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Dual Stage Pressurized storage system

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by afblue, Feb 1, 2011.

  1. afblue

    afblue Feeling the Heat

    Joined:
    Sep 1, 2009
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    278
    Loc:
    Buffalo, NY
    So I have poked around on the forum here now getting ready to have a more comfortable house by moving to a gasification boiler vs a wood burning insert. My dillema is I have somewhat limited space in my basement and my access is rather tight, so I have been weighting my options as far as what I am can get/need for thermal storage.

    The specs on the house are 1922 bungalow, 1750sq ft, with blow insulation walls, and ALOT of windows 39 to be exact!!! The house air change is rather tight, based on a recent energy audit, blower door test, and the FLIR camera shows the house for the most part well insulated. I just have alot of windows, so my design BTU is pretty high. I calculate it at 45K/hr max design load. SO I am looking at a 130-140K Gasifier, with 250-800gal of storage. I will have a combination of radiant loops, Forced air exchanger and some type of DHW. DHW need a back up, and the house back up will be the original Nat gas forced air furnace.

    So my issues for storage are I have a 32" wide door with a pretty big jog to get down in the basement, so for storage my options are
    1) unpressurized storage in basement
    2)multiples of 250gal pressurized tanks in basement
    3)outdoor storage pressurized super insulated R45+
    4) combination of indoor and outdoor pressurized storage

    I would like to at some point in the future add a homemade solar loop, so I need to keep options open for that.

    In my calculations I am looking to have roughly 750gal pressurized storage or 800 unpressurized, so I can achieve 450K BTU if I run the temp of the tank all the way down to the lowest primary heat being the radiant loops. 800 unpressurized or 3 250gal propane tanks in my basement seems to take up alot of space I dont want to give up.

    So the option I am trying to entertain is a 500gal propane tank under my tall outdoor deck. I have roughly 45" clearance and plenty of length, and could dig down a little if needed to get it in place. I have a huge supply of polyiso 3" panels that I can insulate with, and will blow the odd shapes with 2 part poly foam when I get my attic walls redone. The tank would be separated roughly 20-30 feet from the boiler. Would it be feasible to put 500 outside, then have a primary 250gal inside that will heat first, and maintain usable heat for the DHW and forced air exchanger while the outside tank heated up slower. Is my diagram logical how I have all the parts plumbed in?

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  2. Singed Eyebrows

    Singed Eyebrows New Member

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    My vote would be for the 3 250 gallon pressure tanks, Randy
  3. mole

    mole Member

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    Loc:
    Western NY
    I went through this decision making process a couple of years back. Right of wrong, here's my thoughts:

    The three tank idea sounds good on paper, but you'll likely end up with a lot more heat loss than you will with one big tank due to the extra surface area and lines. Heat lost to the basement isn't completely wasted like heat lost from the outside tank. I would make the outside tank a last resort. An unpressurized tank may require about 30% or so more storage volume (for forced hot air) but the footprint for this tank might not be any bigger than the three individual tanks, depending on how you can use the height. The unpressurized tank requires expensive internal heat exchange coils, but the advantage of this type of tank is that you don't need to spend money on a large expansion tank. You also don't need a control loop and pump for your DHW, since it passes through on demand. IF you can save a space in the corner of the tank for a coil for your solar upgrade, you will already have the DHW coils in the tank, so the solar will be a very simple add-on. (you won't have to buy a solar HW tank, or mess with the DHW again, just the solar loop).

    I ended building a 1250 gal unpressurized tank a couple years ago. This past summer I installed 3/4" copper lines from my basement, clear up to the attic while I was gutting and remodeling the 2nd floor bathroom. If I ever finish the bathroom (7mos and counting now!!!!), I'm planning to build my solar panels and install...It's looking like a 2012 project at this point!

    Hope this helps.
    JR
  4. afblue

    afblue Feeling the Heat

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    Loc:
    Buffalo, NY
    The pros and cons of pressurized to unpressurized liners is really taking a tole on me!!!!

    the reality is this are the issues I am having with either side.

    Pressurized cons:
    Used propane tanks are harder to find than I thought
    The expansion tank is going to big and expensive.
    alot of plumbing to link all the tanks, or excavation outside to fit one large enough
    DHW looking to be about $1000
    The biggest I can get inside is multiples of 250 gals
    solar may be limited to using the coil in the DHW tank.

