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condensing hot water boilers

rp Member Posts: 1
some manufacturers of hot water condensing boilers have very small supply and return on there boilers.  Example a boiler that modulates from 25K to 150K has a 3/4" supply and return tapping on the unit. I am confused with flow rates and btu's. any answers


  • SpeyFitter
    SpeyFitter Member Posts: 422
    Don't worry too much about it...

    For a 150,000 BTUH boiler with say 93-95% AFUE efficiency, you should be transferring around 140,000 BTUH's to the water give or take. At a 20 degree delta T at 1 GPM per 10,000 BTUH you're trying to ram 14 GPM at max fire through that 3/4"pipe that goes to the heat exchanger. NO problem.

    You can ram 50 GPM's through 3/4" pipe, you'll just need HUGE circulator power to do it, due to the significantly increased head loss due to the smaller diameter of the pipe combined with the higher flow rates. The pipe will of course also erode much more quickly as well more than likely.

    For whatever reason this is what the condensing boiler manufacturers have specified to work with their heat exchangers, and this is why primary secondary pumping, or low loss headers are almost a fact of life with condensing boilers in a strong percentage of installations, due to the larger head loss of the exchanger itself.

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  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    My W-M Ultra 3

    boiler runs between 16,000 BTU/hr and 80,000 BTU/hr input. It has 1" pipes for supply and return. My contractor thinks bigger is better, so right after the LWCO on the return and the Pressure Relief  Valve on the supply, he has fittings to run it up to 1 1/4". I do not know how much resistance there is in their heat exchanger, but they supply a Taco 007 for the primary circulator. So they think that is enough.
  • bob eck
    bob eck Member Posts: 930
    condensing boilers

    Take a look at Triangle Tube Prestige Solo boiler only and their Prestige Excellence boiler with built in stainless steel indirect water heater that delivers 180 GPH

    their 60,000 & 110,000 BTU have 1" supply and return

    their 175,000 & 250,000 BTU have 1 1/4" supply and return

    their 399,000 BTU has 1 1/2" supply and return

    their heat exchangers have very little pressure loss and on the 60,000 and 110,000 BTU size you can use grundfos ups1558fc. Primary secondary piping is not always needed.

    check them out
  • Unknown
    "almost a fact of life with condensing boilers"

    I STRONGLY DISAGREE Scott K, as bob eck says " Primary secondary piping is not always needed."

    Mine is not P/S, and other of my installations are not also,,, there are options out there other than what you may think. :-)

    On TT P/S does have its place,, but seldom on anything residential.
  • The Steam Whisperer (Formerly Boilerpro)
    edited June 2010
    Larger pump needs

    This is one of the main reasons why I do not use boilers with restrictive heat exchangers.  It's one of those places where there are huge inefficiencies on supposedly high efficiency equipment.  Its great that you can get 95% gas efficiency, but just how much electricity is it taking to get that efficiency?  Start adding up the power of the draft fan and extra large pumps and additional pumps, and I bet your yearly operating costs are higher for a very restrictive condensing boiler than they are for a simple cast iron unit.  For Mod cons I stick with TT, since the heat exchanger are very open, especially on the larger sizes.  The old Hydrotherm Pulse boilers were probably even more efficient, since the draft fan was only used at start up.   Also, due to these problems of power draw and with the upcoming introduction of additional condensing steam boilers, steam will probably soon be eclipsing hot water heat as the most efficient heating system.   Funny how history repeats itself.

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  • Unknown
    edited June 2010
    Right-on Dave(Boilerpro),,,,

    That`s exactly what I meant!

    PS- Some guys are "blow hards" who only believe what a local manufacturer wants them to,,, :-)
  • SpeyFitter
    SpeyFitter Member Posts: 422
    ALMOST, is true...

    Primary Secondary Pumping (Or utilizing a low loss header) - basically anytime you have to have a boiler pump to deal with the higher head loss of the heat exchanger IS ALMOST a fact of life with condensing boilers. I stand by my words. Note I said ALMOST.

    There are lots of situations where it is not a fact of life - such as when you have fixed head and subsequent flow loads. If your control strategry allows you to pump directly through a heat exchanger, indirect, or another fixed head medium, then you can pump directly through the boiler to the load with one pump. Just size the pump for the boilers heat exchanger & load head loss, pick your required flow rate, and voila, you have a one pump system.

