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  1. #1
    Join Date
    Jan 2006
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    32
    Might be better posted here than in Controls which is mostly commercial...

    Does anybody have an idea how much the Tekmar 062 can modulate boiler temps down from the calculated boiler tempeature based on the heating curve with outdoor reset ? My continuous circ Air handler system runs beautifully on cloudy days and all nights with the current input parameters (115F boiler temp for a 70 deg room temp on a 14F design day). But on sunny days, even though the RTU is in the great room where we have alot of solar load (south facing house with a lot of low-E glass) it seems to not have the range to account for the solar gain and the house overheats.
    I've partially solved this by setting the WWSD to 45F and putting the outdoor sensor on the south facing wall of the house. But on a day like today (32F, cloudy) there is still some solar load and the house is creeping up past it's 70F setpoint. When I turn the RTU air sensor off (so now it's just outdoor reset working) the boiler target temperature goes up 5F, so the RTU is pulling that down 5F. Is that it's limit ?

    The RTU is hooked up to a Tekmar 262. I do have a regular heat/cool thermostat for the cooling system that I could use to turn off the boiler demand on the 262. Seems inelegant given the amount of money that I've put into the system.


  2. #2
    Join Date
    Feb 2003
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    1,996
    What type of boiler is this? If your house is warmed enough by solar gain, why have the pumps running at all.

    If the 260 has a min boiler temp, I think it will always fire to at least that temp (+ 1/2 the differential). Are you heating an indirect? The boiler goes to max temp to satisfy the indirect, would still be hot and the continuous circ is heating the AH's when you don't need to.

    Also if this is not a condensing boiler, you don't want to be flowing cold water thru the boiler.

    Don't you have the 262 controlling the circ pump?


  3. #3
    Join Date
    Jan 2006
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    32
    It's a Buderus GA124/17, which puts out 59kbtu gross. Piped to a manifold, no P/S. The AH is 1 takeoff from the manifold, the DHW is the other. Checkvalves on outlet and retun manifolds and at the AH and the DHW indirect pipes at the tank. So no heat leaks. 4500 sq. ft. ICF house with stone walls. Looses about 15kbtu at 32F. Obviously the boiler is sized for the DHW, not the heating load !

    The sensor is actually in a buffer tank, and that allows buffer tank temps down to room temp. So "boiler min" is set to 70F. The boiler itself is protected by a Johnson Controls A419. It interrupts the AH circ pump if the boiler fires and the boiler well temp is below 104F. This is Buderus' recommended lowest operating temp. My firing cycles are typically 20 min every 60-70 min (its a 40 gal buffer) and the A419 stops circulation for less than 2 minutes till the boiler gets to its min temp. Buderus says this is fine...it's a cold start boiler and my burn times are so long that they can evaporate any residual condensation off.

    I do heat an indirect and the DHW pump interlock stops the AH pump both during DHW and for a purge period afterwards that brings the boiler back near 125 F (the DHW setpoint). So there isn't a lot of heat dumped into the AH circuit when it's pump restarts. The boiler has 2.5 gallons, the buffer tank and AH coil almost 20 times that.

    If I arbitrarily jack the RTU several degrees above the current room temp (say 3 F) the Boiler target temp on the 262 shows "---" and the circ pump does go off. But prior to that, there is temperature creep on a sunny day even though the RTU is located in a southfacing room with lots of glass. I could move it a bit more into the sun I suppose.

    It's possible my design temp is actually too high and this is why the RTU can't bring it down enough. Turning off the RTU sensor and just running outside reset, I've now got the buffer tank at 101F to keep the house at 70F on a very cloudy, thurnderstomy 39F day. This is 10F lower than the original design temp would have had it.



    [Edited by odcman on 02-16-2006 at 06:03 PM]

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,875
    What btu is the hydro coil in the air handler.
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  5. #5
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    Feb 2003
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    With my 260 boiler control, I didn't like the auto differential setting which always stayed around 10 degrees. I manually set it to 20. Also didn't like the heat curve as it got colder out. Changing design temp, desired indoor temp (I have the 076 indoor sensor) so the curve was not so steep.

    If the buffer temp sensor is located in the middle of the buffer, you might be getting slightly warmer water at the top of the tank. The slightly extra heat might be overshooting the indoor temp.

    Are you setting a wide differential? I think some of the other temp setting are effected by the diff number. Half of the normal 10 degree auto diff setting might be the 5 degree change you're seeing.

  6. #6
    Join Date
    Jan 2006
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    32
    Yes, I too didn't like the auto differential. After going to all the trouble to put in a buffer tank, it was running around 10 degrees and giving me 2.2 cycles per hour. So I set it to 24 F, which gives me around 1 cycle per hour.

    You might be on to something with the sensor location and differentials. The sensor is in the middle (it's an Amtrol Boilermate 40 gallon indirect). That and the wide differential may be what's causing the overruns.

    It's 39 F today and I've got it running on only outdoor reset for debugging purposes. 100 F buffer tank temp keeps the room temp at 70F. It will be 3 F tomorrow night, so that will give me some range to play with the heating curve. I suspect it needs to be set a lot less than it is at because of the tank stratification and differentials.

