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Free Radiant Cooling – A Closer Look

by Chad Ludeman on June 10, 2008 · 15 comments

in Building Science,HVAC

Two weeks ago we looked at two solar thermal and radiant heating systems by Radiantec. One of the options that Radiantec offers is a very simple diverter valve that allows the homeowner to run the incoming cold water through the radiant floor prior to reaching the hot water heater and fixtures. The diagram below details this setup that advertises free additional cooling in the summer for homes with radiant slabs such as ours.

Free Radiant Cooling Diagram - Radiantec

I’ve had some discussions about this theory with some professionals in the past two weeks and thought we might benefit from a closer look at this setup. First, Radiantec offers test data based on a case study house they built in the 80′s in a DOE Report (pdf). The cooling option is discussed on pages 22-23.

Radiantec Test Case Data & Assumptions

  • 2 story, 1,400 sf home with a 720 sf 5.5″ thick concrete slab on grade foundation
  • Water supply temperature of 55 degrees and a design temperature of 78 degrees
  • Household water consumption of 300 gallons per day
  • Heat exchange efficiency of 90% in the radiant slab
  • 51,667 BTUs extracted per day

100K Adjusted Figures

  • 2 story, 1,050 sf house with a 648 sf 5.5″ thick slab on grade foundation (0.9 Case Study)
  • Household water consumption of 100 gallons per day (0.33 Case Study)
  • 15,345 BTUs extracted per day (0.9 * 0.33 * 51,667)

OK great, but what does this mean. Well I dug up an estimated cooling requirement calculated by a mechanical contractor a few months ago that said we needed just over 22K BTUs of cooling. This was a previous version of the house that had less insulation, poorer windows, over double the lighting and took into account none of the passive cooling strategies. So if we used this old estimate, the free radiant cooling feature would cover 70% of our cooling requirements. I can’t quantify the effects of the passive elements on our cooling load to get a better figure without spending a lot of money on more energy modeling, so for now we’ll have to be happy with these figures.

OK this is all great, but what about the main concern with radiant cooling – condensation? Well the lovely people at Radiantec (I like them more and more as I delve deeper into their websites) have created an FAQ to address this issue and other questions on the cooling feature. They address the condensation issue in two parts:

  1. Since the radiant tubing is embedded in the concrete and not exposed to the air, you will not get condensation on the tubing. Makes sense, but what about the slab itself?
  2. They claim that the system will not create enough cooling to present a whole house moisture problem and go on to say that in very hot and humid climates an air conditioner will be needed and will help dehumidify the air.

This is nice but does not entirely quell my fears of a slimy floor syndrome in the summer. I happened to run into a Mechanical Engineer, Joe, from Bruce Brooks & Associates at a local pub last night and decided to get his thoughts on this issue. He has had some experience using radiant cooling in commercial applications, but like most cool HVAC techniques, it is not widely used in residential applications. I first thought that the fluid in radiant cooling systems would be cooled to a temperature much lower than the 55 degrees we have in our calculations but he said that it is actually only 48 degrees. This hurt my assumption that we would not get condensation due to the higher temperature of our cooling liquid.

Next I asked him if active dehumidification would help in the house as needed. He said it would but was unsure of what degree. Joe then mentioned that they actually simply create an air flow over the radiant cooling surface with fans. I mentioned that we will have multiples ceiling fans in the house and he said that would certainly help and might just be enough.

So in summary, I am now about 95% confident that our combined low-cost cooling and dehumidification techniques will be sufficient enough to keep the home comfortable on all but possibly the very hottest and longest heat waves of the summer. I especially love the fact that we can achieve up to 15K BTUs of free cooling daily but utilizing the radiant cooling detail from Radiantec.

Let us know below if you think any of this sounds crazy…

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

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{ 14 comments… read them below or add one }

1 Ed Gouin June 11, 2008 at 2:23 pm

First, I love this idea. I did look into this for my house. I share the concern of condensation on the slab and have one other concern… A passive system implies a lack of control. Water usage during the warmest parts of the day may be minimal. Sure, you may use 300 gallons of water per day, but when do you use it? At my house showers or baths are usually taken in the morning or in the evening. When do you brush your teeth, shave, etc? When do you run the dishwasher? When are the toilets flushed most? At my house, with a family of five, the answer is primarily morning and evening.

Yes, the slab will be cooled during these times. Yes, heat will be extracted from the house, but when it be when you want or need it most?

Perhaps passive cooling combined with a supplemental through-the-wall or window A/C will provide the cooling you need as well as dehumidification. Dehumidification will also be provided by your ERV.

2 chad June 11, 2008 at 3:11 pm

A couple of things Ed. First, with homes like the 100K and yours that will both be built with SIPs, the time of day that heat is extracted is much less of an issue. The home is sealed extremely tightly and has superior insulation as well so once a comfortable environment is achieved, it will be literally maintained for days before becoming uncomfortable again. In this light, if there is a decent amount of cooling twice a day (morning and night) then this should be more than sufficient for maintaining comfort.

Secondly, we will most likely have some sort of whole-house dehumidifier or strategically place room dehumidifier to aid with any moisture issues. An ERV technically will not dehumidify the air within the home. It will do it’s best to maintain the humidity levels within by blocking or exchanging the high humidity from outside with the (hopefully) lower humidity levels inside.

3 Scott Sanders June 11, 2008 at 3:40 pm

I have been keenly following your progress since this site first game up in a Google Alert filter for “green home” and up until now have had nothing to contribute since all of your research and narration has been extremely thorough.

I do have something to share now , however, since I came across an article from about radiant heating in a high performance home. You may have seen it already, but some of your readers may not have. The article (from way back in 2002) questions the cost effectiveness of a radiant system in a house that is tightly sealed, and especially in one that utilizes passive solar technology.

