Latest / Bozeman Builders Science / Ep. 8: BBS - Radiant Floor Heating
Transcript
- Ian: On the topic of heating systems and those houses that just feel different, radiant floor heat is another one of those systems. If you've spent enough time in different houses, you probably know what I mean. A house with forced air has a certain feel, or I guess lack thereof. A house heated with a wood stove feels another way, as we discussed. And radiant floors have their own feel too, but radiant also behaves differently. You can turn the thermostat up and little to nothing happens for a while. The floor might not even feel especially warm. But then once the system gets going, the room can stay comfortable for a long time after the thermostat stops calling for heat. So I've wanted to understand this a little bit better. Why does radiant floor heat actually feel and behave so differently? And the short answer is that the floor itself becomes part of the heating system. We're using warm fluid moving through tubing to put heat into the floor. And then the floor becomes this huge warm surface that slowly transfers heat into the room. And if that tubing is buried in a big concrete slab, the floor can also store a significant amount of heat. And that's the basic idea. let's break down what is actually happening. For this episode, I'm mostly talking about hydronic radiant heat. And hydronic just means we're using a liquid to move the heat around. I'm going to call that liquid water throughout this episode because it's just much easier to listen to, but some systems use a water and glycol mixture for freeze protection. At the simplest level, we need something to heat that water. Oftentimes, that's a boiler. We then need to move that heated water, which is where a circulator pump. comes in. From there, the heated water gets distributed into loops of tubing that run through or underneath the floor. So if you want a really basic picture in your head, it's this. The boiler puts heat into the water, the circulator pump moves that water, the tubing carries that heat through the floor, and the floor then transfers it into the room. And that's really the whole system from 30,000 feet. Obviously there can be a lot more going on in the mech room, but I think that's enough to start with, because the interesting part is what happens when that heat reaches the floor. If you've got radiant tubing buried in concrete, heat moves from the water through the wall of the tubing and into the surrounding concrete. The concrete closest to that tubing warms first. Then that heat keeps spreading outward. Eventually, instead of having a warm tube buried in cold concrete, you've got a much larger area of warm concrete. And I've actually gotten to watch this happen quite a few times through a thermal camera. Before drilling a suction point in a slab with radiant heat, a radon mitigation specialist needs to identify the location of inner slab radiant lines. Sometimes that is far easier said than done, but thermal imaging goes a long way in helping us see through the concrete. One of the first things I ask homeowners during a walkthrough is whether or not they have radiant heat. If I'm going to drill or core through their slab for radon work, I need to know where those tubes are. Radiant heat Isn't really a problem for us, we just need to find it. So we'll normally ask them to shut that area or zone off for the night before our arrival so the concrete has time to cool down. When we arrive, we turn the heat back on and start watching the floor with a thermal camera. At first, there isn't anything to see, but then the individual loops start appearing as bright lines. You can start seeing the straight runs and then the bends, and in my experience, somewhere around Twenty to forty five minutes in, you've normally got a really useful picture of where those tubes are. But if you keep watching after that, those bright lines start becoming less dramatic. The heat is spreading out in the slab around them. Eventually the floor starts washing together again because now you're not just looking at hot tubing, we're looking at warm concrete. And that's a really useful picture of what a radiant slab is doing. The tubing puts heat into the floor, and the floor then spreads that heat. And eventually the floor becomes part of the thing heating the room. Now, here's where I think radiant heat gets especially interesting. We're usually describing the temperature of a room with one number. If the thermostat says 68 degrees, then the room is 68 degrees. And as discussed in previous episodes, air temperature matters. But your body doesn't only interact with the air around you. It's also exchanging heat with the surfaces. If those surfaces are colder than your body, you're losing radiant heat toward them. And if they're warmer, that relationship flips. If you remember mean radiant temperature from the previous wood heat episode, we're talking about the average temperature of the surfaces around you and how that, combined with air temperature, can affect your comfort. And you've experienced this even if you've never heard the term. That's in part of why radiant heat can feel so different. Two rooms can have basically the same air temperature and still not feel the same. And I've noticed this for years working in other people's houses. Radiant, wood, forced air, they all feel different, and homeowners say the same thing. And I knew they felt different long before I could actually explain why. And that's part of the explanation. Now, radiant floor heat does not only heat through radiation. That's another one of those explanations that sounds good but isn't quite right. Just like the outside of a wood stove, the warm floor heats the air touching it, and that warmer air moves through the room in a form of convection. So we are getting both. There is a radiant exchange between the warm floor and the things around it, and convective heat transfer between the floor and the air in the room. The difference is that we're not primarily heating air at one central piece of equipment. And then blowing it into the room through a vent. We've created a huge warm surface instead. Now we need to talk about thermal mass, because if there's one thing people tend to notice about radiant slabs, it's that they tend to take their sweet time. Thermal mass is a material's ability to absorb energy, store it, and then release it later. You can think about this almost like a battery, and a concrete slab is just a very large battery. And I actually have a really specific memory that makes this click for me. We had one of those