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How Radiant Heating Works Transcript

How Radiant Heating Works

00:06                Hi, this is John Melvin. I’m your host for the How Great Buildings Work podcast. This is a podcast where we discuss all things buildings from construction and build methods, to architecture, engineering, and systems designs. This podcast is sponsored by JM Engineering, PLLC. JM Engineering provides innovated and integrated building systems designs including mechanical, plumbing, electrical and structural engineering, as well as building commissioning services. Market served are commercial, including educational and health care and high-end residential projects nationwide.

00:48                Today’s topic is how in-floor radiant heating and cooling systems work. We will discuss the components of radiant systems, an overview of how they work, and applications where radiant heating and cooling as used. Our guest today is Devin Avalon. Devin is the Business Development Manager for engineering services at Uponor North America. He has over 23 years of experience in the HVAC and plumbing industries, with a focus on high performance and sustainable building design and construction. He holds a Bachelor of Science degree in Mechanical Engineering from the University of California at Santa Barbara and is a registered Professional Engineer in California and Arizona. He is an active member of both ASPE and Ashray, and serves as Ashray membership committee chair, region 10 nominating member, programs subcommittee chair for Ashray TC 6.5, and as an Ashray distinguished lecturer.

01:52                Devin welcome to the show. Tell us a little bit about yourself and how did you end up in the radiant heating and cooling field? (DEVIN) That’s right. You know, I started as a consulting engineer. I worked as a consulting engineer for over 15 years. Worked for a great firm based in Phoenix, Arizona and then ended up managing their operations over in San Diego. One of the things I really enjoyed was seeing the design process from the conceptual design phase all the way through construction documents. And one of the projects I’ll never forget, it was one of the most challenging projects I’ve ever worked on. It was my first radiant heating project and it was for a monastery in Oceanside. And I’ll just say that, that you know, there’s a saying you learn from your mistakes. That was a tremendous learning experience.

02:37                I learned a lot about how to design a radiant system, and I think more importantly, how not to design a radiant system. I had the opportunity about eight or nine years ago to switch gears, if you will. I was offered a position with Uponor to work very closely with engineers, with building owners, with contractors on different ways that they can incorporate radiant heating and cooling systems as part of an energy efficient design solution. So, it’s been something that I’ve been doing it for a number of years. It’s, it’s been an incredibly rewarding experience to be able to work with some amazing engineers throughout the country on some very unique and innovative applications.

03:10                (JOHN) That’s awesome. Yeah. Sometimes those lessons, they’re a very difficult at the time, but stick with you for a lifetime. (DEVIN) There are lessons you don’t forget very easily. (JOHN) Unfortunately, that’s all part of life, but it’s a good thing too. So, before we get too far in, let’s talk about radiant heat transfer. We basically have three modes of heat transfer. We have conduction, convection and radiant. If you don’t mind, give us a little explanation of those three types and how they’re different.

03:53                (DEVIN) Conduction is heat transfer when you’ve got objects in contact with each other. So, you can imagine if you take your hand and you touch a hot coffee mug, that heat will transfer from the coffee mug directly onto your hand because there’s a contact there. Convection has more to do with heat transfer via air movement. So, if you’re looking through your more conventional air conditioning systems, if you will, where you’re actually forcing convection blowing air into a space or even natural convection where you’ve got natural ventilation coming in through based on the differences in density of the air because of the buoyancy, you get that kind of natural flow. Radiant is very different in that unlike convection, radiant heat transfer doesn’t rely on the air itself. It’s actually heat transmitted through these electromagnetic waves, this electromagnetic radiation. And so, you actually can get heat transfer even though you’re not seeing a difference in actual air temperature.

04:58                (JOHN) Got It. So basically, I think maybe for the listeners, an example would be for those of us that live in a northern climate, in the wintertime, standing next to a brick building that has had the sun shining on it when it’s freezing cold, but yet it feels warm standing next to the brick and example.

05:24                (DEVIN) And actually, you know, to take that example, you look at the heat energy that we get from the sun itself is really through radiation heat transfer. The fact that, again, you’re in your northern climate and you can be standing outside next to a building with an overhang and the air temperature is such that you feel very cool and you feel cold, especially if you’ve got that building next to you. But then you take one step out into the sun. Now, even though the air temperature is exactly the same, you feel much warmer because you get that radiation energy from the sun. (JOHN) Sure, sure. That’s good. So, I guess backing up then, where did radiant heating and cooling systems originate? And when roughly? (DEVIN) Sure. Well, you know, there’s evidence of radiant heating system or the idea of using a warm thermal masses that go back thousands of years.

