Flow Calculator

 
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lsayre
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Post by lsayre » Tue. Dec. 15, 2015 7:53 pm

This online flow calculator is great. I never imagined that water flowing at 4 GPM through a 3/4" type M copper pipe (0.811" inner diameter) is only traveling at a lowly 2.4843 FPS.

http://www.1728.org/flowrate.htm

You can solve for GPM of flow if you know the diameter and length of the loop and how long it takes the return leg to get warm once the water starts flowing through it on the supply leg side.

Now I need to time how long it takes each of my zone loops return legs to get hot after its zone valve opens and the circulator kicks on. My zone valves are on my return legs.

For example, the return leg of a 130 foot long zone loop with water flowing through it at 4 GPM should take about 52 seconds to warm up. 130 ft/2.4843 ft/sec= 52.33 seconds


 
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Post by lsayre » Tue. Dec. 15, 2015 8:33 pm

Regardless of pipe ID, Taco recommends at least 2 FPS of flow in order for their air scoops to function properly and effectively remove entrained air. And it seems that 4 FPS is considered the top desirable flow velocity (I'm initially presuming to prevent noise and inner pipe wear).

Flow velocity is calculated by:

FPS flow = (0.408/ID^2)*GPM

Where ID^2 = inner pipe diameter squared
And where GPM = gallons per minute of flow
And where FPS = feet per second of flow velocity

For 3/4" Type M at 0.811" ID
----------------------------------------
2 FPS = (0.408/.811^2) * GPM
GPM = 3.2241

4 FPS = (0.408/.811^2) * GPM
GPM = 6.4482

So for 3/4" Type M copper the ideal flow should lie somewhere between 3.22 and 6.45 GPM.

All of the functions performed by the online calculator I linked to above can be solved by this formula.

 
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lsayre
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Post by lsayre » Wed. Dec. 16, 2015 9:48 pm

Of course once you know your actual flow in GPM you can plug that into my friction head spreadsheet (see thread on this topic) and solve far more definitively for your closed loop system head.

Or better yet, go straight to your circulators pump curve, and know beyond a shadow of a doubt what your systems head is. Then fine tune my spreadsheet to your system by altering the "averaging" 1.5X factor to suit your systems "actual" closed loop vs. straight line pipe adjustment factor.

 
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Post by scalabro » Wed. Dec. 16, 2015 10:01 pm

You really need to get out more Larry.

 
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Post by swyman » Sat. Dec. 19, 2015 6:43 am

So what do I do, shut my circulator off till the pipes cool off then turn back on and time till pipe feels warm?

 
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Post by lsayre » Sat. Dec. 19, 2015 6:53 am

swyman wrote:So what do I do, shut my circulator off till the pipes cool off then turn back on and time till pipe feels warm?
As long as you don't freeze the pipe, that would be one way to gauge your flow in FPS, through which you will be able to understand your flow in GPM. This would have been better done in warmer weather. How accurately do you know your total loops run in feet? Do you know the actual ID for the type and brand of 1" PEX that your loop is using?

 
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Post by swyman » Sat. Dec. 19, 2015 6:54 am

Hey, what if I look at the temp gauge on my return in the basement and the gauge on my return at the boiler. When the furnace runs the temp will drop on the basement gauge and I could start timing then run outside to the boiler and see when that gauge starts to drop then measure that distance? I think that should do it?


 
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Post by swyman » Sat. Dec. 19, 2015 6:56 am

lsayre wrote:
swyman wrote:So what do I do, shut my circulator off till the pipes cool off then turn back on and time till pipe feels warm?
As long as you don't freeze the pipe, that would be one way to gauge your flow in FPS, through which you will be able to understand your flow in GPM. This would have been better done in warmer weather. How accurately do you know your total loops run in feet? Do you know the actual ID for the type and brand of 1" PEX that your loop is using?
I have a 330' tape measure so I could get an accurate measurement. Also have a spare piece of pex but would I have to measure the I.D. of the fittings since that is the smallest diameter?

 
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lsayre
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Post by lsayre » Sat. Dec. 19, 2015 6:58 am

swyman wrote:Hey, what if I look at the temp gauge on my return in the basement and the gauge on my return at the boiler. When the furnace runs the temp will drop on the basement gauge and I could start timing then run outside to the boiler and see when that gauge starts to drop then measure that distance? I think that should do it?
Yes, I believe that will give you a good ballpark of the FPS flow velocity over that distance.

 
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Post by swyman » Sat. Dec. 19, 2015 7:01 am

Sweet, I will see if I can get this done today. Thanks for the help

 
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Post by swyman » Sat. Dec. 19, 2015 8:10 am

Took 1:39 but I need to get exact measurements of ID and length. Looks to be around 4 gal/min but will get specifics in a couple hours

 
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Post by lsayre » Sat. Dec. 19, 2015 8:19 am

swyman wrote:Took 1:39 but I need to get exact measurements of ID and length. Looks to be around 4 gal/min but will get specifics in a couple hours
IF you know what kind of 1" PEX you are using, its ID should be readily available on the internet.

