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Pipe sizing

deek Member Posts: 21

Both my indirect DHW tank and my fan coil have 3/4 inch connections for the the boiler, but according to the flow rates they should be piped with 1 inch.  Is it worth piping with 1 inch to the appliance to reduce mechanical room noise?  Will it make any difference at all?  It will only be a run of 10 feet or less from the manifold.




  • Zman
    Zman Member Posts: 7,542
    Larger size

    I would pipe to the larger size. Your emitters may be designed to handle a higher flow rate than copper piping.It therefore can have smaller piping. You  do not want your pipes to erode due to the excessive flow rates. 
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    3/4 inch connections for the the boiler, but according to the flow rates, 1 inch

    My boiler manual says to pipe the boiler loop (in a P-S piped system) with at least one-inch pipe. The boiler has one-inch fittings. My installing contractor said it works better if piped in 1 1/4 inch, so he had a step up adapter almost immediately after the supply and return  come out of the box at the top of the boiler. The boiler loop includes two closely-spaced Ts at the top. The near boiler piping up there is 1 1/4 inch too, but the pipes to the big zone are one-inch, and to the small zone are 3/4 inch. Those two zones could run at the same time.

    Also, across the supply and return of the boiler loop are two 1 1/4 x 1 1/4 x 1 Ts to go and return from the indirect. There are also lots of isolation valves, purge valves, etc., as required. The indirect has one-inch fittings for the boiler water and 3/4 inch fittings for the domestic water.

    As a home owner, it does not seem to make sense to step up the size, though I assume it does no harm. Because once it has gone through the choke point of the smaller size pipe, it seems you have "lost it." But I have seen this done with other systems, and I have seen it done with gas pipe too. At one location, the gas pipe in the ground comes up one inch, goes through the regulator and the meter, runs a few feet, turns a corner, steps up to 1 1/4 inch and runs 10 feet or so, turns a corner and runs about 6 more feet, enters the building, and across. From there it has two separate one-inch (or maybe 3/4 inch) drops, one to each furnace. And in my father's old house, the piping was simpler to his old coal-fired furnace converted to gas. The gas entered the house, went through the meter (no regulator), went one-inch a long ways to the gas stove, and hot water heater. But a nearly parallel pipe stepped up to 1 1/2 inch and ran a long way to the gas burner for the furnace and stepped down to the burner itself.

    So maybe the same considerations apply to water and gas as apply to electricity. Is this true?
  • icesailor
    icesailor Member Posts: 7,265
    Pipe Sizes:

    "Pipes increase as do their squares".

    That means that it takes 4-3/4" pipes to equal the size of a 1 1/2" pipe. It takes 4 - 1/2" pipes to equal a 1" pipe.

    The manufacturer of the indirect says to use1" pipe (Copper) from the boiler loop to the secondary to the indirect coil. They must know something so I always connect it 1" or larger. Not 3/4".  I see a lot of indirects connected with 3/4" copper. They must know something that I don't know. It's my understanding that it has something to do with how many gallons per minute can flow through a 3/4" pipe and how much more can flow through a 1" pipe.

    It will work OK as 3/4" connected. It would probably work better if the installer followed the manufacturers recommendations and connected it with 1" pipe.. They employ people far smarter than I and if there is a problem, unless you know what you are talking about, those that work for the manufacturer, that know more than you or I, will tell you that it is wrong.

    "here's never enough time to do it right but always time for someone else to do it over"
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    "There's never enough time to do it right but always time for someone else to do it over"

    Where I used to work, I would annoy them by saying: "We haven't time to stop for gas; we're late already." Yet we made the same mistakes over and over again because we skipped the planning stage of a project to save time, and had to spend way more time than proper planning would have taken to straighten out the disorganized mess that results when building something with no plan.

    I know that stuff about 4 half inch pipes being equivalent to one 1 inch pipe. But my question was more like this: if you have a loop with different size pipes in series, does it really matter if some of it is of larger diameter than the rest? I.e, if the connections from the boiler are one-inch, does the fact that the rest of the piping is larger, say 1 1/4 inch, make any difference? I see systems where the piping is not all the same diameter in a loop, so I assume this is the case. And this does not satisfy my intuition -- I recognize my intuition is not always right. I mean, making some of the pipe a larger diameter could lower the velocity of the fluid (feet/second), but would it really affect the flow rate (gallons/minute) if there was a piece of smaller diameter pipe in the system? I am getting the impression that this is true, but I do not really understand it.
  • Henry
    Henry Member Posts: 996
    Sizing tools