backyard pools that was filled with hose water, so that water was cold to say the least. And it didn't help that we consistently splashed out so much that mom was filling it with more hose water every evening. The pool was also shaded for a good chunk of the day, which meant it never got a great opportunity to warm up. We'd only be able to play in it for maybe ten or fifteen minutes before we were freezing. So we'd climb out, shivering. And sprint over to the concrete sidewalk that ran along the south side of our garage. That sidewalk had been baking in the sun, and we'd just lay on it. Oftentimes it was too hot, but it was better than the alternative. The concrete spent hours absorbing energy from the sun, and now that stored heat was being transferred into us. Eventually we'd warm back up and before long we'd be sweating and ready to jump back into the freezing pool. And that's thermal mass in a nutshell. The concrete can absorb a substantial amount of energy, store it, and then release it later when something cooler comes into contact with it, or when its surroundings cool down. A high mass radiance slab is doing the same basic thing. Except instead of the sun heating the concrete from above, We're using tubing inside the slab to heat it from within. And that's why changing the temperature of one of these systems can take so much time. We're not just heating the air. We're putting energy into a giant chunk of concrete. But the other side of that is just as important. Once that concrete is warm, it doesn't instantly stop being warm because the thermostat is satisfied and no longer calling for heat. It still has a massive amount of stored energy and it keeps releasing it into the room. Which is why a high-mass radiant slab can be slow in both directions, slow to warm up and slow to cool down. That's specifically a high mass radiant slab. Not every radiant system behaves exactly like this. There are lower mass systems installed under or above wood floors that can respond much faster. So I don't want the takeaway to be that radiant heat is always slow. The takeaway is, as always, the construction of the floor and the system matters. Once you understand thermal mass, thermostat behavior starts making a lot more sense. Again, the thermostat is measuring the air temperature. it's set to 68, but you're cold, you walk over and crank it up to 75 because you want more heat. But the thermostat isn't a gas pedal. In a typical system, it's making a request for heat. The room is below the set point, so the controls tell the system to start delivering heat to that area. Exactly what happens after that depends on the system. But setting the thermostat dramatically higher doesn't automatically make the slab heat up faster. In most cases, it just keeps the system calling for heat for longer. And with a large concrete slab that matters because the slab itself is storing energy while the room temperature is still catching up. So by the time the thermostat is finally satisfied, you've already put a lot more heat into that concrete than you likely need. The call for heat stops, but the slab doesn't instantly cool off. It keeps releasing that stored heat into the room. That's one reason why a high mass radiant slab can overshoot. The thermostat is reacting to the temperature of the air in the room, while the slab is still carrying heat that hasn't fully worked its way into the space yet. And this brings me to something that I think a lot of people hear when they think about radiant heat. It's also why I don't love blanket advice about a term known as thermostat setbacks. You'll sometimes hear you should never use setbacks with radiant heat. And there's a reason that advice exists. With a high-mass radiant slab, the floor may take a very long time to cool down. So if you drop Thermostat for a relatively short period of time, the slab may still be releasing stored heat for a majority of that time. Then, if the slab does cool substantially, bringing the room back up to temperature can take time because you're not just reheating the air. You're relying on putting heat back into the concrete before that heat eventually makes it back into the air. Now that doesn't mean setbacks don't work, and it doesn't mean they're always a good idea either. It just means that a high mass slab behaves differently from a heating system that can change room temperature or air temperature much quicker. The actual result obviously depends on the system. The floor assembly, the building, the controls, and how it was designed. So I think the better question is one that gets us closer to the actual system at hand. How quickly does my house and my heating system actually respond? Now, talking on the design of the system itself, let's put something on top of the radiant floor, because this is another place where homeowners get told rules that don't necessarily work. I've heard a lot of people say we can't put carpet down here because we have radiant heat. And I've heard the same thing about wood flooring. But neither one is universally true. The thing the heating system cares about is how easily heat can move through whatever you're putting on top of it. Tile and concrete generally give that heat a relatively easy path into the room. Carpet, carpet pad, they add resistance in the form of insulation. And that doesn't mean that the heat suddenly stops, it just means we've made the path harder. Wood can work over radiant too, but wood brings some of its own considerations because temperature and moisture can affect the material itself. So the specific flooring product and the manufacturer's recommendations All matter. But here's the bigger picture. If you replace tile with thick carpet and pad, you haven't changed the boiler. You haven't moved the tubing, but you have changed the path that heat has to travel to reach the room. So that room can behave differently. And it's another good reminder that the heating system in the building aren't really separate things, they're constantly interacting with one another. This gets even more obvious when one room isn't comfortable. Let's say the living room and the kitchen feel great, but one bedroom is always cold. It's tempting to say that the boiler is undersized or isn't keeping up. Maybe, but one cold room doesn't really tell us that. The more useful way to look at it is that this room is losing heat faster than it's receiving it. Now we just have to figure out why. Maybe that room has more exterior wall or larger windows, or a radiant loop that isn't moving as much heat as expected. The floor covering could also be different, and there