How do I determine pipe size for steam to water conversion?

bxwildshot Posted By bxwildshot, Feb 29, 2008 at 1:30 AM

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  1. bxwildshot

    bxwildshot
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    Feb 29, 2008
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    I am planning to convert over from steam to hot water boiler hopefully. I would like to run the majority of the new piping myself but I'm a little confused as to how to figure out what size pipe to use. I realize it has to do with delta t's and flow rates but I am unclear as to how that works. I will be be using the oversized cast iron rads I have and hopefully be running 160 degree water. Can any one explain the math to me in layman's terms? Thanks

    Eric L.
     
  2. slowzuki

    slowzuki
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    Start by reading the hot tech issues on www.heatinghelp.com it will give you a really good idea of where to start.

    Are you single pipe steam currently?
     
  3. BrownianHeatingTech

    BrownianHeatingTech
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    First, you need to figure out how many gallons per minute you will be using for each piping run.

    GPM = btu / (500 * dT)

    (dT is the change in temperature from the supply to the return - you probably want to design for 20 in your case)
    (if you will be running 160-degree supply water, and a 20-degree dT, you need to size the radiators for 150-degree water, since that will be the average temp)

    For example,if you will be running a loop that will dump 20kbtu into a room, and that will all be one zone...

    GPM = 20,000 / (500 * 20)
    GPM = 20,000 / 10,000
    GPM = 2

    So now you know that the 20kbtu loop will have 2 GPM through it.

    You want to stay below 4 feet per second in residential systems, so you need to compare the pipe sizes and flow rates to that. Any pipe sizes that are small enough to push the velocity at that flow rate over 4 feet per second should be dropped from the list.

    Next, you need to figure out the pipe length. Be a bit generous, and account for the head loss in the radiators, as well - it ends up as an equivalent length of pipe at different pipe sizes. Knowing the flow rate, you can find out the head loss for each pipe size at that flow rate and water temperature (use the 150-degree average temp, in this example).

    Check the pump curves for your circulator pump, and find out how many feet of head it produces at the flow rate (2 GPM, in the example), then compare that to the head loss of each pipe size. You want to run a reasonably-small circulator and still be just over the head loss of the loop. There's really no set way to do this, other than to balance the circulator size with the pipe size using good judgment. You don't want to try to cram too much water through too small a pipe by using a gigantic circulator. You also don't want to use 2-inch pex just to use a tiny circulator. Use some common sense.

    If you will be zoning with zone valves, the (single) circulator needs to be sized for the head loss of the worst-flowing zone, and the flow rate of all zones open. This is where bumping pipe sizes up can be important. If the worst zone is going to force you to use a larger pump, it may make sense to pipe that zone in a larger pipe size, even if it cost a bit more for the piping, in order to keep the pump size more reasonable for the other zones.

    Joe
     
  4. slowzuki

    slowzuki
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    There are other piping arrangements possible too, everything can be off the same loops with TRV's and a semi restrictive bypass at each rad. Or as Joe says plus put a pressure bypass around the pump so when zones shut it doesn't crank up the head on the remaining zones. Tons and tons of other options too. You could circ continuously and use a controller on the mix to have even heat. Your rads are oversized so this can be a very nice option to get smaller pipe sizes.
     
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