I have a 40x60 pole barn that I am doing a slab in this summer. I plan on doing a radiant setup. I received a quote for 6 300' loops of 7/8" pex spaced at 16". That seems odd to me, I was under the impression that 1/2" pex spaced 12" would be the norm (2400ft of tubing in a 2400sp ft slab). Less loops sounds fine but the cost of the tubing is much greater and I would assume the pump required would be more, along with having to heat to a higher temp? Anyone have any input on if the 7/8 is overkill?
PEX size for slab
- Thread starter elliottw
- Start date
Rusty Bolt
Well-known member
All-in-all, I think 7/8 on 16 inch centers is one of the normal sort of installs for a pole barn. You'll have to run the numbers to see if that's cheaper than a smaller tube that's spaced tighter.
I went to Blue Ridge Company and ordered a system for my 42x40 stick built. I got 8 x 300' loops of half inch PEX on 9" centers. My boiler is a modulating combo unit (provides hot water). I think they gave me so much PEX because my boiler is large (140K BTU) for the space in order to be able to make hot water fast enough and the extra tube makes it faster to put heat into the slab.
AIUI, with 7/8 tube each loop can put more heat into the slab, so they can be spaced wider. The cross section of 7/8 tube is much larger than 1/2 tube, so the loops could be longer. Fewer loops that are longer should save a few dollars on the manifold. Wider spacing also saves labor - it's a lot of work to tie down the tubing so that it doesn't float during the pour.
I doubt the pump requirements will change much. My system has a Grudfos pump that figures out what speed to run all on its own.
I went to Blue Ridge Company and ordered a system for my 42x40 stick built. I got 8 x 300' loops of half inch PEX on 9" centers. My boiler is a modulating combo unit (provides hot water). I think they gave me so much PEX because my boiler is large (140K BTU) for the space in order to be able to make hot water fast enough and the extra tube makes it faster to put heat into the slab.
AIUI, with 7/8 tube each loop can put more heat into the slab, so they can be spaced wider. The cross section of 7/8 tube is much larger than 1/2 tube, so the loops could be longer. Fewer loops that are longer should save a few dollars on the manifold. Wider spacing also saves labor - it's a lot of work to tie down the tubing so that it doesn't float during the pour.
I doubt the pump requirements will change much. My system has a Grudfos pump that figures out what speed to run all on its own.
I had a lot of different recommendation when i did the research for mine. most often people said the larger the pipe the better the heat transfer to the slab.
most people I talked to recommended 3/4" on 16"
but my place is a little smaller (24x32) and I DIY'd everything so it was cheaper to get 1/2" and it was easier to manage un-coiling and stapleing to the floor.
have not gotten the heat source hooked to it yet.. so I can't say if I did the right thing or just wasted a bunch of money in the slab.....
this is what it looked like laying it out.
most people I talked to recommended 3/4" on 16"
but my place is a little smaller (24x32) and I DIY'd everything so it was cheaper to get 1/2" and it was easier to manage un-coiling and stapleing to the floor.
have not gotten the heat source hooked to it yet.. so I can't say if I did the right thing or just wasted a bunch of money in the slab.....
this is what it looked like laying it out.
Lots of bad information.
Think about what you are doing: heating water and moving that water to a hunk of concrete. Think about the heat in the tubing and how that heat is going to be transferred. Think about the tubing layout -- if the tubes are 4" apart in one slab and 16" in another ..... how is the slab going to heat ? What's the center temp of the 16" spacing going to be vs right over the tube?
The primary reason to use larger sized pipe is -- longer runs. If you look at the transfer rate of the piping .. it's not piping to slab ratio playing the biggest part.
The key to any radiant project is to properly insulate -- doing so reduces the overall heat load. Reducing overall heat load reduces the amount of heat needed to be directed to the slab. Lower output = Lower water temp = lower slab temp = lower cost.
So whats all the piping layout talk?
If you are doing a huge industrial space. First the slab will be properly insulated. It's also going to be heated with constant circulation. Even with some reset in the evening, if not used 24/7, the slab is going to be always getting heat. So, let's think about that wide spaced 7/8 tubing .... how long is it going to take to heat up a huge slab of 6" concrete? A long time ... will the spaces between 16" tubing be hot ? not unless you run it a long time ... and industry keeps it warm ...
With wide spacing -- it's time. You have to run it longer .. and the water must be hotter as you a delivering less heat to the slab ... think about it.
