Modcon buffer tank temperature

Discussion in 'Boiler Forum' started by Buffalobillpatrick, Oct 10, 2011.

  1. Buffalobillpatrick

    Buffalobillpatrick New Member

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    Stonewall Colorado
    I'm planning new Buffer-centric system. Modcon, slab, & 120 gal. Buffer tank.

    Should buffer tank temperature be controlled by ODR, with Modcon set to continuous call for heat
    OR
    Aqustat on buffer tank 90 -> 110* controlling boiler, with thermostatic 3-way valve limiting water to floor 90*

    BBP
  2. Dana

    Dana In the trades

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    What's the point of the buffer if you have high mass concrete slab radiation?
  3. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Minneapolis
    Buffer-centric system? That's a new one.

    The design water temperature for any hydronic heating system, but more especially for a radiant floor heating system is determined by the heat load and the type of radiant panel you are using. I design radiant floor heating systems driven by condensing boilers every day and have only used on a handful of a buffer installations - usually to correct another's mistakes. Reading Siegenthaler's books again are we?
  4. Dana

    Dana In the trades

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    I can't imagine any slab-radiant system no matter how micro-zoned that would require any kind of buffer, let alone 1000lbs of water. It's useful to have a calculator handy (and actually use it) when reading "Hydronics for Dogs" or whatever, eh? ;-)
  5. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Well, if you boiler is twice too big for the application and is bumping off the bottom, but then you're in a whole other world of hurt. Perhaps our mountain man has solar heating in mind?
  6. Buffalobillpatrick

    Buffalobillpatrick New Member

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    Stonewall Colorado
    I still need some input related to my question.
    Thanks
  7. Tom Sawyer

    Tom Sawyer In the Trades

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    Dump the buffer tank, it's a waste of money and labor to install. Notice how the pendulum has swung back the other direction? Twenty years ago everyone was on the low mass mod con train and now we are moving back to high mass boilers as the engineers in Europe have now decided that a cast iron, high mass boiler has greater thermal efficiency. I find great humor in reading articles from 15 years ago written by guys that are now doing a complete 180 in their thinking. I have to admit that due to pressure I have installed a crap load of mod con's in the past few years but always with reservations because in the end it has become pretty obvious that the added cost of the unit and the installation will almost never be covered over the life time of the boiler. Still, we sell them because that's what folks have been told they need. We are always so willing and even eager to throw out the knowledge of the past but when you start reading literature from the early days of hydronic heat you will soon see that those old guys had a pretty good handle on the physics of heat and heat transfer and even though the equipment back then was crude by todays standards; particularly oil and gas burners, the concepts and physics have changed very little. Somebody is always trying to re-invent the wheel but as my old college professor says "physics is physics and only God can change that"
  8. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
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    Location:
    Minneapolis
    Just adding to the confusion.

    I agree with dumping the buffer tank, but the rest is delusion. " high mass boiler has greater thermal efficiency" really? Condensing boilers represent
    the bulk of our business both design and installation and ROI is nearly always guaranteed. Proper application is the key of course.

    All successful hydronic designs start for with a proper heat load which, determines emitters and design water temperatures. All else
    might as well be hot air. There is a formula and procedure for sizing buffer tanks most often -and properly-used for solar applications. But
    I have very rarely used a buffer tank in conjunction with a condensing boiler since they started coming with modulating burners over
    10 years ago. And I have never met anyone who knows how to size a buffer tank that is willing to volunteer the service.

    Still I am amazed at how often DIY people insist including an obvious design flaw into their DIY condensing boiler systems. If you doubt the
    wisdom or intent of these words, please forward a condensing boiler factory drawing of such a system.
    Last edited: Jan 2, 2012
  9. Dana

    Dana In the trades

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    Buffalobillpatrick: Where a buffer is already installed there no advantage to keeping the temp of the buffer any higher than the radiation needs for delivering the load. Higher temperature storage and mixing down to a lower temp only leads to lower combustion efficiency on the mod-con and higher distribution losses. But getting sufficient flow through the boiler loop may be problematic for simply setting the boiler output to the radiation temp requirements. In the absence of real design, and assuming the buffer tank is the point of hydraulic separation between the boiler loop and the raditaion loop(s), you can empirically get there by ignoring the aquastat on the buffer tank and bumping the ODR curves to whatever it takes for the radiation to keep up. The delta-T on the boiler may be on the high end if the flow on the boiler loop is a small fraction of the radiation loop flows, but most can take 50F deltas without stressing it too much. The temp of the buffer tank will fluctuate a bit, but the burner will still modulate and you won't be giving up much (if anything) on raw combustion efficiency, since return water temp & firing rate are the primary determinants of combustion efficiency, not output temp.

    Having the buffer tank there will have little effect on system efficiency unless you raised the storage temp well above the radiation needs, in which case it'll be a net-negative, since it would inevitably raise the return water temp entering the boiler, lowering combustion efficiency.

    Buffering can make sense for micro-zoned systems with low-mass radiation (but 120 gallons? Ever? Probably not). With radiant slabs no matter how cut up into zones it would take a GIANORMOUSLY oversized boiler for it to short cycle on zone calls. If at some point you use the buffer for storing solar input to the system there could be real benefit, if designed well as a system.