 
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Post by swyman » Sun. Dec. 20, 2015 6:43 am

Ok, took 1:39 to go 215'. The 1" pex ID was .825 but the fittings used in this pex measured .6875.....not sure what to use but I would imagine .6875 as that is the smallest passage in the line? SO that would be what either 3.6gpm or 2.5gpm correct? That being the case I am sure I totally have the wrong circulator....just wasting money running it? Now how do I work the pump sizing charts?

 
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Post by lsayre » Sun. Dec. 20, 2015 7:42 am

swyman wrote:Ok, took 1:39 to go 215'. The 1" pex ID was .825 but the fittings used in this pex measured .6875.....not sure what to use but I would imagine .6875 as that is the smallest passage in the line? SO that would be what either 3.6gpm or 2.5gpm correct? That being the case I am sure I totally have the wrong circulator....just wasting money running it? Now how do I work the pump sizing charts?
Stick with 0.825" ID, as this is really a volume related calculation, and your elbows, etc... have little to contribute to volume, whereas your PEX represents the vast majority of the loops volume.

215 feet/99 seconds= FPS = (0.408/.825^2)*GPM
GPM = 3.62

There is nothing inherently wrong at all with having 3.62 GPM of flow. The measurement method employed is crude and is only giving you a ballpark. The error bars here could perhaps be 25% or more to either side.

As the others have told you, your boilers efficiency has absolutely nothing to do with (is totally independent of) your circulators GPM of flow. But you can now gauge your underground (etc...) baseline for heat loss from the flow if you reliably know your temperature drop across the system when no "users" are calling.

For example, if your standard loop temperature drop from supply to return when no users are calling is 5 degrees, and your flow is 3.62 GPM, then:

BTUH = 5 x 500 x 3.62
BTUH = 9,050

Assuming 75% boiler efficiency (the thing all of the others are trying to assist you in achieving):

9,050 BTUH output / 0.75 = 12,066 BTUH input

This requires the burning of about 1 pound of anthracite per hour, or 24 lbs. per day just to overcome the your loops baseline of losses to the ground, etc... It establishes your minimum daily coal consumption target.

So from this I can tentatively conclude that if your no users calling temperature drop is 5 degrees (my guess, you will need to accurately determine this) and when the others have guided you to a fully dialed in system you will know that (for 5 degrees of drop) you will burn only 24 lbs. of coal per day as your baseline. Add to this the unavoidable daily average coal consumption for DHW demand, and call it a baseline of roughly 30 lbs per day.

So if you are presently burning through 80 lbs. or more per day, your initial goal (through the assistance of the others helping you) is to reduce this to 30 lbs. per day. But that would be for a day when the home is not calling for boiler supplied heat and all you are doing is overcoming the losses to the loop and supplying DHW.

Conclusion: If the others who are trying to help you achieve proper boiler efficiency can get you dialed in, you will know that you are dialed in if your daily baseline of consumption drops to within the ballpark of 30 lbs. per day.

Then, from there, your consumption to supply the baseline "user" plus all of your other "users" will be determined by your total heat load requirement, and the daily "heating degree days, or HDD's" for your area. And your measure of this will be the temperature drop across the system when all users are calling.

For example, sticking with 3.62 GPM flow, and seeing a temperature drop of 50 degrees from supply to return with all users calling means:

BTUH output = 50 x 500 x 3.62
BTUH output = 90,500

And BTUH input = 90,500/0.75 ~= 120,650

120,650 BTUH input/ 12,250 BTU/lb. = 9.85 lbs. per hour of anthracite being consumed each hour.

Only your real world total of heat load demand will determine if burning 9.85 lbs. per hour on the coldest day of the year is sufficient to bring heating comfort for that day. But notice that burning about 240 lbs. on only the single coldest day of the year is much better than burning that much every day, as I believe you initially stated that you were doing.

And don,t let the 120,650 BTUH input figure concern you (vs. your previous 500,000 BTU rated wood boiler, this figure itself likely being wildly overstated), as most people find that their true heat load demand (on the single coldest day of the year) is far below what they imagine.

For example, my homes total heat load demand worked out to being only about 38,000 BTUH output (roughly 50,700 BTUH input) on the coldest single day we experienced here in the past 5 heating seasons. I would have guessed much higher if I was only guessing.

 
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Post by swyman » Mon. Dec. 21, 2015 8:58 am

Thank you, that is very interesting and informative! I think over this Christmas break it would be in my best interest to move the boiler into the attached garage.


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