1/2 tubing happens to provide a good balance point -- that's why is most often used. Because of this it is produced in the largest quantities -- larger production = cheaper. 3/8 is more expensive as well ... loop length max of 200'. I use 3/8 tubing for some situation .. it works great ... just can't have as long of loops.
10 loops of 3/8 is not going to need a lager pump vs 8 loops or 7 loops of 1/2. You are circulating water .......... not pumping with a high head.
So think about what you want to do .... if you are going to run constant circulation you have time to heat the whole slab with wider spacing -- but don't expect even slab temp or the lowest temp water.
You are always better off going with more loops with closer tube spacing .. again ...visualize what you are doing ?
With long loop lengths -- what is the water temp going to be at the beginning of the loop vs at the end ?
Radiant is very forgiving. You can have a poorly insulated slab and wide spacing -- just get a big boiler and pump very hot water through the tubing .... you will get heat. William Levitt -- built Levittowns in a few states with radiant .. small house on slabs with copper tubing -- no insulation. Oil fired boilers pumping 180 degree water into the slab -- houses got warm. It also melted snow outside.
Using the 1/2 tubing with more loops will work better ....
Also -- think about the temps and where you will be working. If there is an area where you will be storing vehicles .. use one tubing layout. If there is an area where you will be working .... sitting ... or in that area more often. Use a tubing layout that incorporates an extra loop of tubing within your standard layout -- that way you can raise the temp in that area by bleeding in some extra water to that extra loop.
Visualize what you are doing. Remember the manifold location matters.
FYI -- Inside slabs I typically do 6" spacing ... but I have used 4" spacing when I needed very low temp water in a well insulated space.
Think about what you are doing: heating water and moving that water to a hunk of concrete. Think about the heat in the tubing and how that heat is going to be transferred. Think about the tubing layout -- if the tubes are 4" apart in one slab and 16" in another ..... how is the slab going to heat ? What's the center temp of the 16" spacing going to be vs right over the tube?
The primary reason to use larger sized pipe is -- longer runs. If you look at the transfer rate of the piping .. it's not piping to slab ratio playing the biggest part.
The key to any radiant project is to properly insulate -- doing so reduces the overall heat load. Reducing overall heat load reduces the amount of heat needed to be directed to the slab. Lower output = Lower water temp = lower slab temp = lower cost.
So whats all the piping layout talk?
If you are doing a huge industrial space. First the slab will be properly insulated. It's also going to be heated with constant circulation. Even with some reset in the evening, if not used 24/7, the slab is going to be always getting heat. So, let's think about that wide spaced 7/8 tubing .... how long is it going to take to heat up a huge slab of 6" concrete? A long time ... will the spaces between 16" tubing be hot ? not unless you run it a long time ... and industry keeps it warm ...
With wide spacing -- it's time. You have to run it longer .. and the water must be hotter as you a delivering less heat to the slab ... think about it.
1/2 tubing happens to provide a good balance point -- that's why is most often used. Because of this it is produced in the largest quantities -- larger production = cheaper. 3/8 is more expensive as well ... loop length max of 200'. I use 3/8 tubing for some situation .. it works great ... just can't have as long of loops.
10 loops of 3/8 is not going to need a lager pump vs 8 loops or 7 loops of 1/2. You are circulating water .......... not pumping with a high head.
So think about what you want to do .... if you are going to run constant circulation you have time to heat the whole slab with wider spacing -- but don't expect even slab temp or the lowest temp water.
You are always better off going with more loops with closer tube spacing .. again ...visualize what you are doing ?
With long loop lengths -- what is the water temp going to be at the beginning of the loop vs at the end ?
Radiant is very forgiving. You can have a poorly insulated slab and wide spacing -- just get a big boiler and pump very hot water through the tubing .... you will get heat. William Levitt -- built Levittowns in a few states with radiant .. small house on slabs with copper tubing -- no insulation. Oil fired boilers pumping 180 degree water into the slab -- houses got warm. It also melted snow outside.
Using the 1/2 tubing with more loops will work better ....
Also -- think about the temps and where you will be working. If there is an area where you will be storing vehicles .. use one tubing layout. If there is an area where you will be working .... sitting ... or in that area more often. Use a tubing layout that incorporates an extra loop of tubing within your standard layout -- that way you can raise the temp in that area by bleeding in some extra water to that extra loop.