    With the 262 the heat curve slope is essentially set by chosing the type of terminal you have. I have selected air handler because that is what I have and it is the steepest (rads may be slightly steeper, but also more curved). We'll see if that puts me in the right ballpark when it gets cold.

  7. #7
    Join Date
    Jan 2006
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    32
    Originally posted by beenthere
    What btu is the hydro coil in the air handler.
    I think I have 5 gpm going through the pump in the AH according to the Grundfos and Lifebreath manuals. So assuming that, it will put out 18kbtu per hour at 130F for a 350 cfm air flow rate. That's about what the house loses at an outside temp of 20 F at night.

    To put that in more normal terms where the AH switches on and off with a thermostat, it will do 110 kbtu at 180F and 5 gpm and high speed on the fan which is 1180 cfm.

    It's a big coil...4 rows. So it allows a huge delta T and low water temps.

  8. #8
    Join Date
    Feb 2003
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    1,996
    IMO, the terminal type just sets min/max/design temps which you then can change. I don't think's there's that much temp differential to really create a changed curve (more like a straight line slope!) The other settings just shift the slope around.

    Change the boiler temp design, and indoor temp. Watch the target temps change as the outdoor temp drops.

    Why are you using the 262? The GA124 doesn't have a 2 stage gas valve does it? Don't think they make a GA124/15 just the /17 now. Probably the smallest nat draft boiler Buderus makes. Funny since some American boiler makers do make 40-50K BTU nat draft boilers.


  9. #9
    Join Date
    Jan 2006
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    32
    As far as I can tell by experimenting, the terminal type just changes the allowable boiler target. For example, if I am 1 degree below desired room temp and I set the terminal type to low mass radiant, it sets the boiler target to maybe 8 degrees above where it would normally be. If I change this to air handler, then it sets the boiler target to 20 plus degress over where it wants to be. In either case, if you are in steady state, it picks the same temperature based on the heating curve you set. In other words, an air handler terminal type seems to allow a higher transient deviation from the heating curve than a radiant floor. Makes some sense I suppose based on thermal inertia.

    Buderus actually allow you to change the slope of the heating curve and the intercept with any numbers you want on their Logomatic control, and they note that the numbers need not exactly correspond with the desired temps.

    The 262 is a 2 stage unit control of course, but it does allow the 2nd stage to be turned off. So that's how I am using it. The Buderus GA124/17 is the smallest boiler they make. It does use an inducer motor. Ideally what I want is a modulating condensing boiler that goes from 5kbtu to 60kbtu. This for a 4500 sq. ft. house !

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,875
    http://www.weil-mclain.com/ultra/modelultra.htm


    The ultra 80 will modulate down to 16,000.

    More then you need on the mild days, but can be set not to modulate higher then 40, or 50,000(your choice) in heat, but full input for DHW calls.

    Can you slow the water flow through your hydro coil, this would lower the btu ouotput, if you can get your water temp control to where you need.
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  11. #11
    Join Date
    Jan 2006
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    32
    The Buderus came wih the house, so I won't be replacing it in the next 25 years (or something reasonable anyway !) unless I got a good price for it. It's 9 months old and 85%AFUE, so not too bad, though nothing like their 98% AFUE GB142. My bills are low enough with the ICF house that this distinction is only one of pride as opposed to a lot of dollars in my pocket. About 20 dollars in the coldest months.

    The buffer tank takes care of the oversizing for boiler for heat, and the Tekmar keeps the buffer tank at an indoor/outdoor based reset temp. I turned it down 10 F from where the boiler design temp was set and will see what happens. Seemed OK at the 49F it was last night at 9 pm and kept a 70.6F indoor temp without overheating and with no solar load obviously. I've switched to outdoor reset only to do the calibration. It's super windy and 22 F in blowing snow this morning ! Heading to 5 F tonight.

  12. #12
    Join Date
    Jan 2006
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    32
    Problem solved....sort of.

    I did play with the design temps and found that if I set the design boiler temp to 112 for a 7 degree design day, then the house kept a steady 70.2 over the 12 hours that we've had 10F temperatures so far. Inside design temp is set to 70F.
    I stuck a spare sensor in some 50 F water and then it tells me the boiler target is 90F, which is about where I was seeing it during the warm spell a day or two ago. So the end points of the curve have been well sorted out and the inside temp stays at 70 along both ends of the scale.
    This with the RTU off, so it's only running outdoor reset on the characteristic heating curve.

    When I hooked up the RTU this morning again, it didn't alter the boiler target till the sun came out, and now it's dropping it and the house is still 70F so I think my overheating problems are sorted out.

    However, interestingly, the Tekmar showed a boiler target temp of 120 F for the outside 10 F early this morning. This doesn't correspond to the numbers I have put into the Tekmar (112/7 mentioned above). Even without the RTU, the boiler target temp is higher than what I punched in. So I'm not sure what the numbers I punched in mean, Is there an offset of some sort that I'm not accounting for ? Boiler target temp that the Tekmar shows on its heating cycles seems about 10 F higher than the numbers I'm inputting.

    Any insights ?

    [Edited by odcman on 02-18-2006 at 09:33 AM]

  13. #13
    Join Date
    Feb 2003
    Posts
    1,996
    Max/min temp is not realy max/min. My 260 will go over by 1/2 of the differential setting. So I guess 12 degrees in your case.

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