Tips on designing a snowmelt system

So talk to me about snowmelting. This is for folks with more money than brains, right?

Well, I don't know about that, but snowmelting is commonly used residentially for walkways and driveways, and for parking lots, walkways, handicapped access ramps and dozens of other applications commercially. It's really about safety and convenience.

That's cool. So how do you design it?

Well, if you have the right resources in front of you, designing a snowmelt system is pretty easy. The very first thing you need to decide is what kind of a system you want. Your design criteria for, say, a small residential driveway would be very different than, say, a handicapped access ramp. It's critical to keep the handicapped ramp free and clear of snow and ice all the time. Your goals for the driveway may be a bit different. In the driveway you could design for lower surface temperatures and wider tube spacing.

OK, I get that. Say I want to do a 20 ft. x 60 ft. driveway; where would I start?

First you establish some design criteria. Choose an outdoor design temperature, wind speed and desired surface temperature.

I get to choose these? My heating design temperature is minus 5 ...

Yeah, that's for heating indoors. We're talking about melting snow outdoors. You don't necessarily want to use your heating outdoor design temperature for snowmelting. What you want to do is determine, as best you can, at what outdoor temperatures do you normally get snow or ice. Does it snow at minus 5 degrees? And if it did, would you want to try to melt that snow? Remember, the lower the outdoor design temperature you use, the greater the Btu/hr. snowmelting load you're going to have to deal with.

So you're saying I could design my snowmelt to work at say, 20 degrees F, even though I design my heating system to work at minus 5 degrees?

That's right!

Well, what if it snows at 15 degrees? Will I have an unhappy customer?

You really have to talk to the customer and define his level of expectations. If the customer never wants to see a speck of snow or ice on his driveway and never wants to put a shovel in his hands, then you have a pretty clear set of marching orders: Design the system to vaporize snow before it ever hits the ground. That means you'll want a low outdoor design temperature, a relatively speaking high surface temperature and a fully automatic control strategy.

But if he wants to use his snowblower and then use his system to melt off the snow crud and keep it clear, then you have a very different set of expectations. You can design to a slightly higher outdoor design temperature, a slightly lower surface temperature and a semi-automatic control strategy. It's your call on insulation, although in a small area like we're discussing, insulation ain't a bad idea!

What's next?

Let's assume a 20-degree F outdoor temperature fits the needs of the job. Let's also choose a 10 mph wind speed (cold wind will cause ice quicker than low temperature). Let's also select a surface temperature above the 32-degree F freezing mark - 38 degrees F. We should also choose a supply return fluid temperature differential at this point - the standard is 25 degrees F.

Next, based on the above information, we need to figure out the Btu/hr./sq. ft. heating load. Let me check the charts real quick ... that comes to 69 Btu/hr./sq. ft. In a 1,200 sq. ft. driveway, that's a total load of 82,800 Btu/hr.

The chart also tells me with tubing 12 inches on center, we'll need 107-degree F fluid running through the system.

What do I have to do to make the system work at 15 degrees F? Put in more tubing?

That'll work. Go to 9 inches on center with 107-degree F fluid, and you'll put out 87 Btu/hr./sq. ft., provided you have the heating plant to do it. The chart also says you could leave the tubing 12 inches on center, goose the water temperature up to 123 degrees F and make it work, again, provided you have the heating plant to do it. Now we're talking 104,400 Btu/hr.

But look at it another way. That extra five degrees of leeway comes at a cost of a more than 25 percent increase in snowmelting load and, of course, operating cost. It all comes down to, once again, what does the customer expect?

Since this is a theoretical customer, we'll stick to the 20-degree number.

OK, we need to choose the tubing type and size. For giggles and grins, let's choose a 5/8-inch multilayer composite pipe.

What's next?

Now it's kinda like radiant floor heating. We need to figure out how much pipe we need using simple multipliers based on tube spacing. If the tubing is 12 inches on center, that means there's 1 foot of tubing per square foot of driveway, so we multiply the total square footage times 1. We have a 1,200 sq. ft. of driveway, so we multiply by 1 to determine that we need 1,200 feet of tubing in the driveway. If the tubing was 9 inches on center, we'd multiply by 1.33; 6 inches on center, we'd multiply by 2.

How many individual loops do I need? How long can I run each loop? With floor heating I can run 5/8 inch over 400 feet. It's the same here, right?

Nope. The heating loads, and therefore the per loop flow rate, are very high with snowmelt, so we have to deal with some limits. 5/8-inch multilayer composite can run about 250 feet or so in the snowmelt slab. Depending on leader lengths, it can be run close to 280 feet or so.

Since we have 1,200 feet of pipe in the driveway, we'll divide 1,200 by 250, and come up with 4.8 loops of tubing. Since we can't run 8/10 of a loop, we'll have to run five. Divide 1,200 by 5, and we'll find each loop has an active loop length (the amount of the loop in the driveway) of 240 feet. We'll add the leader length to that.

I'll put the manifold in the garage, so I don't think it'll be more than 5 feet from the driveway.

