A guy was walking along this beach, and he came across a lamp. Taking a chance, he rubbed the lamp and out popped a genie. The genie told the guy he could have just one wish. The guy thought about it, and said he wanted a bridge built from California to Hawaii so he could visit friends without ever having to fly.

“That’s going to be kinda tough,” the genie exclaimed. “The ocean’s pretty deep … then there’s the storms to contend with … and the construction engineering! I don’t think I can do it. Isn’t there anything else you want?”

The guy thought for a minute and said, “I want to know everything there is to know about hydronic radiant floor heating.”

The genie thought for a minute and finally asked, “Would you like that bridge with two lanes or four?”

Does anyone out there know “everything there is to know” about radiant heating? I’ve run across a few who thought they did. But when you get right down it, no one, including those charged with the task of training the industry, knows it all.

Since 1994, I have been fortunate enough to host training classes for upwards of 10,000 people, ranging from contractors to wholesalers to reps — even builders. We’ve seen significant changes in the people who come to our factory-training program. Three years ago, it was common to have a class filled with 70 guys who’d never picked up the business end of a piece of PEX before. Today, a growing number of experienced installers want to know more and take their games to the next level. They come armed with questions — lots of them. Here are some of the most common:

What’s the “preferred” installation method?

In our opinion, there really isn’t one. Every guy out there has his own favorite, but each installation option has its own application. It’s not a matter of which one is the “best.” It’s more a matter of which one best fits the specific application staring you in the face — and which one will give a specific customer the radiant comfort he wants. All installations will get the job done — and keeping an open mind to all options is important.

Sure, it’s faster and easier to install tubing for a gypsum job. And sure, gypsum installations tend to be more forgiving, allowing for a greater “Oops” Factor. Unfortunately, gypsum-based concrete isn’t always readily available in many parts of the country. If it ain’t there, it ain’t there, so it ain’t an option. Again, it’s not a question of which one is the best, it’s a question of which one best fits a specific application.

There’s one surefire way to keep that Oops Factor under control — and one way only: Do the math. When you perform heat loss and system design calculations, you’ll always know what kind of performance to expect out of any radiant system.

What’s the best water temperature to run?

Basically it depends.

It depends on the heat loss. It depends on the installation method. It depends on tube spacing (at least in concrete applications). It depends on the Delta-T. And it depends on finished floor R-value.

There’s no “best” water temperature other than the water temperature you need to satisfy the heat loss. And that’s all based on the variables mentioned above.

Are there limits? Sure — 150 degrees F for concrete applications (per the Uniform Building Code; although gypsum concrete makers generally like to see temperatures below 140 degrees). And 180 degrees for joist heating (with or without plates).

Should you design to 180 degrees when going between the floor joists? You could, but it leaves nowhere to go if something should change, such as floor coverings, new French doors, etc. If you do your design and the water temperatures come up to anything more than 170 degree, you’ll certainly want to re-evaluate your design.

Will you “smoke” the floor with such high water temperatures? High water temperatures don’t always translate into high finished floor surface temperatures. Remember, the higher the finished floor R-value, the more insulation there is between the tubing and the room itself. The required water temperature will have to go up to overcome the increased floor R-value.

The actual heat loss of the room under design conditions is constant, so the Btus delivered and floor surface temperature will also remain constant. However, if you run high water temperatures through a system, and certain areas don’t need that high a temperature, you could certainly run into problems. And that brings us to our next question.

How do you deal with different water temperature in different loops?

