Snowmelt PEX loop max length

WillC

Hi there,

I am designing my snowmelt system and cant figure out what is the max legth for 5/8 PEX tubing. Uponor snowmelt PDF says 225', the Modern Hydronic heating book says 310' and a contractor says 250'.

Cheers

kcopp

As with anything have a proper heatloss. done.

Uponor's C-dam manual has all that included. Shorter lengths are better.

The contractor should have all that done.

HomerJSmith

There are a lot of things to consider.

Snowmelt most often operates between 15-35 deg. This is the temp range in which snow falls.

What % age of snow times and clearance do you want melting to occur. Class 1-2-or 3

There is very little difference in heat delivery between tube size 1/2" to 5/8" or 5/8" to 3'4" foot for foot. Heat delivered (BTUs) is dependent on Supply Water Temp and flow. Pex size has more to do with pumping power and tube length. Are you using Pex A-B-C? Use Pex A with an O2 barrier. I use 3/4" tube for increased flow and low pumping power.

You are going to use Anti-freeze, what % age? Anti-freeze increases the need for more pumping power at a given flow.

How is it to be installed? Sand Bed, Concrete, Asphalt, etc. Pavers going to be the finish?

How is the Pex laid out? Serpentine, Counter-flow. I always use a Counter-flow pattern. What about Pex spacing? Too long a loop length—too long and you could run out heat energy before hot water reached the end of the loop. 250' , I would think is the max.

What are you going to use (insulation) to prevent back and side heat loss?

Is it going to be an automatic of manual sys?

hot_rod

The longer the loop, the more pump power you need. The 5/8 could be any of those lengths. If you need to go long distances from the manifold, 3/4" could be an option.

Keep the glycol below 50% to help the pump sizing.

A snowmelt sizing program would spell out all the size, pump requirement and supply temperature required.

All the sizing depends on how quickly you want to melt the snow.

The Uponor design manual takes you through all the steps for the design.

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

WillC

I am using LoopCAD, the above Uponor manual and also the information about snowmelt systems found in the "Modern Hydronic Heating" book by John Siegenthaler.

Glycol will have to be at 50% as the design temperature of the driveway is -10F

Goal is to have a class 1 snowmelt.

Planning to use PEX A with air barrier. Will be in 6" of concrete and suspended roughly 3" under the top on the concrete. Using R10 GPS foam under the concrete and on the sides. Doing a counter flow pattern. Delta T 25F.

I just finished a design with 3/4" PEX and 9"on center (average loops 310ft). Issues are: the supplied temperature is 161F and circuit head loss of roughly 30 ft (max should be 12ft according to LoopCAD)

I will do another design with 5/8" but 6" on center, this should fix the supplied heat temperature issue.

How do I correct the circuit head loss being too high?

Thanks

GGross

Check your boiler specs, many manufacturers void their warranty over 40% glycol. 40% is more than enough for our systems in the tip of the mitt michigan, and the UP.

You fix the circuit head loss being too high a couple of ways. you can either lower the load (150 btu/hr per sq ft is what I use in my area for ER doctors houses, thats 140 plus about 10% backloss) shorten the loops (this is the "best way" in Loopcad add more loops to make them shorter) and you can base the design on a 30 degree Delta T. Of these choices shortening your loops won't change the design parameters, the other 2 choices are design parameter changes. if you are inputting correct numbers the flow rate, head loss, and supply temperature is how you will determine max loop length/spacing. you usually end up finding that you will need slightly shorter loop lengths than recommended max to get reasonable head loss numbers (remember unless you changed it Loopcad is based on 20 degree delta T, if your pump is under powered chances are you will just run a wider delta T). Also could be your numbers being input aren't all the way correct. I always try to keep the head loss low on residential snow melts so if a pump goes out its one I have on my shelf for the customer.

hot_rod

A Class1 SIM tries to keep up with a snowfall predicted in your area.

So it requires more BTU/ sq ft. than Class 0, so more tube, more boiler, etc.

Note the 3 temperatures on glycols, freeze, slush and burst. 50%is a -80F burst.

It varies from brand to brand, this Fernox at 50% is pumpable down to -20F.

\

here is the correction for the pump, although all this should be in the LoopCAD program?

HomerJSmith

I wouldn't go more than 1 1/2" to 2" below the surface of the slab especially if the loops are 6" O/C. You will get quicker response time. Take care to provide concrete control joints in which counter flow would be a better choice if you can do it. Less shielding where the cuts in the concrete are. If you go with a shallower tube layout be sure and drop the tube down at the control joint and shielding below the concrete cut line which would be about 2" deep.

With a serpentine pattern, you will have a lot of 180 deg turns. With counter flow you will have a lot of 90 deg turns with one 180 deg turn and a more balanced temperature response over the whole loop.

WillC

thank you for the replies.

Regarding the insulation under the snowmelt, with all the snow melting and water potentially running around the concrete driveway, would it be better to use a closed cells foam like XPS instead of EPS or GPS?

hot_rod

Yes you want a closed cell with adequate PSI rating. The 250 you see on some of the blue or pink board indicates 25 psi foam. Also the ground contact rating.

I would not use the white "beadboard"!

Try to find the sheets that are not scored.The scored ones tend to break when you walk on them, unless the sub-grade is perfectly flat.

Derheatmeister

Generally we use 5/8" tubing on 9" centers… Do not know how to easily get 3/4" on 9" centers unless you are doing a header system such a football field would have.. 6" is generally overkill but more is better that less just not for the pocketbook.

hot_rod

I have seen 8D, 6D and Mr Pex claims 4D for their pex.

Bend radius, not the 180 loop end.

3/4 pex is .875 X 4= 3.5" so a 7-8" loop end??

Warm pex, like 80° or warmer makes it much easier to work.