I am not sure if it does or will have any impact on the 100k house, but it certainly interesting and worth the time it takes to get through it.

The article is at
and is titled: “Radiant Floor Heating, When it Does and Doesn’t Make Sense”

4 chad June 11, 2008 at 8:47 pm

Thanks for the article Scott. I just finished reading it. Honestly, one of the reasons we are going with radiant is due to the comfort rather than just the efficiency of it. I do think there are a couple of things that set our specific home apart from what the article is covering.

First, our radiant system is extremely cost effective. We will get the materials for the radiant system for around $1K and install costs will be negligible due to our slab on grade foundation. We already need a water heater, so it could be argued that our heat source is free. No $6K-$9K premiums here. For more info see the Radiant Heat is Affordable post.

A minor detail that I’m not sure I agree with in the article is that the earth can actually act as a good heat sink and increase the efficiency of a radiant system. We will probably leave the center 20% of the slab uninsulated from below to take advantage of this free feature from mother earth.

Lastly, our design is utilizing the most effective uses of radiant cooling that are discussed and even taking them a step further by using free incoming cold water rather than actively chilling it. We are using an ERV to bring in fresh air with controlled humidity levels also as prescribed in the article.

Having said all of this I do agree that by increasing the insulation levels a bit further, we could significantly decrease the heating system complexity and cost. For more on that check out the post on the Passive House Standard for more info on how they are only preheating the incoming fresh air that enters via the ERV with an inexpensive electric coil.

I apologize if you have already read these posts and again thank you for the very informative article. It definitely makes me lean towards trying the Passivhaus techniques in a future home.

Nice, clean site by the way.

5 Ed Gouin June 11, 2008 at 9:21 pm

Not sure this is the right place to post this… regarding increased insulation levels… Have you looked at Murus to supply your SIPs? They are located in Mansfield, PA. They make both EPS and polyurethane SIPS. Their 6″ poly SIPs are R40. They are more expensive than EPS SIPs, but on a $/R value scale, the poly SIPs are cheaper. We are using the R40 panels in our house.

If interested, I can give you contact info for the VP of Sales. They are great to work with.


6 chad June 11, 2008 at 9:26 pm

Thanks Ed. I have looked into their panels but they are just too much for our budget. We are also very happy with the service from our vendor, SureTight.

7 lavardera June 15, 2008 at 4:10 pm

I tried a passive cooling experiment yesterday using a fan and our basement. This tme of year the basement air is usually 10-15 degrees cooler than upstairs, and we dehumidify it so the air is dry as well – very comfortable down there. When you go down the steps you feel like you are stepping into a pool of cool water!

So I placed a Vornado fan – now these are not like regular house fans, they will send a directed column of air a good 20-25 feet. If you have a room without a ceiling fan one of these placed in the room pointing up will turn over the room air just as well. Anyway that fan threw a steady flow of air up the stair well and into our kitchen. The kitchen was about 5 degrees cooler than the living room space following. Both rooms have full shade, and I let it run into the evening when the sun had set. Both rooms also have cross ventilation.

8 chad June 16, 2008 at 3:02 pm

Passive cooling experiments from our readers. I love it!

Now all you’ve got to do is figure out a way to bring more fresh air into your basement and vent out the hot air at your roof for a poor man’s ZED home.

Reference post:

9 Kevin Bourque June 21, 2008 at 10:38 pm

Chad, you don’t need to worry about “spending a lot of money on more energy modeling” if you’re simply looking for answers about these “what-if” type questions. Use RETScreen. It’s a free energy project analysis software made by Natural Resources Canada (disclaimer: I’m one of the developers) that is made to answer these type of questions. RETScreen doesn’t require a huge number of inputs yet still gives you a good idea of the impact of potential changes (e.g. compare “less insulation, poorer windows, over double the lighting and took into account none of the passive cooling strategies” with your current strategy.)

I’d love to see a RETScreen study of the 110k house because then the files can be made available to everyone. Try it out at We even have some research at our lab where they are using RETScreen to evaluate zero-energy homes, etc…

PS – I also am part of the technical support team, so can help with some basic usage of the software. But please e-mail for that because my wife and I are expecting a baby any day now, so I’ll be away from work for a few days.

10 Red Green October 13, 2008 at 2:11 pm

My thought was to lay in a 50′ by 50′ lattice at 10′ to 15′ below ground level and then to pipe it up into the housing using a very small motor. Once inside the house it would go through a radiator with a fan blowing through it. From there you could duct it just like a central air system. I am almost positve it would work in a low humidity setting, not so sure about higher humidity areas of the country though. Any thoughts?

11 Paul Beaton November 23, 2008 at 7:08 pm

How about a in-floor (slab) radiant system that pulls hot water from a solar heater in the winter and from a large in-ground cistern in the summer used primarly for watering plants?

12 chad November 24, 2008 at 12:41 pm

Interesting idea Paul. It would increase the complexity a bit as we would need separate loops most likely and two sets of pumps. Also, we don’t plan to have plants that require much watering so I don’t know how much cooling we’d get…

13 Jim C July 8, 2009 at 12:30 am

Hi, I’m about to start on my Sip project in about a week, I’m kicking around the radiant cooling idea, has anyone implemented this locally in SE PA?

14 Doug Jones November 11, 2009 at 3:55 am

It’s actually a bit better than you suggest- the area of the slab isn’t important, as long as the heat transfer is good. All you really need to know is the mass flow rate, temperature change, and heat capacity of the water. I calculate about 17,000 btu for warming 100 gal/day of water from 55 to 75 F (78 F is a bit optimistic, the slab would be too warm to cool the house much). Free cooling indeed!

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