Is radiant cooling an option?

Mention radiant cooling to most HVAC engineers and you get a funny look. The first thing they ask is “What about the condensation?”

04/01/2008


Mention radiant cooling to most HVAC engineers and you get a funny look. The first thing they ask is “What about the condensation?”

There seems to be an assumption that conventional 42 to 44 F chilled water supply temperatures are used for all cooling applications. In this case, if you used that kind of entering chilled water temperature for radiant panels, they will condense. Chilled water can still provide sensible comfort cooling even at elevated temperatures above 60 F; depending on the cooling terminal system, the ability to provide dehumidification is diminished.

You also can get the benefit of higher chiller equipment energy efficiencies when the chiller, or chilled water plant device, operates at higher chilled water temperatures.

Where the local climate requires dehumidification during the summer, a series arrangement for the radiant panels can be used. Take the conventional low-temperature chilled water through the ventilation air handler's cooling coil for dehumidification, temper the dehumidified cooled air with the heat recovery coil, and use that warmed chilled water as the source supply water for the radiant cooling system.

Or, use a standalone refrigerant direct expansion cooling system for the ventilation air handler to provide dehumidification, and operate a standalone high-temperature chilled water system for the radiant cooling circuit. The standalone refrigerant ventilation air cooling option with a separate high-temperature chilled water circuit adds some costs, but this is generally a climate-dependent issue for warm, humid climate locations to enable more dehumidification to be done. The sub-cooling of the ventilation air and subsequent reheat can be handled by a properly designed energy recovery air handler with desiccant heat wheels and exhaust air heat recovery for the reheat function to keep the total net energy use as low as possible.

Measure heat exchange

Let's start with basics: Radiant heat exchange is a sensible heating/cooling operation with no capability for airborne moisture removal or addition, which is a latent heat exchange humidity control operation. Radiant heat exchange rates depend on the amount of surface area “A,” which is at a surface temperature of “T.” A small radiant heat exchanger area requires a significant temperature difference “T” for a given fixed heat exchange load. A large radiant surface area requires a much smaller temperature difference “T” for the same fixed heat exchange load.

Radiant heat exchange varies with temperature to the fourth power, and falls off by distance squared, and being infrared energy, operates at the speed of light. Radiant cooling is the creation of cooler surfaces around a warmer surface, such as a human body. This allows humans to transfer heat away from us to the cold surface at a higher rate, making us feel cooler. An example would be standing next to a large glass surface on a cold winter day. Radiant heat transfer energy always goes from hotter sources to cooler sources.

Radiant heating and cooling alone is not a complete comfort system for a healthy indoor environment. The three basic human comfort parameters that must be addressed are radiant comfort (40% to 50% of the human comfort factor), fresh air/convection/air movement (30% of the human comfort factor), and humidity control (10% to 15% of the human comfort equation).

Large area radiant cooling ceiling (radiant slab ceilings and full radiant ceiling panel ceilings) generally are operated at 64 F average surface temperature for effective radiant cooling. Because the extensive radiant ceiling has no thermal gradient differences across its face, convective air movement from falling cooler air does not occur. Cooler convective air patterns are associated with radiant cooling systems, primarily at “point source” or small area radiant cooling surfaces like passive chilled beams and suspended radiant cooling panels, which generally are operated at temperatures of 57 to 64 F, and generate convective cool air movement as part of their function and overall cooling capacity rating.

If you maintain the indoor temperature comfort with a radiant heating/cooling system, the air side of the indoor comfort system must supply fresh air (filtered, treated/tempered, humidified/dehumidified) or outdoor air at a rate to ensure healthy conditions, and at an air movement rate that promotes a healthy human comfort, as well as enough make-up air for the building exhaust systems. The fresh air can be supplied at or near room temperature because it does not need to provide heating or cooling, assuming the radiant system is able to deal with the room heat gains and heat losses.

Increase energy efficiency

This presents a significant leap toward lower HVAC system energy use and requirements, compared to using all-air systems to provide room air temperature control combined with the ventilation function. Moving a lot less air saves energy while the hydronic radiant system efficiently transfers energy and transports it around the building to the central plant.

Water can hold more than 3,400 times as much energy per unit volume compared to air, so moving energy around a building with a hydronic system is much more energy-efficient compared to moving large volumes of warm and cool air around the building. While some may assume that central hydronic heating/cooling plants are more costly, the total cooling and heating plant size required for a radiant cooling and heating system is much smaller, mainly due to the higher performance building envelope that also is required to go hand in hand with a radiant slab/radiant cooling system.