could be a control issue or just fewer radiant loops under that portion of the home. The point isn't that those are the only possibilities. It's that the boiler itself is only one part of the system. The building also matters. And sometimes the tubing layout under the floor isn't anything like you'd expect. I had a slab where all the radiant lines basically had to enter the rooms through the doorways. Now I have my assumptions on what happened here, but instead of the tubes being spread out as they move from room to room, they would neck down to make it through a doorway. So we'd have eight, twelve, Maybe 16 tubes packed through that one three foot wide area. And then once they got through the doorway, they'd fan back out. Standing on the finished floor, you'd never assume what was happening underneath you. And that's one of the reasons why I won't guess where a radiant tube might be before we start drilling through a slab. The finished room doesn't tell you what's happening underneath. And the temperature of a room doesn't automatically tell you which component is responsible for under. Or overperformance. Now, I don't want to turn this into a full on hydronics class, but if you walk into a radiant mechanical room, there are a few things worth understanding. And we've already talked about the heat source, usually a boiler. We've talked about the circulating pumps moving the water, and we've talked about a manifold distributing that water into different tubing loops. One thing that's easy to misunderstand is that one loop does not automatically mean one thermostat zone. You might have several loops serving one area of the house all controlled together. And another piece of the puzzle that's worth mentioning is mixing. The temperature a boiler can produce isn't necessarily the same temperature that we want going directly into the floor. Some systems mix hotter supply water with cooler return water to get the temperature the radiant floor actually needs. Other systems can produce that lower temperature directly at the source. And this is where hydronics can get complicated pretty quickly. When I walk into a mech room, I'm pretty comfortable identifying the boiler. I understand the basic supply and return side, and I can recognize them manifold. And when the system is labeled well, we can usually tell what areas of the house those zones are servicing. But once we start asking exactly why every pump, valve, sensor, and control is arranged the way it is, I'm lost and I'm learning right alongside you. So for this episode, the important thing is simply understanding what all of the equipment is trying to accomplish. We need to make the heat, we need to move it, and ideally we can control where it goes. So this brings us to something else that I hear consistently. Radiant heat is more efficient. Well, sometimes, but that question gets messy pretty quickly because people use the word efficient to mean multiple different things. Radiant heat can be very comfortable, but that doesn't mean that it's using less energy. A radiant system may be able to operate with relatively low water temperatures, and depending on the heat source, that can be a real advantage. But that's still not the same thing as saying every radiant house is cheap to heat. The building still loses heat, and however much heat the building loses has to be replaced. If the house loses heat quickly, the radiant system has to keep putting heat back in, ideally faster than it's lost. The equipment producing that heat matters too. And then operating cost brings fuel prices into the picture. So you can have a very comfortable radiant house that Actually costs a lot to heat. And you can have a very well-designed system with a good building envelope and efficient equipment that performs extremely well. The radiant floor by itself doesn't always answer the question. So when I hear that radiant heat is more efficient, I think the more useful response is more efficient than what? In what building? With what heat source? And that's the question that actually gets us somewhere. As always, We need to be thinking about the system as a whole. So if you've got radiant floors in your house, I think there are a few things worth taking away from this. The first is just to understand what kind of system you have. If you've got tubing buried in a thick concrete slab, you're living with a lot of thermal mass. And it's probably not going to respond like forced air. It's not necessarily going to respond like another radiant system installed underneath a wood subfloor either. The second is to stop thinking of the flooring as something completely separate from the heating system itself. If you make a major flooring change, you're changing the heat transfer path. That doesn't mean you can't install carpet or wood. It just means the radiant system should be part of that consideration. And that last one matters. If you're going to drill, core, anchor, or penetrate a radiant slab, you can't assume you know where the tubing is, nor what depth. It's at or the spacing between each loop. Even when I've watched those loops appear with a thermal camera and I'm very confident about where I'm drilling, I still want to know where the supply and the return shutoffs are before that core bit touches the concrete. There's nothing worse than hitting one of those things and getting doused in fluid. Locating the tubing is how we avoid hitting it. Knowing how to isolate the system is what I want figured out in case I'm wrong. And I've seen enough weird tubing layouts to know that there shouldn't be a line there is not the same thing as knowing there really isn't one. I think the easiest way to understand radiant floor heat is just to follow the heat path. We create it at the heat source, we put that energy into water, we move the water through tubing, and the tubing puts heat into the floor. And the floor puts heat into the room. Once you see that journey, a lot of the weird behavior actually starts making sense. But that then raises another question. So far in this series, we've talked about making heat with fire. And moving heat through warm water. But what if we didn't have to create all of that heat inside the house in the first place? What if we got it from somewhere else? What if we could go outside, pull heat from somewhere that's already storing it, and then move it into the building? And then when summer comes, what if we actually run that entire process backwards? That's the basic idea behind a ground source heat pump. And while that sounds cool, most people know it by another Even cooler name Geothermal.