    Unpressurized cons:
    Copper coils $$$$$OMG!!!
    EPDM liner worries of heat and longevity.
    EDPM chemical reaction with copper
    How do I backup the DHW loop? tankless electric?
  5. shortline

    shortline New Member

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    Has anyone ever explored insulating & deep burying (below frost line) a poly septic tank for unpressurized storage with no copper coils in the tank but flat plate hx back at the heat source/load? If the poly tank would stand up to the hot water the biggest hurdle I can think of -- would be finding a fool-proof way to maintain proper water level in tank.
  6. shortline

    shortline New Member

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    Of course a well-drained place for burial would be must to be able to maintain the insulation values.

    Just thinking in cyberspace here!
  7. mole

    mole Member

    Joined:
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    Loc:
    Western NY

    Yeah, there's a whole lot to think about!! I ended up with "paralysis by analysis" for about 3 months.

    Regarding finding a propane tank, I looked in Niagara County at two scrap yards and 4 different propane stores. Found nothing! The guys at the propane stores looked at me like I had three heads when I told them what I wanted it for! That's the main reason I ended up going the non-pressurized route. From what I've read at this site, it looks like there may be a couple of places in Western PA that carry propane tanks if you're willing to transport it a few hours.

    Regarding the copper coil cost, yeah, that's big! If you end up going this route, I happen to have 4 rolls of 3/4" x60' type L copper for making coils. They are a screw-up from my project, which I was going to try to use in my solar collector, but I've decided recently to get rid of them instead. They go for $219/ea with tax at Lowes, but I'll sell them for $150/ea if you want them.

    The big thing about the EPDM is that you can only use it to 170-175F. So for forced air heat, you only have a usable temp range of 140-170 to store BTU's. I can only store 300,000 USABLE btu's in my 1250 gal tank. If my liner ever fails, I will probably look into buying one of those special vinyl liners that "Tom in Maine" is selling. They are good for 190F, so you essentially get 66% more storage in the same volume tank. I think this is the way to go!

    For DHW backup: My DHW branches off of the cold supply and goes into the tank. In the tank, it branches into two parallel 5/8" 100' coils of tubing (grossly oversized for boiler heating, but will have a very close approach temp when I put in my solar DHW). Inside the tank, the two coils return to a 1/2" copper pipe which comes out of the tank and goes to a Watts 70A tempering valve ($5 ebay special). This reduces the temp from 140-170F to 120-130. It then goes to my electric hot water heater. I installed a couple of ball valves at the top of the hot water heater so I can bypass the HWH. 95% of the time I valve it out and shut off at the breaker. By turning two handles and turning the breaker back on, the HWH is back on line. I turn it on sometimes in the summer if I don't want to make a fire (once every 10 days). It could also serve to "top off" the hot water temp if there is a stretch of bad weather when I get my solar DHW running. You could probably do something similar with a gas hot water heater if that's what you have right now.

    Happy pondering.
    JR
  8. Nofossil

    Nofossil Moderator Emeritus

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    Another 'pro' for pressurized (or 'con' for unpressurized) is that you get a good deal more usable stored heat for the same volume with pressurized storage. The exact ratio is hard to quantify, but here's the reason:

    With unpressurized storage, you're charging and discharging through a heat exchanger. When charging, as storage temp starts to approach the boiler outlet temp, the transfer rate through the heat exchanger drops precipitously. It's *really* hard to get the whole storage tank within 10 degrees of the boiler outlet.

    By the same token, when discharging, the BTU transfer rate out of storage drops off as the storage temperature approaches your zone return temperature.

    In both cases, in-tank coils lead to much less sharply defined stratification than you can accomplish with pressurized storage.

    With pressurized storage, there is no intervening heat exchanger. For that reason I think that pressurized storage is preferable if there's a practical way to do it.

    By the way - nice diagram!
  9. afblue

    afblue Feeling the Heat

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    Loc:
    Buffalo, NY
    I actually in the original post have should have tagged the photo saying "the diagram is modified version of an original design drawn by nofossil
    So thank you for drawing it!!!

    So what I was trying to accomplish is a bypass back to the boiler that would allow the 250 tank to charge quickly, and maintain heat for the forced air unit. My ultimate plan is to put european flat panel radiators in the 3 bedrooms upstairs, which is really the only reason I need the forced air unit, everything else in the house is radiant.

    Really what I am trying to maintain with the storage is a about 10-12 hours of heat so the house doesnt drop below 60 before my wife and I get home from work and can reload the boiler. I just know I dont have the luxury to store 1000gals of pressurized storage.

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