Latest / Bozeman Builders Science / Ep. 5: BBS - Winter Heating Series
Transcript
- Ian White: If you set your thermostat to 70 degrees, your furnace comes on, your house warms up, and eventually the furnace shuts back off. Your house is 70 degrees. Then 20 minutes later or an hour later, whatever it happens to be, the furnace comes back on. Nothing went wrong, your house is just losing heat. And I think that's probably the most important idea to understand before we start talking about all the different ways that we heat our homes. Your heating system isn't really creating a temperature and then somehow holding your home there. Your house is constantly losing heat to the colder environment around it, and your heating system has to continually replace what you're losing. So that's what I want to talk about today. And before we start digging into wood stoves, furnaces, boilers, radiant floors, heat pumps, baseboard heat, and everything else, I want to start with the house itself. 'Cause every one of those systems is trying to solve basically the same problem. Can we put heat back into this building as fast or faster than the building is losing it? Let's say it's 70 degrees inside your home and 20 degrees outside. 20 degrees doesn't sound particularly extreme, especially here in Montana. But your house isn't really looking at those numbers individually. There's a 50 degree temperature difference between the inside and the outside. If it's zero outside, now there's a 70 degree difference. And that number climbs even higher if it's say minus 30. There's a hundred degree temperature difference across that building. And that difference matters. Heat naturally moves from warm things toward cold things. The bigger that temperature difference becomes, the harder your house has to work. Or more accurately, the harder your heating system has to work to replace the air that's moving out of the building. That's part of why a house may be perfectly comfortable when it's 35 degrees outside. And suddenly struggle when we get one of those February cold snaps. It doesn't necessarily mean something broke. The heating system is being asked to keep up with a much larger heat loss and a greater temperature difference. And that's where I think it becomes useful to stop thinking only about the furnace, boiler, wood stove, or whatever equipment is sitting in your home. The building itself is a huge part of the heating system. Sometimes the heater isn't really the problem. I had a homeowner a while back who wanted a mini split installed in her living room. Initially the conversation was about air conditioning. It was an older home built in the late eighties and one entire side of her living room was basically picture windows. It was a kind of fish bowl. And she said that room got really hot in the summertime, which made plenty of sense. But while we were talking about the mini split, She also said something along the lines of, I want to make sure this thing has heat too, because the heater doesn't work very well in here during the winter. And that was the interesting part, because the heater may not really have been the problem. You've got this enormous area of older glass separating the room from outside. In the summer, especially with sunlight coming through those windows, that room gained a lot of heat. And during the winter, those same windows acted as the boundary between the warm room and the cold outside. And in her particular case, those windows face south. So on a sunny winter day, she actually benefited from quite a bit of solar heat coming through that glass. But on cloudy days, She said the room was always cold. Same room, same heater, very different conditions. And that's a good example of why comfort problems aren't always equipment problems. Sometimes we're asking the heating equipment to compensate for areas where the building itself may be dropping the ball. I think it's important to talk about how heat actually moves. There are three basic ways heat gets transferred around: conduction, convection, and radiation. I don't want to turn this into a physics class, but understanding those three terms is going to make almost everything else we talk about in this series easier to understand. Conduction is heat moving through direct contact in materials. Think about grabbing the handle of a hot cast iron pan. The heat from the pan transfers into the handle and eventually into your hand. In a house, heat conducts through things like framing, drywall, concrete, glass, and basically every other material that's separating the inside from the outside. convection involves heat being carried through the movement of a fluid. And in houses we're usually talking about air or water. Warm air becomes less dense and rises. Cooler air falls and moves in to replace it. That's natural convection. A hydronic baseboard is a good example. Air around that warm baseboard heats up and rises and And cooler room air moves in behind it. A forced air furnace gives us a different version of convection. Instead of relying only on natural air movement, we have a blower mechanically pushing warm air through ductwork and throughout the home. That's forced convection. And then we have radiation. Radiation is heat transfer that doesn't require the air between two objects to physically carry all of the heat from one place to another. The easiest way I know to think about that is a fire. If you've ever stood near a campfire or a wood stove, you know exactly what radiant heat feels like. You can feel the warmth on your face or your hands