Cv

icy78

Can someone explain Cv to me. I know it's gpm at 60f water at 1 psi pressure drop. How does that relate to a valve I may come across in a system? For instance I may have a coil that is not heating well and it has a 3-way valve for mixing rated at 2.1 Cv. How would I know if that Cv is too high? How would I determine the pressure drop across it ( without ports)? On another discussion I said the 3-ways were a Cv of 1.9 and was told that was probably too high. Why? Is that data to be used only when designing a system? Thanks in advance.

Robert O'Brien

(F divided by Cv)2 x 2.31 = valve head loss

F = flow rate

Cv = Cv of valve

Robert O'Brien

Change the Cv to ft/head and then add to head of system for total head

hot_rod

Lower Cv valves are usually include in coil kits as they have a higher shut-off pressure.

If for example you had a commercial building with a system pump that could develop 20, 30, 50 psi or more differential pressure, you need to assure the ZV can close off against that head.

Dig deep enough and you find ZVs with various Cv choices.

This spreadsheet show how to determine flow, Cv or pressure drop if you know two values.

Be aware of installing a high Cv valve where a low Cv was designed, that leads to noise and possible ghost flow, not enough shut off pressure.

Use a valve with too low Cv and you see high pressure drop.

If it is a control or modulating type of valve, valve authority becomes another factor.

Zman

I think this thread will help your understanding.
http://forum.heatinghelp.com/discussion/160369/valve-selection-valve-authority-rangeability#latest

Low cv valves are appropriate for applications where the valve has high pressure to work against.

In your case, the circulators are having trouble pushing through the restricted valves. It is also difficult to balance a system like yours because the slightest change on one loop has a dramatic effect on others.

In your case, valves with a cv approximately equal to designed flow rate would be more appropriate.

icy78

Thanks, you guys. I've got some reading to do. One more question then. A valve with a higher Cv, say 10, would then be less restrictive and thus allow for example, 2 gpm thru a coil at a much lower head. Am I on the right track?

Zman

You are correct that a cv of 10 is less restrictive. The trick to getting things balanced correctly is to size all the valves to approximately the same cv to flow ratio.
You will run into trouble if some valves have a cv twice the rated flow and others are 1/2.