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# Formula for GPM

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Member Posts: 397
Is there a formula to determine pump gpm, when the pump curve is not available? Something other then a stopwatch, and a bucket? We have a extremely old pump that the manufacturer is un known. No nameplate
Thanks to all.

• Member Posts: 3,626
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You can make a guess by looking at the pipe size, connected load, ΔT, etc., but aside from a true measurement, I think you're outta luck.

Post a pic, someone may recognize it.

• Member Posts: 7,569
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ratio said:

You can make a guess by looking at the pipe size, connected load, ΔT, etc., but aside from a true measurement, I think you're outta luck.

Post a pic, someone may recognize it.

This is a good idea in any event. There is no saying it was sized correctly to begin with.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 7,376
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If you know the btus it's carrying, and the delta T, then use the universal hydronic formula: btus / (delta T X 500) = gpm.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 22,143
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Depending on how accurately, or how badly you want to know
You can rent ultrasonic flow meters, they clamp on the outside of the pipe.
Motor HP size, rpm, diameter could get you a close estimate.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 397
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Thanks to all. I was using the delta td x 500. And will try and get a picture
• Member Posts: 128
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Do it right. Figure out the connected load, BTU's required. Once this is known you can calculate the required flow. Once you have the correct flow you will need to figure out the friction loss through the piping and heat emitters, plenty of rules of thumb and charts to help with this. Once you have the required flow and a good idea of the friction loss it is easy to select the correct circulator.
• Member Posts: 7,569
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If you get to the point where you are looking for a decent SWAG and the load is unknown, plug in the GPM for the pipe size at 4ft/sec and use a delta T of 20. For example 1" copper carries 10.9 GPM at 4 ft/sec. 10.9 (gpm) x 500 (constant) x 20 (delta) =100,900 btu/hr. I have found that the majority of engineers design this way. In some cases, you can check a known part of the original design to verify the original engineer's preference and then apply the formula to the unknown portion.
http://s3.supplyhouse.com/product_category_files/11448-Flow-Rate-Chart.pdf
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 998
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Do it the simple way that works. Get a B&G System Sizer dial. By turning the dial to the approximate net BTUs ( you may do a combustion test, new condensing boilers have the Delta T and combustion rate) plus the Delta T will show you the GPM. No math and I have used it in many litigations.
• Member Posts: 22,143
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Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream