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flow rate through different diameter pipes - as in return to boiler

jhrost Member Posts: 57
I've found some formulas on line for determining flow rate through pipes - sort of complicated by friction and other factors but I would just like a ballpark assess from experienced people . What I am curious about is how much longer condensate returning after the boiler turns off would take to get back into the boiler with different length and diameter of pipe after it drops down to ground level. For instance if you went from travelling through 2 feet of 2" diameter pipe to 5 feet of 1" diameter pipe would that change the time for all the water to get back into the boiled appreciably? Like a few minutes? The boiler is supposed to hold 12.7 gallons of water- though not that much would have traveled out into the system as steam and returned as condensate of course. Thanks for your insights.


    EBEBRATT-Ed Member Posts: 13,433
    There are two kinds of returns, dry returns are above the boiler water line and wet returns are below the boiler water line.

    The return line is sized based on the size of the boiler for a single return. If there is more than one return each return would be sized based on the radiation on each lopp of pipe.

    Wet returns because they carry only water are sized smaller than dry returns.

    We would need to know your boiler and radiation capacity to size the return lines
  • Jamie Hall
    Jamie Hall Member Posts: 21,099
    In a direct answer to your stated example question -- not much. Assuming that both pipes were clean. But... sludge can and does build up in wet returns, and can make a huge difference in timing.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 840
    Wait a minute guys. Pressure drop in piping calculations is based on piping that is full of whatever is flowing. I seriously doubt that except for gigantic heating systems that the pipe is full of condensate.

    Using round numbers, a 100,000 BTU/HR boiler converts 100 lbs of water to steam per hour.

    Water at 212 F weighs about 60 lbs/cubic foot.

    1 cubic foot is 7.5 gallons.

    1 gallon is 128 fluid ounces.

    100 lb X 1 cubic foot = 1.67 cubic foot
    hr 60 lbs hr

    1.67 cubic foot X 7.5 gallons = 12.5 gallons
    hr 1 cubic foot hr

    12.5 gallons X 1 hr = 0.2 gallons
    hr 60 minutes minute

    0.2 gallons X 128 fluid ounces = 26 fluid ounces
    minute 1 gallon minute

    That’s slightly more than 2 @ 12 ounce cans of beer per minute. If you make a second hole in a beer can so air can get in while the water flows out, the can will empty in about 6 seconds through the drinking opening in the can. That translates to less than 15 seconds for 26 ounces to flow out and the opening is way smaller than even the inside diameter of 1-3/8 inches in a Schedule 40 1-1/4 inch pipe. (Aside the outside diameter of the pipe is 1-2/3 inches).

    All this proves is the pipe would not be full, assuming the water cannot return faster than the steam is created.
    JUGHNE Member Posts: 10,564
    If your return is at the floor and always full of water, I guess we would call that a "wet" return.
    The pipe is always full of water, if a gallon flows into one end then a gallon would flow out of the other end.
    Assuming the pipe is relativity clean.

    I think this discussion has come up before. Someone wanted to upsize their wet return pipes to have the system hold more water to overcome delayed condensate return.
    In that case the increased water volume always stayed in the pipe.
  • jhrost
    jhrost Member Posts: 57
    Thanks for the info. It seems then that piping size is not a big issue, and making sure pipes are clean is a more important consideration ?

    That makes me wonder though whether a smaller diameter pipe is likely to clog up more easily than a larger diameter one. I seem to recall from somewhere that smaller diameter pipes have greater velocity even though larger diameter pipes have more capacity - at least in your house drainage system that goes out to the sewer. In the case of this mud leg or wet return into the boiler where water sits and sediment settles out, would there be relatively little difference in sediment building and clogging issues between pipe diameters?

    It seems sort of counter-intuitive to experience with house plumbing drainage where clogs usually occur in smaller diameter pipes , also usually at bends or sometimes traps where water sits?

    Sorry if this is plumbing 101 to you guys but I still can't seem to totally accept the idea the pipe size doesn't matter that much.
    JUGHNE Member Posts: 10,564
    1" would be the smallest recommended size for wet returns.
    You could go 1 1/4" and cover most bases.
    Copper would stay cleaner and easier to clean.

    The main thing would be to put isolation valves on each end and then ports for hose connections on the wet side of the valves. This lets you flush out the piping.
    Especially the lowest point at the boiler to have flush capability to keep sludge out of the boiler.

