“That’s A Magic Number”

Jack Black – math hero!

Math is a really cool thing.  And it helps make sense of this crazy thing we call hydronics.

Got a question today on one of my favorite topics: how to figure out the head loss through an iValve or a tempering valve.

First, we need to understand the concept of Cv.  Cv is to piping components what belly buttons are to people…everybody has one!  More accurately, Cv is an expression of the pressure drop through a piping component.  Specifically, the Cv rating represents that amount of flow needed in order to create a pressure drop of 1PSI, or 2.31 feet of head, across a piping component.

Let’s look at a 1” Taco 5000 series 3-way heating-only tempering valve, used in a radiant floor heating system, which has a Cv of 2.3.  What that means is that at a flow rate of 2.3 gallons per minute across the valve, the valve will impart 1PSI of pressure drop, or 2.31 feet of had loss, on the fluid.

This head loss is in addition to the head loss you’ll calculate through the PEX tubing and hard piping in the radiant system, and will have to be added to all that head for accurate circulator sizing.

Note the Cv of 2.3 is particular to that specific valve.  Different valves have different Cv values.  The ¾” Taco iValve – a 24v motorized mixing valve with outdoor reset –  has a Cv of 4.5, which means that the valve will impart 1PSI of pressure drop on the fluid at a flow rate of 4.5 gallons per minute.  That means the ¾” iValve has creates less pressure drop in the system than the 1” tempering valve.

Different valves, different values.

But how do you figure out the pressure drop when the flow rate you’re trying to pump through the valve is different from the valve’s Cv?  Well, there’s a math formula for that….

(F ÷ Cv)2 x 2.31 = valve head loss

F = required flow rate
Cv = Cv value of the piping component

So let’s say we have a radiant system that requires 6 GPM worth of flow, with a piping head loss of 7 feet.  If we are considering using a 3-speed circulator, you can see we’d be using a Taco 0015 at Low Speed. But we need to factor in the valve head loss, using a Cv of 2.3 for the 3-way tempering valve:

(F ÷ Cv)2 x 2.31 = valve head loss

(6 GPM ÷ 2.3 Cv)2 x 2.31 = valve head loss

(2.6)2 x 2.31 = valve head loss

6.76 x 2.31 = valve head loss

15.6′ = valve head loss

Uh-oh.

You’d have to add the valve head loss of 15.6′ to the 7 feet of piping head loss for a total head loss of 22.6′.  The new operating points for the circulator are now 6 GPM at 22.6′ of head:

That sure as heck takes you out of a standard 3-speed pump, doesn’t it?

You’re looking at a Taco 0011.  It’s a great pump,  Pricewise, however, it’ll be higher than your 0015.

Now, the attraction of tempering valves is that they’re inexpensive and non-electric.  But here’s a clear case of an inexpensive valve forcing you into a more expensive circulator. But what if we chose a Taco iValve – a motorized 3-way mixing valve with outdoor reset built in?  Sure, it’s a more expensive valve, but its higher Cv/lower pressure drop (4.5 Cv) gets you back into a standard 3-speed circulator:

(F ÷ Cv)2 x 2.31 = valve head loss

(6 GPM ÷ 4.5 Cv)2 x 2.31 = valve head loss

(1.33)2 x 2.31 = valve head loss

1.77 x 2.31 = valve head loss

4′ = valve head loss

So now we’re talking about 6 GPM at a total of 11′ of head, which puts us back into the 3-speed 0015, with the circulator running comfortably at Medium Speed.

The “less expensive” valve can lead to a more expensive circulator, while a “more expensive” valve can lead to a less expensive circulator.  If the total is the same, why not give the customer the benefit of outdoor reset?

Too much to think about?  Todd Snider sympathizes…

“Stuck between hope and doubt…”

2 Responses to ““That’s A Magic Number””

  1. Nice job John

  2. Can we make it so that we can just email this site to others like we tweet?

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