What are your thoughts on using radiant heating system in the summer to cool?

Would bury several thousand feet of PEX 4' below the basement slab (need to over excavate anyway).
Sort of a "geothermal" cooling system without the heat pump.
Would just twice a year switch some connections to switch source.

Seems like 50 degree water to cool would be as efficent as 90 degree water to heat.
Radiant system is a warmboard type system in floor with hardwood floors and ceilings (no slab).
House is in Boulder Colorado so humidity is not an issue.  Cooling loads are quite low.

Will condensation be a problem?

Do the math- with similar delta-Ts, heat flows both ways. Several THOUSAND feet of PEX seems like a good enough heat exchanger, but creates quite a bit of head if you intent to pump much volume. You may want to calculate pump load and cut back on the amount of PEX in the ground... (If you need a kilowatt of pump you may be better off with a small air conditioner.) If you can bury the PEX below the water table it'll have much better heat-transfer characterists than if buried in loose dry sand (no different than in a geothermal application.)

Condensation will be an issue only if the dewpoint of the room air is above the anticipated finished floor temp (not the groundwater temp.) Note that the floor will be cooler under rugs, etc- where condensation might create a mold issue. Most of the time that probably won't be an issue in Boulder, but try to dig up some history on average hot-day summertime dewpoints- if they're above 60F a lot you may have some concern. (Poking around wunderground.com data from last July it looks like 98F days have dew points in the 20s -30s only very rarely into the 60F + range. Don't worry!)

Nighttime radiant chillers using unglazed swimming-pool solar heating collectors as sky radiators has worked successfully in dry-air climates like yours. The collector/radiators themselves can get well below the outside air temp on a clear night. It may be cheaper, but more complicated than what you're considering though.

No matter what, if you have sufficient thermal mass reasonable insulation & air tightness, by minimizing daily solar gain with external shades &/or window films, cooling it with even a small amount of earth-coupling shouldn't be all that tough, but then again, neither would cooling it by night ventilation.

I don't think this would be that great of a payoff and it might be a bit cold on the feet for some.

I have our radiant floor plumbed so that I can run the water through it in the summer. We have 2" concrete. We have no condensation problems.

I do not find that it makes enough difference to be cost effective though. We are far better off with cooling the house down at night and shutting it up during the day. The floors do get cool and while I like that on a very hot day, it is not that comfortable for most people.

Dana & jmagill, thanks for your replies!

I am already doing superinsulation, medium thermal mass, airtight, well shaded, night time cooling with ECM whole house fan, etc etc.
I guess I am really just trying to justify the up front expense of the radiant system by using it for cooling also.
Radiant heat would only run 4 months (mid Nov to mid March) in my climate (Boulder, Colorado).

"Several thousand feet" of PEX was just a WAG based on approximately 1,000 lf of PEX in heating system.
Had not thought of pump size! Tossed idea out on this forum BEFORE I talked to plumber.
House is very low heat demand (14-15k btu/hr) and radiant is backup to passive and active solar.

Actually the PEX would be below water table in the summer, so maybe a lot less PEX and a small pump could add some benefit.
Maybe night time whole house fan and radiant cooling during day when closed up?
Might just install PEX, just in case, whole house fan is not enough. Could hook up later.

Thought about night time radiant chiller, but only place for it would be on a roof that would get a lot of late afternoon sun, so I believe it would start out too hot. Also roof is crowded with PV and solar thermal.

Was not concerned about "cold" floors because of hardwood flooring and about 60% of radiant is in walls and ceilings.
Office, rec room and bedrooms are all carpeted because I hate wearing shoes.

Thanks!

If you have the radiant in the walls and ceiling then you should give it a go.

Our house is SIPS and we have cool nights(6500 feet in Wyoming). We find that by cooling in down at night and keeping the south windows covered during the day, we can keep the temp at 66 inside during the day.

Radiant chillers work best on flat to slightly north-pitched roofs, not exactly where you'd be placing stuff designed for maximum solar gain, but if there's really no room I'll take your word for it.

Burying a pool heater or two at a level below the summer-time water table for should provide a lot more heat-exchange surface area for radiant cooling (at much lower pump head) than a big ball o' PEX, and will provide cooling as-needed via thermostat-controlled pump. Predicting it's performance may be difficult without data, (I don't know of any experiments with buried a pool heaters for cooling) but it seems like it ought to work for the relatively light cooling loads we're talking about. Limiting daytime solar gain with shading is always good idea though, no matter what the cooling method.

In a superinsulated house, shade management coupled with night-time ventilation should work GREAT in dry climates where the nighttime temps are reliably below 65F. Awnings on the S side, seasonally installed exterior shades or trellises with vegetation on the W side can kill the gain by quite a bit. Unless you have a lot of east facing glazing it's probably less of an issue since the AM cooling load is typically pretty small and a bit of AM solar gain can be tolerated, may even be welcome after cooler nights, but you'll figure out what works best for your site.

I wouldn't worry about super-chilled floors with this approach unless you're leaving the exterior doors & windows open or not shading your S & W facing windows and it has to run 24/7 to keep up. Sounds like you have a lot of surface area- with the surfaces in the mid-60s it won't feel super-cold even to bare feet, and would keep air-temps well under 80F most of the time. If your groundwater was under 40F and the radiation area so small tha it had to get the surface temps to the low 50s, yeah that could get pretty chilly to the touch. But with 50F+ ground water I doubt you could ever get the floor surface temps below 60F.

The fact that you have radiant walls & ceilings means it can still be effective with higher temp water/surface. Were it only the floor you'd get cool air pooling at the floor and never convecting away, forming an insulative boundary layer. With cool walls & ceilings you'll get improved heat transfer from the convective flow. In a super insulated house with radiant walls & ceilings you will probably be able keep up even at 70F surfaces (how cold is that on your feet, eh? ;-) ) You may find this article useful/interesting:

http://www.csemag.com/article/CA6553395.html

(If office environments with 60% coverage radiant ceilings can be cooled with 64F surface temps, I think you're good-to-go at 70F surfaces given the amount of surface area and low load you're talking! And that's probably achievable with 50F groundwater coupling.)

I have been giving a lot of thought to radiant cooling using ground water in southwest Florida for a couple of years. In fact just today I started a spreadsheet to calculate the amount of heat that could be removed with ground water.

If your water table is so close to the surface you could just put in two wells. Pump cool water out of one and use the other to inject it back into the ground. Be sure it is far enough away from the first so that you don't end up pumping warm water out of the ground. This scenario would also probably be a lot easier than laying out a loop under the slab.

There were comments about shading windows on the east and west. Windows on the east and west walls are bad in just about any climate. They let in a lot of heat mornings and afternoons. Keep in mind that 50 square feet of single-pane glass let in about one ton (12,000) btus of heat that has to be removed again. There is virtually no solar gain on the north-facing windows and south-facing windows can be shaded in the summer with overhangs. Good design is very important. It is easier and probably cheaper to keep the heat out than to have to remove it afterwards - especially when you are using solar panels.

Cold floors are not comfortable to walk on with bare feet. Radiant cooling is usually done with just the ceiling. However, the more surface area that is being cooled hydronically, the warmer the water temperature can be. So, by cooling the walls, ceilings and floors you should be able to remove enough heat without having a cold floor.

There is some convection that takes place when air is warmed or cooled by a warm or cool surface. That, however, is not the primary method of heat transfer with radiant heating and cooling. Every object with a temperature above absolute zero (-459.67̊ on the Fahrenheit scale) emits infrared heat and heat always moves from the warmer to the cooler object. Radiant heating works by emitting infrared waves which are absorbed by the cooler objects in the room and heat up as a result. “Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. ... In a practical situation and room-temperature setting, humans lose considerable energy due to thermal radiation. However, the energy lost by emitting infrared heat is partially regained by absorbing the heat of surrounding objects (the remainder resulting from generated heat through metabolism). Human skin has an emissivity of very close to 1.0 .[1] Using the formulas below then shows a human being, roughly 2 square meter in area, and about 307 kelvins in temperature, continuously radiates about 1000 watts. However, if people are indoors, surrounded by surfaces at 296 K, they receive back about 900 watts from the wall, ceiling, and other surroundings, so the net loss is only about 100 watts.” (Wikipedia, Thermal radiation, http://en.wikipedia.org/wiki/Radiant_heat)

Radiant cooling removes heat by absorbing the heat emitted by other warmer objects and transporting it away in the water.

A BTU is the amount of energy necessary to heat one pound of water one degree Fahrenheit. One gallon of water is about 8.34 pounds so if the water warms up one degree, it has absorbed 8.34 BTUs. If one gallon of water is warmed 10 degrees, it has removed 83.4 BTUs. With this information I’m starting to calculate the possibilities of cooling a home in Florida using ground water and a solar-powered chiller. The system would also include a tank to store chilled water for cooling at night when the sun can’t power the chiller. Imagine the fossil fuels we could save if all our homes were cooled with solar-powered geothermal radiant cooling systems. I find the idea fascinating.

In a superinsulated house, shade management coupled with night-time ventilation should work GREAT in dry climates where the nighttime temps are reliably below 65F. Awnings on the S side, seasonally installed exterior shades or trellises with vegetation on the W side can kill the gain by quite a bit. Unless you have a lot of east facing glazing it's probably less of an issue since the AM cooling load is typically pretty small and a bit of AM solar gain can be tolerated, may even be welcome after cooler nights, but you'll figure out what works best for your site.







_