# PEX BTU Ratings



## mattd860 (Mar 11, 2011)

I just want to make sure I'm doing the right thing here before I dig myself a hole that I can't get out of. I have a 2400 sqft home which includes 700 sqft in the finished basement that never gets heated. I just purchased a Harman SF-160 Wood Boiler rated at approx. 90,000 BTUs to heat a max 2200 sqft home. My plan is have the boiler piped in series with the oil boiler to heat the DHW coil in the oil boiler. Down the road when I get a storage tank, I will pipe the wood boiler in a parallel loop with the oil boiler. Additionally, the new wood boiler is being placed in the garage, about 30ft away from the oil boiler located in the basement. So the loop between the two boilers is about 70ft (35ft supply + 35ft return). The circulator pump is a grundfos UPS15-58FRC with a max flow range of 17GPM and head range of 19ft. 

The boiler manual suggests 1-1/4" pipe between the two boilers so I bought 1-1/4" PEX tubing. However, the pex tubing is NOT 1-1/4" ID, it is more like 1-1/16" ID. 1-1/4" copper pipe can handle around 160,000 BTU's/h but 1" copper can only handle 75,000 BTU's/h which is less than my boiler is rated. 

So what are the ratings of the PEX pipe? Are BTU/h ratings based SOLELY on pipe size or is pipe material also a factor? If pipe material is a factor, does PEX tubing flow more or less than copper? If pipe size is the sole factor, then what kind of MAX BTUs/h should 1-1/16" pipe be able to carry??

Thanks


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## in hot water (Mar 11, 2011)

11- 12 GPM in 1-1/4" pex so about 120,000 btu without a problem.


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## barkeatr (Mar 12, 2011)

I Just spent a lot of time researching this,.  Most stuff i found that was credible says that pex has higher flow rate for the same actual measured I.D., its due to the flow of the pipe and better flow through fittings. basically they said there is at least equal flow between one inch pex with one inch copper even if the actual measured I. D. of the pex is smaller.  

the right size pex will leave you with more options as far as a circulator...less cost and better durablity, less power usage. 

good luck


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## afblue (Mar 13, 2011)

http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

I always reference this document, its has flow rates for different size PEX. PEX-AL-PEX, and copper, but its also a planning guide for the next step in your project, head pressure and the pump.


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## SteveJ (Mar 13, 2011)

Kind of interesting that they reference Type M copper.

With Type L copper, the 0.75" max flow drops from 6.5 gpm to 5.4 gpm - see thread https://www.hearth.com/econtent/index.php/forums/viewthread/72618/

Also, copper.org recommends keeping the velocity below 3 ft/s for temps above 140F.


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## heaterman (Mar 13, 2011)

A Wilo S21, or grundfos 15-58 *on speed 1* will move all the btu's your Harmon can produce given that diameter and length of pex.

Nice job on selecting the 1.25" pex. Your pump will love you and operate with minimal electrical requirement all the days of it's long and happy life.


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## mattd860 (Mar 14, 2011)

heaterman said:
			
		

> A Wilo S21, or grundfos 15-58 *on speed 1* will move all the btu's your Harmon can produce given that diameter and length of pex.
> 
> Nice job on selecting the 1.25" pex. Your pump will love you and operate with minimal electrical requirement all the days of it's long and happy life.



Good to know. I have the pump screaming on speed 3 (highest speed) right now. I guess I will lower it.


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