Calculating vs. Estimating…

A new blog feature…The Song of the Week!

Every week we’ll post an obscure but interesting song from the Barba iTunes library.  To start things off, here’s a classic from the original King of Song Parodies, the late, great Allan Sherman.

A true classic...

You know him best for “Hello Muddah, Hello Faddah.” Here’s a hilarious collection of mishigas (look it up) called “Schticks of One, Half a Dozen of the Other.”


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Press to play!

And now, back to the business of head loss….

Last time we discussed the easy way to estimate head loss.  The key word here, of course, is estimate.  For a quick review, click here.

That’s a pretty simple way to estimate head loss in a piping circuit.  Do that and you’ll be safe (as long as you’ve sized the pipe properly).  You’ve given yourself some “fudge factor.”  A lot of fudge.  With nuts.  Extra sugar.

We’re talking FUDGE!

There is a more accurate way to calculate (as opposed to estimate) head loss. Do the first two steps in exactly the same way: measure the run and then multiply by 1.5.  Now, instead of multiplying 1.5, you need to look at the following chart found in the Copper Tube Handbook from the Copper Development Association (download here):

Click to enlarge

This chart gives you the pressure loss per foot of pipe for Type K, L and M copper at different flow rates, with pipe sizes ranging from ½” to 2”.

Now, the sample job we’ve been working on calls for a flow rate of 6 gallons per minute (look here for how we came up with that).  We also calculated last time that the longest piping run in our system was 140 feet.  So let’s do our thing…

140 × 1.5 = 210 total developed length

The first thing we’ll do differently from the easy head loss method is to actually calculate the flow rate for that individual zone.  Looking at the chart, note the pressure loss (which is expressed in PSI, not feet of head – more on that in a few paragraphs) is based on the actual flow rate through the zone. We’ll need to use the Universal Hydronics Formula to figure that out.  Let’s say the longest zone on the job has a load of 38,000 BTUH.  Remember we were also using a Delta T ofv300, as we’re using a mod-con boiler and panel radiators:

GPM = 38,000 ÷ (30 × 500)

GPM = 38,000 ÷ 15,000

GPM ≈ 2.5

So the required flow rate for that zone is just a shade over 2.5 GPM.  Let’s look at the chart a little more closely….

Click to enlarge

On the left you see flow in GPM and on top you see pipe sizes and types.  Just for fun, let’s try both ½” M and ¾” M.

The pressure loss at 2 GPM for ½” M is 0.024 PSI per foot of pipe.  At 3 GPM the pressure loss is 0.051 PSI per foot of pipe.  Since our flow rate is 2.5 GPM, we’ll split the difference and call it 0.038 PSI per foot of pipe.  Next, we’ll multiply that number by the total developed length of 210’

210’ × 0.038 PSI/foot = 7.98 PSI pressure loss

To turn that number into head loss, we’ll multiply by 2.31 (1 PSI of pressure loss equals 2.31 feet of head):

7.98 × 2.31 = 18.4 feet of head

So our circulator would need to produce 6 GPM while overcoming 18.4 feet of head.  We could do that if we needed to, but it’s very likely we’d encounter some velocity noise along the way.  Instead, let’s consider ¾” M.  The chart says the pressure loss of ¾” M at a flow rate of 2.5 GPM would be roughly 0.0065 PSI per foot:

210 × 0.0065 PSI/foot = 1.4 PSI pressure loss

1.4 x 2.31 = 3.2 feet of head

Quite a bit lower, ain’t it?

And remember, this is the worst-case zone (we don’t need to add the head loss of all the zones together. It’s a parallel piping system.  If the circulator can overcome the head loss of the worst case zone, it can overcome all the others). Overall, we’d need a circulator that could deliver 6 GPM at 3.2 feet of head.  Last time, when we estimated head loss, we were looking at a circulator that provided 8.4 feet of head.

That’s 2-and-a-half times more head pressure than we need.  We wound up selecting a Taco 007 the first time, but knowing what we know now, would our circulator selection change?

By looking at the chart, we’d be fine with either a 005 or a 006.  However, since those circulators are more expensive and not as commonly stocked at the 007, the most likely choice would be the 007.

So why not stick with estimating?  Good question!  There will be times when estimating may cause you to select a circulator larger than what you need, which adds cost to the job and may lead to velocity noise in the system.

Also, a job such as this nicely demonstrates the value of a variable speed circulator, such as the Taco 00-VDT.  The VDT is a variable speed circulator that changes its speed based on the amount of BTU’s taken out of the fluid at a given point in time, which is reflected in the system Delta T.  If the Delta T gets smaller than design, in this case, 300, the circulator will slow down so it provides the proper flow rate.  If the Delta T gets wider than 300, then the system BTU demand is going up (meaning it’s getting colder outside, or more zone valves are opening), then the circulator will speed up to provide the proper flow rate.

By doing this, the circulator always provides the proper flow rate for the system, always provides the widest possible Delta T at the boiler (important for efficiency), always eliminates possible velocity noise, and uses minimal electricity.  All in all, not a bad thing…

Let’s finish with some questions for you…

  1. How do you select circulator for your jobs?
  1. Does the difference in actual vs. estimated head loss surprise you?
  1. Did you enjoy the Allan Sherman tune?

Also, here’s  the intro to the Hot Wheels Cartoon, circa 1969, featuring the very cool Jack Rabbit Special….[youtube][/youtube]

Next time, more on steep curve circulators vs. flat curve circulators…

5 Responses to “Calculating vs. Estimating…”

  1. michigas……….definition: rednecks from the upper penninsula part of michigan (which could really be a part of the real definition since they are all crazy and chaotic from up in that area any)

  2. Thanks John!

    I really appreciate these blog’s and learn alot that I can share with our field technicians!!

    Steve, The “rednecks” in the U.P. have some of the best snowmobiling short of the big mountains. Most of them are quite honorable people.


  3. I concur with Hilpipre, (finally SOMETHING we can agree on), these are great tutorials. Kudos to Taco and Barba. My guys get to go through these with me on Tuesday and Thursday mornings during in-house training. Concise and easily digested. ( just like my breakfast). Geez, JB, you ought to think about doing this for a living.

  4. Thanks guys….

    Rocky — ready for a live get together? Can do a powerpoint/video conference for your guys…up to five computers can log in…don’t need to have ooVoo…although a webcam puts a face to a name. Lemme know!

    And did you guys like the Allan Sherman tune? That’s the important question!!!


  5. JB,
    Would love to. The next few days are a little up in the air. My wife and I have to fly to Seattle for some medical tests. I don’t know exactly when we will be back in good ol’ Bearflanks. Would love to finally do this with my guys, but I will have to let you know on the dates.

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