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Garn Tank and Supply Temp ?????

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Deere3720, Jan 27, 2013.

  1. Deere3720

    Deere3720 Member

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    Ok I have had my Garn up and running since Dec. 16th 2012. and so far we are loving it. However the last two days the house has been getting cold. This is what I have noticed:

    1. The house is calling for heat and the pumps are working.
    2. The baseboards are luke warm.
    3. The toe heater that we have in the kitchen is NOT turning on.
    4. The tank temp in the Garn is 170 and the Suppy is 115.

    So what I don't understand is why the tank and the supply temp have such a big discrepancy. The Garn is completely insulated and the supply temp sensor is about located on the black pipe about 8 feet after it existed the Garn. The sensor is wire tied to the black pipe and is wrapped in insulation.

    Any help would be greatly appreciated.

    Thanks in advance

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  2. maple1

    maple1 Minister of Fire

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    Air bubble/ air lock in the piping somewhere?
  3. heaterman

    heaterman Minister of Fire

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    If you notice this at the end of a"cycle", in other words, some period of hours after you have fired the Garn, it is more than likely a matter of stratification giving the false impression that you have lots of high temp water left in your storage.
    Bear in mind that the "tank" sensor is located at a higher level in the water column than the water temp being "seen" by the supply sensor. This can give you a misleading sense of the water temp actually available to the system.
    Try firing on a little closer time frame to keep the whole tank up to temp. 115* will not heat anything but a radiant floor application. For baseboard you are probably going to have to maintain 140*+ in the Garn.

    That's one scenario. The other that you may have going on would happen only if you have the possibility of a loop between the supply and return piping on the Garn like a mixing valve before you get to your baseboard. There is virtually no reason for a mixing valve on a Garn unless your system requires low temperature water.

    Lastly, if you have a heat exchanger between the Garn and your house system it may be sized incorrectly.
  4. Deere3720

    Deere3720 Member

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    Thanks for the input. I have another tank sensor located in one of the holes in the front of the Garn and this is connected to a Rancho Controller. It reads close to the sensor loacted on the manhole. It sometimes reads less and I think it is because of its location like you mentioned. So I believe that the water temp has to be close to what these two sensors are displaying.

    I have radaint floor in my barn so I do have a mixing valve. I don't think that this is the porblem because it was working fine and plus I got the piping design from my local rep Chris. I can send you pictures of my piping as well as the piping diagram that I followed.

    The HX is a 50 plate. I also got that from Chris.
  5. heaterman

    heaterman Minister of Fire

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    Is the mix valve before or after the HX?
    You may still be seeing stratification in your Garn to the point where the water actually hitting the supply line is markedly lower temp than what the sensor is seeing. I have measured temp differentials of over 40* in less than 6" height on the water column inside the tank under low flow conditions. (low flow being less than 15GPM)
  6. Deere3720

    Deere3720 Member

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    The mixing valve is before the HX.

    I have never seen my flow go over 10GPM. This is the pump that is in the Barn.

    Is there a way to mix the water in the Garn so the temp stays more equal?
  7. Deere3720

    Deere3720 Member

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    The Mixing vale is teed off supply before the pump.
  8. deerhntr

    deerhntr Member

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    Capture.JPG Deere,

    I'm with heaterman. If you are at the end of your cycle, you are seeing stratification. Here is a pic of my cycle temps yesterday. You can see when the Burn maxed out last night, both supply and tank top were at ~200F. When I stoked it this morning, the supply(Which is ~12" below the tank top sensor) is ~20F below the warmer tank top water.
  9. Deere3720

    Deere3720 Member

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    Here are two pictures that might help.

    Attached Files:

  10. heaterman

    heaterman Minister of Fire

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    So the heat exchanger is after (downstream) of the mix valve? In other words, the supply from the Garn goes through the mixing valve and then to the heat exchanger?
  11. Holley

    Holley Member

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    Here's Jim's piping plan. I just spoke with Jim and you guys nailed it..it's clearly a stratification issue. I believe Jim first saw this happen after activating the garage radiant slab portion of his system. In addition to the added load, the slab is returning 80 to 90 degree water to the GARN increasing the stratification effect. Jim has generous underground piping to his house and I a while back I suggested he might want to try reducing the pump speed on the house run. It heated fine and that would have increased the temperature drop across the HX also lowering the house return water temp a few degrees too.

    This stratification thing is a double edged sword. A stratified GARN will provide several more hours (depending on load) of operation from storage but it can be harder figure firing times and control a backup boiler. In Jim's case the system is set to switch over to oil when his GARN hits 140 but the sensor well is too high. We'll need to lower the sensor/well to correct this. We could push up pumping rates out of the GARN to reduce stratification but that would be a shame.

    Chris Mohler piping.jpg
  12. henfruit

    henfruit Minister of Fire

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    Chris, If he is running his oil system where does his return water come ?
  13. BoilerMan

    BoilerMan Minister of Fire

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    Closely spaced T's. The flow from the zone circs makes the boiler into an injection loop, with the "existing 007" on the oil's return.

    TS
  14. BoilerMan

    BoilerMan Minister of Fire

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    My question to the smarter people than I........Why the Alphas on the Garn and rediant side, if they are set to fixed (high and med per the diagram)? Aren't the savings from smart circs from their variable speed capability?

