5 Ways Radiant Heat Is Better In The Ceiling

5 Ways Radiant Heat is Better in the Ceiling

By John Seigenthaler

The Upside of Radiant Heat in Floors

Ask almost anyone in the heating trade about radiant panel heating and they’ll probably start describing tubing embedded in floors. Radiant floor heating is by far the biggest part of the radiant panel market. It’s an excellent approach in many projects ranging from residential all the way up to heavy industrial applications. However, it’s not necessarily the ideal solution in the coming generation of low-energy-use houses.

“Radiant floor heating is by far the biggest part of the radiant panel market.”

When viewed only from the standpoint of heat source performance, the low operating temperature of a bare concrete slab with closely spaced tubing (6-in. to 9-in. spacing) is very beneficial. A well-insulated house on a design day may only require supply water temperatures in the range of 85° F to 90° to maintain the interior space at 70°. Condensing boilers, solar collectors and hydronic heat pumps all love to operate at these low temperatures and show their gratitude by operating near the upper end of their performance range.

The Downside of Radiant Heat in Floors

That’s the good news. The downside is two-fold: First, the average floor surface temperature required of a heated floor in a well-insulated house is only a few degrees above the room temperature. You can estimate this average floor surface temperature using Formula 1.

“The downside is two-fold: First, the average floor surface temperature required of a heated floor in a well-insulated house is only a few degrees above the room temperature.”

Formula 1

Where:

TS(ave) = average floor surface temperature (°F)

TR = room air temperature (°F)

q = upward heat flux from floor (Btu/hr./ft2)

For example, imagine a house with 2,000 sq. ft. of heated floor area and a modest design heat loss of 30,000 Btu/hr. The required upward heat flux under design load conditions is: (see above)

Assuming the room air temperature was to be maintained at 70°, the average floor surface temperature would be: (see above)

This temperature is at or slightly below normal bare skin temperature. In such a case, heat would be conducting from the foot or hand to the flow, as shown by the infrared image in Figure 1. During most of the heating season, the floor surface temperature would be even lower, perhaps around 74° when the outdoor temperature is 35°.

“True, the floor is still warmer than it would be with convective-type heating. But it may not be delivering the “barefoot friendly” effect so widely advertised as a benefit of radiant floor heating.”

True, the floor is still warmer than it would be with convective-type heating. But it may not be delivering the “barefoot friendly” effect so widely advertised as a benefit of radiant floor heating. The fact that the room is still maintained at 70° is unlikely to placate the unmet customer expectations of warm-to-the-touch floors.

The other drawback is thermal response. Low-energy-use houses are especially susceptible to rapid temperature changes from internal gains. In many new homes, this is further exacerbated by above-average passive solar heat gains.

These characteristics don’t bode well for high-mass heat emitters, such as heated concrete slabs. Spaces will quickly overheat when the sun comes out and much of the solar gain will be lost through the ventilation necessary to keep the house from turning into a sauna.

Low-energy-use houses need heat emitter systems capable of rapidly changing their rate of heat delivery. Think Jet Ski rather than oil tanker. One good candidate is a low-mass radiant ceiling panel.

“Heated ceilings deliver more than 90% of their heat output as thermal radiation. They “shine” thermal radiation down into the room much as a light fixture shines visible light downward.”

5 Ways Radiant Heat is Better in the Ceiling

Heated ceilings deliver more than 90% of their heat output as thermal radiation. They “shine” thermal radiation down into the room much as a light fixture shines visible light downward. They offer several benefits:

  • Low thermal mass. Low-mass radiant ceilings can quickly warm up following a cold start. They are ideal in rooms where quick recovery from setback conditions is desirable. Low mass also means they can quickly suspend heat output when necessary. This helps limit overheating when significant solar heat gain occurs.
  • Higher heat output. Because occupants are not in contact with them, radiant ceilings can be operated at higher surface temperatures than radiant floors. This allows greater heat output per sq. ft. of ceiling. For example, a ceiling operating at an average surface temperature of 102° releases approximately 55 Btu/hr./ft2 into a room maintained at 68°. This is almost 60% more heat output than a radiant floor with a mean surface temperature limit of 85°.
  • Not affected by changing floor coverings. It’s probably safe to say the days of shag-carpeted ceilings are over. Ceilings are the least likely surface of a room to ever be covered, especially by anything with high thermal resistance. Thus, the output of a heated ceiling is very unlikely to be compromised by future changes, such as surface coverings or furniture placement.
  • Warms objects in the room. The radiant energy emitted from a heated ceiling is absorbed by the surfaces in the room below. This includes unobstructed floor area as well as the surfaces of objects in the room. The upward-facing surfaces tend to absorb the majority of the radiant energy; the top of beds, tables and furniture are slightly warmer than the room air temperature. The surface temperature of floors below an active radiant ceiling will be slightly warmer than they would be if the room were heated by convection.
  • Easy to retrofit. Radiant ceilings are usually easier to retrofit into existing rooms than are radiant floors. They add very little weight to the structure and require minimal loss of headroom.
Radiant Ceiling Vs Radiant Floor Heating

Radiant Ceiling Vs Radiant Floor Heating

Tom Tesmar is a legendary pioneer in the introduction of Euro style modern radiant heat to North America.  We think Tom says it best.