without waiting for all that air between you and the fire to warm up first. And that idea is going to become really important as we get further into this series. I think another distinction that's worth understanding is the difference between insulation and air sealing because they get lumped together all the time. I came across an analogy while researching this that I actually really like. Think about insulation like a sweater. A sweater slows down the transfer of heat away from your body. And that's essentially what insulation is doing in your home. It's adding resistance to heat flow. But now imagine standing outside on a cool Windy day in nothing but a sweater. You can have a really good thick sweater on and still get cold pretty quickly if the wind is moving through it. That's where the windbreaker comes in. Air sealing is the windbreaker. Instead of only slowing down heat transfer through materials, we're also stopping the unwanted air from moving through cracks, gaps, penetrations, and joints in the building. And that's an important distinction. Heat can leave your house even when the actual indoor air doesn't go anywhere. Energy can conduct through a wall or through a window, but heated indoor air can also physically escape through openings in the building while cold air comes in to replace it. Those are different mechanisms and ideally we're controlling both. I think it's important to think about heat loss before you think about the utility bill. When we talk about newer homes being tighter or more energy efficient, I think most people naturally translate that into money. This house is efficient, so my heating bill should be lower. And that's absolutely a reasonable way to think about it. But I think there's another way of looking at it that's more useful for what we're talking about here. A more efficient building is losing heat more slowly. If you improve the insulation in part of a home, you're increasing the resistance to heat flowing through that assembly. And if you improve the air sealing, you're reducing unwanted air movement through the building enclosure. Either way, your heating equipment now has less lost heat to replace. And that can all translate into lower operating costs. Whether a specific insulation project financially pays for itself, how long that may take, and whether it's actually worth doing depends on the house, But the physics is much simpler. Reduce the heat loss and you reduce the amount of heat your heating system has to put back in. And this brings us to something that I think is pretty interesting. There's a difference between heating the air in a room and heating the things around you. You can walk into two rooms that are both technically 70 degrees and have them feel completely different. Maybe one has a huge window next to you. Maybe one has a cold exterior wall, or you're standing on a warm floor and there's a wood stove across the room. Your body isn't interacting only with the air temperature. You're also exchanging radiant energy with the surfaces around you. That's part of why sunlight coming through a window can make you feel warm even if the room's air temperature hasn't changed. It's part of why standing near a wood stove feels different from standing underneath a forced air register. And it's part of why a radiant floor can feel so comfortable. an easy way to think about this is with a thermal camera. If you've ever seen one, you already understand the basic picture. Warmer surfaces show up differently from cooler surfaces, typically red and blue, because the camera is detecting infrared radiation coming from those surfaces and translating that information into a picture that we can actually see. we're still constantly exchanging energy with everything around us. And that's a big part of comfort that gets missed if we only look at the number on the thermostat. So why are there so many different ways to heat a house? That's really what this whole series is going to be about. We've come up with a bunch of different ways to solve basically the same problem. The house is losing heat and we need to put it back. For a very long time we did that with fire. Then we figured out better ways to control the fire and get more useful heat out of it. We developed wood stoves, furnaces, baseboard heating, radiant floors, Heat pumps, and plenty of variations inside of all those categories. Some burn fuel, some use electricity, some actually move heat from somewhere else instead of creating it directly. Some use air to distribute the heat through the building, while others use a fluid. Some of those things heat up super quick and others use a tremendous amount of thermal mass and respond slowly. Some of those things primarily change the temperature of the air, while others make the surfaces around us a much bigger part of the comfort equation. But every one of them is interacting with the building around it. The best heating system in the world has to deal with however much heat that particular house is losing. And I think once you understand that, all of the other heating systems start making a lot more sense. So over the next several episodes we're going to start pulling apart the different ways we've figured out how to heat our homes, how they actually work, how they move heat around the building, why some feel different than others, where they're really good, where they may have limitations, and where the house itself fits into all of it. And if we're going to talk about how humans heat buildings, there's really only one logical place to start fire.