Is radiant cooling a more efficient alternative to forced air?
Dhous5
| Posted in Energy Efficiency and Durability on
I am planning to build a home in Phoenix, Arizona, and have been looking into creating ceiling tiles that act similar to radiant floor heating. From the information I’ve gathered humidity is a main reason against it. However Phoenix is very dry with a low dew point.
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Daniel,
I am not aware of any data showing that radiant cooling is more efficient than conventional cooling. Even if a radiant cooling system is more efficient, it has a serious downside: the high capital cost of installing the necessary equipment.
Here's what the DOE has to say about radiant cooling:
"Although potentially suitable for arid climates, radiant cooling is problematic for homes in more humid climates. Most radiant cooling home applications in North America have been based on aluminum panels suspended from the ceiling, through which chilled water is circulated. To be effective, the panels must be maintained at a temperature very near the dew point within the house, and the house must be kept dehumidified. In humid climates, simply opening a door could allow enough humidity into the home to allow condensation to occur.
"The panels cover most of the ceiling, leading to high capital expense for the systems. In all but the most arid locations, an auxiliary air-conditioning system will be required to keep the home's humidity low, adding further to the capital cost. Some manufacturers do not recommend their use in homes.
"In addition, the limited U.S. experience with radiant cooling creates concerns about the quality and availability of professionals to install, maintain, and repair a residential system. Despite these caveats, there may be cases where radiant cooling is appropriate for homes, particularly in the arid Southwest."
In general, moving heat using air as the fluid medium is more energy intensive than using water as the medium, so in an idealized situation the radiant cooling approach is more efficient. Also, with ducted air-handler driven systems (no matter how efficient the blower), there are inherent room-to-room pressure differences created by the air handler that drive infiltration leakage, even with perfect ducts. ( These infiltration drives don't occur with ductless mini-split heat pumps.)
The real-world has less-than-perfect hydronic pumps, and SFAIK very few (if any) radiant cooling systems have fully modulating compressors & pumps optimized for load tracking at minimum power (which is a hallmark of most ductless heat pumps). In large commercial systems where chillers can take advantage of heat that might otherwise be wasted can be incorporated there may be a models efficiency advantage using chilled panel radiant cooling over modulating ductless air source, but the net efficiency is (as always) in the hands of the system designer. At the single-family residential level I don't see that becoming a particularly cost effective approach, even though it's technically possible.
"( These infiltration drives don't occur with ductless mini-split heat pumps.)" With the consequence of uneven temperature distribution. Consider a very large single room with windows. The heat loss through the windows will make the temperature near the windows cooler during heating operation than locations distant from windows. A ducted system can force air movement and bring the temperatures closer. There is no pressure differential ( and consequently no infiltration penalty) if the return path is unrestricted!
In a reasonably high-R home (or even a IRC 2012 code-min home) temperature distribution across a mere room (even a pretty big 'un) isn't usually an issue with a mini-split. Even in a sub-code house with single-pane glass that would be a rare condition. Mini-splits DO force air movement with real cfm across the room- many of them even have louvers that auto sweep side to side whenever the blower is working (usually a feature than can be overridden, if so desired) so that every far-corner & side has at least some directed air, and mixing is pretty good- higher during high-load (=higher modulated cfm on the blower) than during low load, and during those lower loads is when you'd have lower delta-Ts to worry about.
Methinks that criticism/risk is largely unfounded, and anecdotal evidence from my relatives now heating with ductless, having retired or mothballed their ducted air heating (one propane fired, the other resistance electric) is that with the mini-splits drafts and cool pockets have disappeared in the large common areas with the mini-split heads- it's a more consistent temperature over both time & space.
But if you have measured data indicating these are outliers I could be convinced otherwise. The data on air-handler driven pressure differences driving infiltration are in, due to a lot of good work by the folks at the Lawrence Berkeley National Labs and PG & E over the past decade or so (which led directly to Manual-D duct design & leakage testing requirements under CA Title 24.) The return path impedences of ducted systems are never zero, even if the impedance across the room is zero, due to the lengths of duct between the room & air handler on both the supply & return side. Even with the most perfect duct design, changing the air flow in any one room at the register will unbalance the system. Zoned ducted systems have fairly substantial balance changes when zones coming on/off line, and the infiltration-drive consequences have been measured in multiple studies.
My Thoughts:
The issue seems to be cost. No well designed packaged systems that optimize the efficiency for residential needs exist. And the issue it goes after could be solved more effectively otherwise.
I have read of some experimental night time radiated cooling where you circulate the interior water panels to an outdoor radiator on the roof to cool the water. A low pressure pumps runs until the temp difference is equalized. During the day the interior water absorbs the heat. The space needs to be humidity controlled. Only works in climates with extreme day / night temp differences and little humidity.
Probably only cost effective in a DIY/tinkering approach - you basically just buy a truck load of pex.
If you cool the water with some sort of chiller, air based units are more than likely desired as they are instantly felt. Only if you have super cheap off peak energy would it possibly be worthwhile.
You accomplish the same much cheaper and efficiently with a whole house fan as long as you don't live near a coal power plant or pig farm i guess.
I can't think of what the true name is for the system right now, but installing underground piping made from clay or other similar materials can be used to cool air coming into a house. Install a fan at a high point to draw hot air out and this will in turn draw air from outside through the underground piping thus cooling it.
Thanks for all the input. I do plan on adding a whole house fan but not using it when it's 115 outside.
Would it make sense to have a system that was integrated into the water system. For example water comes in cold from the utilities, makes a circuit (possibly manifold system) then proceeds to sinks, shower, ext..