Maximum linear feet of baseboard heat with 5/8" PEX

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Old 05-06-12, 08:51 PM
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Maximum linear feet of baseboard heat with 5/8" PEX

I am looking to add baseboard heat to an existing zone for a single floor addition. I've completed a heat loss calculation using the SlantFin tool as recommended in other threads and came up with 14' of additional baseboard heat needed for the new space. My current system uses 5/8" PEX and I currently have 34 linear feet of baseboard radiator in the zone. I've found maximum recommended lengths for 1/2" PEX (25') and 3/4" PEX (67'), but the calculation does not appear to be linear and I haven't found the number for 5/8" pipe. Can I safely add an additional 14' to the existing loop?
 
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Old 05-07-12, 05:03 PM
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Are you sure the numbers you got for 1/2 and 3/4 are correct? I think PEX is slightly smaller ID than copper, and the numbers you posted are exactly what one would spec for copper...

And you are sure that you have 5/8 pex? You read this from the stamping on the tubing?

OD of 5/8 is around 0.75 and the ID is a bit over 1/2. (brands differ a bit)

The ID of 1/2" COPPER is also a bit over 1/2" ... (more tightly controlled)

Normally 1/2" copper is spec'd at about 15K BTU ( 25' of fin-tube at 600 BTU/F )

The same for 3/4" copper ( ID of 0.811 ) 40K BTU ( 67' of fin-tube at 600 BTU/F )

SOOOO, your 5/8 PEX is close to 1/2" COPPER, and with 34' already installed, you are already pretty much over the limit.

The reason the calculation isn't linear is because it's based on a circle. Good old PI comes into play. It's all about the AREA of the circle.

came up with 14' of additional baseboard heat needed for the new space.
Good... yer on the right track then. But at what water temperature is this calculated?

Reason to ask is this:

As the water flows through the system, it will give up heat as it passes through the baseboard sections. This means that you may in fact have slightly COOLER water than you calculated for.

If the new area will be downstream of the existing, you should compensate for this by using a slightly lower water temperature and perhaps adding a couple feet extra baseboard.

If the new area is UPstream of the old, you may find that the existing area will be a bit cooler than desired.

I think I would suggest a home run back to the boiler manifold.
 
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Old 05-07-12, 10:36 PM
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Yup, the 5/8" is stamped right on the PEX (OD looks to be 3/4").

I spoke to a local plumbing supplier today and they told me that the maximum for 5/8" PEX is roughly similar to the maximum for 3/4" copper (67'). It sounds like your saying that the maximums for PEX are actually less that that of copper, so now I'm really confused.

The water temperature I used for the calculations was 180 with a 20 degree maximum drop.
 
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Old 05-08-12, 12:30 PM
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5/8 " Pex will flow around 4 gpm with a 4 ft/sec flow rate.
This would yield a head loss of about 15' per 100' of pipe (in addition to your convectors).
4 GPM = 40,000 BTU
at 500 BTU per foot = 80 feet of rad.
That is by the book engineering. There may be factors that prevent you from getting these results.

You will need to ensure your circulator can overcome the total restrictions in the circuit.. Tubing, convectors, valves, fittings, boiler etc...

I suspect 14 additional feet would not be a huge burden, but if your system currently is at 20 degree delta t and the pump os not trimmed by a valve then you really don't have any more head left in the circulator to overcome the additional baseboard.
 
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Old 05-08-12, 03:55 PM
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I dunno TO... if the ID of 5/8" PEX is just slightly more than that of 1/2" copper, I don't know that I'd treat is the same as 3/4" copper.

at 500 BTU per foot = 80 feet of rad.
Most fin-tube is rated at 600 BTU / FT which would yield 67 feet.

Let's look at it this way:

If we normally spec 40K BTU max emitters (67') for a 3/4" COPPER system why would one presume that a SMALLER tubing ( 5/8 PEX with 0.5xx" ID ) can be used for the same heat output?

Even 3/4" PEX is STILL smaller than 3/4" COPPER.

COPPER TYPE M

..........ID
1/2....0.569

3/4....0.811

BARRIER PEX

..........ID
5/8....0.574

3/4....0.677

See what I mean? 5/8 PEX is durn near exactly the same ID as 1/2" copper.

How can one expect to use 5/8 for more than say 20K BTU total?
 
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Old 05-08-12, 04:23 PM
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Not that I have to tell you, but it's all about flow and restriction.
If your willing to push the water, then you can move more BTU.

As a designer you need to look at how long the run will be, and how much it costs to flow the circuit.
We all have been used to oversized pipes and not using the circs to there fullest.

It does not take much of a circ to move 4 gpm at 15 FOH. a 15-58 on high will do it.
Taco 008 is close.

All things being equal, if the delta t is 22 or so who is gonna complain as long as the room is warm.
 
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Old 05-08-12, 06:28 PM
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True all that, agreed...

I haven't 'run the numbers', and sure you can push 4 GPM through .574 ID tubing, but my gut feel is telling me that's too fast... maybe 6-8 FPS, no?
 
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Old 05-09-12, 08:57 AM
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I just did it quick when I did it and based it on 5/8" ID but it was only 4.137 FPS...

Just checked, watts 5/8 pex has an ID of .584.. so the velocity would be slightly higher.

At 4 GPM the velocity is just under 5 GPM which is still quite acceptable given that it's pex pipe and its for heating.

It is closer to the top of what I would do, but I would be ok with it myself.
Our Delta T's here in NA are less than what Europe uses (about 15 C) and they have been using hydronic heating for a bunch of years, so if the delta T made it to 23 F or so due to the additonal rads one would be able to live with it.

If adding radiation, add it to the return leg if possible and add a few feet to compensate for lower water temps...
 

Last edited by TOHeating; 05-09-12 at 09:17 AM.
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Old 05-10-12, 05:12 AM
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If we normally spec 40K BTU max emitters (67') for a 3/4" COPPER system why would one presume that a SMALLER tubing ( 5/8 PEX with 0.5xx" ID ) can be used for the same heat output?
Are the heat outputs calculated based on the nominal size or the actual? I would assume nominal because the actual ID of copper pipe changes based on wall thickness. The OD needs to stay the same for the fittings to be interchangeable.
 
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Old 05-10-12, 03:18 PM
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The amount of heat you can pull out of a particular pipe size loop is based mostly on the fact that we try to design for a fast enough flow to keep air from accumulating and slow enough to keep the pipes from getting scoured on the inside and eventually developing pinhole leaks... and to not make 'noise'. This is about 2-4 Feet Per Second.

With a velocity of 2-4 FPS flowing, you need to figure out the area and volume of the pipe to determine how many GPM will flow in the pipe at that velocity. For 3/4" copper, 4 GPM is about in the middle of this range.

If one is designing for a 20 delta T, an easy number to remember is that you can push about 10K BTU for every GPM.

So 3/4" pipe at 4 GPM is good for (roughly, more or less) 40K BTU.
1/2" is about 15-20K, 1" is like 70-80K

There really is enough 'slop' in the numbers that the wall thickness of the pipe is irrelevant. I mean, the difference between 2 and 4 FPS is pretty huge... but that's what the design is really all about, controlling the velocity of the water in the pipe, and everything else comes after that.
 
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