Infloor Pex - 288sq ft. One 300' loop or two 150' - what's better?

[MrModus]

Hello all,

I've been reading through the forms and have seen a ton of advice that a 1/2" pex loop shouldn't exceed 300'. Totally get that. I have a 20' x 14' pad I want to do for a shed/garage. My question is, should I have a single loop, since it will be under 300'. Or is it better somehow to have two 150'(ish) loops instead? I'm struggling to find anything definitive on it, other than exceeding 300' is bad.

And I guess a second question is, does it matter what the loop design is? I know it needs to be continuous, 12" apart, secured every 3' etc. If I just use a serpentine design down the 20' length will this be ok?

Thanks!

 

[PoorUB]

Run one loop. I have done loops larger than 300 feet in 1/2" fairly often. It is not the end of the world if you go over, just a recommendation. We did a 10,000 sqft shop, all in 1/2" all with roughly 400 foot runs in 1/2". The owner of the building wanted to see if it would work and it did, just fine. (I worked for the guy, and he did a lot of floor heat)

As for layout I would prefer to make a couple runs around the perimeter, then fill in the middle.

 

[jack stand]

Since there's no second chance, and all it costs is a manifold with another pair of ports -$20? I'd follow recommendations without hesitation. I'm sure it will work, but it may take days to even out the slabs temperature or in the worst case, leave you with cool areas. Isn't this a big part of radiants feature or benefit?
In your fairly small space, wrestling a 300' coil to "lay" properly would be another consideration for me.

 

[MrModus]

Thanks both. The single run will be under 300' since it's only a 14' x 20' slab. So if there's no real benefit to doing two 150' runs then I think I'd just prefer the one.

@PoorUB Any particular reason for circles around the perimeter first?

 

[PoorUB]

Run the hottest part of the loop where you really need the heat. I should have added this is for outside walls. It might not make a big difference, but this is the way I was taught from Rahau.

Also you can fill in the center and cheat the 12" spacing in the middle of the floor if you run short. We have done a lot of floors with 18" spacing in the middle of shops and not one complaint.

 

[Toolfool]

Two shorter loops will be more efficient.

 

[PoorUB]

You might get more heat into the floor faster with two loops, but more efficient? In which way? Also, just for an example, if it takes 10,000 BTU to heat a space why use 20,000 BTU? More is not always better.

Which floor heat manufacturer recommends 150 foot, 1/2" loops? None that I am aware of.

 

[jack stand]

Also, just for an example, if it takes 10,000 BTU to heat a space why use 20,000 BTU? More is not always better.

It doesn't work that way.
He may have "spent" the usable heat in his water in the 300' loop long before the time it reached......
Cold (or overly heated) areas can be the result, for what cheaping out on under $50 for a 2 loop manifold?

 

[PoorUB]

Why does every floor tubing manufacturer recommend 300 foot loops for 1/2" tubing if you say it doesn't work efficiently?

 

[yeldogt]

What is the heat source?

Radiant is very forgiving ...... it's also impossible for most people to compare one setup of vs another. Most people say -- I did this and it works great. My neighbor did this and it works great ... vs what?

Think about what you are doing .... think about the hot water going into/ thorough a slab. What is going to happen with one loop of Pex vs two ? If you pump hot water into the 300' -- what's the temp coming out the other end? What about the 150' .... what one is colder?

The 300' is colder. The two loops will deliver more heat to the slab faster. What if you did 4 loops of 150 and did the loops at 6" -- how much more heat (BTU's) can you deliver to the slab ?

Does it matter ? Maybe ....

When you have outdoor reset incorporated into the system and it on constant circulation ---- eventually you get to a fairly stable slab with longer loops and wide spacing. But it's not going to be as stable as a slab with more loops -- or a slab with tighter spacing.

If you want the slab to be more responsive -- do the two loops. The tiny cost of a few fittings and valves is worth it.

I always do 6" or 8" spacing in my slabs depending on the application -- inside a house always 6" and it is noticeable --- you get a faster response

 

[MrModus]

Thank you all for your thoughts on this, I appreciate it. You've all given me a lot to think about and provided some great insight and logic. I think I will do the two loop approach purely based on the responsiveness consideration.

Thanks so much!

 

[andyvh1959]

I like the logic of how much heat is effectively put into the slab with two loops versus one loop. In my 24x28 shop I ran two loops, each starts from the midpoint on the 28' side along the perimeter and then back and forth on 12" spacing to meet up 12" from the other loop returning to the manifold. Plus if you have two loops can you give one slightly more flow priority than the other? For me the north loop routes closest to the OH door end of the shop, versus the south loop routing closer to where I'll be doing most of my work. If I can adjust the heat to slightly higher to the south loop than the north loop I can get better heat where I'll feel most of it.

 

[yeldogt]

I like the logic of how much heat is effectively put into the slab with two loops versus one loop. In my 24x28 shop I ran two loops, each starts from the midpoint on the 28' side along the perimeter and then back and forth on 12" spacing to meet up 12" from the other loop returning to the manifold. Plus if you have two loops can you give one slightly more flow priority than the other? For me the north loop routes closest to the OH door end of the shop, versus the south loop routing closer to where I'll be doing most of my work. If I can adjust the heat to slightly higher to the south loop than the north loop I can get better heat where I'll feel most of it.

Most people don't look and space and .... Think. Look at the space and "see" the tubing ..... that's where you are going to get the heat. Put more tubing into one spot -- not shocking you will get more heat there.

What you did was smart ... when doing a house -- where do you need more heat ? When doing a garage where do you need less heat ? If you run more tubing in a bathroom -- it's going to be warmer. If you run less tubing -- or better "zone" the Pex in the area ... you can lower the flow.

Most people just start running the PEX around ... without thinking about the space. You can always do a full coverage and then run an extra loop in the area where you often work ... a simple turn of the manifold or a zone valve gets you extra heat only where you want it.

In most garages there is no reason to heat under the cars as much as back where the benches are -- if you zone or set them up correctly it's easy to do. Save some money on running costs if using propane.