Back to Basics: A Study of Heat Transfer, Pipe to Pipe
Editor's Note: "Back to Basics" is a column that will run periodically in PME reviewing the basic principles of plumbing engineering.
I recall a high school science teacher of mine asking the class a question about heat. The question was this: "A 1" wooden rod and a 1" steel pipe were placed outside on a snowy day. If you were to grab both of these, which one would be warmer?" Why, the wooden rod, of course, was the answer given by the near-unanimous group of 17-year-olds.
I must admit I was part of that group. Since those early days, I've learned a few things about heat transfer. The accurate answer to that question, however, is the same now as it was then: both objects are the same temperature. The question wasn't which one felt warmer, but which one was warmer. The steel felt colder because it was more conductive--that is, it moved energy away from our hands faster than the wooden rod did.
In the world of radiant floor heating, I have found a similar, and growing, misconception. Most misunderstandings center around two main issues: tubing and applications. I have heard people claim copper is more efficient than PEX, or PEX is more efficient than EPDM. Others claim slab applications are the only way to install a radiant system. But what actually is the truth? Is one system or product really the best, or more efficient?
To answer this we need to understand how heat is transferred in a radiant floor. We have all heard the phrase "Heat moves from hot to cold." This is true, but it's more important to understand where the heat is coming from and how it gets into the room.
To help illustrate the mechanics of heat transfer, the following example will be used: a typical 20 ft. x 20 ft. room has a heating intensity of 25 Btu/hr/ft2 and a desired room temperature of 70 degrees F. Our calculations show the room will not meet our design heat load if the floor surface temperature is below 80 degrees F.
The floor in this case is a radiant panel, and it is transferring energy into the room because it has an elevated temperature of 80 degrees F (remember, hot moves to cold). Does it matter what the floor covering is? No. Does the floor covering affect the room's heat loss? No again. The room is going to have a 25 Btu/hr/ft2 load and must have an 80 degrees F floor surface temperature regardless of the floor covering. The same can be said for the floor construction. We can have a slab, thin slab or a frame floor, and the same requirement holds true: the floor surface temperature needs to be 80 degrees F in order to satisfy the heat loss of 25 Btu/ft2.
Now, that isn't to say floor construction or floor coverings are not important--they are. Floor construction will dictate the path--direction of least thermal resistance--that the energy will take from the radiant tubing to the floor surface. Floor coverings are part of the floor construction. They play a part in this directional heat transfer, as well as helping determine the minimum required fluid temperature.
Floor Coverings and Temperature ChangeHeat moves to cold in a very predictable manner. And the greater the temperature change (DeltaT), the greater the energy movement through a given material. Another way to look at this is, the greater the amount of energy needed and/or the higher the material resistance, the greater the DeltaT needs to be to transfer the energy through the material.
If we change the floor covering in our example room from carpet to tile, the required energy is still the same, but the conductivity value of the floor covering has changed. We still need an 80 degrees F floor surface temperature, but we can now achieve the required heating intensity with a smaller DeltaT (a lower supply fluid temperature, assuming the rest of the floor construction has remained the same). Regardless of whether EPDM, PEX or copper is used, the required fluid temperature for any one of these would be reduced when we changed from carpet to tile.
Okay, we've got floor coverings...covered. Let's move on to look at what happens to our design if we change tubing options and how these affect system efficiency.
Tubing and System EfficiencySince the room's heating intensity is the same (system load is dictated by room construction, not floor covering), it's important to know that what affects efficiency is the back and edge loss, which are mostly influenced by the surface temperature of the radiant piping. The higher the pipe surface temperature, the higher the back and edge loss. If the pipe surface temperature is the same, then the back and edge losses are the same.
There are several tubing options on the market today, and each one will call for a slightly different design condition based on the given use. Let's say that our room calls for a 25 Btu/hr/ft2 load and a floor covering of carpet with an 80 degrees F surface temperature. The first thing we need to do is to determine what the required fluid temperature needs to be to properly heat this room. From there we can determine panel efficiency.
To determine the required fluid temperature, we need to work from the floor covering back to the tubing. In our sample room, the required floor surface temperature is 80 degrees F. Taking into account the floor construction has a known conductivity value, the EPDM, PEX and copper piping will need to have the same outside surface temperature in order to maintain the required 80 degrees F floor surface temperature.
Likewise, for the systems to have equal heat output, the tubing skin temperature for EPDM, PEX and copper is, and has to be, the same.
However, equal surface temperatures do not necessarily mean equal supply fluid temperatures. The reason why is directly related to the DeltaT through the tubing wall. Figure 1 shows various characteristics of four pipes used in radiant floor heating.
Piping thermal resistance values are calculated using the equations found in Chapter 6: Panel Heating and Cooling, 2000 ASHRAE Handbook--HVAC Systems and Equipment. Once the equivalent tubing conductivity value is calculated, a corresponding temperature drop through the tubing wall can be determined (Figure 2).
EPDM tubing will require a higher supply temperature because it has a slightly higher material thermal resistance and wall thickness. When compared to PEX, EPDM will require approximately 8.68 degrees F (13.0980-4.418 = 8.68) higher fluid temperature than PEX in the same application.
A fixed DeltaT helps us to avoid delivering more or less energy than needed, potentially causing the room to overheat or under heat. This means we need to have the same tubing surface temperature whether we use EPDM, PEX or copper. Same surface temperature indicates the same back and edge loss, meaning the panel efficiencies remain the same, even though the supply fluid temperatures vary.
EPDM does require a slightly higher supply temperature than PEX, but the difference is minimal. Recent incomplete "research" that has positioned EPDM tubing as having 20%, 30% or even 40% less efficiency, simply isn't true--these are misconceptions. The reality is that EPDM requires only 5-6% higher water temperature than PEX in a typical application.
The Bottom LineThe bottom line is it's important to design a radiant system for the components being used, and more importantly, install a system according to that specified design.
So, like the example with the wooden stick and the steel pipe asked by Mr. Webster (my high school science teacher, not the dictionary guy), "Which pipe is warmer in a radiant floor?" The answer is, "All pipes are the same temperature."