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Thread: Feet head resistance - 25 y/o boiler?

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    Question Feet head resistance - 25 y/o boiler?

    Hi.I currently have a 1986 Trianco oil fired boiler. I beleive the model is DB125. I've done *tons* of searching and have been unable to come up with any details on this unit other than the ID plate on the front which says it is 125000 BTU, 108000 BTU net and that it is likely a steel boiler.

    I am in the planning stages for the addition of an indirect water heater. At the moment I am trying to properly size the circulator pump for the indirect zone. The problem is I need the resistance (feet of head?) for the boiler itself and honestly do not have a clue what to use for this in the calculation and doubt I am going to find anything in print for this boiler. As much as I like to do things right, I may have to just take a guess at the feet head for this boiler. I was wondeirng if anyone might know that what the typical feet of head resistance is for a 25 y/o steel boiler of this size?

    Thanks very much for any insights you can provide

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    In the trades Dana's Avatar
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    The head on a cast iron or steel boiler is typically small relative to what it is on the coil in the indirect, but as indirect heat exchangers have improved it's not always the case. But if you assumed the head on the boiler is zero and size the pump for the head on the indirect + plumbing + 25% it'll usually be good enough. If you're cutting it close you can assume the boiler's head to be something like 1-1.5 feet at ~10gpm, (in most cases it's less than 25% of the total.)

    The gpm specs for getting the first-hour hot water gallons into the indirect aren't always sufficient to keep the boiler happy. On an old-skool cast iron boiler need enough pump to keep the delta-T on the boiler within spec- shoot for 25F or less unless you know for sure it can take more.

    An output of 108KBTU @25F delta-T is a flow of (108000/25F=) ~4320lbs/hr, which is (4320/60=) 72lb/min, which is about (72/8.34=) 8.6gpm. If you sized the circulator to be able to deliver 11gpm+ against the head of just the indirect + plumbing you'll be good, but 10gpm might be cutting it close if it's a low-head indirect. There's little downside to erring the high side on flow- high flow=higher turbulence=better heat transfer efficiency in both the boiler and the indirect. You pay a bit more in electricity, but duty cycles are small. The bigger cost will be in the pump itself.

    A primer on the subject lives here:

    http://www.comfort-calc.net/circulator_sizing.html

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    Dana,

    Thanks for the great info. Really appreciated. I'll need some time to process it. May come back with a follow-up question...

    p.s. I had seen the primer you refer to. In fact, it is what prompted me to ask the question.

    Thanks again!

    -Matt

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    As long as the indirect is not a half a mile from the boiler a Taco 007 will work just fine. Do not reduce the inlet and outlet pipe size at the indirect. Your Trianco is steel. We used to sell them by the trailer load.

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    In the trades Dana's Avatar
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    Peter: Out of curiosity, what is the typical delta-T tolerance of steel boilers vs. cast iron? (A lot of CI beasts specify 30F or 40F max operating delta- I pulled out 25F to be conservative.)

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    Depends on the manufacturer, though most steel boilers are less tolerant of large temperature differences due to thermal expansion. If the numbers don't meet up we will pipe a bypass with a ball valve on it from the feed to the return to ensure we don't get thermal shocking. It was a big problem early on with System 2000 boilers to the point that they eventually tapped the bottom for an aquastat control to keep cold water from returning.

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    Moderator & Master Plumber hj's Avatar
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    Typically, the only "head" pressure you have to be concerned about is caused by the piping and any resistance through the heater itself, and if the heating water is in the shell, and not the coils, which would be typical, then even that is minimal. You would have to have an unusually complex system to require computing head resistance and ways to overcome it.

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    Head loss thorugh boilers is a measure of the resistance to flow through the boiler, expressed as head loss and added to to the piping, valving and controls. Cast iron boilers generally have a higher head loss figure than steel boilers only because the passages are typically more narrow though some newer steel boilers such as the EK1 from system 2000 have narrow passages also. Normally for residential calculations it is not a problem but when you start getting into bigger commercial stuff or multiple boiler installations the figure can become and issue.

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    Thank you again for all the help and above insights. Just getting back (was away) and had some time to do some calculations. I'm just following the indirect manufacturer recommendation of properly calculating resistance in order to properly size the circulator to ensure that the recommended flow rate for the indirect is maintained. I was hoping someone could check my work. All calculations were taken from this article on circulator sizing (actually locate on this web site) http://www.comfort-calc.net/circulator_sizing.html. I first calculated EFP and then Feet Head. Thanks in advance for any additional insights/help/guidance you'd be willing to share.

    I first calculated Equivelant Feet Pipe (assuming 1" copper):

    16' Copper (est.) x .042 = .672 EFP
    1 - Tee (side port) = 4.5 EFP
    4 - 90* Elbows = 10 EFP
    2 - unions = 1.5 EFP
    -----------------------------------
    16.67 EFP

    16.67 EFP x .04 = .67 Feet Head

    Superstor Ultra SSU45: 7.9 Feet Head @ 10gpm (recommended flow rate)

    If I assume a 1.5 Feet Head for the boiler that would be:

    Boiler = 1.5
    Plumbing = .67
    SSU45 = 7.9
    ---------------
    10.07 Total Feet Head @ 10gpm

    If this is correct then using the the Taco Pump curves as shown here http://www.comfort-calc.net/Piping/Taco_pump_curves.JPG in order to maintain the Superstor's 10gpm recommended flow rate I would need at
    least Taco 0012 assuming as recommended above that it is better to have more flow rather than less as both the Taco 007 and 0010 appear to fall below the recommended spec at this resistance as also shown here: http://www.taco-hvac.com/en/products...nload_id=10672 and here: http://www.taco-hvac.com/en/products...nload_id=10678
    Last edited by mattk22359; 02-28-2010 at 02:40 PM.

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