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Getting my tank to temp problem

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by EricV, Nov 2, 2007.

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  1. EricV

    EricV Feeling the Heat

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    Hi Guys

    I'm new here but real glad to have found this site with a bunch of folks that think alike.

    I have Tarm Solo 40 with a homemade 1000 gallon tank (might be a bit less with the insulation etc.)

    The boiler works great, I'm very pleased with it. I can quickly and easily get it up to temp (180 plus)

    In my tank I built a heat exchanger from 200' of 3/4 copper coil. This is fed with 1" pex from the boiler.

    I can fire the boiler and only get my tank to about 150-152 degrees.

    The boiler water is plenty hot and I have a Taco pump that is rated at about 16gpm.

    Any idea where my weak link is?

    Thanks in advance.
    Eric

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    How about some more details, Eric?

    How is the hx positioned in the tank?
    Where does the water from the boiler enter the hx and where does it exit?
    What's your method for getting the heat back out of the tank?
    How are you measuring the tank temp?
    Does the tank temp hit 150 and then the boiler gets up past 180 and go into idle mode?
  3. Corey

    Corey Minister of Fire

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    It sounds like you have plenty of copper and decent flow. As the difference in temperature (dT) between your boiler and your tank gets smaller - ie boiler is 180F and tank is 152F = 28F dT, the heat transferred between the two is less. This means two things:

    1) The warm-up will follow a logarithmic curve. For instance if it takes 30 minutes to go from 70F to 110F, it may take 30 more minutes to go to 130F, 30 more to get 140F, etc. This brings the question - how long have you fired to get the tank to 150F? - you may need to just wait longer.

    2) As the tank heats up and you get the lower heat transfer, you also start loosing more heath through the insulation. You may simply be hitting a point where the heat transfer between the 180F boiler water and the 150F tank is equal to the heat lost through the insulation.

    I'm not so much into the boilers, but the hot tub industry claims that a lot of energy is lost as evaporation at the water surface. If you homemade tank as a large evaporative surface, that would be one key place to look into stopping the evaporation and add insulation.
  4. EricV

    EricV Feeling the Heat

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    How about some more details, Eric?

    How is the hx positioned in the tank?

    It's two coils. The 1" feed comes into the bottom of the tank and splits into 2 3/4" coils. The water flows from bottom to top.


    Where does the water from the boiler enter the hx and where does it exit?

    The water flows from bottom to top.


    What’s your method for getting the heat back out of the tank?

    Same coils

    How are you measuring the tank temp?

    Digital thermometer at top of tank


    Does the tank temp hit 150 and then the boiler gets up past 180 and go into idle mode?
    Yes.


    The piping comes from the tarm, to the oil boiler so the house and DHW get first priority, then to the bottom of the tank, then back to the tarm.

    Thanks
    Eric
  5. EricV

    EricV Feeling the Heat

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    The tank has 2" Celotex on all sides, top and bottom. Plus 3/4 ply on sides, but that isn't really any insulating value.

    Eric
  6. Eric Johnson

    Eric Johnson Mod Emeritus

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    Since you're using the same coil for heat storage and recovery, you must have a way to reverse the flow, no?

    My only thought initially is that by the time the water gets to the heat exchanger, the temp has dropped enough so that your delta T is lower and it's harder to charge up the tank. I'm sure you've considered that, however.

    What is the temp of the water entering the heat exchanger, on average, when you're trying to charge up the tank?

    Is there a way to bypass the water heater and oil boiler and just deliver water from the Tarm to the tank to see how that works?

    Have you tried (or should I say, can you) crank the boiler output temp up 10 or 15 degrees to see if that makes a difference?

    Any evidence that your insulation isn't working? Is your basement getting hot or is it noticeably warmer in the vicinity of the tank?

    And I assume the tank is sealed somehow so that you're not getting evaporation out of it.