    Even if the coil is 3/4, it is the amount of flow required through it and the flow restriction created (head) that determines the actual connecting pipe size. Here is a few good tools:

  • Zman
    Zman Member Posts: 7,542

    To really get to the bottom of this we will need more info. The brand of the indirect and the flow rates we are discussing.As well as the model circulator you have. A manufacture of an indirect may have figured out that they can safely push 8gpm through a 3/4" pipe. Maybe the exchanger is stainless. That does not mean you should try this with 3/4" copper. You pipes should be sized based the flow rate recommended for the material being used. Not only  is going beyond 5 ft/sec noisy it also will cause pipe erosion and require to go with a larger circ than necessary.


    The "real world" flow rate of a circuit is based on the point where the head loss curve of the circuit meets the the performance curve of the circulator. With a large enough circulator you could push 100 gpm through a 3/4" pipe. Not a good idea but possible. The circulator does not know it is supposed to stop at 5 ft/sec it just does whatever it is designed to do.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEI
    SWEI Member Posts: 7,356
    mixing sizes

    Pressure, flow, and pipe friction are regularly used analogies for teaching about Voltage current, and resistance.

    Friction loss is proportional to the length of the pipe as well as the size.  Reducing the size of a pipe for a short distance will not necessarily "lose it" but it will have an effect.  The exponential terms in both area and friction formulas will indeed make this effect drastic at some point (e.g. splicing a full-flow 4 inch water pipe to a 1/2" nipple.)  Just like with electricity, a long run can have a far larger pipe size (larger wire gauge) than would normally be used for that amount of flow (current) in order to reduce losses.  This is commonly seen in gas lines, where the meter outlet immediately necks up -- sometimes to a pipe which is 3-4X the outlet diameter.
  • deek
    deek Member Posts: 21
    Specs for my system


    The tank is an RTV-52-L bradford white ss tank.  My circulator is a grundfos 15-58 FC 3 speed.  The manual for the tank doesnt specify the piping but the specs are given at 8 gpm which is 5 fps with 3/4 inch pipe.  The system has pretty low head loss so I'm thinking speed one will be good (8gpm@3 feet). Also I am using a traditional viessmann ecd 140 (vitogas 050) boiler with 104500 DOE Jetted for the rockies.  The DHW tees off before the primary circ and is set up for priority.

    I guess the more I think about the more I think I should be OK with 3/4 inch pipe, but all your feedback is appreciated.

    Love the site.  Thanks for all the advice.
  • Zman
    Zman Member Posts: 7,542

    I had in my head that 3/4" at 5ft/sec was 7gpm. You are correct that it is more like 8 gpm. It looks like you have a low head indirect. You should be fine with 3/4" There would be no harm in 1",personal preference. I like to tweak the circ speed after I fire it up based on delta T. Speed 1 sounds right.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    System optimization ...

    Engineers go through trial and error "iterations" or calculations to determine the best, optimal pipe/pump configuration. If you jump up one pipe size to 1", what is the required parasitic cost of operation? What is the velocity? (Remember, there is a minimum velocity required to move air in suspension in a downward flow configuration).

    You can go with the 3/4" because the actual run time of the DHW system is minimal compared to space heating needs, so the tube erosion potential is minimal, and the noise generated would be confined to the mechanical room. But, if you can go with an even smaller pump with even less energy consumption compared to your first choice, then the long term benefit is less parasitic power consumption for the life of the equipment.

    In these days of ever increasing electrical cost and environmental awareness, things like going one pipe size larger, and one pump size smaller make good sense.

    The easy thing to do would be to use a larger pump to overcome the inherent pressure drop at the expense of electrical consumption.

    Very few people put as much thought into it as you are. Most people would look at the 3/4" pipe and assume that that was the size of pipe they should run, which is not necessarily true.

    Where abouts in the Rockies are you located? I'm in Denver.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • hot_rod
    hot_rod Member Posts: 21,820
    a graphic

    from the next issue of I-dronics # 11 which covers DHW.

    This explains how to select a circ that best matches the operating point. With multi speed circs and more pump models offered you should be able to find a circ that is near perfect for most pumping applications.

    The key is knowing the pressure drop thru the circuit, heat exchanger, etc.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • deek
    deek Member Posts: 21

    I made a small error in my last post. 3feet head loss@8gpm is just my piping and valves.  I need to add to the the indirect at 6.2 feet so according to my pump curve speed 2 should work.

    Thanks again.


    Calgary, Alberta
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