What size pex tubing do you recommend?

MichaelT

This fall I will be tearing out the concrete for 3 car wash bays and redoing the radiant floor heat lines with O2 barrier PEX. The old concrete is falling apart and needs to all be replaced.

What size pex would be best? I was thinking to do 1/2 ID but also know that 5/8th inch pex delivers more BTUs efficiently.

Each bay is 33 feet by 18 feet. There are 2 bays west of the boiler room and one bay to the right of the boiler room.

This is just a snow melt system. I'm leaning towards doing 6 inch on center spacing but also considering doing 9 inch spacing.

Thank you for all the help!

hot_rod

For an ice melt system you would want 3/4" pex, 9" on center.

What size boiler do you have to run this?

Call; it 600 sq ft times 125- 150 BTU/ sq ft = 75- 90,000 btu/hr per bay

GroundUp

Tube diameter has little to no effect on BTU output per SQ FT if properly installed. 1/2" is cheapest and easiest, and with only ~1800SF, 5 loops per bay at 250ft each w/ 6" OC is going to be more than adequate. There is absolutely no reason to run larger diameter, and anything wider than 6" OC would be a mistake.

Alan (California Radiant) Forbes

So, each bay is 600 square feet x 125 BTU/square foot = 75,000 BTU. And you think that would work with ½" PEX?

hot_rod

You need to move a lot of BTUs of cold glycol in a SIM. Uponor suggest 5/8 or larger tube. 1/2 maybe for some stair treads where loop lengths are very short, you need a tight spacing.

Assuming the bays are covered you are designing more of an ice removal or anti-icing protection system, no real snow load to speak of.

So the tube and spacing as well as loop length dictates and SWT/ gpm dictates how many BTUs you deliver.

All the info about SIM design here.

https://www.uponor.com/getmedia/50a684b6-091d-4044-b8f7-c06f7705668c/snow-and-ice-melting-design-and-installation-manual.pdf?sitename=USA

GroundUp

https://forum.heatinghelp.com/discussion/comment/1862120#Comment_1862120

125 BTU/SF is more than double what's needed to prevent ice buildup inside a wash bay (I own a 2 bay car wash and have installed radiant floors in roughly 20 others as well). With that said, yes; 1/2" would easily cover 125 BTU/SF. Tubing diameter matters not; tubing quantity is what makes the difference. 1/2" at 9" OC will put out roughly the same amount of BTU as 3/4" at 9" OC, the only benefit to larger diameter is slightly longer loops.

Alan (California Radiant) Forbes

What delta do you design at?

hot_rod

1.25 gpm, 200’, 1/2” loop with 40% PG around 20’ head

300’ of 3/4 is about 1/2 that at 2 gpm

pex manufacturers will do a design for you based on your location and needs

GroundUp

With a cold start SIM system, a 20* delta is largely ridiculous to get any amount of BTU output regardless of tube size. Space heating, sure- but a cold start snowmelt will almost never hold a 20* delta while also doing its job properly. 50+* is not uncommon and there is absolutely no downfall to doing that, quite the opposite in fact. Maximized BTU output with minimized upfront cost and the highest possible efficiency, all great things.

With that said, car wash systems typically don't need more than about 40-50 BTU/SF to keep the pads clear unless they never close the doors. Any respectable car wash closes down at about 0*, so my suggestion of 1/2" at 250ft is only a 15* delta at .8 GPM per loop, yielding 50 BTU/SF. This is not rocket science, guys.

Alan (California Radiant) Forbes

Thanks for your patience in dumbing it down for us addled brainers, @GroundUp.

GroundUp

https://forum.heatinghelp.com/discussion/comment/1862197#Comment_1862197

As I'm sure you are aware, tube diameter is not really relative to BTU output. A 1/2" tube at 100* will put out an almost identical amount of BTU to 3/4" at that same 100* in the same slab. 1/2" tubing at 6" spacing will put out considerably more BTU than 3/4" at 9" spacing, assuming the same temperatures which obviously vary by flow rate. Pressure drop in 250ft of 1/2" at .8 GPM is about the same as 375ft of 3/4" at 2 GPM so while there is potential to carry more BTU with the same pressure drop and fewer loops, larger diameter tubing does nothing for actual BTU output after the dust is settled if spacing and delta are equal. I would personally rather fight 1/2" at 6" spacing than 3/4" at 9" spacing, but maybe that's just me.

Now if it's a large space like a 5000 SQ FT driveway with a 150 BTU/SF load, I'm all aboard the 3/4" train. In the case of a car wash however, there is not a chance I'm wrestling 3/4" when 1/2" is better from all aspects.

hot_rod

Missing from this discussion is an accurate load. Is it 50 or 150 btu/ sq ft?

50 btu/‘ is a leaky or uninsulated home in New England🧐

A hand wash bay without any doors, a wind tunnel, probably 100 btu or more.

Location and typical winter temperatures goes into a design

Large pex allows more gpm.= more btus transferred. That is why pex comes in multiple sizes

You get one chance to get the tube right. Compromise and go with 5/8

GroundUp

Yes, we would obviously need more details if it's not a regular car wash bay that gets shut down at a given temp, but that does not negate the fact that larger diameter tubing does not equal more BTU output per square foot. 1/2" at 6" spacing offers more BTU output capacity than 3/4" at 9" spacing, no matter how you slice it.