    TS
  15. heaterman

    heaterman Minister of Fire

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    To clarify, the system return in the house flows through the HX from left to right looking at the drawing, picking up heat from the Garn while passing through, and then on to the primary secondary tees above the oil boiler. The oil boiler itself would be considered a secondary input circuit.
  16. heaterman

    heaterman Minister of Fire

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    VS pumps use less energy than traditional pumps even in constant mode. They also have far more starting torque than a similarly sized wet rotor so there are other advantages to using them than just the variable speed part of the equation.
  17. Holley

    Holley Member

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    The Alpha smart pumps are magic. As Heaterman said, even in a constant mode they use much less power than a conventional wet rotor pump. And they display both power consumption and approximate flow rate..all for only about $60 more than than their conventional equivalent. On Jim's system either the GARN pump is running and the oil boiler cold or the oil boiler and pump are running and the GARN pump is off. The two are isolated from each other.

    I often draw systems without the P/S with the HX located in the fossil boiler supply line. ( In this case it would have been just to the left of the air eliminator/exp tank.on the drawing) But if the owner doesn't mind the extra work, isolating the oil boiler with the P/S arrangement and letting it go cold, prevents heat loss that would continue to occur from the oil boiler drafting. I wouldn't do that on an older oil boiler...particularly one well known brand (whose name will go unmentioned) that was prone to cracking and section leaks.

    A properly sized hx has very low pressure drop and piping it directly into the supply side of the main system loop saves a pump, isolation flanges and maybe a pump relay. If the line size to the house was marginal I'd put the hx out in the remote location with the GARN and pressurize the run to the house. That way you could run a higher head pump to the house and avoid the pump cavitation issues we run into with unpressurized high head pumps.

    Speaking of boiler heat loss from drafting, I recommend that GARN owners with vertical flues add a motorized intake damper to their GARN. Dectra now has that damper available on their web site. The new GARN digital control has contacts to operate that damper and adding one to an older GARN is simple enough with the addition of a relay. If you have ever brushed out the heat exchanger tubes on a hot vertically flued GARN you know how well it drafts...no vacuum necessary..the draft sends most of the dust right up the stack...without the blower.

    Chris
  18. Clarkbug

    Clarkbug Minister of Fire

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    Just a dumb question, but wouldnt you need another pump on the house side of the piping to the heat exchanger? If your house alpha is running, dont you need something to push the hot water from the Hx into the hydro separator?
  19. Kemer

    Kemer Member

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    Chris and Heaterman
    If you had to guess how much improvment can I expect to gain with the new dampner? It seems with my setup(vertical Garn) if I don't use it I loose it
  20. heaterman

    heaterman Minister of Fire

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    Kemer I really can't answer that with a $$.$$ number because there are many factors that enter into that equation. Let's look at the btu's that could be escaping out the flue.
    Assume that the entering air (intake side) is at 20* and assume that the leaving air going out the stack is at the water temp of 180*. That would give us a 160* rise.

    If the flue is drafting 75 cfm (no way to tell without measuring draft) that would give you the following; 160 x 75 x 1.08 or 12,960 btu's per hour. (1.08 is the constant for calculating btu/hour with heated air)

    That's a lot of heat going up the stack.

    In applications that can use the horizontal flue Garn, this is the main reason I like to go that way.
  21. heaterman

    heaterman Minister of Fire

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    The Alpha you see in between the expansion tank/air scoop and the 4 house zones circulates everything in the house. Given that the return is piped through the heat exchanger all the heat is picked up there when the Garn is running. When the Garn is off the oil boiler (and it's own circ) are heating/moving the water through the closely spaced tees to the left of the air scoop.

    Chris has the oil boiler isolated from the house loop via those closely spaced tees to let it go cold when not needed. This is easily done with the appropriate relays.

    BTW Chris, a tip of my hat to you for a really nice piping layout. That's about as good as it can be done and the owner is going to enjoy great economy and service from that design. Kudo's to you and a couple of Huzzah's besides :cool:
  22. taxidermist

    taxidermist Minister of Fire

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    Speaking of boiler heat loss from drafting, I recommend that GARN owners with vertical flues add a motorized intake damper to their GARN. Dectra now has that damper available on their web site. The new GARN digital control has contacts to operate that damper and adding one to an older GARN is simple enough with the addition of a relay. If you have ever brushed out the heat exchanger tubes on a hot vertically flued GARN you know how well it drafts...no vacuum necessary..the draft sends most of the dust right up the stack...without the blower.

    Chris[/quote]


    Holly can you give us a link to the damper? I can not find it on their site.

    Thank you,
    Rob
  23. bpirger

    bpirger Minister of Fire

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    FWIW, given this recent cold spell, I've been looking at 35-40 offsets between the Garn front well temp sensor and what my supply is in the house. Typically I see 20ish. Running the radiant during this 0C stuff is causing the stratification to show. I have noticed lately that, say 12 hours after the burn, the digital controller on the Garn's FLUE TEMP is about exactly what my supply is inside the house.

    Yes, this is rather irritating, and using this sensor to kick on alternative heat would NOT work very well. I think this is the biggest "complaint" I have with the Garn. We couldn't really be much happier. Love the beast.
  24. Clarkbug

    Clarkbug Minister of Fire

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    D'OH!

    I just realized looking at this again that I was thinking the Hx was the line at the bottom of the diagram, and the actual Hx was a hydronic separator, like the caleffi unit. Upon further inspection and coffee, I see that the graphic at the bottom is the buried line, not a heat exchanger.

    Sorry for the obvious question :)
  25. TCaldwell

    TCaldwell Minister of Fire

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    I have a type k thermocouple in the first section of class A about 1ft behind the garn to monitor flue temps, and about half way through storage temps the flue temp is very close to the supply water temp. With a turbulator in the last pass, the cleanout plate thermometer at high burn will be 150degf higher than the thermocouple. I agree with heaterman, unless you have low water temp emitters, or poorly insulated undergroung pex, there is not much sense in mixing the water at the garn. At higher setpoint temps you get robbed twice, as most mix valves allow a small amt of return flow in the design temp mix and secondly at the hx.

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