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Go with the flow: Getting the flow rates right with hydronics

July 14, 2017
Hydronics: of, relating to, or being a system of heating or cooling that involves transfer of heat by a circulating fluid (such as water) in a closed system of pipes. (Source: Merriam Webster.)

The human body is absolutely amazing. The average human has 60,000 miles of blood vessels. For every pound of fat you gain, the body creates seven miles of new blood vessels all by itself. No wonder I’m tired. I’m busy creating seven miles of new blood vessels all the time.

Over your lifetime, your heart will pump 48 million gallons of blood (about 2,000 gallons per day). To get an idea of how much blood that is, if you opened your kitchen faucet to full blast, it would take it 60 years to flow as much water as your heart does in its lifetime. To put it another way, that’s 120,000 bathtubs full (or a million barrels) of blood.  

And if the body springs a leak, most of the time it will repair itself. If it doesn’t, well…

Arteries to capillaries, then capillaries to veins, and it starts all over again. There are 300 million capillaries in your lungs alone.

Your heart will beat 2.5 billion times during your life: faster when you exercise; slower when you are resting. All by itself. Fortunately for you, your birth certificate doesn’t come with an expiration date, so you don’t know when the warranty is going to run out.

The fascinating part of all this is that your body uses fluids (mostly water) to do all this work. It uses fluids to heat, to cool, to transport nutrition, to carry away waste, to transport oxygen, and to redistribute energy. It has its own filtration system and its own wastewater treatment facility. All built in. 

And it does all this work with only one pump, four valves and some pipe. We can learn from this. Which brings us to hydronics.

Hydronics: of, relating to, or being a system of heating or cooling that involves transfer of heat by a circulating fluid (such as water) in a closed system of pipes. (Source: Merriam Webster.)

Sounds a lot like the human body, doesn’t it?

Water vs. air — no contest

Why does water work so well, anyway?

One reason has to do with the hydraulic nature of water. Pascal’s law (1650 A.D.) says that pressure in a liquid is transmitted equally in all directions. For example, if I put water in a closed vessel and apply pressure at any point, the applied pressure will be transmitted to all sides of the vessel equally and instantly. So if I give water a little push on one end, water pops out the other end instantly and with very little energy expended.

The Swiss physicist Daniel Bernoulli, who came along about 100 years after Pascal, added that a decrease in pressure will cause an increase in the speed of a fluid occurring simultaneously throughout the length of the pipe. Reduce pressure, move water. The Swiss are so clever! It is also important to note that it was the Swiss who also invented the holes in cheese, which greatly reduced their calorie count and cut their shipping costs considerably.

The second reason water works so well is its ability to store heat. This we can measure with specific heat, which is the amount of heat required to raise the temperature of a quantity of a substance by one degree.

Every substance has a different specific heat. For example…

·      Water has a specific heat of 1. It takes 1 BTU to raise 1 pound of water 1°F.

·      Air has a specific heat of 0.24. It takes 0.24 BTUs to raise 1 pound of air 1°F.

Water, therefore, has a higher specific heat than air. Let’s use Chart 1 to compare the “energy transport capability,” or ETC, of water to air, calculated by multiplying the specific heat of a substance by its density (pounds per cubic foot).  

·      For water, if I multiply its specific heat (1) by its density (pounds/ft3), it will equal an ETC of 62.4. That means that a cubic foot of water can transport 62.4 BTUs of heat that has been raised 1°F.

·      For air, multiplying its specific heat (0.24) by its density equals an ETC 0.018. That means a cubic foot of air can transport 0.018 BTUs of heat when raised 1°F.

If you divide 62.4 by 0.018, you get 3,467. Which means water is 3,467 times better at carrying heat than air. So if I wanted to transport 1 BTU in 1 bucket, I would need to have 3,467 buckets of air to do the same thing.

That’s why they call it “forced air.”

Nobody is going to carry 3,467 buckets of anything, much less air. They would need to be “forced” to do it. Think about this for a moment. Don’t you make insulation by trapping air? Then why would you use insulating material to transport heat? You’d need the I.Q. of a sweet potato to even consider it.

Practical example

Let’s imagine you’re in your living room, binge-watching “Gilligan’s Island” to determine who looks better, Mary Ann or Ginger. You get cold because you haven’t moved in two days. You need a couple of buckets of BTUs. You store your buckets of BTUs in your boiler down in the basement, so you want to transport them up to the living room.

You call out to your personal concierge, Mr. Thermostat, and say, “I need a couple of buckets of BTUs up here straight away!”

Of course, Mr. Thermostat is, in actuality, just a switch on the wall that is incapable of hearing, so you have to get off your sofa and turn the dumb thing up. Click. Mr. Boiler now sends a couple of buckets of BTUs up in a ¾-in. pipe. If it were air, you would need an 8-in. x 14-in. duct to do the same thing. Moving heat is effortless with water; not so much with air.

So let’s go a bit deeper. How many buckets of BTUs do I need and how many buckets of BTUs can I send through a pipe anyway? You can’t put five pounds of potatoes in a three-pound sack, right?

Since all pipe has friction, our job will be to overcome the friction of the pipe. I can’t make the water flow too fast, or it will erode the pipe. I can’t make the water flow too slow, or it will allow entrained air to come out of solution. Usually, water flow should be no slower than 1.5 feet per second, and no faster than 4 ft./sec. for copper pipe and 8 ft./sec. for PEX pipe. (It turns out PEX pipe can take quite a beating in the erosion department without leaving its post.)

To determine the actual flow rates — the number of “buckets” — I must first do a heat-loss analysis to calculate how many BTUs I need. Then I can plug that BTU figure into a flow-rate formula to determine required flow rate in gallons per minute (GPM):

GPM = BTU/hour ÷ (ΔT x 500).

The ΔT (or “Delta T”) is the difference of temperature of the supply water verses the return water — typically 20°F. Let’s assume my BTU requirement is 100,000. The formula and its calculation would be:

GPM = BTU/hour ÷ (ΔT x 500)

GPM = 100,000 ÷ (20 x 500)

GPM = 10

In short, I would need 10 gallons per minute. Now, what size pipe would I need to transport 10 GPM? The formula comes from the brother/sister physicist team from the Duchy of Grand Fenwick named Ben and Ilene Dover. It goes like this:

Din =

Din = inside diameter of the pipe

V = 4 feet per second for copper, 8 feet per second for PEX.

GPM = 10 in this example.

If you plug those numbers into the formula, you would get 1.1-in. for copper and 0.71-in. for PEX. Rounding off our calculations, we could use 1-in. copper or ¾-in. PEX.

We’ve got the right flow and the right pipe — we’re now ready for the next step!

Those are just a few thoughts on “going with the flow.” I would like to hear your thoughts on the subject or any other ideas you might have. Feel free to contact me at the email address shown at the very end of this article.

To finish this installment, I came across another very interesting number while doing research on the human body. Did you know that your body sheds 1.5 million skin cells an hour? Where are they going anyway?

Which reminds me … we really need to talk about indoor environmental quality (IEQ) soon. Buildings are getting sicker by the year. Best regards and happy heating.

Steve Swanson is the national trainer at Uponor Academy. He actively welcomes reader comments and can be reached at [email protected].

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