Underfloor Radiant COOLING? Anybody?
My freind’s dad is a wingnut type HVAC installer who has convinced a couple of people to use their underfloor hydronic heat systems for cooling by piping chilled water. Is this reasonable? I have several questions that he has never been able to satisfactorily answer:
1. How effective is a cold floor for cooling a room?
2. How does one avoid condensation issues? We’re here in Missouri, and today the temperature is going to hit 96 with 60% humidity. The dewpoint is often 75-80 degrees. How do you cool anything with that?
3. What sort of flooring could you use in this scenario?
4. Humidity control: In our area, it’s not so much the heat but the humidity that makes things uncomfortable. Assuming a tight and well insulate structure, you’re gonna be exhausting air anyway for fresh air changes, but how do you get the humidity out of the intake air that’s getting sucked in at (for instance) 96 degrees?
I have many other ???s, but hopefully somebody here might have some insights.
Is underfloor cooling feasible, or is this guy just crazy?
Replies
Radiant cooling is reasonable, but like radiant heating only more so, thourough knowledge and planning is critical. When all of the details are considered, the option seldom makes sense compared to other options. To answer your questions:
1. Very effective. Understand in all cooling and heating systems, the intent is to cool (or heat) people, not rooms. Seems like splitting hairs, but its a very important distinction when doing so via radiant heat transfer.
2. Dewpoint control. You have to dehumidify air in a space that is going to have cold surface cooling. Air has to move across a cold coil (or a dessicant) to remove moisture or the cold floor/wall/ceilings will sweat. At 75 degres and 50% relative humidity, the dewpoint is 63 degrees. This works in most cases because floors colder than that are not comfortable. Vapor transmission control is also very important.
3. Water resisten, non-slip. Terra cotta tile works well. Stained concrete.
4. See #2. If you are going to ventilate in addition to cool, thats more moisture to content with. It has to be removed. First step is preconditioning via and ERV.
What other ??? do you have. You asked the important ones, excect maybe how do you produce cold water to make this happen? (Small scale chillers are not real common and are expensive). Some will point you to the Europians that have successfully done everything we have not. Do not believe them, or anything posted by Frenchy!
You say vapor transmission is important. You're talking about water vapor transmission through the entire building envelope, right? One thing I've always had trouble with in our region (Kansas City, Missouri) is how to plan wall assemblies for climate. I mean, we get plenty of heating days and low humidity in winter and plenty of summer cooling days and lots of humidity there. So how/where do you do you vapor control? That's maybe a different discussion.Obviously, one needs to plan to avoid condensation, but when it does happen, I assume that it occurs on the exposed surface of the floor. Or will the water show up somewhere where you can't see it to know there's a problem? Say, in between the concrete/gypcrete and the subfloor decking? I can imagine a nightmare where your subfloor rots out from above...ERV: That's going to take the cold (dry) stale air, run it over a bunch of plates to cool the hot incoming air, right? During that cooling process, the hot air is going to lose some ability to hold moisture and create condensation on the conducting plates. Would that take care of all of it? Or is there some way to further dehumifdify the incoming air and what would that device be?Chilled Water: Assuming a ground source heat pump for heating water, is it possible to use the same to produce enough chilled water? How does one supplement the chilled water if GSHP isn't enough?
Condensation is the killer. I've seen with my own eyes radiant cooling in a ceiling where the ceiling become a condensing surface. Water droplets, sagging drywall, etc.I've heard of radiant cooling in the floors, carpeted areas became moist and mildew/moldy. Tiled areas became slick. Hardwood cupped.
Wow, you've really sold me. When you can you start?
No worries, a small bag of desiccant in each corner of the room and we'll be good to go. I've got 1,338 more things to do on my "to do" list, after that my schedule is open and I'm all yours.Job 1,338 I'm tackling tomorrow. Grouting 1800 sqft of slate tile. Don't hold your breath...<g>
I mean vapor transmission through the enevelope. Vapor barriers and vapor control IS a subject of a different discussion, covered at various times by those here that care, ad naseum. I heat for nearly 6 months and cool for less than six weeks. My vapor barrier is on the inside of the envelope. But the point is, to be able to control humidity well enough to consider cold surface cooling, uncolled vapor transmission though the building envelop cannot occur
Condensation forms on the cold surface, but then goes .... you imagine the nightmare accurately.
Lets be clear on some terms. Energy Recovery Ventilator (ERV) is a simple, relatively ineffective (30 to 40% effective) total (senseble and latent) energy tranfer unit with a fixed plate/cartidge type of exchanger like RenewAire. Heat Recovery Ventilator (HRV) is a fixed plate ait-to air heat exchanger that is sensible only, no moisture is transfered between air streams. HRVs are usefull primarily in Nataoriums, Gyms and the like, where humidity is to be expelled, not recovered, The third type doesn't really apply to residential, but is an Energy Recovery Unit (ERU), which is a larger more complex version of the ERV with a rotating dessicant wheel and very high (80% plus) effectiveness. The ERV will transfer some of the moisture and some of the heat energy from the hot humid incoming air to the cooler, dryer exhaust air. It will do this on a molecular level and no condensation will occur. The RenewAir core and other similar versions like Venmar, transfers water vapor and heat through the media while keeping the air streams separate.
Further duhumidifcation by means of a coil. You have cold water run it through a coil before the floor (this would be a good parctice for the heating season as well) and bring the air dewpoint to at leats 5 degrees below your cooling surface temp. You would need to do this to the air in the house as well as the ventilation air. Sources of moisture in a normal house include respirations, perspiration, cooking, showers, plants, aquaria, leakage, etc, etc...
There are small chillers available. A knowledgable DIY mad scientist type, like Junkhound could make a small split system into a water chiller without too much work. In the lower 48, GSHP/WSHP are generally sized to meet the cooling load as the worst case. They should be able to meet the load.
I'm just wondering if the whole idea of radiant cooling is even feasible in today's world. One concern I've always had with radiant heat in my climate is that you still have to go to the expense and complication of installing equipment, controls, and ductwork for forced air cooling, so radiant heat is really a luxury. Now, if you could eliminate the majority of the ductwork and the bulk and expense of air handlers and condensers and such while making your hydronic installation work double-duty for cooling too, well then maybe I'd have a legitimate argument about the "greenness" of the system.The project I'm thinking about is for a new construction / infill lot house for a vacant lot in my neighborhood. The exterior and interior details would be thoroughly grounded in the architectural and historical roots of the neighborhood, but the construction methods and mechanical systems would be as efficient and up-to-date as possible. I'm thinking 2x4 / 2x2 mooney construction with closed cell or closed cell / densepak hybrid insulation. ICF foundation. GSHP. Etc. Etc. Tight. This would be my primary residence, but it also needs to be able to be sellable at some point in time.
I looked at this issue extensively when building my own house: infloor heating/cooling vs forced air systems. While there are some successes out there, they are very few and far between. Feasible? Not in any practical sense. Only as an oddity or a science project, IMPO.
A well done version of a forced air system is very comfortable, provides an easy means to ventilate, humidifiy, dehumidiy and filter the air in your house as well as provide continuos slight air movement for turnover. This requires a variable speed air handler and a well concieved and implemented duct system.
$3/sf can get you a good system, installed, more for GSHP depending on the field details. You can add direct ventilation with a barometric damper (a skuttle ducted in to the reurn of the air handler works great for this) for positive building pressure control or your can have a neutral system using an ERV. You can add a side stream (partial flow) inline dehumidier as well.
For $10/sf you can get a basic zoned infloor hydronic heating system with a high efficiency boiler, no cooling and no air treatment. Gas to gas a furnace is as efficient as a boiler, but the hydronic system distributes heat more efficently (for less energy costs). Same for a ground source heat pump system. Triple that cost if you want to have cooling and decent Indoor Air Quality provisions - a real bargain.....not.
I've been involved in this industry for some time and if cost and resale were important, a good forced air system is the choice every time. The comfort available from a well done RFH system is better that that of a FA system, if you spend a great deal of the winter nekid or at least barefooted. Otherwise, the differences usually cited are only based on the best vs the worst (and one regular poster here has a magical house and infloor heating system, but thats a special case!). I have electrically heated tile floors in bathrooms and laundry rooms and the like in my house and its a wonderful thing to have, but it is certainly a luxury.