    If buried under the floor you want to protect the pipe from contact with dirt or concrete.
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 840
    I don't know how the spacing in my calculations got messed up. See the .pdf below
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 840
    Don't be so hard on yourself. Go back to what I said about how much condensate is returning to a boiler. Even if your boiler is twice the capacity of what I used in the calculations, it's still a relatively small flow rate. A faucet can flow a lot more than what your boiler flows, a toilet flushes a "blob" of water to make sure the solids make it down the hill, through any traps and hopefully out of the house.
  • Precaud
    Precaud Member Posts: 369
    JUGHNE said:

    The main thing would be to put isolation valves on each end and then ports for hose connections on the wet side of the valves. This lets you flush out the piping.
    Especially the lowest point at the boiler to have flush capability to keep sludge out of the boiler.

    On each end of what? I'd love to see a photo or drawing that shows an example of what you're describing.
    1950's Bryant boiler in a 1-pipe steam system at 7,000 ft in northern NM, where basements are rare.
    JUGHNE Member Posts: 10,564
    Lets call the condensate return pipe that is always full of water the "wet" return.
    The return piping that drains down into the wet return could then be called the "dry" return.
    These are probably at the end of steam mains to drain condensate back to the boiler......thru a wet return (low pipe below water line) or the dry return (hanging from the ceiling and sloping down to the boiler) continues back to the boiler room.

    I would put a full port ball valve where the dry drops into the wet piping. Just a ways above the water level of the boiler. Then just below that valve I would cut in a tee with a hose bib fitting.

    Then at the boiler I would put a ball valve on the vertical riser that goes up into the Hartford Loop.
    At the 90 below the vertical that connects into the horizontal wet return, that 90 would be a tee with a full port 3/4" ball valve with hose adapter and cap.

    Then you take a garden hose to connect to the far end hose bib (double female coupling) and shut off that isolation valve.

    Then put a drain hose at the 3/4" boiler ball valve.....isolate the boiler from the returns.
    Connect the first hose to a source, (water heater drain or laundry hose bib).

    Then wash fresh water thru the wet return for cleaning.
    The most sludge will be at the base of the Hartford Loop.
    If wanted you can reverse the flow of water to push/pull sludge back and forth.
    EBEBRATT-Ed Member Posts: 13,433
    1" is the smallest recommended for a wet return. One reason is that a smaller pipe could plug up easier. Weather you run 1", 1 1/4" or 1 1/2" the labor is about the same and the material cost difference is slight.
  • Precaud
    Precaud Member Posts: 369
    Thanks for your description, @JUGHNE . It's as good as a picture :)

    So opening the far end of the wet return, attaching a waterhose to the boiler draincock, and forcing water in through the boiler and out the wet return end is not sufficient for a flush?
    1950's Bryant boiler in a 1-pipe steam system at 7,000 ft in northern NM, where basements are rare.
    JUGHNE Member Posts: 10,564
    edited March 2020
    That method might work if you had no blockage in the return.
    If pressure built up in the boiler then water would rise in the piping up to the radiators/vents.
    Or if enough pressure built up to open your 15 PSI relief valve then that would be another mess.
    Water/sludge pushed up into air vents, relief valve and controls could cause problems.

    IIWM, I would want the return flushing process isolated from the boiler.
  • Precaud
    Precaud Member Posts: 369
    Understood. Doing it my way would be a low-pressure flush only.

    I'll probably be cleaning mine out manually this spring. This weekend I found a 2" round wire brush on a 40" stem that the prior owner left behind. It'll be messy but it should do the job.

    Adding those valves and tees to my system would be quite a project. It will probably wait until the boiler gets replaced.
    1950's Bryant boiler in a 1-pipe steam system at 7,000 ft in northern NM, where basements are rare.
  • dopey27177
    dopey27177 Member Posts: 885
    Where true condensate lag exists the use of water seals and some piping will remove the condensate lag condition.
    The drawing shows the corrective measure for a dry return.

    This can also be done for a wet return. In the wet return the water seal is not needed.

    Enclosed also is how riser drips can be used to cure other problems and or add additional radiators to a building provided the boiler has the capacity for additional heating needs.

    Three pages are taken out of my book steam the perfect fluid for heating and some of the problems.