A great question, and a fundamental one in quality design. If the water temperature swings on a specific manifold are no greater than 15 degrees, there really isn’t a problem as long as the floor coverings and installation methods are consistent. But if the swings are wider than 20-25 degrees, you have to do something about it. Basically, you have three options:
  • In gypsum concrete, move the tubing closer together in areas with higher water temperatures. By doing this, the supply water temperature needed to provide the appropriate Btus per square foot for that area will go down. For instance, a room requiring 20 Btus per square foot — with a carpet and pad with a combined R-value of 2 and tubing 12 inches on center — needs 147 degree supply water. If you move the tubing to 9 inches on center, the required water temperature drops to 125 degrees.
  • In a joist heating application (tubing suspended from the subfolder without aluminum heat emission plates), simply adding plates can lower the water temperature. For example, at 15 Btus per square foot with a finished floor R-value of 2, the required supply water temperature is 150 degrees. Adding plates, the supply water temperature drops to 137 degrees.
  • In any application, differences in supply water temperature in different areas can be made irrelevant by simply zoning those areas separately using telestats. For instance, let’s say an upper floor has three bedrooms with carpet, and each room requires anywhere from 135-145 degree supply water temperature. The bathroom, however, requires only 105 degree water. By zoning the bathroom separately with a telestat and thermostat, and running 145 degree water though the entire manifold, the bedrooms will heat adequately, and the bathroom will simply heat up faster than it would with 105 degree water. The floor won’t scorch because there’s an unbreakable link between room air temperature and floor surface temperature. If the thermostat is happy, it’s going to stop flow through the floor, and the floor won’t get any warmer.

Do I Need Those Plates?

Tubing can be installed between the floor joists with or without aluminum heat emission plates. You can achieve the same Btu per square foot output with both methods. However, using plates means you can achieve that same output with supply water that’s 15-20 degrees lower in temperature. Depending on your control strategy and heat source, that may be a benefit. If that water temperature stays the same and plates are used, the actual output of the radiant floor will increase by about 3-4 Btus per square foot.

Many contractors are in fact combining joist heating and plates, using plates along the perimeter of a room — especially when there’s a lot of glass. In concrete applications, it’s always a good idea to run the supply water directly to the outside wall first. Then, if there is a lot of glass, run the tubing closer together for the first 3 feet or so. You’ll get more output where the heat loss is greatest.

When installing between the joists, this setup is impossible. The solution — install joist heating in the room, but install plates along the perimeter. The net results are the same.

Another side benefit of using plates is you’ll have a bit more leeway in a job if a finished floor covering should change. Again, plates lower supply water temperatures by 15-20 degrees when compared to joist heating with output remaining the same. There’s a bit more Oops Factor with plates. But as stated earlier, proper heat loss and design procedures will keep that Oops Factor under control no matter what installation method you choose.

What would happened if we simply put more tubing in the joist bay? Would that boost output? As a tubing manufacturer, we certainly hoped so. Unfortunately, our testing showed running more tubing in a joist didn’t help output all that much, certainly not enough to justify the extra effort and expense.

What’s better — hanging the tubing from the floor joist or stapling it right to the subfloor?

Well stapling tubing right to the subfloor certainly is easier and quicker. But think for a minute about what you’re trying to accomplish. It’s comfort, first and foremost, isn’t it? When tubing is stapled to the subfloor, you have a tendency to get hot spots. It’s called “striping.” The floor surface temperature is noticeably warmer where the tubing is stapled to the floor. By suspending the tubing, the hot spot problem is sharply reduced. To make it go away completely, use plates since they will disperse the heat uniformly though a larger square foot are.

Both methods will deliver heat to the room. But the question must be asked: Which way is better when it comes to delivering the radiant comfort people are expecting from the price they are paying?

How do I overcome the price objection?

OOOOWEEE, this one is the biggie, isn’t it? The price question is usually at the top of everyone’s mind. The solution has to start in fully understanding how radiant floor heating works, why it’s different from the alternatives and how it is uniquely able to provide the comfort that other types of heating systems simply can’t. Once that’s digested, then comes an understanding of the sales process.

If it’s been said once, it’s been said a thousand times: “I’m a contractor, not a salesman!” I was a contractor, too, and have been told that my prices were too high … way too high … ridiculously high … and damn near criminal. The basic fact of life is that every price is too high unless the person making the decision knows what they’re getting for the money, i.e., “What’s in it for me?” If you have two heating system proposals side by side, and one is for $5,000 and other for $10,000, and if, in the eyes of the decision maker, they both do the same darn thing, guess which one he’s going buy? That’s not people being cheap, that’s a consumer decision based on not enough information. If, however, that person understands what he’s getting for his extra five grand — and sees the benefits — then he is much more likely to make the decision for the better system. It’s not automatic, but at least you have a shot — whereas before you had none.