Edited 6/25/2009 4:38 pm by Tim
Well then, if your numbers are remotely close, then obviously hydronic makes no sense except for the super rich.Reminds me of the period I went through back in architecture school when I was interested in high-thermal-mass wall construction and then learned that with lightweight, high-R construction you could get more efficiency for an order of magnitude less money.Kind of sounds like the PV solar myth: Great in concept, just not practical in reality unless you've got lots of $$$.
Well, you know how the old say goes, "Generalizations are generally wrong." Anyone can dispute the specifics of the numbers, but the relative costs should hold up.
New technology is not always better, but warrants close scrutiny to determine the potential. Infloor heating gained a lot of popularity when cross linked polyethylene tubing hit the market in full force. I sold miles of the stuff. In the large McMansion houses economies of scale made it more attactive.
I don't know the details, but this recently built community centre in Vancouver uses in slab heating and cooling. As a public project I doubt the budget was very high.
http://images.google.com/imgres?imgurl=http://www.egodesign.ca/_files/articles/blocks/5937_gleneagles_community_centre2_james_dow_patkau_architects_inc..jpg&imgrefurl=http://www.egodesign.ca/en/article.php%3Farticle_id%3D259%26page%3D2&usg=__ysbzfehqLJTiHcqTpbDggJV0K08=&h=512&w=348&sz=53&hl=en&start=1&um=1&tbnid=Q3dBfulzVI6-9M:&tbnh=131&tbnw=89&prev=/images%3Fq%3Dpatkau%2B%252B%2Bcommunity%2Bcentre%26ndsp%3D20%26hl%3Den%26safe%3Doff%26rlz%3D1T4GWYE_enCA316CA213%26sa%3DN%26um%3D1
I agree with most of what you are saying, but I have no idea where you are getting "triple" the cost of adding cooling and IAQ to a radiant system.at most, it would cost the same as the air system you are not putting in for heating. There is no getting around a ducted system, it's true. but it's still just a ducted system, nothing magical happens there that suddenly makes the air component cost more just because you had a hydronic heating system. that component can cost less without cooling, if all you need is IAQ ducting. But with cooling, you basically are looking at the cost of a furnace vs the cost of a hydronic system.radiant is definitely more comfortable than air, though very well designed air in a very well insulated envelope can, in some cases, get close. In a typical home though, typical to date at least, not likely to happen. Not to be partisan, but the "few and far between successes" on the economic side vs hydronics apply on the comfort side vs air as well.Basically it boils down to this: in modern residential construction, you need ducts for fresh air. From there, you basically have 3 choices:1. Mini splits for cooling, hydronic for heating, leave ducts dedicated for IAQ.
2. Design Ducts for cooling. Keep heating hydronic. Add IAQ ventilation to cooling system.
3. Do it all via duct system.I will note that while radiant floor itself can run anywhere from $5 to $15/sq ft in most whole house cases depending on methods used and labor costs, in modern construction you can keep to the bottom end of that range quite easily using lower cost methods when the heat loads are moderate. I have designed, for example, radiant ceiling systems that are nearly competitive on a first cost basis with a comparable air system. Same for slab on grade systems, and in some cases constant circ radiator systems.But you do still need the ducts, usually.-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Rough estimating on the numbers. I expect people with better numbers to correct mine. Please do so.
We agree on the comfort issue and the ventilation issues. RFH is more comfortable heat but lacks a means to cool and ventilate.
I have yet to see a radiant system that is close to the first cost of a similar features forced air heating and cooling system.
In our home of 2700 sf, a window air conditioner cools for a buck a day in these 90+ temps with high humidity (NW Oh.) Our Electric Utility has been called High. The house is well shaded, 1st floor built into the hill. 2' overhangs. The open floor plan (with open to the second floor great room) keeps the temp pretty balanced. It is not real cold down and warm up.
We musta got lucky. Today it was sunny and 94, indoor temp 74 with 45% humidity. I've got the conditioner set at Low Cool.
You concentrate on the envelope, place it in the woods-partially in the ground and it gets a bit easier to remain comfortable. The radiant infloor heat along with a masonry heater pays dividends on comfort and economy in the winter. The passive solar angle makes it possible easier.
I know we got lucky.A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
1. Very effective. Understand in all cooling and heating systems, the intent is to cool (or heat) people, not rooms. Seems like splitting hairs, but its a very important distinction when doing so via radiant heat transfer.
Yes, very effective ... but your assumption is a bit off even though the result is the same. Radiant heat/cooling is a little bit of a misnomer in these applications. At low temperature deltas of a radiant floor, it doesn't actually heat you. Although it does reduce the body's tendency to radiate to/from a cooler/warmer surface, it is a small part of the comfort equation. True radiant thermal transfer has to have a high temp deltaT to work. The biggest benefit of radiant floors is that is the part of the building your body is in contact with the most and a warmer/cooler floor is certainly more comfortable to walk on.
I know it's a bit of semantics and I'm not criticizing your words ... just adding some food for thought. Maybe I'm splitting hairs now, but it is important to understand the true physics of the systems.
Mean Radiant Temperature. If you know what this means, your good. Else, you sound like Frenchy. BTW, I stated no assumptions. The descriptions I use to a layman are simplistic to the degree necessary. I was not attenmpting to baffle the OP but to simply express the challenges associated with the type of system about which they asked.
Speaking of simplicity, this statement: "True radiant thermal transfer has to have a high temp deltaT to work." is indicative of a critical missunderstanding. Do you wish to clarify? I believe you understand more.
Good thread. I have in floor radient heat in cement. I have wondered about the same thing.
I also put in a air to air heat exchanger with fresh air dusts to all rooms and pull air out of bathrooms and kitchen.
I got the sheet metal dude to put in a large box in the fresh air side, to which someday I hope to put in an A coil and run well water through and water the lawn and garden.Years ago I was in and old farm house and they had a coil in the forced air system, and again they ran well water through it, It kept the house very comfortable and the garden was great.
Anyone who has ever been in a basement know the concept works, but the cost of a chiller plus a dehumidifier probably exceeds the cost of a good minisplit
"I also put in a air to air heat exchanger with fresh air dusts to all rooms and pull air out of bathrooms and kitchen."Thats the problem with HVAC, it's dusty unless you change the filters.
I know you were keeping it simple. Respectfully so.
For a radiant object to 'heat me up' requires a high temperature object. For example, a campfire on a cold night. Ditto the sun and earth.
For me to lose less heat to a surface, requires a much lower temperature. With a cool slab, I will radiate more heat to it than one at say 90+ degF. I think this is what you refer to as mean radiant temperature? (my terminology recall is admitedly a bit rusty).
The fundamental problem with a radiant system is that ultimately you still heat the air in the room to satisfy the room thermostat. The net energy [savings] at the same setpoint is zero. Is there an energy benefit? Yes! IF you turn the stat down a few degrees ... which you should be able to do to achieve the same comfort level ... you will save some energy. Why can I turn down the stat? Again, standing on a cold slab is noticeablly less comfortable than one 40 Fdeg warmer.
As I recall, the radiant heat transfer between objects is related to the absolute surface temperature (in Rankine) difference, to the fourth power. Small differences can transfer substantial energy.
Noticeable heat, as experienced near a fire, is intense compared to the small but comfortable exchanges used in good comfort heating applications. Half of the process is not transferring heat to the person, its preventing heat from transferring from the person. Comfort can be realized not only in being warm, but also in not feeling cold.
Mean Radiant Teamperature (MRT) is a thermally descriptive property in which an environment is characterized, in relation to a warm body (say one at or about 98.6 degF) to determine radiant heat transfer.
Energy savings for RFH systems are minimal, but are realized in several manners. With modulating/condensing boilers operating at low temperatures, you convert gas into hot water at about 96% efficiency. Moving heat around the building from the boiler with hot water and a fractional horsepower circulator uses less energy than doing so with a fan moving air. Lower inside air temperatures can be as comfortable, there by lowering heat loss through the envelope.
There is a Canadian study, I think about 10 years old that found that that, in use, radiant heated houses used the same amount of energy. And IIRC it was maybe a percent or two more.But they admit that the study did not have a lot of good controls. And that they where starting a new research project. I think that I saw that last year. And probably no results will be published for a year or two..
