“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?

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?

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?

• 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?

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?

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?

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.