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Head Loss Calc for 1/2" PEX Line with Elevation gain

Dean707 Member Posts: 2
I'm completing a radiant heating system that was partially installed by the prior owner of my home. He ran 1/2" PEX to the top floor of the house and connected it to 4 Messana Ray-Magic ceiling panels. He did this because there was no way to expand the existing (forced air) system to this part of the house (it is very old, for starters). The panels have been pressure tested but otherwise never used. I have designed a single loop system but am uncertain about pump sizing. The Messana panels are low flow (0.22 GPM for optimal efficiency) but the system will be supplied from the garage which is fully 40 feet below the installed panels. How much should I adjust my head loss estimate?

Thank you! Dean.


  • Ironman
    Ironman Member Posts: 7,376
    edited June 2019
    None, if it's a closed loop. What it takes to go up is cancelled by what goes down.

    What you will need to do is make sure you have .434 psi of cold static fill pressure per foot of elevation + 5 psi for a safety cushion. Also make sure that the circulator is "pumping away" from the PONPC (where the expansion tank connects to the system).
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    The head loss element doesn't change with elevation. Remember that the pressure lost going up there will all be regained coming back down (think roller coaster). So all you need to consider is the usual -- total length of pipe in the longest loop, plus all the various fittings and so on.

    What does change is the static pressure to which you set your expansion tank. The static pressure difference between the basement and the high point is about 18 psi in this instance. Conventionally, for good operation, one wants to add about 5 psi to the pressure from the elevation difference -- or in this case, a total of 23 to 25 psi as the setting for you pressure in the basement.

    Now... this is rather close to the normal pressure setting of the pressure relief valves for heating systems (30 psi). So... several cautions, since you are pushing the envelope a bit. First. Pump away from the expansion tank. Don't even think about any other pumping arrangement. Second, oversize the expansion tank. That static pressure is with the system cold and the pumps off. What you don't want to happen is to allow the pressure to rise to 30 psi when the system heats up. Therefore, select the pressure tank size in the usual way -- and double it. Don't skimp. Third -- and related -- install reliable pressure gauges at the tank inlet and at the inlet and outlet of the pumps. Once you get the system operating, you won't need to check them much, but they're going to be handy on setup. Don't skimp here, either. Fourth, if we assume that you are going to use a pressure reducing valve to connect to the domestic water system -- quite likely -- don't skimp on that, either. You want a valve which reliably operates to maintain the set pressure, and you want to set it rather carefully.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,517
    4 panels @.22 gpm each piped in series=4x .22=.88 gpm That's how much water the pump needs to move. Elevation make no difference in a closed loop as @Ironman & @Jamie Hall pointed out.

    To figure the head the pump has to pump against measure how many feet of pex are installed both supply and return. Find out from the manufacturer what the flow resistance of the pex is at .9 gpm. (they usually give you the resistance in 100' of pex) so if you have 75 feet of pex multiply the resistance by.75, if you have 150 feet multiply by 1.5 etc.

    Whatever you get for total resistance multiply that by 1.5 to allow for fittings. To this add the resistance of (4) panels at .9gpm. This will give you a safe total to size the pump.
  • Dean707
    Dean707 Member Posts: 2
    Guys, I sincerely appreciate your thoughtful responses. Thank you.