When is Residential Radiant Cooling Right?

Posted on:

April 10, 2015

While hydronic radiant cooling is gaining acceptance in commercial buildings, many contractors still avoid residential radiant cooling, because of the perceived complexity and challenge of managing condensation. Recent advances in smart controls make the superior comfort and efficiency of radiant cooling a viable alternative for high-performance houses, even in humid climates.

Typically designed in conjunction with radiant heating, radiant cooling systems circulate cooled fluid through the same network of PEX pipes used for heating. This creates cooled floor, wall or ceiling surfaces that absorb sensible heat energy from the space.

The radiant system handles some to all of the sensible load while a downsized forced-air system provides fresh air and dehumidification. This significantly reduces the demand on the forced-air system and the overall energy consumption. Operational energy and cost savings vs. forced-air-only systems are typically 30-40 percent, with a slightly higher initial investment.

Identifying Appropriate Projects
Radiant cooling works best in a tightly sealed house, such as those designed to achieve LEED® certification. The ideal house for radiant cooling has:

fixed windows or operable windows with contact relays;
hard-surface flooring (not carpet or wood);
concrete floors, walls or ceilings where PEX pipes can be embedded;
multiple control zones that respond to local loads throughout the house;
a source of cooled water (e.g., chiller or geothermal heat pump);
a downsized air system providing dehumidified fresh air; and
smart controls that integrate the HVAC systems.

If this sounds like a light commercial building, it’s not far off. In fact, many of the houses that are feasible for radiant cooling are large enough to be considered light commercial buildings.

While there are some locations, such as the very arid southwestern US, where buildings can be safely cooled year-round without advanced controls, throughout most of North America, radiant cooling is not for the typical house with a single thermostat.

Selecting Floor and Ceiling Construction
To absorb heat energy into floors without making them too cold for comfort, there must be minimal barrier to heat transfer. Radiant cooling is not recommended with carpet or hardwood flooring. Bare concrete, tile or stone floors are ideal. Likewise, exposed concrete is the idea ceiling construction.

Mitigating Condensation
Simply pumping cold water through radiant surfaces could be a moist disaster; it’s essential to have techniques for avoiding condensation. Since the coolest point in the fluid’s path is the entering water temperature at the manifold, this is the point to monitor. Ensuring the entering water temperature is at least 3°F above dewpoint effectively mitigates condensation.

In a hybrid radiant-forced air system, a smart controls system is used to monitor relative humidity, increasing air cooling and dehumidification if the dewpoint gets too high, and possibly modulating radiant fluid temperature so that pipes and surfaces stay above the dewpoint, while keeping occupants comfortable. Modulating radiant fluid and surface temperatures, a well-known technique in radiant heating, may be applied here to provide the required cooling capacity.

A typical radiant cooled space will use 75°F air temperature with no greater than 50 percent relative humidity, yielding a low 55°F dewpoint temperature. To maximize system capacity, supply water temperature is set between 58 and 60°F.

Optimizing Cooling and Heating Design: Not as Complex as You Might Expect
It is easier to emit heat from a floor than to absorb heat back into a floor (and the reverse for ceilings), so a few design adjustments are required to optimize the same piping network for both modes.

Radiant cooling systems in poured floors or ceilings typically use 5/8 to 3/4 in. PEX pipes at 6 to 8 in. pipe spacing and higher flow rates than radiant heating systems, with a typical fluid Delta-T between 5 and 8°F. While this will require more pipes and larger circulators than heating-only systems, this also results in a more efficient radiant heating system, with faster response.

Radiant cooled floor capacities of up to 16 Btu/hr-ft² can be achieved under optimal conditions, with more typical capacities in the 8 to 12 Btu/hr-ft² range. Cooled floor and walls are very effective at absorbing solar gain through nearby windows.

Are You Ready for Residential Radiant Cooling?
Consumers considering radiant heating often ask, “So how do I cool the space?” Under the right conditions, radiant cooling not only provides superior comfort, but can also be the most cost-effective answer.

With high-performance custom home building on the rise again, you might want to reconsider residential radiant cooling technology. It could be the thing that sets your company apart and makes the home the talk of the neighborhood.

Before starting your first radiantly cooled home, you’ll want to seek out a manufacturer that has the know-how to design the system and the right controls to deliver all the benefits that your customers expect.