 psi which explains why I gained pressure so fast. Since I found this issue I decided to see if I can get by with 10 psi in my tanks we will see how it goes. I will keep everyone posted thanks for all the help
  16. arngnick

    arngnick Member

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    So lesson learned I guess...I set the tank pressure to 10psi and refilled the water and purged any air that got into the system. At 190 degrees I was only at 16psi. Looks like this is going to work for me now and I still have some room to go lower in psi since my highest point is only 9 ft. Thanks again for all the help!

    "No wonder I was having trouble...I essentially had no expansion capacity"

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