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Indirect Water Heater as heat exchanger for unpressurized storage?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by mikefrommaine, Jun 5, 2012.

  1. Figured I would tap into the brain trust.

    Has anyone used an indirect water heater as a heat exchanger for unpressurized storage? I am planning to add storage to my system this summer and for a lot of reasons unpressurized makes more sense.

    I piped my system using the Tarm diagrams (plumbing example solo - 4) and planned to use "example F" to add my storage. I have purchased most of what I need except for the tank and copper for a heat exchanger.

    http://www.woodboilers.com/admin/uploads/public/WoodBoilerPlumbingSchematic1211web.pdf

    I happen to have two Triangle Tube TR 120's sitting in the basement not being used and started thinking that I could use one for a heat exchanger in place of a copper coil. The unpressurized 500-1000 gallon tank will have to be at least 30-50 feet away from my wood boiler and primary loop. The indirect could be next to the boiler or next to the unpressurized tank -- not sure which would be most efficient.

    I know I will need to add another pump to the system to make this work, but I'm not sure how I would control it. Should I let it run 24/7? (doesn't seem efficient) I keep coming up with overly complex solutions in my mind that would equal the cost of just buying the copper. Is there some super efficient pump that could be set to maintain an equal temp between the indirect and bulk tank?

    Let me know if you think I'm barking up the wrong tree. diagrams.png
    Anyways thanks in advance.

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  2. hobbyheater

    hobbyheater Minister of Fire

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    Personel preference is that I like simple!
    Have had a copper coil in unpressurized storage for 30 years; not a penny in maintenance to date.
  3. tom in maine

    tom in maine Minister of Fire

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    Hi Mike,
    I am not up on the specs on the Triangle Tube tank hx, but the concept will work.
    You will need to prime the pump that is feeding from the tank but that is a simple process. I would just tie the two pumps together to come on when needed.
  4. Just realized I posted the wrong diagram --- This is how my system is currently set up

    Attached Files:

  5. This is what I was planning on following.

    Attached Files:


  6. I might be wrong but.... based on the above diagram --

    If there is no call for heat in the house and the wood boiler is at temp than ZV1 is closed and pump C-3 is charging storage.
    If there is a call for heat and storage is at temp than ZV1 is open and pump C-1 is drawing heat from storage.

    So there are two pumps that charge/draw from storage -- so I can't just tie the pumps together -- maybe I could use relays and if either C-1 or C-3 is operating than the "indirect" pump turns on?
  7. Eric Johnson

    Eric Johnson Mod Emeritus

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    I like simple, too. I've always thought a cast iron radiator or series of stacked ci rads would make a dandy non-pressurized heat exchanger. I don't think corrosion would be a problem in any event, but if you treated the water, it would probably be a non-issue. Certainly cheaper than most of the alternatives.
  8. Bob Rohr

    Bob Rohr Minister of Fire

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    Yes an indirect tank is just a heat exchanger. the Triangle have a nice flow rate to move a lot of BTUs. But a plate HX may be a lot less $$. Use the tanks for storage or solar input.

    hr
  9. jebatty

    jebatty Minister of Fire

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    I agree on the concept but I question whether that it will perform to the level of a plate HX or a coil HX designed for the purpose. Indirect hw heaters often are spec'd at a Side B delta-T of about 70F, to raise well/supply water at 50F to 120F with boiler Side A water perhaps as high as 180F; also at a particular gpm flow rates Sides A and B. If your application varies from these specs, performance may be substantially less than what you want.
  10. Bob Rohr

    Bob Rohr Minister of Fire

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    Good question. I have never seen that tank performance listed at different ∆ T.

    With a plate HX their simulation programs allow you to change temperature flow rate and approch temperature. This allows you to accuratly size a plate HX to any application.

    Generally it is a square foot calculation. If the tank in tank indirect has the same square footage of surface as the plate style performance should be close.

    Plate HX are fairly well designed with corrugated surfaces, and ideal exchange design. Considering the size, performance and wide range of available configurations, hard to beat a plate HX.

    hr
  11. I still cant get this idea out of my since I already have the spare tr 120. It has 42 square feet of heating surface.

    Attached Files:

  12. Assuming the tr 120 would work as a heat exchanger -- I am envisioning a hybrid pressurized/unpressurized storage scheme based on the this tarm diagram.
    --I would use the inner tank of the tr120 as pressurized storage.

    -- I would then circulate water between the outer tank and two unpressurized propane tanks.

    -- To control the circ I would use a flow switch
    -- whenever water is flowing into the pressurized tr 120 the circ would be on. This would equalize the temps in all the tanks.

    The advantages I see are lower costs - no welding, no need for an expansion tank and I wouldnt have untagged pressure vessels as part of my boiler system.
    The cons I see -- unproven concept, is the heat exchanger big enough? Can I get the unpressurized storage hot enough? My house doesnt start to warm up until the boiler is above 165.

    So good concept or hairbrained?

    Attached Files:

  13. Bob Rohr

    Bob Rohr Minister of Fire

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  14. Bob Rohr

    Bob Rohr Minister of Fire

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    Check with TT, you may need to have pressure on both sides of the vessel? It's just a heat exchanger and should work functionally for your application, maybe the tech support folks at TT would be the ones to ask about your exact application. Sometimes warranty is void if it is not installed or pressurized to their direction. Never hurts to go to the source to ask.

    hr
  15. I know the owners manual states that the inner tank must be filled and pressurized before filling the outer tank. I assume that is to keep from collapsing the inner tank. Since the inner tank can support itself when full I'm guessing that if the outer tank has water in it all the time things should be ok...