06:19                You can go back to about 3000 to 5,000 BC in areas like China and Korea where they were taking this basically the hot flue gas from a kitchen range or a stove and circulating that warm air on the underside of the floor and then exhausting it through a chimney. In doing so, they were able to warm that floor surface and create very comfortable environments. So, we’ve seen that there’s evidence of that that goes back thousands of years. Fast forward a little bit into say 500 BC, you see the Greek and Romans. It’s again a very similar approach where they’re taking hot flue gasses and routed them on the underside of these Roman bath houses, if you will, to create this very warm environment. So, it’s this idea of using this thermal mass to warm or cool spaces.

07:11                It’s not a new concept. Not at all. It’s just what we’ve seen particularly in recent years is this idea of where we’re not using hot flue gasses anymore, but now we’re circulating water and very strategically managing a slab temperature in order to drive heat transfer in one direction or the other. (JOHN) Sure. When did we first start to see radiant systems being used in the United States? (DEVIN) Well, there is evidence of that same kind of Korean Aandahl type system with the hot flue gases, as early as the mid to late 1800s. There actually are records of civil war hospitals where they were using this type of system again to create a warm space. (JOHN) Okay. Very interesting. Occasionally we will see a project that is from the, the 1950s and a lot of times they’re abandoned, we’ll see that they have copper, pour it into the slabs and over time they had stopped working.

08:18                (DEVIN) Yeah. And you know it’s funny you mentioned that we’ve actually seen, from that era, from the 1950s and such different guides and advertisements on the benefits of using radiant heating and those systems incorporated copper tubing, as you had mentioned, embedded into that concrete slab, which at the time was very efficient. It was a very nice way to create this warm environment. But again, with copper being embedded in that slab, over time, it starts to deteriorate. You start to get material build up within the copper tubes and kinks and pinhole leaks and things like that. So, it’s something that, that overtime was not to a reliable system. (JOHN) Yeah. Not a great long-term system. (DEVIN) As a matter of fact, you know, I had mentioned that that a project that I did in Oceanside, the monastery, I had made the mistake of relying on kind of our company’s old boiler plate specifications that actually called for the use of copper tubing.

09:19                And that was one of the big issues when the contractor was trying to install it, is just trying to work with the copper tubing. You’re, you’re talking about a network of tubing that that’s at six or nine inch on center and bending it without kinking it or causing any problems with the tubing itself post to a significant challenge. So, the industry in general has really moved away from copper to a cross linked polyethylene tubing. (JOHN) And so that’s what’s referred to as Pecs, correct? (DEVIN) Yes. So pex is polyethylene crosslinked and there are different types of pex that are manufactured. So, I’d certainly encourage the listeners to do some investigation into what works the best and also take advantage of the resources that are available from the various manufacturers in terms of picking the right system and designing it accordingly.

10:16                (JOHN) Sure. That good to know. So how do radiant systems work? Take us through that of a typical house or a commercial installation. What are some main components in that that makes it a radiant system? (DEVIN) So, the system itself is actually quite simple. We’re taking advantage of the thermal mass, whether it be a concrete slab or even a wood floor. And we’re essentially controlling the surface temperature. As long as we can create a temperature difference at Delta t between the surface temperature in the space temperature, we’re always going to be driving heat transfer in one direction or the other. So, to use an example of a radiant heating system, basically what we would do is we would have a network of tubing, say in this case, embedded into a concrete slab. And then we circulate water through that network.

11:11                And by controlling the temperature of the water we’re circulating, we’re able to control the temperature of the slab. If that slab temperature is, say warmer than the space temperature, then naturally by the second law of thermodynamics, we’re going to get that radiation effect from that slab to the space. So, if we have your typical, let’s say you’ve got a typical space it at 70 degrees, if you’ve got a slab that say is 80 degrees, it will create this very warm, inviting environment. The same thing can be said for a radiant cooling system. And, I know that it may be sort of a misnomer to call it radiant, but at the same time, it’s the same principle where we have, again, a network of tubing embedded into this concrete slab and now we’re running chilled water. So, with chilled water, we’re able to create a cooler slab temperature. And again, second law of thermodynamics. Because of that temperature difference, we’re actually able to absorb the heat that’s in this space and create a cooler environment.

