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pressurised systems and non pressurised systems, whats the difference?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by ihookem, Feb 19, 2009.

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  1. ihookem

    ihookem Minister of Fire

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    I was looking into an OWB but an leaning towards a woodgun because it's a gassifier and is ok to put outside and has a big firebox with a 26" log capacity. I was told it has to be pressurised but I don't know much at all between the two. Can anyone give me advise on the two systems?

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  2. Nofossil

    Nofossil Moderator Emeritus

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    Pressurized means that you don't need a heat exchanger between the boiler and the rest of your system. Since there's no ongoing supply of dissolved oxygen in the water, components (including the boiler) will last much longer, and water treatment is minimal to nonexistent. As far as I know, all indoor systems are pressurized. Any indoor boiler can be installed in a suitable outbuilding - many have done it. I assume that you've read up on the advantages of gasification. If you aren't planning on heat storage, you might want to at least design the system with the option of adding it later.
  3. Northwoodsman

    Northwoodsman New Member

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    A pressurized/closed boiler system operates at approx. 15 psi vs. a non-pressurized/open system which operates without pressure. Because the water in a pressurized system is not exposed to the atmosphere, it does not require the water treatment that a typical non-pressurized, open system does. Also, a pressurized boiler, because it is not exposed to polutents and oxygen, will typically last much longer than an open system.

    The next question you have to ask yourself is do you want to go with a stainless steel boiler construction or a regular steel boiler construction.

    There are several threds on the forum that discuss the pros and cons of each but I think you'll find the majority would choose regular steel over stainless.

    Stainless steel has a very bad habbit of cracking at the welds due to its extremely high expansion rate at elevated temperatures (thus building up internal stresses in the metal itself) and thus many people have experienced cracking at the welds of stainless boilers and thus subsiquent leaking.

    Finally, when going with a gasifiaction boiler it is extremely critical to have adequate thermal storage. This storage allows the boiler to operate at its peak (90%+) efficiency and also reduces the amount of times a wood boiler must be filled with wood. I have an EKO40 gasification system with 1,000 gallons of storage and have been extremely impressed with the systems' operation and minimal wood consumption. Even on cold days (0-10 F) I am able to go approx. 18-20 hours before having to start a new fire and charge the thermal storage tanks back up. Two weekends ago when our temps in northern Michigan were up to 55-60F I was able to go 30-35 hours before making a fire. Thus, the thermal storage is extremely important in the spring and fall and allows the boiler to run at optimum efficiency where as a boiler without the storage will cycle rapidly and thus loose a large portion of its efficiency.

    I am heating a 2,500 sq. ft home at 73 F, 76 gallons of hot water for showers and a 350 gallon hot tub.

    (BLATANT ADVERTISING PLUG REMOVED - MODERATOR)

    Thanks,

    NWM
  4. Piker

    Piker Minister of Fire

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    Dittos to what has been previously posted about closed systems.

    There are some boilers on the market that do not require thermal storage... although it is very desireable for the shoulder seasons as stated above. The flexibilty of not absolutely having to have storage allows you to install systems in stages. Plenty of people put their boiler in and run it for a year or two without storage, and then add it later. I believe Nofossil has done this, as am I this season. If you plumb the provision for storage in when you first install the boiler, it's pretty simple to just hook it up and open a valve when you get around to getting the tanks in.

    cheers
  5. ihookem

    ihookem Minister of Fire

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    So a closed system is better but must be more expensive right? I got a quote for a Woodgun 100 w/ stainless and it's 10k and the dealer said it would be another 8k for the install so i'm looking at 18k if he does everything. I can make the shed,dig the trench, pull the pex between the joists but it will still might be at 14k. At this point I won't get my money back for 15 years. I can even take the 14k and get 350 dollars in interst and that pays Januarys heat bill to boot. There has to be a cheaper way and still get hydronic heat. A forced air is cheaper and works but how much cheaper? The only thing I know about hydronic heat is that you use pipes to carry btu's and it is a very warm 72 degrees and forced air natural gas is a very cold 72 degrees. Does this quote seem high? This guy knows everything about every furnace I asked about (he said he will install any furnace or OWB. don't matter to him but he likes the Woodgun the best but likes EKO and Orlan Ekos almost as much.
  6. Piker

    Piker Minister of Fire

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    Installation prices between closed and open systems aren't really that different. If you already have a boiler system installed in the house, the closed loop ad-ons are usually cheaper because no heat exchange is necessary, and the rest of the system is already there.

    You could probably save some money if you bought and installed an indoor gasifier, providing you have space and a chimney for it... No ditch, no underground pex, no extra building. That having been said, you will likely still not get a turnkey gasser system installed for much less than 10k no matter what. The cost of the equipment to install the boilers is expensive.