The proper application of radiant cooling that ensures that you will not get condensation requires the of use large surface areas, like ceilings, walls, floors, or combinations of all three surfaces. This keeps the mean radiant cooling surface temperatures as high as possible above dewpoints, yet still provides an effective amount of cooling capacity.

Based on the nominal heat gains in a conventional office space, and with 60% to 70% of the ceiling used as the radiant cooling surface, a 64 F surface temperature would be able to provide effective cooling to the occupants in that space. With an average ambient indoor air dry bulb temperature of 76 F, the corresponding maximum relative humidity at which condensation will start to form on surfaces colder than the dewpoint temperature will be approximately 64% relative humidity (RH) (dewpoint temperature of 63.5 F/ wet bulb temperature of 67 F).

Considering that ASHRAE Comfort Standards require maintaining less than 60% RH levels for indoor comfort, there should be zero risk for condensation on large radiant cooling surfaces that operate at 64 F or higher. Also, testing of radiant cooling panels have shown that it would take more than 8 hours with a surface temperature held at 14 F below the ambient dewpoint temperature before visible drops were apparent.

If the ventilation air system for the building can be designed to ensure that the indoor ambient RH is below 50% at all times (a requirement for good indoor comfort), then the lower limit on the radiant cooling dewpoint temperature would fall to around 57 F.

This is a large change in potential radiant heat exchange rate when dealing with temperature to the fourth power. Lower the indoor relative humidity to 45% at an ambient air temperature of 76 F, and the dewpoint falls to 53 F. That's a reasonable margin of safety to operate the radiant cooling surfaces down to 62 F. This can be observed on a psychrometric chart to see the envelope of performance that radiant cooling surfaces can work within, without risk of condensation.

Outside air

What about opening windows and infiltration of humid outdoor air during the summer? That should be a concern, but considering that the amount of untreated humid outdoor air that mixes with the drier indoor air leads to a blended ambient RH, and the length of time it takes to form condensation, even light fogging of a cooled surface, then transient humidity spikes of a few hours can be tolerated. Condensation formation is a slow phenomenon, and can be responded to easily by conventional controls, so it's easy to prevent damaging condensation formation by use of fail-safe controls.

To ensure that the condensation risk is minimized, there are relatively inexpensive condensation sensors that can be used to reset the cooling water supply up a few degrees to be slightly above the dewpoint to insure that condensation can't form.

There will be a slight loss of some radiant cooling capacity, but a good engineer always has a back-up plan, right? Use the required ventilation air supply as a second-stage cooling medium, using a re-cool coil for that zone to lower the ambient air temperature to deal with the room comfort for short periods of time. The ideal complementary ventilation air supply system that should be coupled with a radiant cooling system is a low-level air supply method like underfloor air distribution (UFAD) or low-level displacement ventilation (DV) terminals to make the most of the combined systems advantages.

The basic premise here is that the radiant cooling system is providing close to 100% of the sensible cooling load in the space, therefore the ventilation air only needs to be supplied at virtually “room temperature” (i.e, around 70 to 72 F). If a low-level UFAD or DV system is used, lowering that small volume of ventilation air down to 67 F would provide an additional 10 to 14 Btuh/sq. ft of space cooling capacity.

Using the ventilation air supply as your second-stage cooling source can provide a quick response to fast-acting solar transients at perimeter areas, as required. Ideally the building design team should try to reduce and eliminate those pesky fast-acting solar gain transients in the first place, but where they do exist, the air system can provide a fast-acting response. The HVAC designer will then have to ensure that the radiant cooling system is adequately sized for the “steady-state” cooling requirements, and then the air system would be sized for the basic minimum ventilation air supply plus whatever excess air is needed for any peak transient cooling loads. A variable air volume air valve could be used, or a re-cool coil to lower the air supply temperature of the basic ventilation air volume.

If the room cooling was being provided by a traditional overhead all-air fully mixed air system, at least 1 cfm/sq. ft to 1.3 cfm/sq. ft of cooling air would need to be circulated. For the same heat gains in that same room, with most or all of the sensible cooling being done by the radiant cooling system, then the only supply air needed is the basic 20 cfm/person of ventilation air supplied at “room temperature.”

That is less than 0.20 cfm/sq. ft in a normal office occupancy, maybe as high as 0.30 cfm/sq. ft in a densely occupied office space. As pointed out above, even that small volume of air, when supplied at low levels in a UFAD or DV style, can provide additional boost cooling to supplement the radiant cooling system for transient short-acting heat gains (meeting rooms, localized solar gains, etc.).

