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Author Topic: Opinions on using pex pipe less than 1" inside diameter  (Read 2506 times)

jemmunro

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Opinions on using pex pipe less than 1" inside diameter
« on: September 26, 2017, 05:48:25 PM »

I've read many posts from members of this forum strongly advising to stick with using pex pipe that is no less than 1" inside diameter to prevent restriction of water flow.

One of the dealers that we are considering purchasing an OWB from wants to use Rehau pex pipe which has an inside diameter of .875 which if I understand correctly, means that the inside diameter of this pex pipe is 1/8" smaller than 1". 

This dealer told us that it makes no sense to worry about staying with 1" inside diameter for the pex pipe because the inside diameter of the piping used inside the house & other buildings has an inside diameter of less than 1". 

Any thoughts or advice? 

Thanks.


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mlappin

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #1 on: September 26, 2017, 06:17:54 PM »

It depends on your end heat requirements. My old set up I was using standard 1” pex (7/8” ID) this worked just fine for just DHW (domestic hot water) and a furnace HX (heat exchanger). However whenever I tried to run my snow melt return temperatures quickly plummeted and the boiler couldn’t keep up.

Regardless of the water temp, it takes flow to get large amounts of BTU’s delivered to the final source. A simple way to look at it, temperature makes the heat, flow decides how much heat can be made.

My new setup uses Logstor (slightly larger than one inch) and all 1” copper pipe in the basement. I can run the dishwasher and both washing machines while the house is calling for heat, the snow melt is on and the wife is taking a shower with no issues of return temps dropping a large amount on the return to the boiler.

What your dealer is talking about is most likely fine if your just looking to heat your DHW and heat the house. If you plan on adding on to the house, adding a hot tub or pool thats to be heated or snow melt in the future that will run off the house circuit then stepping up to a larger underground pipe now would be cheaper in the long run rather than having to deal with issues from insufficient flow later.
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Smokeless

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #2 on: September 26, 2017, 06:27:23 PM »

Takes more energy to push the same BTU threw the smaller pipe over 20 hrs it adds up.
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E Yoder

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #3 on: September 26, 2017, 07:56:58 PM »

Maybe you mentioned in another thread about what type of heating system you currently have? Forced air is pretty easy to get by on smaller lines by oversizing the coil to raise air temps and running lower return water temps.
Copper baseboard is not so easy and needs more flow..
Every system requires a different flow rate. There is no one size fits all. We use 7/8" ID piping all the time for some pretty big heat loads, but it's all forced air.
As the flow rate drops the difference between supply and return temps must get larger to maintain the same BTU rate. We don't know if your system can tolerate that without a few more details.
« Last Edit: September 27, 2017, 06:01:23 AM by E Yoder »
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RSI

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #4 on: September 26, 2017, 10:30:40 PM »

The overall length of the pipe is one of the main reasons for the 1" ID or larger. If it is a very short run then it won't matter as much unless you have a large heat load.
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heat550

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #5 on: September 27, 2017, 01:27:13 AM »

Here is some info to digest . That might help. Others can check my math. I think it's pretty close.

Heat550
« Last Edit: September 27, 2017, 01:29:32 AM by heat550 »
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E Yoder

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #6 on: September 27, 2017, 06:07:04 AM »

I agree with you RSI. Length is big one.

Your math looks correct heat550, assuming the design was around a 20 Delta T.
When we get into larger heat loads we tend to design for a 30-40 degree delta to move more btu's with lower flow. But that so much depends on the system.
I'm not trying argue at all, just love to talk about hydronics.  :)

« Last Edit: September 27, 2017, 06:11:59 AM by E Yoder »
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heat550

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #7 on: September 28, 2017, 01:01:44 AM »

Lots of variables and everybody systems. Makes sense longer the runs the more it change things . And big variable is the design of the boiler and recovery time . Outside temp barometer wind speed. List goes on and on .. :thumbup:

Heat550
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mlappin

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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #8 on: September 28, 2017, 09:11:36 AM »

I agree with you RSI. Length is big one.

Your math looks correct heat550, assuming the design was around a 20 Delta T.
When we get into larger heat loads we tend to design for a 30-40 degree delta to move more btu's with lower flow. But that so much depends on the system.
I'm not trying argue at all, just love to talk about hydronics.  :)

I try to stay away from that large a Delta T unless the homeowner is absolute on the fact they won’t be expanding or requiring more heat for anything in the future. I try to stick to a 20 degree Δ T if possible unless it become prohibitively expensive for larger pipe and pumps.

In the future at our house looking at heating a whirlpool tub once thats installed, also have considered adding heat tubes to a new front porch when we do that for keeping snow off it or just for comfort in the late fall. Have also considered adding another plate exchanger just to run well water thru then running that to an outdoor hydrant we hook the pressure washer to, I have a steam cleaner, however its miserable yo use in the summer and when its cooler the steam makes it practically impossible to see anything, a hot water pressure washer would work well enough the majority of the time.
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Re: Opinions on using pex pipe less than 1" inside diameter
« Reply #9 on: September 28, 2017, 05:35:25 PM »

Good point.
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