Radiant cooling: It’s coming!
Hydronic contractors who are laying tubing for infloor radiant should consider making their jobs "cooling ready" for when radiant cooling controls soon become available.
Carol Fey | May 01, 2011
Radiant cooling is in about the same place that infloor radiant heating was 25 years ago — it's coming! A few pioneers are already enjoying it. Some thinkers are noodling it. The rest say it's just not practical. Yet hydronic contractors who are laying tubing for infloor radiant should consider making their jobs "cooling ready" for when radiant cooling controls soon become available.
One of the obstacles to selling hydronic heating has been "what about cooling?" If radiant cooling can be incorporated into the distribution system provided by hydronic heating, selling hydronics will become easier.
Do you remember the obstacles to infloor radiant heating? They seem pretty silly now. Some believed that heat from the floor would burn people's feet. Heat and inevitable leaks would ruin flooring. If you made the floor temperature low enough not to cause the first two problems, you wouldn't have enough heat, which was another problem. Add to that sun coming in the windows, making the room too hot.
Of course it turned out that if radiant heating was addressed as an exercise in moderation and control — moderate temperatures and good controls — it would work quite nicely. And moderate temperatures and precise controls are exactly what will make radiant cooling the next great idea for the hydronics business.
There is a myth that radiant cooling can be used only in arid climates. The fact is that radiant cooling has been successfully done in one of the most hot and humid climates in the world — Thailand. The Bangkok airport boasts 1.6 million square feet of radiantly-cooled floor. It is used in a very practical application — to cool areas where there is high solar heat gain through windows onto floors. According to ASHRAE, solar heat gain through large areas of glass can be up to 70% of the cooling load, especially with southern exposure.
Because of the real threat of condensation, the key to successful radiant cooling is to control supply water temperature by the dew point of the air, not by air temperature. To avoid condensation, the surface temperature must always be above the dew point. Dew point is the temperature at which water condenses out of air. A well-known example is dew on the lawn in the morning. When the temperature of the grass falls below the dew point of the air, the grass gets wet. The more moisture in the air, the higher the dew point.
The ASHRAE Journal reports that the Bangkok airport supply water temperature is 55°F, return 66°F. Floor surface temperature is 70°F. With a dew point of 50°F, there is no condensation.
The balance of the cooling load is picked up by a traditional air conditioning system. Where then is the energy savings? Savings from radiant cooling comes in a surprising way, through reducing the transportation of air. According to ASHRAE the savings comes from not having to transport air in order to deliver cooling. A significant amount of the cost of conventional air conditioning, 37%, is to power blowers. In contrast, only 5% of the cost of cooling goes to the distribution of radiant cooling.
Most radiant cooling in use today is found in commercial buildings. Yet there is a growing number of successful residential applications. Al Wallace of Energy Environmental Corp. specializes in home design and energy efficiency. He promotes the benefits of radiant cooling for both energy efficiency and comfort.
"The difference between properly designed radiant cooling and conventional air conditioning is like night versus day — absolute comfort to where you do not think the cooling system is on," Wallace says. "As with radiant heating, the noise and drafts of air movement are removed. There are no diffusers in the way of décor and cleaning. Al uses his own radiantly-cooled platinum LEED home to sell to prospective clients. He has two other sites incorporating some radiant cooling features, and two homes in construction, which implement full scale systems."
With packaged radiant cooling controls still in the development phase, Al is pioneering controls himself, along with controls expert Ray Blum of Dahl Plumbing and Heating Supply. They use the approach of providing real time humidity control and dew point tracking and then adjusting water temps to provide optimal cooling.
Wallace comments, "Actually this can be achieved using a variety of individual controls if you know what the limiting factors you are controlling. But I think the best approach now for the contractor community is to plan on dedicated radiant cooling controls being there in the near future. Contractors can begin to implement the infrastructure now with correctly spaced in-floor tubing. With a reversible ground source water-to-water heat pump (GHP), their clients will be well positioned to incorporate for radiant cooling within a year or so."
Both Al and Ray believe that the key to success is in the controls — the ability to sense changing dew points and adjust water temperatures accordingly.
"It is definitely doable," Blum says. "With the right mechanical and controls design it can be achieved. No one makes a canned out-of-the-box control yet, but it is coming. And so is the manufacturer training to support it."
"It can be complicated, but so is your car," Blum summarizes.
And don't worry about radiant cooling being a risky new idea. The Turks did it back in 7000 B.C.