Is pressure loss a good thing ?

  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. russ_watters

    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. 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).
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