    But when you have a load, such as that which utilizes actuators or zone valves, such as zoned radiant floor loops, panel radiators, or baseboards, where the head loss may be constant (or it may not) and your flow rates will constantly change, you can not just as easily pick a pump to pump directly thorugh the boiler on MOST mod-cons. Yes, the TT IS an exception, but 98% of mod-cons out there (note my use of the word "ALMOST") are not or no where near the TT design, but instead utilize a coil around a burner, whether it be a Viessmann, IBC, Giannoni, or other similar condensing heat exchangers, all of which have relatively high head loss.

    Also, the head loss curves of your typical heat exchanger might not match that of your heat distribution, and subsequently it makes picking a variable speed pump, or adjusting a differential bypass, even more of a headache if you are trying to pump through the boiler into your constantly changing heat distribution network. And too boot, many mod-cons require a minimum flow rate otherwise they might shut down. Subsequently, I think I have proven my point with the use of the word "ALMOST" in it.

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  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669
    hot water boiler

    I think he is talking about an on demand hot water heater.

    small inlet and outlet pipes are fine due to the high temp rise. 3 to 5 GPM at 70* rise.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited June 2010
    Pri/Sec LLH

    Is really not needed on any mod/con providing your boiler circ can overcome the combination of head loss through the boiler and system. I can do the same with a Viessmann providing I don't exceed the recommended flow rate of 6.2 gpm or have less than 1.7 gpm on the smaller size boilers up to 120,000 and they also only use a UPS15-58. If my head is a little too much I can always use a UPS26-99.

    The fallacy that triangle is the answer to not having to pipe pri/sec or use a LLH is just that a fallacy.  Triangles install manual does state that it is recommended that pri/sec is used just as Viessmann recommends the use of LLH's in their manual. When you don't use pri/sec on that TT are you turning off the freeze protection as required? Wouldn't you then be turning off a function of protection that the customer may need. 

    To answer the original posters question. The outlets are small because of longer delta-t's across the boilers. You size your near boiler piping using the same required practices as we do in any heating system or as recommended by the manufacturer of the boiler you are installing. For instance, Burnham gives you in their Alpine Manual the require primary piping size for each boiler where as others don't and you have to do the math.

    I prefer LLH's over pri/sec because of the advantage of not getting temp drops across my zones as multiple zones open as you get with pri/sec. From a design standpoint it gives me better control and confidence when setting a system side heating curve. Others may like the added dirt removal/air separation. I'm a Viessmann guy and have to use LLH's with their Vitodens 200 boiler as there is a temp sensor that goes into the LLH and ensures low return temps back to the boiler.

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  • HDE_2
    HDE_2 Member Posts: 140
    edited June 2010
    Many will come out that way

    No you will start to seem more boilers that way. For instance the 175M Rinnai is 3/4, the Navien combi 199M is 1".

    Don't confuse 20 degree system DeltaT with the boiler needing it also, thus toss out the idea that 1 GPM = 10,000 BTU's, when talking about boiler output, that only applies to systems and boilers that see full system flow with a desire of maintaining 20 degree DeltaT. Yes the smaller flow boilers can't go beyond their flow capabilities of 4-6 GPM that will produce 40,000 to 60,000 BTU's when directly connected to radiation but, beyond that the additional BTU's up to their net rating is accomplished by higher Delta T operation via Primary /Secondary piping or hydraulic separators.

    To calculate mixed temps or return water, use the following example:

    Pretend you have a 4 GPM, 175,000 BTU Rinnai condensing boiler, 3/4" supply and return piped primary/secondary or connected to a hydraulic separator.

    Boiler is set/operating at 140 degrees,

    Secondary zone loop may be 14 GPM radiant floor, 70 degree return water temp.

    14 GPM (zone) + 4 GPM (Boiler) is 18 GPM combined, thus primary loop size would be best 1-1/4" to remain below 4 FPS.

    (140 Boiler temp X 4 GPM) + (70 Secondary return temp X 14 GPM) =  1,540

    4 GPM primary flow + 14 GPM secondary flow = 18

    1540 / 18 = 85

    85 degrees is the mixed temp downstream of the secondary taps

    140 degree boiler temp - 85 = 55

    Meaning 55 is the boiler operating DeltaT

    4 GPM boiler flow x 500 (8.34 x 60) x 55 boiler DeltaT = 110,000

    At this point, that Rinnai boiler with 3/4" taps, flowing 4 GPM through the unit increased to 1-1/4" primary pipe would be transferring 110,000 BTUH to the 14 GPM secondary loop.

    Thats how it is done
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