    I'm not sure about a Tarm, but I would think that if you boiler gets up to temp and goes into idle mode and stays there while the other components on your system are calling for heat (tank, oil boiler and hw heater), then obviously it's producing more heat than your various heat exchangers and/or piping can handle. If, on the other hand, the Tarm chugs right along and you can't get the other components up to temp, then that's a different problem.
  7. EricV

    EricV Feeling the Heat

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    Since you’re using the same coil for heat storage and recovery, you must have a way to reverse the flow, no?

    No reverse. I put a line between the feed and return so when the wood boiler is off a second pump will pump water from the tank to the oil boiler and return, in the same flow pattern as when I'm firing the wood.



    My only thought initially is that by the time the water gets to the heat exchanger, the temp has dropped enough so that your delta T is lower and it’s harder to charge up the tank. I’m sure you’ve considered that, however.

    I thought about that and i made sure there was no call ofr heat or DHW while I was trying to charge the tank. I'll put a thermometer probe on the feed and retrun at the tank this weekend and see how hot the water is entering.



    What is the temp of the water entering the heat exchanger, on average, when you’re trying to charge up the tank?

    I'm pusing 180 to 190 out of the boiler.

    Is there a way to bypass the water heater and oil boiler and just deliver water from the Tarm to the tank to see how that works?

    no

    Have you tried (or should I say, can you) crank the boiler output temp up 10 or 15 degrees to see if that makes a difference?
    Yes, cranked to over 2300 and it goes into overheat mode and shuts down, but not much change in the tank either.


    Any evidence that your insulation isn’t working? Is your basement getting hot or is it noticeably warmer in the vicinity of the tank?

    And I assume the tank is sealed somehow so that you’re not getting evaporation out of it.

    No, insulation is looking good. no excess heat in basement, some but the pipes aren't insulated yet.


    I’m not sure about a Tarm, but I would think that if you boiler gets up to temp and goes into idle mode and stays there while the other components on your system are calling for heat (tank, oil boiler and hw heater), then obviously it’s producing more heat than your various heat exchangers and/or piping can handle. If, on the other hand, the Tarm chugs right along and you can’t get the other components up to temp, then that’s a different problem.


    it will go into idle mode for a while. When the fire box cools there is a sensor that shuts down the system so the fan will not come back on and I have to reset that manually. so I'm sure there is plenty of heat being generated.

    Thanks
    Eric
  8. leaddog

    leaddog Minister of Fire

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    Like cozyheat said it will take along time for you to get it from 160 to 180. if you are measuring the temp at the top the bottem could be alot less. When I have my tank charged to 180 at nite when I go out in the morning it still reads 170 but that is the top reading. It will take almost a full load of wood to get it fully charged to 180 as it has to heat ALL the water. The tank gage won't move until all the water is at 170. My eko80 would cycle on that last 10degrees until I put a brick over one of the nozzelsbut now the heat exchangers can keep up. In the cold of winter I don't think that will be a problem as I will be drawing heat so the hx should be able to keep up.
    leadog
  9. Nofossil

    Nofossil Moderator Emeritus

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    Here's a graph showing my EKO 25 (80,000 BTU) heating an 880 gallon tank last night. This shows what you might expect - you'd have to scale this for the difference in boiler output and tank size, but you can see temperature differences and tank heating rates. If your numbers are a lot different, perhaps you have a flow restriction somewhere.

    The tank has three sensors - from top to bottom, the green, orange and brown lines. The black and magenta lines are the EKO outlet and inlet. The two horizonatl lines near 60 degrees are digital signals - the top one is baseboard demand, and the bottom one is the zone valve that allows heat to go to the storage tank.

    The circulator is a Taco 007, and the heat exchanger is a 50' coil of 3/4" copper with a row of parallel 1/2" pipes near the tank top and bottom for some additional surface area. Flow during heating is top to bottom. The tank is 48" high and insulated with a minimum of 8" on the sides and something like 16" on top, mostly foil faced foam board.