    If you are only using $1000 per year worth of fuel for heating right now, your payback will definitely be long term. Don't forget to factor in time and/or money spent acquiring firewood. I believe it would still be a practical investment for you over the long haul.

    Don't forget that the added comfort of a radiant system is worth 'something.' On top of that, people who buy gassers don't usually tend to keep their homes cool when they can keep them 75 for minimal cost. Our home is heated with both forced air and radiant floor heat. If we put the temp above 72 in the radiant zones, it's almost uncomfortably warm... while the forced air portion of the home is always set at 74 when we are home. It's cozy all the time... and if you're in to cozy, then cozy has value.

    Again, take your time and don't get frustrated at preliminary numbers. Do the research and find an installation that gives a payback that you are comfortable with. There really is something out there for everyone.

    cheers
  7. fabguy01

    fabguy01 New Member

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    ????? everyone seems to think that a pressurized system is not open to oxogen getting into the water. How can that be? H2O is water part oxogen part hydrogen if there were no oxogen in your system you would be heating with hydrogen :bug:
  8. Piker

    Piker Minister of Fire

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    Closed systems will still corrode, but at a much slower rate than open systems. I think the difference is the amount of O2 that is suspended in H20. I think... not a chemist here.

    I have been considering the idea of putting some anti corrosive agent in all the systems I install just as a precaution... most of the HVAC guys around here don't do that. Not sure if the precaution is warranted or not.

    cheers
  9. stee6043

    stee6043 Minister of Fire

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    Ummmm....there is no "loose" oxygen in a water molecule. It's bonded with hydrogen to form water. It's not just sitting there chillin' with hydrogen waiting to split.

    Your theory above would be similar to assuming table salt is likely "HIGHLY FLAMMABLE" because it has sodium in it. Sodium alone is a highly reactive metal when exposed to water. But I think you'll be hard pressed to find someone who has experienced spontaneoulsy combusting table salt when they threw a dash in a pot of water....
  10. Der Fuirmeister

    Der Fuirmeister Member

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    Have you found one near by to look at? I asked Pat at the AHS factory where I could go look at one in WI and got no response. Same question for Jim at Garn.....no response. Both received my inquiry ten days ago (confirmed email reply), but must be too busy to respond to a potential customer.
  11. nic89

    nic89 New Member

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    I thought you would still have heat exchangers in your pressurized tank that go to the house? Keeping the boiler water separated from house water.
    That way your oxygen in your water in the boiler would be driven off in time due to the heat..
  12. Nofossil

    Nofossil Moderator Emeritus

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    Not typically. In a pressurized system, the water in storage is the same water that's in the boiler. Domestic hot water is in a smaller and separate tank.
  13. racinrick

    racinrick New Member

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    what type of boiler life difference are you speaking of between pressurized systems and non-pressurized? I've got two rough numbers from two different contractors for a system for my house, one with pressurized tanks and one for non-pressurized and the price difference is substantial (6-7k).
  14. tom in maine

    tom in maine Minister of Fire

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    Boilers are not going to see the difference, since they are both pressurized and de-aerated. Both storage systems will not affect the life of the boiler.
    An unpressurized tank offers you the convenience of getting the storage into a basement that is already built that does not have any other easy access for getting a propane tank or other large vessel into that location. Of course, a site built tank can be welded up in a basement, but if it is a pressure vessel, it's life expectancy might not be the same as a professionally built pressure tank.
    I will leave it to others to comment on the longevity of used propane tanks. I would assume they should last.

    Our (unpressurized) tanks have been around in one form or another for about 25 years. They are rebuildable, if that was necessary and are easily moveable--once emptied.
  15. Singed Eyebrows

    Singed Eyebrows New Member

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    Hi Ihookem; I wouldn't worry about the stainless steel welds cracking in a Woodgun as you have been warned about. They have been welding 304 for about 25 years. Yes others may have troubles with this. Have you seen the number of stays in a SS Woodgun, it's four times as many as my old Energymate & this was ASME. I do not hear of welds cracking in the SS Woodguns, at least not recently.You are in Wis. as I, IPS tanks will weld up a pressure tank very reasonably if you go this route. My friend in Milwaukee just bought a SS E140 Woodgun with 4 of the better options & although it isn't hooked up I'm sure you could look at it. I also know of a E140 SS Woodgun that has been running in Wisconsin for about 3 years & the owner is very pleased with it. I saw him load green wood in it & all there was just some steam out the chimney on startup. I am quite sure you could look at this. / Also, Der Fiur Meister, if you PM me I can give you the phone numbers of the 2 Woodgun owners after ok'ing it with them. I guess I got to sell Patricks boilers for him, Randy
  16. Duetech

