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# Can I use 1/2" pex for the supplies and returns for Slant Fin Jumbo heating element?

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Member Posts: 29
edited July 2022
1/2" Pex maxes out at 600 BTU/foot. But the Jumbo element states 790 BTU at 1 GPM. How is this possible? I can run 3/4" pex if needed

• Member Posts: 2,835
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How much baseboard will be on line ? Series system ? What type Pex ?

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• Member Posts: 29
edited July 2022
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From a manifold. 3' regular element, then to 5' Jumbo element, then back to manifold. Two separate rooms. Uponor hePex. 1' hePex from boiler to manifold.
• Member Posts: 4,109
edited July 2022
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You can safely run 15' of the H-1 Jumbo element with a 1/2" PEX supply and return at a 20°ΔT.

1.2 gpm x 10,000 = 12,000

12,000 ➗ 790 = 15' of BB
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 29
edited July 2022
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If I wanted 840 BTU at 4GPM, would I need to upsize to 3/4"? But then, would everything else on the manifold have to be 3/4"? I have a 3-speed circulator by Grundfos.

You can safely run 15' of the H-1 Jumbo element with a 1/2" PEX supply and return at a 20°ΔT.

1.2 gpm x 10,000 = 12,000

12,000 ➗ 790 = 15' of BB

• Member Posts: 4,109
edited July 2022
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Yes, you would exceed the recommended velocity of 1/2" PEX.

If you go with 3/4" PEX for that one loop of BB, all the other loops would be independent. That is, 1/2" loops are fine.

But if you max your pump, the other loops will be effected and you might pick up some cavitation noise.

It sounds as though there's not enough BB in those bedrooms?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 29
edited July 2022
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Yes, you would exceed the recommended velocity of 1/2" PEX. If you go with 3/4" PEX for that one loop of BB, all the other loops would be independent. That is, 1/2" loops are fine. But if you max your pump, the other loops will be effected and you might pick up some cavitation noise. It sounds as though there's not enough BB in those bedrooms?
This is for a laundry room and a bathroom in the new addition.

For the laundry room, I’m okay with 3’ of baseboard because I get hot when I am doing chores haha. And I dont have any wall space.

The layout of the bathroom and the framing below will cause me to split the heat into 2 runs. The laundry 3’ BB and bathroom 5’ BB are physically back-to-back so I can run those two BB is series. The bathroom will have other BB in other locations due to the layout and framing. Bathroom total area is 100 sqft. I can mount a panel radiator elsewhere to bump up the BTUs in the bathroom. Or a heated towel rack
• Member Posts: 4,109
edited July 2022
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I can mount a panel radiator elsewhere to bump up the BTUs in the bathroom. Or a heated towel rack
Perfect. You can keep your velocity nice and slow. Good hydronics.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 9,981
edited July 2022
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Did you do a heat loss and figure out if you need that much heat in that room?

The 600 btu/ft for pex if if you are using the pex as a heating element, if you are using it as the emitter in a radiant application.

In this case you are concerned with the flow needed to get the output you need from the emitter you are connecting to it, not with what can radiate from the tubing.
• Member Posts: 29
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mattmia2 said:
Did you do a heat loss and figure out if you need that much heat in that room? The 600 btu/ft for pex if if you are using the pex as a heating element, if you are using it as the emitter in a radiant application. In this case you are concerned with the flow needed to get the output you need from the emitter you are connecting to it, not with what can radiate from the tubing.
I did not do a heat loss. I just googled “how many BTU required per sqft in MA.” It said 60 BTU / sqft. So, according to that, I need 6000 BTU. Then since it’s a bathroom, I want it hotter, so I just added a few more hundred BTU.

Would there be any downside of me running ALL 3/4” pex from the manifold? Other than more difficult of snaking it through the house. More electricity from the circulator? I’d get a little bit more BTU according to SlantFin.
• Member Posts: 4,109
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There's a Slant-Fin app that lets you figure the heat loss of your rooms. Spend some time to do this so you can feel good about how you heat the room.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 22,505
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You are getting some bad info 60 btu/ sq ft required in MA?? You need to do the room by room heat load specific to your building. Even 1/2 that is a high load, 30 btu/ ft

Use a delta P circ with any zoned system and velocity noise will not be an issue.

Have you ever worked with 3/4 Pex?

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 29
edited July 2022
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There's a Slant-Fin app that lets you figure the heat loss of your rooms. Spend some time to do this so you can feel good about how you heat the room.
That’s a pretty good app. I just got done with my first floor in the app. I’m not sure what to enter as “outside temperature” so I put 0 F.

Based on that, the ball park 60 BTU/ sqft is close. I’m short a couple feet of baseboard but I am limited by the layout of the house.
• Member Posts: 29
edited July 2022
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I have never worked with any Pex. But my plumber left some short scraps of 1/2” and 3/4” uponor pex. I can feel how tough the 3/4” is to bend and manipulate.
• Member Posts: 4,109
edited July 2022
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Is it AquaPEX or hePEX? Do not use anything without an oxygen barrier. It will say so on the tubing.

Sixty BTU per square foot? You must have single-glazed windows, two outside walls with no insulation. That's pretty high.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 29
edited July 2022
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Is it AquaPEX or hePEX? Do not use anything without an oxygen barrier. It will say so on the tubing. Sixty BTU per square foot? You must have single-glazed windows, two outside walls with no insulation. That's pretty high.
HePex.

I must be inputting the inputs wrong in the app then. What “outside temperature” am I supposed to input? I am inputting 70 F for inside.

Okay the 100sqft bathroom says it needs 2940 BTU/hr. That’s 5.5ft of regular SlantFin.
• Member Posts: 4,109
edited July 2022
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What's your closest large city in MA?

8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab
• Member Posts: 29
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Boston Logan Airport
• Member Posts: 1,245
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You need to enter the AHRI design day temp for your region.

• Member Posts: 29
edited July 2022
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Thank you PC7060.

I'm confused about "infiltration factor". I believe I have 2 possible choices and either one will make a huge difference.

1) "Windows and Doors weather strippped > Rooms with windows or exterior doors, two sides" 0.18
2) "Rough construction > Rooms with three outside walls" 0.68

Both of those options apply to me....