12:10                (JOHN) Sure. And I think one extreme example on the radiant cooling is anyone who’s ever gone and stood on an ice rink. Yes, you may get cold feet that’s from conduction, but the rest of it feels very, very cool. And that’s because of the radiant cooling in fact.

12:30                (DEVIN) Exactly. It’s the fact that you’ve got this large surface area that’s at a much lower temperature and because it’s absorbing all that heat energy and drawing heat away from your body, it leaves you with this cooling sensation.

12:44                (JOHN) So what, you talked about air temp or floor temperature, surface temperatures. What are some of the ideal, like for heating, what is considered a comfort temperature or maximum surface temperature we’d want to see the floors? Does it vary between a concrete floor and a wood floor or carpet?

13:10                (DEVIN) Yes. As you’re approaching the design of a system, you’re going to maximize the effectiveness of the system by maximizing that temperature difference. So obviously the higher the temperature, the more capacity you’ll get from the system on a heating side. But having said that, I think everyone enjoys kind of having that warm toasty feeling when they’re walking on the floor. But there is a limit as to how warm you can get that floor. If you look at Ashray standard 55, which dictates the factors for thermal human comfort, it’ll say that typically you want to keep your surface temperatures no higher than about 84 degrees. So, 84 degrees, that’s your typical surface temperature. Now having said that, there are other limitations that you need to be mindful of, particularly if you have, as you had mentioned, carpet or wood flooring because there are wood flooring manufacturers and there are carpet manufacturers that will want to limit how high that temperature can be – so you don’t risk damaging say the wood or the adhesive that’s part of that carpet. So, I’ve seen recommendations for wood flooring and for carpet where they try to limit that surface temperature to about 80 degrees. So you’re not aggressive but you’re still getting a lot of capacity out of the system.

14:36                (JOHN) And then how about on the cooling side of things, what would be considered too cold of a temperature for bare feet?

14:43                (DEVIN) So, for a bare feet, I guess it’s a bit subjective. You look at Ashray Center 55, they look at normal footwear. So, if you’re in a commercial application, say an office building, a museum, an airport, then they typically say you want to keep it no lower than about 66 degrees. So, 66 to 68 degrees, again, if you’re managing systems where you’ve got bare floors you probably don’t want to be that aggressive. But you’re trying to maximize that temperature difference between the surface and the space and that’s where you’re getting your capacity. Having said that, one of the things that often comes up when you’re considering radiant cooling solutions is that risk of surface condensation and this idea that if you chill the floors or if you make the floors too cool, then you’re going to get condensation on the floor, which could cause a host of problems.

15:44                You can have the occupants slipping on a floor or you can get mold growth, or a lot of different things that you definitely want to avoid. So, in addition to that kind of 66-degree lower limit, you always want to make sure that you’re above that dewpoint. In most cases it’s really not an issue, only because as a system designer evaluates that indoor space and they look at both temperature and indoor relative humidity, the dewpoint is often well below that 66 degrees temperature. So, it’s really not a problem unless you’re unable to manage relative humidity.

16:28                (JOHN) That’s a great point. 66 degrees is really not that cold. And a typical forced air system may be putting out air at the low 50s. So where are some areas where radiant systems are typically used in the commercial world?

16:57                (DEVIN) In the commercial world we tend to see more uses in large open spaces. A lot of installations in say, airport terminals, in lobbies, museums where you’ve got this large open mass. And you’re able to manage those spaces very comfortably. However, if you look at a lot of the different case studies that have been done by Uponor or by any other manufacturer, you will see a lot of applications, lobbies, areas with high direct solar heat gain, because of the ability to absorb a lot of that energy. But what we’re seeing more and more is, as engineers, as building owners, as contractors get more and more familiar with radiant systems and they recognize the opportunity not to create a very comfortable environment, but to create these manageable spaces with reduced energy usage.

