Designing a driveway snowmelt system

[Rob Boyle]

Hey gang, I'm in the design phase of of a heated driveway project.

I'm in the Mtns of Colorado, so lots of snow to melt. Estimate my needs at 135 btu per sq ft, 500 square feet of driveway. BTU requirement of 67500 BTU

The home has two 75 gallon natural gas water heaters for the DHW, rated at 76,000 btu each. These are standard water heaters with b vent exhaust.

Doing the math, it may be feasible to use the 2 existing water heaters to heat the drive, using heat exchangers and pumps as needed. Would like to hear opinions on this option.

The most reliable and more expensive route will be to add a condensing boiler that is dedicated to the purpose of driveway heat.

Some boiler system questions.
1. Can the glycol loop be direct from outside slab to boiler? Do I need a mixing bypass, mixing valve or heat exchanger? ( can most boilers handle high concentration glycol mix for our 30 below temp extremes?)
2.Can the new high efficiency boiler be used in conjunction with the 2 75 gallon water heaters to make them more efficient? Perhaps add a pump loop and a heat exchanger to them?

 

[Plumber69]

Might be cheaper to buy a gold snow shovel

 

[Sponsor]

 

[Reach4]

Might be cheaper to buy a gold snow shovel

What, and miss the environmentalist award?

 

[Jadnashua]

Your first issue is to decide which class of snowmelt you want. One that will keep the snow off regardless of the rate of snowfall; one that will melt it most of the time with maybe a slight slush for higher rates; or one that has the luxury of taking its time. That determines the amount of heat you need.

You need a slab versus asphalt, or you'll end up with a wavy driveway. It also needs insulation underneath and drainage accommodation or you'll end up with ice buildup along the sides. The locale will also be much annoyed with you and your liability will go up if you can't direct the snowmelt somewhere that it doesn't flow onto the roadway, washing away any salt or chemicals they may have applied, and leaving a skating rink at the end of your driveway.

You'll want to incorporate a moisture sensor so the thing doesn't decide to run just because the temperature is below freezing when there's no snow or ice to melt.

Deciding on the spacing of the tubing and optimizing the loop lengths so it can create an even heating is also important. A manifold can work on some systems. The slope of the driveway also will play into the layout and drains. Hooking into the storm sewer may be prudent, otherwise, the accumulation of snowmelt can create dams and put the water where you may not want it.

IOW, to do this right takes some engineering for a successful outcome. It's more than just determining how much heat you need.

FWIW, most conventional water heaters aren't designed for the duty cycle a snowmelt system may require. Some could handle it. Any boiler should.

Keep in mind that adding antifreeze will decrease the heat transfer, so your heat calculation will need to account for that...you'll need more heat into the system than you calculate to get that real output to the driveway. There's a limit on how hot you can run water through the tubing encased in your slab or you'll degrade the concrete and the loop length needs to ensure the water is still hot enough at the end to do its job. Overpumping adds a lot of power and creates more friction in the pipes, lowering the flow.