"Flow Rate, Head Loss, Chart!!!"

I wrote an article for a trade magazine a couple of years ago about circulator selection.  The inspiration came from a real conversation at a training class, in which a student asked about the easiest way to select the right circulator for a job.

“That’s easy,” says I.  “After you calculate the heat loss (see previous blog posts), you determine the flow rate, figure the head loss and then look at the pump curve chart.  Then you pick the one that best fits the job.”

Pretty simple, right?

“No no no….that’s not what I meant.  I mean what’s the easy way…”

Gang, that is the easy way.  Heat loss, flow rate, head loss, chart.

The guy wasn’t discouraged, though.  He kept after it, wanting to know if there was a simple rule of thumb.  Fortunately, there is a simple rule of thumb:

Heat loss, flow rate, head loss, chart.

He made one last try, asking about those new 3-speed pumps.  Shouldn’t that circulator work for every job?  Wouldn’t that make circulator selection easy?

Heat loss, flow rate, head loss, chart.

Over the past couple of weeks, we’ve shown you the basics in performing heat loss calculations manually.  It’s a pretty simple process, and the only math you’ll need are the four basics – addition, subtraction, multiplication and division.  No sines, cosines or square roots. And trigonometry isn’t even part of the equation! (yuck, yuck – I love math humor!)

Picking the right circulator – even for a small residential job – is a key element in making a job work to the customer’s expectations.  The right circulator is big enough without being too big.  It makes the water go round and round quietly and in sufficient quantities, and it doesn’t make other parts of the system, like zone valves and such, go bang, clang or boom.

First, you have to calculate the required flow rate using the Universal Hydronics Formula, which states:

GPM = BTUH ÷ (ΔT x 500)

GPM is the required Gallons per Minute flow rate needed to deliver the required amount of heat at a given point in time.

BTUH is the required amount of heat at a given point in time.

ΔT is Delta T, or the designed-for temperature drop in the zone or system.

500 is a math shortcut, representing the weight of one gallon of 100% water (8.33 lbs) times the number of minutes in an hour (usually 60) times the specific gravity of the fluid (water has a specific gravity of 1) times the specific heat of the fluid (water has a specific heat of 1).  If we multiply that out, it looks like this:

8.33 x 60 x 1 x 1 = 499.8

Let’s call it 500. If you’re using glycol, check out the bottom of this post!

So let’s say we want to size a circulator for a zone valve job.  We need to know the heat loss for the entire structure under design condition (the “coldest” day of the year).  We know how to perform those calculations after reading earlier posts (click here to review!).

Let’s assume the total load for a 2,600 square foot home in Prior Lake, MN (purely hypothetical, mind you) is 88, 297 BTUH.  And let’s say, for giggles and grins, I want to design a panel radiator job with 4 zones.  And since I’m using a mod-con boiler, I want pretty low water temperatures, so I’m going to design the system with a maximum supply water temperature of 150 degrees, and I want to design around a 30-degree system Delta T.  And because I like to live dangerously, I’m going to use 100% water, with no glycol.

Let’s do some math:

GPM = 88,297 ÷ (30 x 500)


In math, you always do the calculation in parenthesis first:

GPM = 88,297 ÷ 15,000


GPM = 5.886

Let’s call it 6 gallons per minute.  In other words, my system will need to flow 6 gallons per minute of 150-degree water through my system to deliver 88,297 BTU’s under design condition.  The water temperature returning to my mod-con boiler on the coldest day of the year should be in the 120-degree neighborhood, if I size my circulator properly!


Now that we know the required flow rate, we can size the boiler main piping and then determine the head loss through the system.   That’s the next installment.

Fun with Glycol!

500 is the multiplier for 100% water.  Glycol complicates things.  First of all, it weighs a little more per gallon that does water, and it has a different specific gravity and specific heat, depending on the concentration of glycol used, and the specific brand of glycol used.

For instance, Cryo-tek Original at a 100% concentration gives freeze protection to -220F, and burst protection to -800F.  However, it weighs 8.7 pounds per gallon, has a specific gravity of 1.04 and a specific heat of .908.  If you multiplied that out, you’d get something interesting:

Specifications for Cryo-tek

8.7 x 60 x 1.04 x .908 = 492.9…

Let’s call it 493.  Not much different, is it?  If we used that number to calculate the flow rate for our job, it would look like this:

GPM = 88,297 ÷ (30 x 493)


GPM = 88,297÷ 14,790


GPM = 5.97…


Let’s call it 6 GPM.  No difference.

Cryo-tek Original is already diluted and ready to use (so are many other brands.  I’m not promoting Cryo-tek, it’s just the one I found online.  Your favorite glycol will have similar information available).  However, Cryo-tek AG (for Arctic Grade) is not diluted.  If you used that stuff right out of the jug, it would weigh 8.78 pounds per gallon (not so different), have a specific gravity of 1.054 (again, not so different), and a specific heat of .681 (whoa, now THAT’s different!):

8.78 x 60 x 1.054 x .681 = 378

Now let’s calculate the flow rate:

GPM = 88,297 ÷ (30 x 378)


GPM = 88,297÷ 11,340


GPM = 7.78…


Let’s call it 8 GPM, compared to 6 GPM.  May not sound like much, but it’s a 33% increase in required flow, which will also result in a higher head loss for the system.  And that will impact circulator selection.

It pays to know what you’re using!


3 Responses to “"Flow Rate, Head Loss, Chart!!!"”

  1. John,

    I am trying to determine the head loss of a mono-flo system that is using 1″x1/2″ B&G monoflo tees. I have not found any data that gives me the pressure drop through those tees. Can you help?

  2. HI Dan …

    I have a chart that we use in the Compleat Boiler Room class that gives the equivalent feet of pipe for several different fittings…A 1″ Venturi is equal to 14 feet of straight pipe. I’ll post a copy of the chart on your profile page on The Neighborhood. Thanks..


  3. I think you have an error in your glycol calcs.

    The specific gravity is by definition a comparison of the weight of a fluid to that of water at the same conditions. So you can either use the density of glycol directly (e.g., 8.7 lb/gal), or use the density of water along with the specific gravity of glycol (8.33 lb/gal x 1.04 = 8.7 lb/gal).

    Your examples included both the density and specific gravity of the glycol, which understates the required flow.

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