17:56                We’re seeing that space expand. And some of the more recent projects that we’ve worked on have been areas where we’re not just limiting the radiant system to the lobby or conference space or a large open space, but actually incorporating as part of the primary heating and cooling strategy in areas like offices, like schools where the radiant system is base loading both the heating and cooling. And then the secondary airside system is used to kind of trim that load. So, we’re seeing more and more use of it. Certainly, as I mentioned before, I think that it’s very important that a lot of forethought goes into how we want to zone the systems, how we want to control the systems in order to optimize it and make sure that it really achieves the goal that they’re looking for, but it can be very effective.

18:51                (JOHN) Are there limitations to where radiant heating and cooling can or can’t be used? (DEVIN) Well, you know, I’d mentioned before the capacity of the system is going to be a function of that Delta t between the surface temperature in the space. And you know, we talked before about wood flooring and carpeting. There can be limitations based on the type of flooring that’s used. You can imagine, if you’re trying to create a warm surface but you’ve got thick carpet on top of it or for instance in a gymnasium you might have a rubber flooring, something like that. Those materials tend to have a very high r value. So, it can be challenging to get to the surface temperature you’re looking for with that floor covering. So, I think one of the key things when you’re designing a system is to really evaluate what the capacity expectations are versus what the anticipated flooring is going to be.

19:54                Because those two things go hand in hand and you want to make sure that it’s compatible. If for instance, the architect, has flooring in mind where again, going to that day example, it’s thick rubber flooring or thick wood flooring. It may not make sense, right? The other areas where you want to be careful is if you’ve got say a tenant space or a space that could be subject to high turnover over the life of the building. Then you want to be careful with that. I’d mentioned before, the key strategy is to base load heating and cooling with the radiant system. But at the same time, if you’ve got say an open office plan that you anticipate over time, they’re going to be a series of tenant improvements where walls are going to be moving, that really limits the opportunity to do any sort of zoning. Because the tubing is embedded in the concrete slab, it’s really not something that you’re going to be able to change very easily. So there are instances where if they anticipate that high degree of turnover, it may not make the most sense because it doesn’t offer as much flexibility as say an air side system.

21:10                (JOHN) Right. Yeah. And that that would make sense. Typically, when a radiant system is designed, it’s designed with the plant walls in place and that makes an awful lot of sense of having a big open space. It’s designed as one big open zone and then you come in after the fact and put walls up through the middle. Then you’ve created more spaces and still only have one place to control it from.

21:37                (DEVIN) We have seen instances where, because you’re using the radiant as the base, the base heating and cooling system, it’s still very manageable because you can still trim the load with localized airside systems. There’s something that you do need to be careful with. And then also, you’d mentioned the idea of introducing new walls. That’s another thing that you need to be careful with because again, you’ve got a network of tubing that could be as little as one inch of overpour. And now if you’re trying to anchor walls into that floor system you want to make sure that you don’t inadvertently puncture the tubing network. So again, something else to be mindful of.

22:23                (JOHN) Okay. Are there any extreme cases of where radiant heating and cooling has been used that you have seen or heard about throughout your career where heating has been used in an application where someone might think, Gee, I didn’t realize they would have heating there, or the same thing with cooling.

22:42                (DEVIN) Yeah. You know there are a lot of unique applications that we’ve had the opportunity to be involved with where we always think of this idea of controlling that thermal mass really for indoor occupant comfort. But some of the more interesting applications we’ve seen are where we’re actually taking that same strategy, really just to control surface temperatures. One example is a stadium, where they were having issues with snow accumulation on the roof. So, this is an installation where we were able to incorporate a snow melting system where we’ve got warmed panels on the roof structure that are able to melt that snow and keep that roof clear of snow. That’s on the heating side.

23:31                An interesting application that I’ve seen on the cooling side is there’s actually a racetrack in Colorado. So, you can imagine kind of two lanes of a drag strip. What they were finding is depending on time of day, one of the lanes would have sun on the lane and another lane would not. And they were noticing that there were differences in how the cars were performing. So, we actually incorporate a radiant cooling strategy so that that the facility could actively manage the temperature of each lane to make sure that they were the same and didn’t have a competitive advantage. So, there are a lot of very unique applications and examples of how this type of strategy can be used not only for indoor comfort but for other means as well.