    Duetech Minister of Fire

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    There is no loose oxygen in the molecule H2O but in the water mass extra oxygen molecules can be present just like dissolved minerals and various other chemicals like pollutants. Fish and aquatic creatures that use oxygen would otherwise render H2O into<maybe, either CHO or just CH and lakes would get explosive. Many ground water sources readily have extra oxygen (sub surface drainage) and rivers and streams, via turbulence, mix extra air into the water stream. Heat will often cause that extra air to gather to the side of a pan before the water boils. Oxygen robbing water treatment chemicals have a very usable place in a boiler system.
  17. Gooserider

    Gooserider Mod Emeritus

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    Let me attempt to clarify this a oxygen question a little bit...

    The way we usually encounter water, it is exposed to air, and has a certain amount of air dissolved into it, including the approximately 21% O2 that is part of normal air. Tap water, well water, and pretty much any other water you would use to fill a boiler will have this dissolved air in it to a greater or lesser extent.

    In a pressurised system, the water in it is sealed away from contact with the air, and two things happen - the first is that the combination of heating and cooling cycles, plus various mechanical devices in the boiler circuit remove the dissolved air from the water. The second is that any remaining free oxygen will start to attack the iron in the boiler and plumbing and start to corrode it - however since there is very little oxygen, the amount of corrosion will be minimal, just a thin film. That corrosion (aka as "rust") also chemically removes the free oxygen from the boiler water. What remains is all bound in the water molecules as H2O, and so long as the Ph of the boiler water is in the right range, it's pretty much chemically inert, and won't cause any further problems. Adding an oxygen scavenger to the boiler water is helpful in this process, but not absolutely essential - the more important function of a boiler additive would be to adjust the Ph of the water in order to keep it in the right range.

    In an open system, the water is exposed to the air, and as such will continue to absorb air into solution, thus constantly replenishing the supply of free oxygen. This free oxygen can and will attack any iron or steel it finds in the system as it circulates, causing additional rust - this rust can lead to either blockages in passages, or eventual failure of system components - Therefore in a non-pressurised system should be made with as few ferrous metal components as possible, and the use of an oxygen scavenging water treatment is much more important.

    The same problem with oxygen happens with any top-up water added to a pressurized system, which is why the standard advice is to avoid draining and refilling a system as much as you can, and also to minimize the water additions... Keeping free O2 out of the boiler water is also why one should only use O2-barrier type PEX in a boiler system.

    Hope this helps understanding...

    Gooserider
  18. racinrick

    racinrick New Member

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    VERY informative. For once, something actually made sense to me. What is the typical life expectancy of a boiler with each system?
  19. Gooserider

    Gooserider Mod Emeritus

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    Varies a great deal - depends on the boiler, details of the system, the care given, and so on, however I've seen numbers thrown around that suggest on the order of 20-30 years for a pressurised system, considerably less for a non-pressurised. This can be somewhat compensated for by design if a unit was intended to be run non-pressurised - for instance Garns are claimed to have very long lives if properly installed and maintained. OTOH, some fossil fuel units can supposedly be destroyed by excess oxygen in a matter of months...

    I would tend to say that the basic guidelines should be never to run a system other than the way it was designed to be run - pressurising a non-pressure designed boiler would be dangerous at best, running a pressure designed boiler non-pressurised is likely to greatly shorten its life. Secondly, if running non-pressured, you need to be fanatical about maintaining water quality. Last, whatever sort of system it is, take all possible steps to limit O2 infiltration - use barrier PEX on all systems, don't tolerate seepage that leads to adding more makeup water (you should very seldom, if ever, need to add make-up water to a pressurised system once it is fully up and running, if you do, find out why and fix it) In a non-pressurised system minimize the area of any vents while still allowing the unit to breathe, consider adding some type of 'sealant' to the exposed water surface, such as parafin or non-flammable oil of some sort (if safe to do so)

    If I was designing a system, I would try to go pressurised as much as possible, though I would consider strongly using non-pressurised storage with either a heat exchanger or pressurised coils in the tank - somewhat easier to build, and getting the storage out of the pressure loop will greatly reduce the size / cost of the required expansion tank...

    Gooserider
  20. btwncentres

    btwncentres New Member

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    Hi trying to find time to go through past forums...any one and only book out there that would be helpful?....also it seems that with open storage HX's must be used...is it totally wrong to have say a 2000 gallon open storage and pull water directly from it to your building/bulidings ? reasons why? think I can think of some but there certainly seems to be some tricky stuff involved....
    thanks......
  21. Gooserider

    Gooserider Mod Emeritus

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    You are correct - any time you go from pressure to non-pressure there must be some sort of heat exchange interface - this can be a flat plate exchanger, a "sidearm" unit, or a set of coils in the storage tank, and possibly other options...