    Hope this helps.

    Attached Files:

  10. Eric Johnson

    Eric Johnson Mod Emeritus

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    That's really helpful, nofossil. It gives me something to compare. It's fascinating to see the relationship between the various measurements, especially how they all drop off and converge around midnight, but the top of the tank just soars along on its own. I guess that's what it's all about. Cool. You could operate one of these things for years and never be able to get the insight that this graph provides.
  11. Nofossil

    Nofossil Moderator Emeritus

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    I'm glad somebody else besides me likes this stuff. One of the things that surprised me is the whole thermal stratification phenomenon. Look how little the middle of the tank changes during the whole cycle. All the action is at the top and the bottom.

    It's also interesting (to me, at least) to look at temperature differentials. Once the baseboards are done, it's all tank performance. Hot water is going into the top of the tank at essentially the temperature of the boiler outlet. When it returns to the boiler, it's a bit hotter than the bottom of the tank, but not by much. My HX is doing its job. My EKO can heat water by 25 to 30 degrees. If I'm producing 80,000 BTU/hr, then it must be flowing about 7 GPM. When I have a few minutes, I'll do the calculations and see how much heat I put in the tank between about 9:30 and 11:15.

    I have to be careful or I end up spending way too much time on analysis.

    "Honey, are you on the computer doing boiler calculations again?"

    "No, just looking at some porn sites."

    "OK."
  12. EricV

    EricV Feeling the Heat

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    That is very interesting data.

    I have a son in the hospital so I'm not so quick to respond, sorry.

    Tonight I am trying an experiment. I swapped the lines on the tank so I'm putting the hot boiler feed into the top instead of the bottom.

    I am noticing my boiler is going to idle then shuts down after the firebox probe gets below 195. So I'm wondering if the termover is flowing properly.

    We'll see how the experiment goes tonight and I'll post tommorrow night.

    Thanks for the help guys.

    Eric
  13. EricV

    EricV Feeling the Heat

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    Well the experiment didn't work, only 140 in the tank.

    So, I think I need to seperate the in bound and outboand. I'm going to get a flat plate heat exchanger and use that to charge the tank and use the coils I have to return to my oil boiler.

    We'll see how that goes.

    Thanks
    Eric
  14. Nofossil

    Nofossil Moderator Emeritus

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    You need more data - something is wrong. Beg / borrow / buy / steal a non-contact infrared thermometer. Sears has them - I think they're around $70. Make sure the temp range is wide enough - some cheap ones have a narrower range.

    Put a piece on masking tape on the copper tubing at the inlet and outlet of the boiler and tank. Masking tape is necessary because you can't get a valid infrared temperature from bare copper. Use the thermometer to get readings of all four locations at three points in time:

    1) When the boiler first gets going and water starts circulating through the tank
    2) When the boiler is cranking and the tank temperature is increasing nicely
    3) When the boiler is still going and the tank temp has stopped climbing.

    You have enough copper. Something is wrong. With this data, maybe we can help figure it out.
  15. EricV

    EricV Feeling the Heat

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    Great, I'll get one of those units in the next day or two and I'll collect some data

    Thanks for the help.

    Eric
  16. Nofossil

    Nofossil Moderator Emeritus

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    Here's a picture of mine. These are great toys, and indispensable for any self-respecting pyro ;-)

    [​IMG]
  17. EricV

    EricV Feeling the Heat

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  18. Nofossil

    Nofossil Moderator Emeritus

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    This is the one I have. The one you reference seems to be a new model. Specs seem OK, but I have no experience with it. I'd look at both and play with them if possible.

    http://www.sears.com/shc/s/p_10153_12605_03450466000P?vName=Tools&keyword=infrared thermometer

    Hope things are going well for your son - how old is he?

    I've stood outside the hospital and watched a helicopter bring my son in - don't ever want to do that again.
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