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Author Topic: Gpm= BTU  (Read 3429 times)

BigAlsc

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Gpm= BTU
« on: October 13, 2015, 11:34:49 AM »
Just having a hard time with calculations on flow rate of my system. I have a grundfos 26-99 pump with what I calculate as 28' of head with 1" pex and a dt of 20°. Should be around 7 gpm but that should only be 70,000 btu. Should need 200,000 for the house. I must be missing something, maybe the ft/sec is more than 4. How do you determine that? Or is the w/w plate exchanger where I'm gaining?
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hondaracer2oo4

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Re: Gpm= BTU
« Reply #1 on: October 13, 2015, 05:31:52 PM »
Did you use this? http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf  That is what I used and walks you step by step.
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BigAlsc

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Re: Gpm= BTU
« Reply #2 on: October 13, 2015, 05:42:24 PM »
Yes that's what I used but it shows 1" pex at 7.5 gpm at 4 ft/sec, but not sure where 4 ft/sec comes from. And that only equals about 75,000 btu which is no where near enough to heat my house but it does. Also it doesn't give pex fitting length equivalent, so I rough estimated based on copper.
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hondaracer2oo4

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Re: Gpm= BTU
« Reply #3 on: October 13, 2015, 06:20:28 PM »
7.5 gpm is the max efficient gpm that you can push through 1 inch before it starts to get unruly in the pipe. You can flow more but you will need alot more power to make smaller and smaller gains. You can probably push 25 gpm through 1 inch but the pump would be ENORMOUS! This is the reason I ran 1 inch pex lines each way because I didn't want to undersize my BTU needs not knowing what the house was really going to demand since it is a very old house and I had no previous data to go on other then the 200k btu oil furnace. Do you really need 200k? What on earth are you heating??? My 2600 sqft house in New Hampshire that is 220 years old needs about 90K per hour on a -15 degree day.
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BigAlsc

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Re: Gpm= BTU
« Reply #4 on: October 13, 2015, 06:45:59 PM »
Heating 3200 sqft plus uninsulated basement/ crawl space that's open to house. Half the house is early 1900 not the greatest insulation and other half newer and decent insulation and just OK on windows( double pane Anderson but not a fan). Just going by 200,000 gas boiler that was hereon btu requirements.
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hondaracer2oo4

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Re: Gpm= BTU
« Reply #5 on: October 13, 2015, 07:06:04 PM »
Already have the lines in the ground I assume? If so you'll need to go with a big pump. Something like the bell and gosset nrf36. Try to shove as many gpm through it as possible. Once you get going for the season you can check your delta t at full load.
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BigAlsc

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Re: Gpm= BTU
« Reply #6 on: October 13, 2015, 07:29:43 PM »
Yes, everything existing. Just trying to figure out the gpm. Has a little trouble keeping up when really cold. Will just have to run it into a bucket and time it to get a close estimate.
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hondaracer2oo4

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Re: Gpm= BTU
« Reply #7 on: October 13, 2015, 07:43:45 PM »
If you have been having trouble keeping up when its cold that is probably because your delta T is to big. Do you have any way to measure your delta t across the HX? You'll want to try to  get that to around 20 as you have read. Looks like your only pushing 3 gpm with that 26-99. Looks like you should get around 6-7 with an NRF 36 with 28 feet of head. That would probably fix your issue.
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RSI

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Re: Gpm= BTU
« Reply #8 on: October 13, 2015, 09:31:49 PM »
A PL36 would be better than an NRF36.
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BigAlsc

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Re: Gpm= BTU
« Reply #9 on: October 14, 2015, 03:59:41 AM »
I think I understand now. Use the rough gpm number with the HL number to determine the pump and then the HL number gives you the actual gpm. Also I was using the wrong grundfos pump chart ( different beginning letters). The chart was for single speed not 3 speed.
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NaturallyAspirated

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Re: Gpm= BTU
« Reply #10 on: October 16, 2015, 06:16:25 PM »
28' of head is pretty significant, equivalent to about 220' of 1" PEX at 7.5GPM.  If that's indeed the loss you have, you will be dissipating about 72,750 or so BTU with a ΔT of 20°F

For 1" PEX, and the rest of the previously assumed numbers you could run up to about 9.8GPM, to stay at or under 4f/s velocity.  You will need a pretty stout pump however as your head would increase to about 44'. You would be extracting about 95,000BTU.   :thumbup:

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mlappin

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Re: Gpm= BTU
« Reply #11 on: October 16, 2015, 10:03:53 PM »
4-6 ft/second of water flow is around the ideal number, too slow lets say around 2 ft per second and you may never get all the air out of the lines, you start approaching 8 ft per second of flow and it can become noisy and cause erosion of any metal fittings.
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BigAlsc

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Re: Gpm= BTU
« Reply #12 on: October 17, 2015, 07:00:27 AM »
Thanks for all the info everyone. Just a rough estimate on head, ordered 200' of pex, may have cut off a few feet here or there, I think 6 pex 90s, 1 male and 1 female pex adapter, 2 pex ball valves and a thirty plate exchanger. Didn't know head loss on fittings so just a rough estimate.
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