24:24                (JOHN) Yeah, that’s fascinating. I’ve read of another example too about the cooling of, I don’t know if it’s installed or not, but a beach in Dubai. They were planning on cooling the beach so people could actually get into the water.

24:43                (DEVIN) Kind of along the similar lines is what have you seen a lot of instances of turf conditioning where a lot of different stadiums use it as well. They’ve got a network of tubing really underneath the turf to maintain a certain temperature to ensure that the ground doesn’t freeze, and that they can keep the field green and alive.

25:06                (JOHN) Yeah. Well that’s fascinating. So, let’s talk about why we’d use radiant heating and cooling rather than a conventional forced air system. Are there some efficiency benefits with a radiant system over a forced air system?

25:26                (DEVIN) Yeah, absolutely. And that’s actually why we’re seeing more and more use and more interest in the use of radiant systems, particularly in commercial applications. It’s because of the opportunity for a higher efficiency systems. You look at some studies that have been done on radiant systems in their application. You can see that if you compare a radiant system versus a, say a commercial airside system, the studies have shown that that a radiant system can be between 17% to 43% more energy efficient. That was one study that was done by Lawrence Berkeley National Labs. Other studies have shown that radiant systems can be over 50% more energy efficient. There was a recent grant that the California Energy Commission granted that was really to study radiant systems. And the reason why they were interested in studying radiant systems is because they found that over 50% of the net zero buildings that were installed in California incorporated some sort of radiant heating or cooling strategy. So, there is a lot of opportunity for radiant systems. Again, we’re talking about a system where we’re circulating water through that network of tubing instead of using a lot of fan motor horsepower to blow air. We’re using pumps and circulators. And because the heat transfer capacity of water is so much greater air, we’re able to more effectively and efficiently move those Btus and create these comfortable spaces.

26:59                (JOHN) Right. Then along those lines, a boiler will operate at a much higher efficiency because of the lower water temperature it’s having to produce. And conversely, on the cooling side, there can be some savings realized with operating a chiller with a higher return water temperature. How about some health benefits? Do you see any health benefits of using radiant heating and cooling over conventional?

27:33                (DEVIN) Yeah, absolutely. We see a lot of different health benefits mainly because we’re taking a system where we’ve got that warm or cool slab and creating this very comfortable environment without having to rely so heavily on air that we’re blowing through a duct work or through the fusers. We hear a lot about different issues with indoor air quality. Certainly, ventilations important in managing. But at the same time, if we’re relying less on that air system to deliver that cooler warm air, we can create an environment that is healthier.

28:14                (JOHN) And along that line, one of the ways is to use what’s called a dedicated outdoor air system where the radiant system is providing all of the heating and cooling and then all of the air being delivered is just the ventilation air, which is being exhausted to the outside. So, we’re not recirculating germs in one place. I can see this being a huge benefit in a school. I like to think of schools as germ factories. If we’re exhausting the air rather than recirculating it through, then hopefully we’re delivering a cleaner air stream to the students.

29:03                (DEVIN) And schools, that that’s a great example because you’ve got a space where you’ve got relatively high occupancy, so you’ve got high ventilation and you’re able to manage that very efficiently with a dedicated outside air system. If you contrast that to say conventional air only system, you’re really relying on that one system to provide not only ventilation but then you’re sensible in your latent cooling and heating capacities. We’re essentially decoupling those. So, you’re right, we’re able to manage the temperature requirements within that space with the radiant system and then rely on the dedicated outside air system to provide the optimal amount of ventilation air. (JOHN) Right. Well, Devin, thank you so much for being a guest today. Where can people go to learn more about you and Uponor?

30:00                (DEVIN) Sure. Where they are a lot of great resources available online. I point you to our main website, which is If you’re in the engineering side of things, then we have a or so we’ve got a lot of websites, a lot of different resources. I certainly encourage listeners to reach out to the local sales reps who are in the markets to help, because, you know, when we start looking at how these systems are designed, how they’re installed, at the end of the day, they’re relatively simple, but they need to be done properly, if you will. So, having the resources, taking advantage of the resources available will really help ensure a smooth job. (JOHN) Well, that’s great. Well, thank you, Devin.

30:54                Thank you for listening to today’s podcast. If you’d like more information, see our website at or on Facebook and Instagram at JM_Engineering. Our show notes will be posted on our website at Thank you.

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