Great, that means each loop will be about 250 feet long, including leaders. Now we need to think glycol, flow and pressure drop. A 40 percent glycol-to-water solution is pretty standard and should protect the system to around minus 10 degrees F.

To determine flow, we'll use the proper flow charts for glycol percentage and Btu/sq. ft. load. Let's see, with tubing 12 inches on center, 69 Btu/hr./sq. ft. and 40 percent glycol, we find that we'll need 1.44 gallons per minute per loop. With five loops, that comes to 7.2 gallons per minute for the entire job.

Next is head loss, right?

Right! Let's find the pressure loss chart for 5/8-inch multilayer composite and a 40 percent glycol solution. We look up the flow rate of 1.44 gpm/loop and 107-degree F water temperature, and come up with a head loss per loop of 6.6 feet.

So my circulator will have to do 7.2 gallons per minute at 6.6 feet of head, right?

The flow is right, but you'll have to add the pressure drop of the supply and return piping and of the heat exchanger to that 6.6 feet of head. A B&G System Syzer wheel will help you with the supply/return piping. I can help you with the heat exchanger.

To size heat exchangers, we go to yet another chart. I could do it with a flat plate heat exchanger with 16 plates, or one with 26 plates, to produce 82,800 Btu/hr.

The 16-plate model, while cheaper, adds 5 feet of head to the boiler side circulator, and more than 7 feet of head on the snowmelt side. Remember, we already have 6.6 feet of head through the tubing itself.

The 26-plate model adds less than 2 feet of head to the boiler side, less than 3 feet of head to the snowmelting side, and imparts less friction loss. If you keep an eye on circulator sizing and possible velocity issues, the 26-plate model may make more sense, even though it costs more.

You talked earlier about semi-automatic and fully automatic controls ...

Well, a fully automatic control system requires no human interaction. You have two sensors - an outdoor air sensor and a moisture sensor in the driveway. If the master control senses it's cold enough to make snow or ice, but there's no moisture, nothing happens. If the moisture sensor senses rain, but it's 45 degrees out, again, nothing happens. But if it's cold enough and moist enough, then bingo, away we go!

A semi-automatic system requires an actual human being to turn the system on. If you know it's going to snow, or if you see the first few flakes starting to fly, you turn the system on. The master control lets the system ramp up to its operating slab and water temperatures, and then the system runs a programmed length of time, anywhere from 30 minutes to 18 hours, whatever you select. It shuts off automatically.

Why wouldn't you choose fully automatic?

Depends on the needs of the job. A small walkway and/or driveway would be just fine with semi-automatic, especially if the homeowner plans to use his snowblower to get rid of most of the snow, and use his snowmelt for safety. If, as we said earlier, the homeowner doesn't want to lift a finger to get rid of his snow, then fully automatic is the better choice.

Can my system melt 2 feet of snow?

That depends on how fast the snow's falling and how moist it is.

You mean if it's that heavy, wet widow-maker snow, it might take longer to melt than the fluffy stuff?

Yep. There's other stuff to consider, as well. How cold is it? Is the wind blowing? If semi-automatic, at what point is the system turned on? Is the slab insulated? Is it idling?

What do you mean, idling?

Using the right control, you can have a slab maintain a surface temperature of, say, 28 degrees F all winter long. Then when it's enabled, the slab temperature only has to rise 10 degrees. If you let it go dead cold, then the slab will have to jump from whatever that temperature is all the way up to 38 degrees. It takes longer.

But idling uses a ton of energy. This decision goes back to customer expectations, doesn't it?

Very good, my friend.

Do I need a special boiler?

You can run snowmelting off any boiler; it's simply a matter of whether the boiler is big enough to handle the load. If you're using a cast-iron, noncondensing boiler, you'll need a properly sized heat exchanger to protect the boiler from thermal shock.

The boiler used to heat the home may also be used for your snowmelting system, provided it's big enough.

So, let's say I want to use one boiler for the entire job - radiant inside, an indirect hot water tank and snowmelting for my walkway. How big does the boiler have to be?

When it comes to sizing a boiler for heating, you size it for peak load, right? Let's say the house-heating load under design conditions (minus 5 degrees F, for example) is 75,421 Btu/hr. Assuming domestic hot water priority, and assuming the domestic hot water load is less than 75,421 Btu/hr., your boiler will need to be at least that big, but not much bigger. Pick the brand you like and choose the next size up that will cover 75,421 Btu/hr.

When you design snowmelt, you design not for the heating design conditions, but for the outdoor temperature at which you usually get snow or ice in your part of the world. In the previous example we chose 20 degrees. You base the choice on local weather considerations, practicality and customer expectations.

So how big does the boiler need to be?

I'm getting to that! The information we have should tell us the peak boiler load isn't at minus 10 degrees F; it's really at 20 degrees F. We have the 82,800 Btu/hr. snowmelt load, and then we need to recalculate the heating load at a 20-degree-F outdoor temperature instead of minus 5 degrees F. If we do that math and come up with, say, 56,566 Btu/hr., then the boiler needs to be able to supply 139,366 Btu/hr. - enough to handle both the snowmelt and heating loads.

You could also do it with two separate boilers, if you wanted to.