Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is pressure loss a good thing ?

  1. Nov 10, 2006 #1
    In a normal water based heating system (boiler, radiators etc) I'm told that having a high pressure-loss over the radiator valves is a good thing? I'm just having trouble seeing why, since I always considered pressure-loss as the enemy (head-loss etc). Why is this good?

    Also, I hear that there should be a certain amount of loss over balancing valves? Supposedly because it eases measuring?
  2. jcsd
  3. Nov 10, 2006 #2


    User Avatar

    Staff: Mentor

    You are correct: without a pressure drop, there is nothing to measure (or adjust) when you are balancing.

    We're doing a project right now where my boss accidentally selected the wrong size circuit setter (a 3" instead of a 2") and when I tried to balance the flow through it, our hydronic manometer could not give steady readings because the pressure drop was too low.
  4. Nov 10, 2006 #3
    Depends on what you are doing.

    Two position (open and closed) water or steam, a low pressure drop is desired. Only require a drop of 10% of available pressure.

    Modulating temperature control, such as Outdoor Reset, using a 3-way valve, the drop wants to be low, maybe 20% of available pressure.

    Proportional control through a radiator or coil, the pressure drop wants to be higher, as you said. Industry standard is 3, 4, or 5 psi drop across a water control valve in a hydronic system. Or it can be equal to the pressure drop across the coil being controlled.

    Proportional control in a 15 psi steam system requires 80% pressure drop for good control.

    So, low pressure drop if you want to redirect or just stop the water flow (two position and modulating 3-way).

    Higher pressure drop required if you need to just reduce or change the amount of flow (proportional control).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook