Avoiding the Drop

One of the things I actually get paid to do in my office is run radiant heat load calculations for one of the product lines I represent. See, in my day-to-day work, I’m a sales representative for about ten different hydronic heating product lines, one of which is a radiant tubing company. Typically, we do heat loads for residential houses and driveways; however, one of the product lines I represent has been involved in some major projects. Projects like sports field turf conditioning. With all the weather postponements over the weekend, I figured I’d take a look at what, exactly, goes into a turf conditioning system and what they’re supposed to do.

First things first: let’s talk about why you would want to condition a field in the first place. The main reason is NOT to melt snow (although that can be a nice side effect); it’s to keep the turf loose and soft. See, when it gets cold, soil (like everything) freezes; turf conditioning is designed to keep the soil heated and to prevent that freezing from taking place. According to Ken Mrock, the head of the grounds at Chicago’s Soldier Field, this is accomplished by keeping the rootzone warm:

“By keeping the rootzone beneath the grass at a temperature well above 32°F, we will make sure the players have a safe and relatively soft playing surface, even in sub-zero conditions.”

In the past, this goal has been accomplished with copper piping. The problem with copper piping – especially hard copper – is that, as the ground moves, the copper can kink and break. Also, depending on the composition of your soil, certain soldering techniques have run into problems (some areas require silver soldering to be used to prevent the soil from breaking down the solder joint).The piping will then be filled with a mixture of water and propylene glycol (usually a 60-40 or 70-30 water-glycol mix); the glycol is there to prevent the water from freezing, because freezing buried copper pipes is generally considered to be a bad, bad idea.

In order to counteract both of those problems, modern systems have tended to gravitate towards using PEX instead of copper. PEX is a cross-linked polyethylene pipe; you can use it for plumbing and heating, and you’ve probably seen it if you’ve perused the plumbing aisle at Home Depot. There are three kinds of PEX, however, each with slightly different properties; the one most commonly used is PEX-A, which is freeze resistant. As water freezes in the piping, the piping can expand; once it’s thawed, it then returns to it’s original shape. Other PEX varieties (PEX-B and PEX-C) don’t offer this level of protection (there’s a lot involved with why; it’s entirely based on how the PEX is crosslinked), and aren’t used as frequently.

Whether you’re using copper or PEX, the next step is going to be laying your material down. Typically, you’re going to throw a bed of insulation down, cover that with gravel, and then run the piping, keeping two criteria in mind: the loops all have to be the same length (generally around 300′-350′), and the piping should always be 6″ on center. The soil bed is then placed on top of the tubing, resodded, and everything’s set from the install side.

It’s not the end of the design, however. One of the most famous use of an undersoil system was when the one installed at Lambeau Field failed the day before the 1967 NFL Championship Game; we now call that game the Ice Bowl. The issue there had nothing to do with copper joints, but (ironically) with the cold: the -13°F temperature in Wisconsin that day were far below the design conditions for the system, and as such the system couldn’t keep up with the demand.

Which brings us to design: it’s important to know just how cold you can get, because you’re going to need to install some big ass boilers to handle the heat load. If you’ve determined that you need 10 BTus per square foot on a typical Premiership pitch (which is 130 yards by 74 yards) on a “design” or “degree” day. You’re going to need a 750,000 BTU/hr output boiler to do that (your house, by comparison, probably requires around 30,000-50,000 BTU/hr, depending on how big it is and how well-insulated it is). This varies by climate; in Green Bay, for example, we’re talking about -8°F with 22 MPH winds (Miami, by contrast, is 50°F with 18 MPH winds). Systems are only sized to provide heat for when the temperature and wind are above that number. Or you can put it another way: anything below that temperature and windspeed, and you’re not going to get your 10 BTUs per square foot, which leads to the system “not working”.

Incidentally, though, that’s not the only reason the thing can fail. You’re talking about throwing about 30-40 miles of something down, and pushing a bunch of water through that. Inevitably, there are tons of issues that can arise; anything from a bad pump seal to a bad valve can cause this thing to have “striping”, or areas that aren’t being heatd. Since it only takes one or two frozen spots to declare a pitch unplayable, small problems can prevent the whole system from working the way it was intended.

Now, in terms of snow: the turf is heated, so in theory snow will melt when it falls. The difference between snowmelting a field and snowmelting, say, a driveway is the material the pipe or tube is buried in. In concrete, the entire tube is surrounded by dense material; this is a fantastic way to transfer heat. Soil, however, is nowhere near as dense, and as such is nowhere near as efficient at conducting heat. Since the soil’s only being heated to keep the soil from freezing anyways, there’s not really an efficient transfer of heat to the snow; if you’re getting 2″ over a short period, the system can’t handle that. That’s why most turfs are still covered; the snow still needs to be dealt with.

Also: this is EXPENSIVE to run. Really expensive. You’re basically concentrating enough heat to warm an entire commercial building onto an entire pitch, with no thermal mass to retain the heat. That’s the other reason for the tarp: it helps to keep the heat in the soil, rather than have it evaporate in the air.

So that’s what’s up with turf conditioning (at least, to the best of my knowledge, since we don’t have tons of experience with it). Of course, there’s a lot of other factors that can play into this, but those are the broad strokes, and the reasons you don’t really want to put it in your yard.