Combination of radiant heat and a permeable driveway in northern utah.

mike99

I’ve been contemplating my driveway for the past year. It’s 80 feet long with a 20-degree pitch. Recently, I spoke with a contractor on the East Coast who has successfully installed heating below Truegrid permeable pavers and achieved good results.

At first glance, heating through gravel-filled grids without insulation below might seem counterintuitive. However, this method may have some significant advantages. One key benefit is that every drop of melted snow or ice seeps below the surface into the sub-base, reducing surface water accumulation and potential refreezing. Additionally, while traditional systems rely on heating an entire concrete or asphalt slab, the heat in this system can radiate more efficiently through the gravel, potentially leading to faster and more uniform melting.

I believe this approach could be more effective than it initially appears, but I’m interested in hearing other perspectives. What are your thoughts on this method? Have you had any experiences with similar systems or insights on its efficiency and practicality?

GGross

Don't fool yourself into believing that system is more "efficient". you are heating the outside it is never efficient, and without underslab insulation you are transferring more heat down than you would with insulation. Heat never "radiates" more efficiently through gravel either, heat transfer works better with more surface contact, gravel by default will have less contact, air (between the gravel) acts as an insulator, impeding transfer. The benefits are purely for your comfort, not dealing with the resulting water from the melt for instance, or for install costs.

mike99

Thanks for your input. You make some good points about heat transfer and efficiency, but I think there are some other things to consider.

Yes, heating an outdoor space isn’t as efficient as heating indoors. However, using Truegrid permeable pavers has some unique benefits. For example, when the snow melts, the water goes straight into the ground, which means it’s less likely to freeze again on the surface. This is really important for safety, especially on a steep driveway. While the system might use more energy, I think it could be more effective at keeping the driveway clear and safe.

About the lack of insulation, you’re right that some heat will go into the ground. But this setup helps spread the heat more evenly through the gravel, which I believe might melt the snow faster than heating a solid slab. The air between the gravel pieces does slow down heat transfer a bit, but it also helps water drain quickly and could help the heat spread faster.

What do you think about these points? Has anyone tried similar systems or different methods to solve these problems effectively?

hot_rod

With a steep pitch it seems the water could run away without needing to "soak in"?

Here is a table showing the thermal conductivity of various materials.

The energy transfer for snowmelt is largely conductive. So no cheating thermodynamics.

The tube likes to be encased in the concrete to transfer best.

Same with under slab insulation. Uninsulated slabs take much longer to ramp up, also. So that works against a residential system that starts with snowfall.

We come across this same riddle when snowmelt tube is under sand and pavers. It can work, but at a cost.

mike99

Bob, I had to read a lot to begin to understand what is displayed in the data. However doesn't that show that gravel will conduct the heat much more effectively?

For instance, copper is.… • Copper: 385 W/(m·K)

In the case of sandwiching heat between Gravel above and some gravel below, wouldn't that mean the heat would conduct outward up and down, but down would be insulated by the earth's extremely low conductivity?

hot_rod

it’s the air space around the aggregate that limits the conduction and heat transfer.

Concrete, blended with stone, sand, and Portland form a fairly solid mass.

when you chop up a slab that has radiant tube installed, you’ll find the concrete adheres to the tube, not quite as well as rebar, but little if any air space around the tube.

In gravel it depends on the size of the aggregate and size of the air space around the tube and rocks.

mike99

I see. The same as me tightly holding a pex pipe vs just barely touching it in a few places. That makes sense.

DCContrarian

Efficiency in this context is going to mean how much heat you put in versus how much snow gets melted. Inefficiency is heat that gets put in and doesn't melt any snow. Where does that heat go? The air, the ground, and everywhere in-between that's not the snow.

My intuition is that the best efficiency is with a layout that maximizes direct contact with the snow and minimizes parallel paths. Imagine a concrete slab with insulation below and along the sides. There's no place for the heat to go but up, and if the slab is covered in snow all of the heat goes into the snow and the snow is in direct contact with the source of the heat, with only a highly conductive material between them.

The proposed design seems to offer a lot of places for heat to go other than the snow and a fair bit of resistance between the source of heat and the snow.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1804333#Comment_1804333

Set a pan of water on a convection cooktop, then lift it up 1/2" to demonstrate the power of conduction transfer.

jumper

Does heat in ground dissipate or build up? Tried using ground under parking lot for heat rejection from A/C. Did that accumulated heat melt snow? Some winters.

mike99

https://forum.heatinghelp.com/discussion/comment/1804359#Comment_1804359

Right. Same reason my mom's old air filled cookie sheet make the cookies less burnt.

Geosman

I would be concerned regarding the 20-degree pitch and if the plastic Truegrid panels will hold their position on such a steep incline. I feel it's best you get some good advice from the folks at Truegrid before venturing on this slope. I've worked with some steep inclines and had to have them done with concrete and cross ribbed for traction with the radiant tubing embedded on wire mesh in the concrete. The concrete needed to be keyed into the slope periodically to also help prevent it from becoming a slide plane and moving down slope. In my case even adding insulation under the concrete was an issue and it was better to step bond it onto the rock surface.

Does it use more energy to melt snow? Yes but at least the delivery trucks can navigate up and down the slope and the customer has access to and from the home despite the difficult driveway.

Zman

I suspect your driveway is 20% (20' rise over 100' run) rather than 20 degrees (36' rise). None the less a steep driveway. I suspect that those pavers create some pretty intense pressure points downward when loaded with vehicle weight. I would be concerned about tubing damage particularly if the gravel is angular.

As far as how well will it melt and how efficient will it be. Snowmelt is inefficient, snowmelt without insulation is horribly inefficient. I think it will melt snow well, as soon as the gravel gets a little wet. You see this with paver systems where the tubing is in sand. The system is a little sluggish when the storm starts but once it is rolling it melts well.

LMacNevin

The talking point is really effectiveness, more so than efficiency. A hydronic snow melting system might consume less energy than a snowplow, but waiting for the sun to melt the snow is the most efficient. What really matters is "will the design work?". With hydronics, we can usually overcome imperfect design or installation with brute force - hotter fluid and higher flowrate. The actual heat transfer from tubing to gravel is very complicated to predict and calculate. Someone with ANSYS FEA software could model it, but that would still be just a guess.

I appreciate the design challenge - probably 50% of the heat input will transfer to the cold frozen earth below the pipes, especially during the first 1-2 hours of operation, so this means you need to double the size of the heat source, pumps, and pipes for a given upward heat flux. In other words, if you think the system needs 150 BTU/hr per square foot to operate, size the boiler, pumps, and pipes for 300.

If you proceed, please be careful to not embed the tubing (assuming PEX or PE-RT) directly in sharp gravel that will cause point loads and gouging, as this will probably damage the tubing over time and may even restrict flow through pinching. I'm not sure how to prevent this, but hopefully the landscaping contractor knows how.