Visualize what you are doing. Remember the manifold location matters.
FYI -- Inside slabs I typically do 6" spacing ... but I have used 4" spacing when I needed very low temp water in a well insulated space.
yeldogt, that information is helpful. I will have R17.5 on the side of the slab and R10 underneath. I also thought the less loops and wider spacing might not be what I want (albeit larger tubing). I was planning 8 300' loops of 1/2 originally.
stingry
Well-known member
Use the 1/2 inch on 12” centers. In all the research I did before I installed my radiant floor, 1/2 pex seemed to be the norm for residential and smaller shops, 5/8 and larger for commercial applications. I used 1/2 inch in my 3800 sq ft shop. It is an incredible form of heat. Uniform, comfortable, quiet and very economical.
Cheers
Steve
Cheers
Steve
More tubing = even heat w/ lower temps. Match the layout to the space and how you will use it. It's a tiny additional investment -- only get one chance.
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As I said above -- radiant is forgiving. I have never done a project with all 12" centers in concrete slab. Will it work .. yes ... is it typically best to simply lay out a space with 12" random zones ... not in my experience. Again -- it's how you are going to use the space. And, also the heat loss in the space.
A garage is also a bit more forgiving ---- so the 1/2" 12 centers is not a bad starting point. More tubing -- closer spacing is never going to hurt. The industry has also moved to less tubing because early on doing specific layouts added cost and the industry wanted radiant to be more competitive. Also -- with American style boilers running hotter -- less pipe was a work around.
Today, the water temp control is not a problem -- both pex and manifolds are cheap. It's normally only a few hundred dollars to tighten up the loops and do a specific layout.
Not if you use that kind of isolation:
took me 1hour alone to install 5 200' run in my 28x44 garage last year..
1/2" - 12" spacing, counterflow run
Craig Balzer
Well-known member
From this on the following website: https://www.ambientfloorheat.com/pages/how-to-plan-a-radiant-system-design
The goal is to have the fluid come out of the tube within 10-15 degrees of the temperature it went in at and move at a slightly turbulent flow as it passes through. The relationships below and on the next page work well although deviations can be acceptable if adjustments in pump sizing are made. Call Ambient Floor Heat for technical assistance if these recommended lengths cause inconvenience.
(the table will not copy/paste -- use above link for diameters of tubing and lengths of each loop)
I'm planning on using 2' x 4' crete-heat panels (288 of them) in Colorado Springs currently priced at $13.50US per panel
The goal is to have the fluid come out of the tube within 10-15 degrees of the temperature it went in at and move at a slightly turbulent flow as it passes through. The relationships below and on the next page work well although deviations can be acceptable if adjustments in pump sizing are made. Call Ambient Floor Heat for technical assistance if these recommended lengths cause inconvenience.
(the table will not copy/paste -- use above link for diameters of tubing and lengths of each loop)
I'm planning on using 2' x 4' crete-heat panels (288 of them) in Colorado Springs currently priced at $13.50US per panel
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Looking over the various specifications on that site -- it's a wide range. Doing the simple math upfront is essential. Heat load and required BTU per square foot for any building w/ any kind of heat.
The radiant has to be designed for the space -- with the understanding that the lowest temp water will give you the highest efficiencies. Radiant has the ability to provide long term savings ..but only with low temp water and the correct boilers.
If you notice they list 8-12 spacing ... over time there has been a movement for wider spacing to make radiant more competitive ... it will still work ... but you don't get the ability for lower water temp in an average building ... and you will not get the low delta they talk about with that spacing and 300 feet.
It's a shame because doing closer spacing and giving some thought to how you will use a space and making custom layout to match can make for a much nicer space with only a bit more $
I still like to use a full VB under the panels --- it's another one of those items that cheap
86turbodsl
Well-known member
I've run radiant in several buildings and run all the numbers multiple times. This is what works best in a pole barn:
1. 1/2" tubing, 250-300ft MAX
2. 12" spacing in center, put an extra loop at 6" on the outside to help with the edge loss.
3. 1' of tubing per sq ft of building floor.
4. Insulate your floor
On a building your size, which is almost exactly the size of my building, it will work great.
The heat is very even. Large tubes on wider centers can make the heating feel spotty at times.
1. 1/2" tubing, 250-300ft MAX
2. 12" spacing in center, put an extra loop at 6" on the outside to help with the edge loss.