    Books - generally the one book that gets pointed at is Modern Hydronic Heating, 2nd Edition by John Siegenthaler, P.E. - It is regarded as the "Bible" on the subject of hydronics, and while probably less ambiguous than the Bible, it is similar in size, complexity, and being a tough read... Also it's a relatively expensive book. There are other books that also get pointed at, but with far less concensus, most noticably the titles by Dan Hollohan of HeatingHelp.com fame...

    As to your question about pulling water directly from a storage tank for heating, in theory it would work, but it's probably a less than optimal design....

    1. You will be using oxygenated water - this means that you need to either avoid ferrous components like cast iron circs and other fittings in that circuit (and the non-cast iron versions of these tend to be MUCH more expensive) or plan to spend more on chemistry to keep the corrosion under control.

    2. It means you would need to heat up all the storage before getting useful heat to the load - can be a problem if things have cooled down for any reason. Putting the storage as just one of the loops in a pressured system can let you give priority to the house first, then worry about heating the storage after the other loads are satisfied.

    3. Because of the lack of oxygen, pressured systems are very sterile from a biological standpoint - little or nothing will grow in the system water. In a non-pressured system, even with the high temperatures, you can sometimes get some really strange "wildlife" growing in your system, with the potential for getting thick enough to clog up the plumbing = expensive and difficult repairs... Because of the many metals in the system and other issues, it can be a challenge to stop such growth by use of chemistry.... If you just have a tank w/ coils in it, any growth is mostly a non-problem as long as it doesn't stink... If you have a plate exchanger based setup you would have more of a concern, but still it's a case of the less plumbing exposed, the fewer potential problems...

    4. The more you circulate the water, the greater challenge it would be to maintain good stratification - it can be done, it just increases the complication of the tank plumbing design...

    Bottom line - If I had to do a "blank sheet" setup where I needed to do an open storage tank, I would choose a setup that put coils in a tank of non-circulated water, but otherwise kept the number of distinct heating loops to a minimum*. (Others might choose other options) I feel that this is something that gives maximum flexibility in storing and extracting heat, maximizes stratification, and is the least likely to cause problems.

    *meaning that I would keep all the plumbing I could in one pressurized zone - I would probably end up with three independent sets of fluid -
    1. The pressurized water circuit (boiler, house heat loads, DHW heating, storage heat exchange, etc. which could be configured in multiple zones and other interconnected loops),
    2. The glycol circuit (Solar heating panels, garage heating, and any other freeze prone items that would be better with the use of a glycol antifreeze solution instead of plain water)
    3. The storage tank - non pressured, non circulating

    Gooserider
  22. 91220da

    91220da Member

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    Hi Guys,
    Any reccomendations on where, or what type, of oxygen scrubbing additive to buy. I am running a non pressurized open system (loop) to my pressurized oil furnace in the basement through a flat plate hx. I went with the open system because my wood boiler is in the garage with no make up or recovery water availiable. I have a 55 gallon expansion/holding tank as a resevoiur and the loop is filled with about 30 gallons of water glycol mix. The glycol supposedly contains a corrosion inhibitor but your discussion got me interested in the oxygen scrubbing addative ?????
  23. Gooserider

    Gooserider Mod Emeritus

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    Nothing specific, might be worth talking to some of the Garn guys or the OWB folks since they run open by design... Some stuff I've seen suggested that it may depend on your individual water chemistry - there are supposedly labs out there that will (for a price) analyze a sample of your boiler water and recommend an optimal chemical mix, but I don't have any details on who / where they are...

    Gooserider
  24. fabguy01

    fabguy01 New Member

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    http://www.woodboilersolutions.com/
  25. pybyr

    pybyr Minister of Fire

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    My boiler is not open (though my storage will be), and, on the advice of a number of pros, I went with "8 way boiler chemical" in my system

    www.rectorseal.com/files/172/ds8way.pdf

    I got it at the local F.W. Webb, and it was very modestly priced. A couple of quarts treated my boiler and all associated plumbing.

    I would tend to think it would work OK in your application, too, as long as you occasionally draw samples or look at it through a sight tube to see that the indicator color still looks good. It is recommended for steam as well as hydronic systems, and steam systems are, for all practical purposes, "open."

    I found in talking to some local installers (regular heating guys, not wood-boiler-specific) that it seems common that they just use regular domestic water, with no treatment. For my money, after a lot of $$$ and sweat putting in my Econoburn, I didn't want to skimp on $13 +/- of chemicals to extend the useful life of my investment.
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