William the Geezer, the sequel to Billy the Kid - Shoe
There is definately savings. Many have boasted 'huge' savings in the past that we now realize is largely a bunch of BS. If you have true radiant heat in e.g. a warehouse or semi industrial setting where the alternative is a sore attempt to heat air, then, yes, you can see huge savings (if properly done). But for radiant slab heat, the savings is largely directly proportional to the stat setpoint. I think you can, on average, turn the stat down 3-5 Fdeg to achieve the same or better comfort levels (IMO). If you don't turn down the stat, you largely have no savings (maybe pump vs. fan energy as Tim pointed out).
Yes, I have worked in an aircraft hanger with overheat radiant gas units. They are great for that kind of application..
William the Geezer, the sequel to Billy the Kid - Shoe
Well said IMO.
Well, that's how ice rinks work, by chilling a slab until it is cold enough to freeze water.
Now, whether a residential cooling system can get you enough cooling for that to work is another question entirely.
Down here in 180-190 day cooling season territory, a person might think such things would be common. And, we are seeing some wacky suggestions for beating the heat, since it's been over 100 all week, after a week of 98-100 last week--about 8-9 degrees above normal.
First, you can't "radiate" "cold". No such thing as "cold" just that absence of heat. That's your first tip-off.
So - what the underfloor system is providing is a "sink", or place the room heat can be transferred to. If there's no air movement in the room, the cold air will just fall to the floor and stay there. If the air gets colder, so what? The closer it gets to the temp of the coolant, the slower the heat transfer rate (a shallower thermal "hill"). Things some distance above the floor, like your head, aren't gonna notice.
You can imagine walking through the room like a shallow pond, your feet kicking up whorls of cold air that quickly fall back to the ground.
Condensation - yes, of course that will happen. So mildew, buckling, dampness, all those things.
Forrest
Often when we get the first blast of humid air after a cool spell my garage floor and/or patio will have wet surface from the condensation.
If you decide to try this either supply everyone with spiked golf shoes or get a Romba and outfit it with a squeegee, suction, and holding tank.
As far as vapor control goes you are right. We are in a Mixed Humid climate.
Go to http://www.buildingscience.com and look up their recommendations.
Basically they like one surface, and better two, to be any to dryout any mositure that gets in the wall and condenses.
That means no impervious surfaces such as poly.
And also to control the way that moisture can get into the walls. And most moisture moves with air movement. Keep air out of the walls and most of the problem is solved.
And the dense pack cels is a good way to go. It stops air flow. And if a small mount of moisture does get through and condenses it can dry through the cels and DW.
William the Geezer, the sequel to Billy the Kid - Shoe
The concept is solid ... the practical application is more difficult. Especially in a humid cooling season climate like yours. You'd have to limit the floor temp to e.g. 65 degF plus (depending on the dew point of your house).
I made a provision for doing that in my new house. Planned to simply dump excess energy out during the night w/ e.g. a coil on the roof.
Floor finishes. I'd only consider tile or concrete myself or something similar (terrazzo anyone?).
Approach carefully ... Control of temps is key.
I can see where radiant cooling can be attractive in that you already have the tubing in place for heating so why not use it in the summer too....
Since I have recently installed radiant heating in my house, inslab and staple up I also was curious about radiant cooling.
If you go to Uponor's website they actually have a section on radiant cooling and several test cases. A couple of which are big commercial buildings in NYC, which is hot and humid in the summer. So it does work.
I am fortunate to live in an area that does not get humid mostly...but this is what I've figured out.
My system already has a ton of thermo-stats and individually controlled loops. If the t-stats where just a little bit more clever and measured relative humidity then its a simple bit of math to get the dew-point temp. The t-stats just make sure you keep the indoor air 5 deg hotter than the dpt and you would have no condensing issues. This would work great in hot-dry areas like NV.
For midwestern type humidity levels you would also need a seperate way to dehumidify the air in the house. This is where it gets to be silly-how are you going to dehumidify an entire house without running ductwork everywhere to circulate all the interior air thru a dehumidifier? If there was a cheap-easy way to dehumidify the air then it could work.
The answer I came up with is that radiant cooling seems really only viable in hot-dry climates. In these climates eveyone cools with a swamp cooler which is super cheap to run compared to AC. But if you already have the tubes installed for heating then I think it would make sense to put them to use during the summer months.
Daniel Neumansky
Restoring our second Victorian home this time in Alamdea CA. Check out the blog http://www.chezneumansky.blogspot.com/
Oakland CA
Crazy Homeowner-Victorian Restorer