Heated Driveway: BTU/SQ FT Calculator?

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Old 01-17-18, 08:46 AM
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Heated Driveway: BTU/SQ FT Calculator?

I plan to attempt a DIY heated driveway snow melt system this summer. I've been reading all I can, including this helpful thread:

https://www.doityourself.com/forum/b...-driveway.html

In the thread, there are several references to calculating the required BTU's per square foot for this application, but I'm struggling to find a calculator for "outdoor" applications, everything is for indoors. I know I need to shoot for AT LEAST 100 BTU/sq foot, but I'd sure like to calculate it exactly if anyone can point me toward a resource. I'll be using ~6" thick granite cobble pavers (which I know have a thermal conductivity of 2.0-4.0), which will of course be placed over some time of insulation: either a sand bed containing the 3/4" PEX, which will sit on 1" rigid foam board, or I may use one of the insulated radiant panel products. Still researching. The driveway itself will be about 1400 sq ft.

Any help is appreciated; I'm sure I'll have more questions as this progresses.
 
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Old 01-17-18, 09:10 AM
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I searched online for "heated driveway btu calculator" and found the information you want. Below is a quote taken from this website.

Raising the Temperature of Snow- In order to begin melting the snow, the temperature must first be brought up to 32 degrees F. The specific heat of ice is about .51 BTU/lb. Since snow is made up of ice, a pound of snow will require about 0.51 Btu to raise it one degree F. If 8 inches of snow weighs 5 pounds and is at 22 degrees F, Then it will take about 26 BTU per square foot to raise the snow itself to just below the melting point (32 degrees F).

Phase Change- The single most significant factor affecting the amount of energy required to melt snow is the energy required to change the water in snow from a solid to a liquid. This “phase change” is brought about by adding what is termed the “latent heat of fusion” to the mass. The latent heat of fusion for changing Ice to water is a whopping 144 BTU per pound. This being the case, Once our example of 5 pounds of snow has been raised to 32 degrees by the addition of 26 BTU per square foot, we will need another 750 BTU per square foot to actually melt the snow.
 
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Old 01-17-18, 09:15 AM
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I also did a little reading and thought this link was interesting, if you haven't seen it.
https://www.phcppros.com/articles/19...l-you-in-class

I'll be watching.

Bud
 
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Old 01-17-18, 09:33 AM
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Bud9051,
I had run across that article, very helpful!
Thanks!

PilotDane,
I'll need to read through that one again, but 775 BTU/sq ft sounds outrageously high. Thanks for the link though!
 
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Old 01-17-18, 01:08 PM
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From what I read of that link, "outrageous" is expected with a snow melt system. Ironically it sounds like anything less can cause other issues like wet surfaces that freeze. Once the heat is used it has to be maintained to avoid a small amount of snow from being more than a dusting that blows away.

Bud
 
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Old 01-17-18, 01:24 PM
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I have a customer/friend with a cobble stone S driveway on a hill. All the negatives needed for minimal snow traction. Driveway is standard width wide by approx 150' long. There is a 100A panel that feeds just the driveway. +20kw of under driveway heating cables. The system must be on for several hours before the snow starts.

I watch the house in the winter. I've gotten stuck there for hours waiting for the cables to warm the driveway once it's frozen over. I got stuck halfway up one year.
 
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Old 01-17-18, 04:42 PM
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Most people aren't aware of the energy required to make water change state (solid to liquid, liquid to gas). If you draw a graph of energy in vs temperature the line gets quite flat at freezing and boiling. It's why a cooler full of ice works so well. You can warm ice pretty linearly but to get it to melt is a big step up. It's the same for boiling water. You pour the heat to it and the temperature rises predictably but then it just stops at the boiling point and it takes a lot of additional energy to push it over the edge.

Also keep in mind the energy required depends on how much snow you want to melt and how fast. You could design your system to melt up to 1" of snow per hour or 3". Maybe your system is only capable of melting 1" per hour and those once in ten year storms will just have to be dealt with manually or you just wait a few hours for the heating system to catch up.
 

Last edited by Pilot Dane; 01-17-18 at 05:02 PM.
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Old 01-18-18, 08:45 AM
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Bring your checkbook.
Sid
 
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