    Condensing high mass boilers can have steady-state thermal efficiencies higher than mod-cons if they're very well insulated, and in systems if they have sufficient mass (and controlled such) that they're effectively short-cycle proof they can have higher as-used AFUE on some designs. High-mass condensing boiler designs are essentially "buffer tank & boiler in one" for low design-load applications that makes them somewhat idiot-proof from a system designers point of view. But with high mass radiation, or a low-mass but reasonable low-temp system design a mod-con would be at least as efficient. (The only non-condensing high-mass boilers I know of coming out of Europe are well-insulated oil-boilers, but they're no match for a decent gas/propane mod-con in a good system design.)

    There are limitations to modulation- below an optimal flow on the fire side there are laminar flows on the HX and the stack temp goes up, yet at higher fire higher flows there's also higher losses. With most current mod-cons the optimal operating point is between 1/4 and 1/3 of full-fire. A bang-bang condensing burner set up to always run in it's optimal range, and with sufficient buffered mass to keep it from short cycling at part load can out-perform a mod-con in some applications, but would it beat it by enough to really matter? We're talking sub-3% differences in annual fuel use, and the real limitations will be in the system design & implementation, not the mass of the boiler.

    Of course, with high-temp radiation that always operates above the condensing point there's no real return on a condensing boiler, and a well insulated cast-iron piggy with 86-88% thermal efficiency can equal the performance with cheaper equipment, but that's not what we're talking about here (is it?)
  10. Tom Sawyer

    Tom Sawyer In the Trades

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    Hey, don't shoot the messenger. My company was the first company in the north east to install System 2000 boilers when they came out along with the early Thermo-Dynamics mod cons and many many others over the years. We still sell and install Viessmann as well as Buderus and Baxi units also. It all seemed so logical then to dump the old high mass, cast iron pigs that we had been using for a generation or more but you have to remember here that when we are talking high mass, cast iron now we are talking about highly efficient 2 and 3 pass boilers like the Buderus Logano series or biase' and others that are not of the same vertical flue designs like the old Weil Mclain 66 and gold series as well as Peerless JO's and HB Smith 8 series which are essentially single pass vertical flue design. Personally, I have a Buderus BE series CI, high mass boiler with the Logamatic control hooked to very large cast iron radiators that are piped individually back to a manifold.
  11. Buffalobillpatrick

    Buffalobillpatrick New Member

    Messages:
    38
    Location:
    Stonewall Colorado
    Adding a bunch of low mass zones. About 5,000 ft2 of "medium temperature" cheapo heat transfer plates under subfloor on upper levels of house.

    Time Delay T-T control idea
    ODR will control the boilers upper output limit on fire & thus the buffers upper temperature.

    The "medium temperature" zone switching relays end switch will pass through a 24vac Delay-On-Make Time Delay Relay in series to boiler T-T (say set to 10-17 minute delay, testing required) This should prevent any short cycling.

    This will allow small "medium temperature" zones to draw heat out of the buffer, possibly without firing boiler, if thermostat is satisfied before time delay expires.

    This will also allow larger "medium temperature" zones to draw heat out of the buffer before firing boiler.

    A basement slab "low temperature" zone will draw heat out of buffer without setting a call for heat. Buffer is large enough where it should never drop to 75* before a medium temperature zone calls for heat.

    Feedback?
  12. Dana

    Dana In the trades

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    Ever considered doing the math?
  13. Buffalobillpatrick

    Buffalobillpatrick New Member

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    Location:
    Stonewall Colorado
    Hi Dana, I have read many of your excellent informative knowledgeable posts. Thank You

    On another site PeterNH did some math:

    "3x50, 150 gallons of water to heat up.
    I can't imagine, even with a boiler that big, especially throttled down by ODR, that you would have any need for a time delay.
    Besides as far as i understand it, short cycling, refers more to short on cycles, not necessarily a short time between reasonably long on cycles.
    At full 130k output, 150 h to 130 low:
    Without any load at all, it would take about 12 minutes to heat up the 150g's of water.
    With a 20k btu load it will take about an hour and 13 minutes to drop 20*
    A 40K load, will take, 37 minutes to cool off 20*.
    But then with a 40k load it will take 37 minutes to heat the tanks up again.
    Not exactly short cycling, imo.
    Additionally.
    Presumably, the ODR wouod also widen the differental as needed, in warmer weather, thus extending the cycle times."

    My response:

    "Thanks Peter for doing that math.

    I'm unsure of the AM10 ODR controllers algorithms.

    Time delay relay cost only $22

    When it comes to boiler cycles "longer IS better"

    My goals:

    1. Heat this LARGE house, with its pex & manifolds already installed, with this boiler as efficiently as reasonably possible, while minimizing expense, using parts on hand.

    2. Determine how to manage a large buffer tank, using ODR as high temperature limit and how to call for heat from boiler using its T-T input.

    3. Prevent boiler short cycles, when small low mass zones call for heat, which is exaserbated in shoulder weather conditions. Draw buffer temperature down without firing boiler when possible."
  14. Buffalobillpatrick

    Buffalobillpatrick New Member

    Messages:
    38
    Location:
    Stonewall Colorado
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