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Is pressure loss a good thing...?

by TSN79
Tags: loss, pressure, thing
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Nov10-06, 12:33 AM
P: 357
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?
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Nov10-06, 05:31 AM
P: 22,305
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.
Nov10-06, 10:16 AM
P: 1,591
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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