Reducing noise from heat transfer plates within radiant panels.

Figure 1

Last month we discussed the “physics” that create the expansion/contraction noises from aluminum heat transfer plates in radiant panels. This month we’ll look at what can be done to eliminate these sounds.

Anything that reduces differential expansion between the tubes, plates and floor materials, or slows the rate of temperature change between the materials, will improve the situation.

Let’s start with a discussion of materials. My specifications always call for PEX-AL-PEX tubing whenever a tube-and-plate radiant system is installed. The coefficient of linear expansion for PEX-AL-PEX tubing is very close to that of aluminum, and about 1/7 that of PEX tubing. That’s because the aluminum layer in PEX-AL-PEX tubing is the “dominant” structural element within the composite tube, and largely controls its rate of expansion. Because PEX-AL-PEX tubing and aluminum heat transfer plates expand at very similar rates, there is less opportunity for stiction and, thus, less chance for ticking noises.

Another issue you can partially control through design is the rate at which water temperature changes within the radiant panel circuits. The use of outdoor reset control, which calculates the required water supply temperature to the circuits based on current outdoor temperature, goes a long way in this respect. Instead of “slamming” hot water into a cool tube and causing its temperature to change 50 to 80 degrees F in a matter of seconds, outdoor reset control may only change the water temperature 0 to perhaps 5 degrees F per hour in response to outdoor temperature change. Although this very gradual temperature change doesn’t eliminate expansion or contraction of materials, it does reduce the possibility of those materials making audible ticking noises.

Figure 2

Fastener Finesse

Another installation detail that affects expansion noise from tube-and-plate radiant panels is how staples, screws or other fasteners are placed.

In an above-floor tube-and-plate system, you should only fasten the aluminum plate on one side of the tube groove, as shown in Figure 1. This allows the plate to open slightly as the tube is pressed in place. It also allows for subsequent lateral movement of the plate as it is heated. A couple of light-gauge staples are adequate to “tack” a 24-inch aluminum plate in place. The final covering layer, be it nailed-down hardwood or a thin plywood cover sheet, provides all the clamping force necessary to hold the tubing and plates flat.

When plates are stapled up to the bottom of a subfloor, it’s important they make good contact with that subfloor. I suggest a minimum of eight equally spaced staples on each side of a 6-inch wide by 24-inch long aluminum plate to ensure it’s tightly pressed against the subfloor. Any air gaps between the plate and subfloor will decrease heat transfer.

In both above-floor and below-floor installations, leave an expansion gap of at least 1/4 inch between the ends of adjacent aluminum plates. This is more than sufficient to accommodate lengthwise movement of the plates.

It’s a good practice to minimize situations where nails for hardwood flooring are driven through heat transfer plates. By installing 6-inch-wide, single-tube plates on 8-inch spacing, you create a 2-inch gap between plates, as shown in Figure 2. This allows two nails to be located every 8 inches along the edge of hardwood flooring without shooting nails through the plates. When using 14-inch-wide plates, it allows for two nails every 16 inches, as shown in Figure 3.

Although either detail should provide plenty of nail-holding power for well-dried wood flooring, my preference is for 6-inch plates at 8-inch spacing because closer nail spacing generates less unusable short lengths of flooring.

Figure 3

It’s also important to remember that floors can produce “squeaking” sounds due to shrinkage of the subfloor or wood sleepers. This shrinkage creates slight gaps between the subfloor and underlying framing. When someone steps on the floor over such a gap, the subfloor moves down, and the shank of a nail in the vicinity may rub against the subfloor, resulting in a squeak.

Many framers counter this by using a glued subfloor system. Construction adhesive is placed at the top of the floor joists prior to fastening down the subfloor. The fasteners then “clamp” the subfloor down while the adhesive sets. The result is a tightly bonded subfloor/framing assembly with little opportunity for differential movement and, thus, minimal potential for squeaks. This detail is especially relevant for heated floors because elevated temperatures accentuate wood shrinkage. Radiant panel installers should recommend it to builders to avoid finger-pointing after the fact.

It’s also prudent to use construction adhesive between sleepers and the subfloor in an above-floor tube-and-plate system. Two light beads of construction adhesive down the length of the sleeper should be sufficient. An alternative is to apply adhesive to the backside of the sleepers using a notched trowel, and immediately fasten it in place.

Figure 4

I also prefer the use of plywood or OSB (oriented strand board) for the sleepers vs. solid sawn wood. The latter is more prone to lateral shrinkage and cupping, especially if it’s not well-dried.

Finally, in the case of radiant walls and radiant ceilings, we’ve successfully used contact adhesive rather than mechanical fasteners to secure aluminum heat transfer plates to strips of foil-faced polyisocyanurate foam as shown in Figure 4. Again, 6-inch plates on 8-inch centers provide a 2-inch-wide strip where drywall screws can be driven without piercing the plates or coming close to the tubes.

When the assembly is done, the plates are clamped in place by surrounding materials. The rubber compounds in the contact adhesive provide a slightly elastic bond that can move with the plates without creating any noise. The solvent-based contact adhesive we have used on several such systems is the same adhesive used to bond laminate countertops, and can withstand temperatures up to 200 degrees F, far higher than they will ever go in this type of system.

One of the key benefits of a properly installed hydronic radiant panel system is silent operation. Proper material selections - combined with control techniques that gently steer temperatures up and down - can assure this is achieved. Although Snap, Crackle, and Pop are great mascots for Kellogg’s Rice Krispies, you don’t want them hanging out in your heating systems.