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Re: Please Check my calcs

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Posted by HeatPro on July 01, 2006 at 21:59:31:

In Reply to: Please Check my calcs posted by Eichler on July 01, 2006 at 18:11:23:

No, it is not reasonable nor the way to calculate head loss for parallel loops.

The head loss for circulator calculation is the resistance of the longest of the parallel loops at the flow of all the loops.

If you have 6 loops 250 feet long, the head required is the head of 250 feet of 1/2" tubing at 0.5 gpm in each of them. So the head required in each of the 1/2" tubes according to the table is half of 1.2 gpm @ 250 feet which is less than 6 ft head. ALL of the 6 circuits still need only 3.2 gpm at 250 feet. Though the table only goes as low as 6 feet head, you need less head than that. A Taco 007 circulator will handle 3.2 gpm @ 10' head, so is far more powerful than actually needed.

You are not feeding the circuit of one 250 foot length into the end of the next 250 feet ciruit six times- they are not in SERIES, they are in PARALLEL, so only need the LONGEST of ONE of the lengths in parallel to establish the resistance for the TOTAL gpm.

Your 3 loops of 2.4 gpm is less than the loops above, Each loop requires .8 gpm - easily handled, so the Taco can still handle BOTH groups of tubing. A Taco can handle 5.6 gpm @ 9 feet head, which is still much more than the less than 6 feet head required for each loop.

The same with the single loop of 150 feet length for 1.2 gpm in the bath,
so the Taco can handle the ENTIRE HOUSE requirement of 6.8 gpm with its ability to handle that flow at 8 feet head.

The ERROR here is NOT UNDERSTANDING what PARALLEL means and what it means to the flow of the water. ALL the water divides its flow of 6.8 gpm up between 10 loops, so an average of .68 gpm flows in each one against a maximum distance of 250 feet. One ordinary residential circulator can handle an ENTIRE 70,000 btuh heat load when using loops no longer than 250 feet.

Putting a high-head circulator like the UP26 will destroy it in a year when it doesn't have a resistance to work against and it runs below its lowest allowable flow.

Take a look at the Knight 150 to see what its resistance actually is - if it is 15 feet head just to get through the boiler - it is something NOT to use. The 14.7 kPa flow is NOT a pump requirement. You are actually flowing 7 gpm, so the head requirement of the Knight is MUCH lower, in fact, it is NEGLIGABLE.

ALSO, you are using a radiant system, so don't expect a fast heatup. You aren't going to get 40 btuh from the floor per sq ft unless you make the floor so hot that you can't stand on it.

I have no idea why you are using a 136,000 btuh boiler for a 70,000 btuh heat load. Perhaps you are thinking you'd be able to 'get away' with having the boiler off until February, then heat the house up in a short time - don't count on it.

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