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# Gravity flow...

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Member Posts: 6
Out of curiosity how do you calculate flow rate (gpm) on a gravity boiler?

• Member Posts: 7,379
edited October 2010
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Use the Universal Hydronics Formula:

Btu/h = Gpm x 500 x Delta T.

In your case, work it in reverse. Take the boiler output Btu/h divided by Delta T Divided by 500 = Gpm.

Do it with the boiler and piping at operating temp. as the flow rate becomes greater with temp rise.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 2,398
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That

"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 7,379
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About the suppliers "Depends" tops 'em all.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 6
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thanks!

Thanks ironman, I appreciate that. I have been in the hvac business 6 years now and I am currently learning more about hydronics, making the transition from primarily working on rooftop units. Hydronic heating and cooling seems way better! haha, more intresting!
• Member Posts: 6
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Gravity flow...

one more question...why does working the universal equation backwards work? How does that get you to the answer?
• Member Posts: 2,398
edited October 2010
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If

you know the boiler output at a given time (not always easy to do!), and you know the Delta-T between supply and return, you have pretty much all you need.

One key point would be to take the temperature measurements with an accurate thermistor setup on a multi-meter, knowing that the surface temperature of the pipe lags a bit, but all things being equal, surface temperatures should be accurate enough. It is the difference we are after.

Say you have a boiler firing at 100 MBH input and 80% efficiency, assume 80 MBH output. (80,000 BTUH). At the same time, you measure the supply pipe at 174.3 degrees F. and the return pipe at 168.2 degrees F. Delta-T is 6.1 degrees.

The "500" is an approximate constant based on gallons over an hour's time, that would be 8.33 pounds per gallon times 60 minutes, which will get you to 499.8, close enough. Even the weight per gallon varies with density/temperature, but still, close enough.

So let's use the numbers:

80,000 / (500 x 6.1) = 26.23 gpm. Not uncommon for an old gravity system with large pipes.

Now, a practical question for you: Are you trying to replicate this flow rate adding a circulator to an old gravity system? Or is it just a curiosity of what is going on in a system under true gravity operation?
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 6
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I asked this question because in this hydronic heating class im in right now someone asked our instructor the same question and our instructor wasnt 100% sure how to do it so he told us to try and find the answer before the next class so here we are. thanks for your help!
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So....

did you get an "A" ?

"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 5,853
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Smart student...

Luv your screen name as well :-)

Tell your instructor that there is only one correct answer to ANY hydronic heating question. That answer is "It Depends..."

We invite you to bring your fellow students and instructor to the worlds most expansive hydronic question resource, The Wall!!

Welcome.

ME

There was an error rendering this rich post.

• Member Posts: 1
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gravity flow

your not working anything in reverse, your just putting the unknown on the left, works with any formula.

btuh=gpm x 500 x delta t

or

gpm = btuh / 500 / delta t

learn one formula and transpose.

whatever you do to one side of formula , do to the other to get the unknow by itself.
• Member Posts: 6
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Thanks

Yea I got an A thanks to you guys haha. Dont worry I gave this site the credit!
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