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Garn water temp - difference between front and rear?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by foxt, Jan 28, 2009.

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  1. heaterman

    heaterman Minister of Fire

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    I thought it would upset the apple cart a bit also Jim. That 1400 is moving some serious GPM through the system and the thermocline in the Garn seems oblivious to it as the temp readings from bottom to top showed. I was really surprised that more blending wasn"t taking place.

    I'll ditto the comment regarding the control setup and system being designed to work with variable water temps coming from any type of storage. That's why the method of heat transfer is just as important as the unit supplying the heat. The lower a person can use the better.

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  2. foxt

    foxt New Member

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    Jim,
    Thanks for that, but I have a question: my piping is not p/s, but rather a single loop that feeds the house via fphx directly; if I put the circ control sensor on or in the supply piping, won't I see a drop in indicated supply temps when the system is idle because the water in that pipe is cooling down? Or is there enough conduction between the water in the tank at the supply port, and water in the supply pipe say 24" away? How close to the supply port itself do I need to get with that sensor?

    If I had known that the front of the tank is an unreliable location for a temp sensor (who woulda thunk it, especially since the Dectra folks supply you with a temp gauge for that same position), I would have rigged it so that I could put a deep thermowell right into the tank at the supply port and be done with it. Next time ....

    In addition to the other options suggested by folks (1-wire, etc), another option I am considering is to submerge a sensor in the tank, at the manhole, at the same elevation as the supply port. I recall reading the details on how to accomplish what I need somewhere on this forum ...

    Tom
  3. heaterman

    heaterman Minister of Fire

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    Ideally, the best location for that sensor would be to install a Tee right on the supply port and use a well in either the side or the bull of it depending on how your particular piping dictates.
  4. foxt

    foxt New Member

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    Ok, thanks for that. That's what I expected to be the right way to do it, and probably would have done from the start if I had known I shouldn't use the front tap. Those two easily accessible front taps and the Dectra-supplied gauge for that location lulled me into just going with what seemed to be the intent and using that location for the control sensor.

    Given where I am at, I think I'm inclined to live with it the way it is until the end of this heating season. I was hoping I could avoid draining down the tank and the little bit of repiping that would be required. Current setup is not ideal, but at least now I know what is going on and that there's nothing I can do to draw down those last btu's above the supply port.

    If you get a chance, I would be interested in Martin's thoughts on the location of that front temp gauge. Is it placed in that particular spot vs any other spot on the front wall for any reason other than easy line of site? Given our exchange here over the past few days, would he be inclined to relocate it so that it's at the same elevation as the supply, or did he consider that and reject it for some reason? I know that the design has been refined over the years, and I assume that at this point everything has a reason for the way it's designed ...

    Tom
  5. Jim K in PA

    Jim K in PA Minister of Fire

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    Tom - Getting at those "last" Btu's may not be worth the effort. Given Steve's estimate that the layer above the upper port is only about 200 gallons or so, even with a delta T of 60, you only have about 100k Btus there to draw down. Depending on your heat load, that may give you a coupla-three more hours of draw time, but that's about it.

    You also may not need to change the piping next year. Live with the system for this season, and then see how comfortable you get with it. Honestly, the only thing you really need to do is get rid of the 24/7 pumping. Use a Zone Valve Controller (assuming you have ZVs) with an end switch and have the pump turn on when a zone calls for heat. Mine works fantastic this way. Pretty simple too, and you do not need a temp sensor in the tank, piping, or anywhere else. Just use the front temp gauge for reference when determining if a burn is needed.
  6. heaterman

    heaterman Minister of Fire

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    I doubt if there will be any change in future model in regard to the location of that tapping. I'm guessing the new control being worked on right now is going to either eliminate it or make it a moot point. Just guessing but based on what we talked about this past spring in Minnesota, that's my gut feeling.
    I have no idea if there is a specific reason for the current location that is operation related. Personally, I tell my customers that it's just there for reference and not meant to be anything other than a rough idea of where your tank temp is.

    Jim brings up a good point and that is to go the winter with it and learn a few more things about your system and the garn before you make any changes. There are many wrong ways to do hydronic piping and control but there are also many different ways that will work very nicely.
  7. RowCropRenegade

    RowCropRenegade Feeling the Heat

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    I can't wait to get into piping.

    Brings up a thought, how about two supply ports. one front, one back? An extra 100k of btu is at least one more standby hour. :)
  8. foxt

    foxt New Member

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    Whoops Jim, I'm not pumping 24/7 in the GARN loop. The tekmar that runs my distribution system decides when to generate a boiler demand. When it does so, the aquastat on the front of the GARN determines whether the demand can be met by the GARN, or if the backup fossil boiler needs to fire. If the GARN temp is above setpoint (which is really set 30* above my actual setpoint due to this thermocline thing), the GARN circ is enabled upon a boiler demand, otherwise the oil burner is enabled. I would imagine that this is the logic behind many controls that include a backup boiler?


    I don't think I will use the front gauge to determine when to fire. It's reporting the water temp at the end of the probe, period. I agree that, during a burn when the tank is mixing well, it does give a fair representation of the tank temp. So I will use it to ensure that I don't OVER fire (coming up on the last half of a burn, if the front gauge is at 190* or higher I won't put in another half load of wood). However, the actual supply temp is what I really need to use to determine when to fire.
  9. heaterman

    heaterman Minister of Fire

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    I don’t think I will use the front gauge to determine when to fire. It’s reporting the water temp at the end of the probe, period. I agree that, during a burn when the tank is mixing well, it does give a fair representation of the tank temp. So I will use it to ensure that I don’t OVER fire (coming up on the last half of a burn, if the front gauge is at 190* or higher I won’t put in another half load of wood). However, the actual supply temp is what I really need to use to determine when to fire.