3. 1' of tubing per sq ft of building floor.
4. Insulate your floor
On a building your size, which is almost exactly the size of my building, it will work great.
The heat is very even. Large tubes on wider centers can make the heating feel spotty at times.
I think "best" should be qualified. What you describe is "best bang for the buck" . with constant operation -- well insulated -- this will work well. I still do closer tubing because I'm an efficiency nerd and I do not want to "feel" the floor. The extra band is a good idea in many buildings.
Most buildings have different uses going on --
86turbodsl
Well-known member
Yes, of course. I should have qualified that. My apologies. Maybe "typical best" should have been the qualifier.
Sorry if I came off too "wanky" ......
I did my first radiant almost 30 years ago -- while radiant was not new by any measure ... it was just getting started in the USA with PEX. Even PEX had been around for 20+ years ... we also did not have the controls or low temp capable boilers in the USA. At least in a consumer situation. In my case it all came from Germany -- and many in the industry used a combination of german and old USA books on the subject. German engineers don't go for standard formulas ....
My point with the above. Everything was figured out -- heat loss per room. Radiant output at different water temps -- etc. Things that are very important and are still not being done in the USA. Try getting a simple whole house heat loss done.
As the radiant industry matured -- the retailers came up against the perceived complexity of "designing" a system and the real cost differences VS forced air -- other hydronic. It was easier to fall back on a simple formulaic layout -- that also by the way cut install costs and did not cause too many problems on the operational side. Tubing and manifolds being much more expensive back then. We also had much cheaper energy.
In Germany -- the systems are about energy savings while still providing comfort .. higher upfront costs are expected .... but, the energy savings must occur. They design for low temp water.
Today, PEX is cheap and so are manifolds -- designing a better pipe layout is easy and adds very little to a system ... but, the old ways in the USA stick.
I ran into the issue on my recent project -- "This is how we always do it". They don't know why. Also -- many installers .. have never lived in a building with radiant. Same with the new VS HVAC equipment -- many installers stay away -- but have no real experience living with a great system.
It's less important in many utility buildings -- but knowing and understanding how do make a multi use building space comfortable with different temps is not hard. Many people build garages with spaces they know will just hold vehicles -- others will be working space and others may be used when just siting or working at a bench. Adding tubing w/o changing water temps can provide various temps throughout a space for very little money.
Fix Until Broke
Well-known member
yeldogt - You mention that a low temp system is more efficient - Can you explain what constitutes low temp vs high temp and why a low temp is more efficient?
finn
Well-known member
I assume he is referring to the efficiency curves of condensing boilers. Lower condensate temperature transfers more heat out of the exhaust stream, in general.
I have run water at 85 degrees -- or so with outdoor reset. Once a floor it at temp for a home I shoot for a bit over 10 degree delta. 15 is fine.
With other types of radiant -- underfloor / wood the temps will be a bit higher and the delta on the higher side.
Often for a garage they say 20 .... again -- it not about working.
As I said above -- I'm a bit of a nerd regarding radiant. I have been invited to see systems before and after ...etc. You can tell the wide spaced layouts as the outdoor temps drop and the water temp increases to keep up ... especially if they run them at lower room temp set points.
Take a simple rectangle. Many people don't think about the heat loss of the building and just lay out the tubing. Building loose heat around the edges -- windows and doors w/ glass. Its easy to lay the tubing to get the hotter water to those areas -- also thinking about cabinets etc.
lower temps = less waste out of slab --- less waste above 7'. It's not a huge number ... but ... it's also more comfortable.
Fix Until Broke
Well-known member
Thanks - taking this concept to two extremes...
If you ran 1 big tube in the middle of the room, it would need really hot water to heat the building (kind of like the overhead radiant tube heaters).
On the other end of the spectrum - if you ran 1/2" tubing 1" on center (touching) across the whole floor, you could get by with 71F water to keep the room at 70F
Neither is practical - just illustrating the concept. The intent is to find the right balance of performance, cost, installation, practicality, etc.
Thanks again
If you ran 1 big tube in the middle of the room, it would need really hot water to heat the building (kind of like the overhead radiant tube heaters).
On the other end of the spectrum - if you ran 1/2" tubing 1" on center (touching) across the whole floor, you could get by with 71F water to keep the room at 70F
Neither is practical - just illustrating the concept. The intent is to find the right balance of performance, cost, installation, practicality, etc.
Thanks again