Thermodynamics is not just a good idea….it’s the law. The output of a radiant floor is limited in two ways. First, the maximum surface temperature of a radiant floor is limited to around 87F (about 45 Btu per square foot). If the floor is operated above this temperature, the occupants are likely to complain of uncomfortably hot and sweaty feet.

The Benefits of Radiant Heat in your Ceiling vs the Floor

Secondly, radiant floors are limited by the amount of energy that can penetrate highly resistant floor coverings, such as certain carpeted and wood floors. Often, these materials limit the actual output to less than 20 Btu per square foot. Unfortunately, these beautiful and thermally resistant floor coverings are often used in the more elegant rooms, with large windows and high heating loads. Radiant floor designers often must stretch to meet the load such rooms. Also, floor coverings are likely to change significantly over the life of the building, leading to the comment “It worked until the owners placed a thick Persian rug over the wood floor”.

Radiant ceilings can easily operate at surface temperatures up to 100 F, delivering in excess of 55 Btu per square foot. Since ceilings are typically constructed of gypsum based sheet rock, they offer very little resistance to thermal transfer. Unless designers regress to the 1960’s, and resume putting shag carpeting on the ceilings, it’s likely the output of the ceiling won’t change during the life of the home.

“There seems to be some ingrained misunderstandings of the concepts. Someone will say that “heat rises, therefore you’ll have a hot head and cold feet”. Not true.”

In any discussion of radiant ceilings and comfort, there seems to be some ingrained misunderstandings of the concepts. Someone will say that “heat rises, therefore you’ll have a hot head and cold feet”. Not true.

Addressing the “Heat Rises” Myth

Heat doesn’t rise. Hot air rises. In radiant systems objects of mass are heated without heating the air. In fact, there is typically more hot air rising with a radiant floor than with a radiant ceiling. This is because air molecules that come into contact with the radiant ceiling already occupy the highest strata. In radiant floors, the cooler molecules sink and come into contact with the warm floor surface and rise as their density changes with heat, driving the convective forces that cause stratification.

Under normal conditions neither radiant floors or radiant ceilings heat the air to an uncomfortable level like in forced air systems, however.

“There are no cold floors in radiant ceiling projects.”

Another myth about radiant floors and ceilings involves the surface temperatures that are achieved. Just as the overhead rays of the sun are absorbed by the beach sand, radiant ceilings warm the floor. There are no cold floors in radiant ceiling projects. You can put a piece of Tahiti in your living room with radiant ceilings. Unless you believe tropical island comfort requires the flow of underground lava.

I also hear radiant ceilings criticized for what is termed “shadowing affects”. This is the belief that legs under tables or desks are shielded from the radiant heat and are, therefore, cold. Radiant ceilings, like floors both radiate and re-radiate. The invisible heat rays emit from the heated surface to other colder unheated surfaces. The total affect of all of this bouncing of energy is very even heat distribution.

If a person is seated at a table near an outside wall, there could be some shadowing from above with a radiant ceiling, just as there might be shadowing from below with a radiant floor when seated on a sofa near the outside wall. In most situations, shadowing for radiant ceilings or floors is negligible. More significant, however is the possibility that objects placed on the radiant floor will impede the flow of energy. Area rugs, and some furnishings such as certain sofas and beds can reduce the usable floor area, increasing the Btu load per square foot, and, perhaps, exceeding the system capability. Radiant ceilings are not subject to these types of problems.

Cost Benefits of Radiant Heat in Ceilings

Radiant ceilings accelerate fast, when needed, to meet a big change in heating load. They dissipate energy fast as well. The responsiveness of radiant ceilings makes them excellent for modern controls, placing energy where it is needed when it is needed, and achieving superior comfort and efficiency. Some high mass radiant floors are sluggish in that they take a long time to accelerate to meet the load.

In modern heating systems, too much emphasis has been placed on energy efficiency of the heat plant while wasting energy on poor distribution. This is like putting a Viper engine in a Yugo. Far more heat energy can be salvaged by not wasting it in poorly controlled buildings, than can be achieved by squeezing another Btu per hour out of a heat plant. There are some pretty interesting solutions to heat distribution problems by using radiant floors in conjunction with radiant ceilings.

“Without a doubt, radiant ceilings cost far less than radiant floors.”

In this way the floors are not required to meet the entire load, yet are conditioned to provide comfortable surface temperatures. The ceilings are heated, where necessary, to take on the severe conditions and give everyone involved the peace of mind that the system will keep up with the heat loss under severe conditions. Without a doubt, radiant ceilings cost far less than radiant floors. In most are heated, where necessary, to take on the severe conditions and give everyone involved the peace of mind that the system will keep up with the heat loss under severe conditions.

Without a doubt, radiant ceilings cost far less than radiant floors. In most cases they cost less than half of a radiant floor. Lower cost means more opportunity. They take less effort to design and install. Radiant ceilings are perfect for retrofit situations. It is very inexpensive and easy to lower a ceiling to accommodate the radiant ceiling, but difficult to raise a floor.

Radiant ceilings are not optimal over a concrete slab placed on the grade of the earth. Radiant floors are best for these situations. I also prefer radiant floors in rooms with smooth surface floors, such as bathrooms,  where occupants are often barefoot. It is a real nice touch. But when the going gets tough, the heat losses are high and the floor coverings are plush, radiant ceilings cannot be beat.”