    And there in lies the reason to use a tee at the outlet port of the Garn for the sensing location. In all honesty, it would be impossible for Garn or any manufacturer to cover all the different ways and methods of installation their product could be connected in the field. There are just too many variations in systems and control strategies. So, one does it by trial and error (like me) or you rely on or hire and import the expertise and knowledge from someone else's head.
    Looking through the ongoing description of your system here, I don't think you did to bad at all. You just have a slight mismatch of control vs product function parameters which can be corrected.
  10. foxt

    foxt New Member

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    Thanks for that Steve. I'm appreciative of all of the help from you and others on this forum.

    I don't want to give anyone the wrong impression - I am very happy with the GARN and the performance of the system. This temperature issue has been the only question mark, more so because the reaction I was getting was that I was seeing something unique to my system. Now that we've hashed it out and have a theory about what is going on, I am content. I'll spend some time taking some more measurements/observations, especially since I am curious to see how long it takes that thermocline to develop and rise to the supply port.

    And now that this is all captured in this thread, hopefully others who are in the research or design phase of their project will benefit from this ...

    Tom
  11. Jim K in PA

    Jim K in PA Minister of Fire

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    Hmm. Sorry- I looked back at your first post in this thread from January and you stated that the pump was running 24/7, and I did not recall that you had changed it.

    Not sure which Tekmar you are using, and I am not fully versed in all their equipment, but I recall that some of their controls have a means of starting the pumps for a pre-prpgrammed run time in order to get stagnant water out of a line. If you use a strap on sensor on the supply piping and let the Tekmar run the pumps for 30 seconds before going to logic control, you will have actual water temp reading to then determine if the oil furnace kicks on. Just trying to save you the work of draining and adding the Tee with the temp well.

    My backup system is simple, but not automatic. I have two valves and a toggle switch to throw, and I am back to oil heat. After going through a season I am now much less worried about/desirous of having an automatic switchover. However, I will be incorporating such an automatic backup system to directly heat the GARN via propane once I add a greenhouse to the system.
  12. foxt

    foxt New Member

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    That's a great idea. I picked up a tekmar controller to dedicate to variable speed control of the GARN circ based on deltaT at the fphx, but ditched that when I ran into the thermocline issue. It's been sitting on a shelf with the hope of one day going back into service. I recall that it also had this purge feature, and may use it for that instead of the variable speed. Have to think on that one for a while. Thanks for the idea ...

    Automatic backup was a big deal for me. I bet there were half a dozen times last winter when the backup kicked in for me, and I wouldn't have planned ahead for it. But I was learning, and only had a half season to play with it. Or maybe I just go skiing for the weekend more than you do :) We run on the GARN the first day or so that we're gone, and then the oil kicks in until we get home. Haven't been away for a long enough stretch where the GARN really cools down, but I've been thinking that I should look into the pool-heater backup option and ditch the oil backup. Not to hijack my own thread, but which propane-based backup were you considering?

    Tom
  13. Gooserider

    Gooserider Mod Emeritus

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    No reason you can't add a backup system to heat the Garn directly, but IMHO it's a really bad idea... The only reason a Garn or other boiler + storage setup uses that big tank of water is to deal with the difference in the intermittent and variable heating demands, and the short term, steady and high heat output of a solid fuel fire. A fossil burner can turn itself on and off rapidly to deal with the difference, so in most cases it doesn't need or benefit from storage. If one does the backup to heat the storage, then in order to deliver heat to the load, a large amount of energy has to be spent in bringing the storage up to temp, or keeping it there, before any heat reaches the load.

    Consider the case where you get delayed for some reason, and don't get around to building a new fire for 2-3 hours after the Garn dropped below it's minimum working point - The backup will come on and work at heating all that storage mass back up to the minimum working point, for only a few minutes of actual heat delivery. If you had the backup set to deliver the heat directly to the load, then the backup only runs enough to keep the load warm...

    This is especially the case in milder weather - presumably the backup control would need to turn on the heat to the Garn as soon as the Garn dropped below the minimum, just so that the Garn could supply loads as soon as they called. But in mild weather they might not even call in a two-three hour period, so you would have heated the Garn for no benefit, where if you had been doing direct heating of the load, the backup would never have fired at all...

    I really can't see ANY circumstance where there would be benefit to having the fossil backup warming the Garn instead of going directly to the load.

    I think the best approach is to plumb the backup in parallel with the Garn, so that one or the other supplies heat to the load, but not to the other boiler. (Possibly there might be a need to do a "last ditch" backup circuit to allow the backup to supply a small amount of heat to the Garn if needed to prevent it from freezing - but that should almost never be needed considering how long it would take that much water to cool...)

    Gooserider
  14. Jim K in PA

    Jim K in PA Minister of Fire

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    Goose - with all due respect, you are making assumptions about a system that has not been built yet, nor which I have provided specific piping or heat flow details about. Based on my single sentence, your reaction may be understandable, albeit a bit over the top. Rest assured, I will NOT try and heat the entire storage mass of the GARN before or during a heat call event via a propane fired backup source. The capacity of the supplemental propane unit I will be using far exceeds the demand anticipated. The loads will be satisfied first, and then "excess" heat will be sent to the storage tank, but only during a call for heat, and/or to protect the GARN from freezing. Without completely hijacking Tom's thread, the control logic will be such that temp as well as dwell time will be considered before the actuation of the backup heat source. It will not be an accidental or occasional function.

    Tom - you are correct, I am no skier! :bug: That is not my idea of fun. We rarely go away during the winter. I will post up a description of my propane pool heater backup in a seperate thread at another time.
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