    I'm not too worried about the warranty since I bought these used and dont have a lot of money into them. I don't think I am covered under the lifetime warranty as the second owner. But I will check with TT to see what they say.

    What do you using an open expansion tank above the propane tanks -- that way the 'unpressurized' outer tank would be under 2-3 psi?
  16. henfruit

    henfruit Minister of Fire

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    Just go with a closed sysytem and be done with it.
  17. Bob Rohr

    Bob Rohr Minister of Fire

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    If you go this route I'd use the inner stainless tank on the "open" side of the system. Open to atmosphere systems with ferrous components are not a great idea, rust never sleeps!
  18. I know that it would work to do that. And probably be easier; however, I am a foster parent and have annual inspections performed by the state fire marshal. So I am concerned about using untagged pressure vessels. Not sure what the code actually states but I know if the fire marshal found two pressurized propane tanks connected to my boiler he would either have a heart attack OR evacuate the house and call in the bomb squad. At the very least he would red tag it all.

    I have installed everything to code and have all the necessary permits and inspections. And so far everyone is fine with the installation. Dont want to rock the boat too much.
  19. I think I will need to use the inner tank for the closed/pressurized side based on the warning in the manual.

    Rust will be an issue, but I think with water treatment and an elevated open expansion tank it can be controlled? I am leaning towards trying this. Figure worse case I can just buy a flat plate exchanger if I need to.

    Attached Files:

  20. SmokeEater

    SmokeEater Feeling the Heat

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    Agreed on the "rust never sleeps". I bought a new TT 100 and use it as a small buffer for my pellet boiler. I also plumbed in a 1 gal/h oil boiler to the TT and heat the outer jacket with either or both. The TT has 2 outlets on top, so I plumbed a recycling loop pumped with a stainless pump through a flat plate HX and use the inner tank a the primary heat source. I also teed into that loop to get my DHW and put a thermostatic valve on that outlet. Trouble that I had was that - dummy me - I piped that supply loop to the HX with black iron and so my DHW is rusty because of the oxygen in the inner tank. But I can see that if infinitymike uses the outer jacket in an unpressurized storage with the propane tanks, rust will be in the picture in a big way because the outer tank material in the TTs is carbon steel. Go pressurized!
  21. ewdudley

    ewdudley Minister of Fire

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    I've always liked the Heatmor idea of using large inner-tubes as elevated gas-tight atmospheric pressure expansion devices.



    If you could get your atmospheric pressure expansion device six feet or so above the highest point in the system -- whether it was an open atmospheric pressure expansion cistern or a sealed atmospheric pressure bladder -- then you could do away all the heat exchangers, pumping, controls, and complexity. All you need is an unobstructed fat pipe from the boiler up to the expansion device and a failsafe method of venting steam up top in the event things get sideways.
  22. jebatty

    jebatty Minister of Fire

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    Interesting. I had a Heatmoor in use for 9 years, prior owner of property in use for several years before that. Boiler was at a slightly lower elevation than one story house, but no problem with anything. The Heatmoor info shows that the bladder maintains pressure of 2 psi. I regularly used sodium sulfite as the only boiler treatment. The boiler water was essentially clear, no evidence of rust from boiler corrosion, all plumbing was plastic or copper. I had to replace the bladder once. The Heatmoor has a weighted vent/valve that blows at excessive pressure (greater than 2 psi about).

    Might a partially inflated tube (12 psi) in a truck or tractor wheel/tire serve as an expansion vessel? Haven't done the research, but it should then be possible to calculate expansion capacity and acceptance volume.

    I would guess that an inner tube is not an oxygen barrier, so there may be a corrosion issue. As mentioned, simple use of an oxygen scavenger (sodium suflite) plus checking for adequate pH (8-11 is normally mentioned, check your boiler info for mfr's recommendation) might take care of that, as my Heatmoor had no visible evidence of corrosion after 10+ years of service. BTW, I also sold the Heatmoor for a generous sum after I retired it.

    All of that said, the Tarm with storage performed much better, was easier to use, burned about 1/3 the wood that the OWB burned, and was smoke free, while burning at most only once per day (6 hours or so) vs two or more large loads of wood per day for the OWB. With the OWB I even had people stop by thinking the place was on fire due to the huge amount of smoke the OWB spewed out.
  23. How big a deal will the corrosion be?

    If I left the tanks open, with enough room for expansion, plumbed in anode rods, used a water treatment and kept an eye on ph etc... Will there be problems?


    I can also add an anode rod to the outer tank of the indirect.
  24. JP11

    JP11 Minister of Fire

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    Before I messed with all of this double tank idea.. maybe just a conversation with the fire guy who does the inspection?

    I mean you're only talking boiler pressure.. He could tell you to install a couple of overpressure drains and be happy with that. Wouldn't hurt to ask. Collect some pictures from this site as an example of what you want to do.

    You might find that they are happy with your homework, and approve it in theory before you do the install.

    JP
  25. If I do go pressurized I probably could get the local inspectors to sign off. But the state fire marshal office is very rigid when it comes to the codes. Last inspection I got written up for having a grill on my deck -- something that every house in my neighborhood does. Every year they find something not to code and the house is only five years old!

    When I needed to build a wheelchair ramp for my son I wanted to build a simple ramp. It would have been fine according to the town inspector. I made the mistake of asking what the FM' s office wanted. And they required me to meet ADA standards -- same as a commercial building. It ended up being cheaper to buy a porch lift (mini elevator)

    I've gone round and round about this in my head. I know it would probably be better to go pressurized, but if I can get close to the same performance without it all rusting away in two years then I don't mind a little extra piping and a second pump. Not having to buy a big expansion tank more than offsets those costs.

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