Control your surroundings

The fundamental key to enabling radiant heating and cooling systems to work well is to control the building envelope thermal loads first so that the radiant systems can work within their effective surface temperature ranges for the radiant surface areas being used. The goal is to reduce or eliminate high-peak transient thermal loads in a room, so the room cooling is limited to people and equipment heat gains. Heat gains from lights can be eliminated from the room cooling loads through the use of low-level air supply and stratified air extraction systems like UFAD and DV systems.

Radiant floor heating systems are limited to a maximum recommended surface temperature of about 85 F for comfort (the average human skin surface temperature). Radiant floor systems also suffer from furniture masking and floor covering variations that can act as insulators, requiring higher radiant heating fluid temperatures to get the right finished floor surface temperatures. Radiant ceiling heating surface temperatures can be much higher because there is no direct contact to them, but the radiant emitter surface temperature versus distance to the human (and other surfaces) becomes a critical design element.

Small surface area heaters, such as low- and high-intensity gas-fired radiant heaters, are like miniature suns, and the closer things are to them, the hotter the objects get. Ask the how someone felt when paint peeled off the roof of his restored classic car after he installed some “low-intensity” overhead radiant heaters in the garage that operated at a high surface temperature.

Consider the limitations

Radiant cooling systems likewise are limited in their effective temperature range; floor cooling systems are limited to minimum surface temperatures of about 65 F for direct human contact comfort (and the issues of floor coverings and furniture masking also apply). Radiant ceiling surface temperatures are limited by the average controlled humidity of the indoor air, and generally can be as low as 62 F as a design limitation.

The following summary represents the radiant heating and cooling limitations for effective heating and cooling loads:

Radiant heating floors limited to 85 to 90 F = 35 Btuh/sq. ft output of radiant surface

Hydronic radiant heating ceilings, up to 200 F = 250 Btuh/sq. ft of radiant surface

Radiant floor cooling maximum output at 65 F = 12 Btuh/sq. ft of radiant surface area

Radiant cooling ceilings output at 63 F = 25 Btuh/sq. ft of radiant surface area

So, the object is to “reverse engineer” the building design to make sure that your maximum cooling load would be no more than 25 Btuh/sq. ft where you intend to use a radiant cooling ceiling (chilled ceiling) system, or 12 Btuh/sq. ft for a radiant floor cooling system.

For hydronic radiant heating systems, the only real limitation is for radiant floor heating systems to keep the room heating losses below an average of 35 Btuh/sq. ft of the room being served. This can be accomplished easily by an integrated design team, with the mechanical engineer working hand in hand with the architect to get a high-performance envelope designed—toolbox items like exterior sunshades, double-skin facades, high-performance sealed windows with triple-paned or better thermal performance, minimized thermal bridging at the perimeter envelope details, and other similar approaches.

Generally, windows are a weak link in terms of allowing high solar heat gains, and excessive heat losses. Policing window thermal performance and exterior solar shading can help reduce the room heating and cooling loads down to within the radiant heating and cooling system design limits. The more windows you want to use, the higher the thermal performance required of those windows, and similarly watch their solar orientation and exterior shading to keep solar heat gains reduced. The inside surface temperature of the glass is a critical comfort factor for the mean radiant temperature of the room as well.

While better wall and roof insulation, with reduced building infiltration, is a big help to reduce heat losses and gains in a building, the windows then become the next big element to pay attention to for indoor comfort. If the perimeter transient loads are reduced or eliminated, then the room cooling load is mostly “steady state” from equipment and people density. That's an ideal fit for radiant cooling and low-level air supply, so that half of those remaining heat gains can be carried off in thermal plumes through the stratified high-level air in the space that is exhausted out of the building. The radiant cooling can deal with the people cooling loads while the equipment heat gains are primarily convective thermal plumes. The small amount of radiant heat being given off by the warm equipment also can be offset from the radiant cooling panels/ceiling. It's all about mean radiant temperature control for human comfort.

Radiant cooling systems can be designed to work effectively in nearly all applications and most climate zones without fears of condensation, provided good design approaches are followed with complimentary fail-safe controls. Very low building energy use can result, primarily from the improved building envelope, as well as from the use of hydronic-based radiant heating and cooling systems.


Author Information

McDonell develops institutional and sustainable projects, including complete mechanical design, design execution through to construction services, and site inspections. He specializes in very low energy semi-passive building systems including radiant cooling, displacement ventilation, and high-performance building envelope design.




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