COMMON HVAC MISCONCEPTIONS
Air is Empty Space
Not true. Air is matter – we just can’t see it. One cubic foot of air on Earth weighs an average of about 0.075 pounds. That might seem pretty light, but in HVAC it really adds up when the blower system for our heating and cooling system has to move a required volume of it. Let’s say that you have a 4 ton AC system (48,000 Btu/h) to cool your home. The blower system needs to move 400 cubic feet per minute per ton of cooling. So that’s 4 x 400 = 1600 of those .075 pound cubic feet of it per minute. So the blower is moving 1600 cubic feet x .075 lbs = 120 pounds of air per minute.
Yes, heat does rise. But the misconception most folks have is that it’s the only direction it’s capable of moving in. In reality, heat transfer is governed by the difference in temperature between two things, i.e. it moves from hot to cold. The greater the difference in temperature between them is, the faster it flows from hot to cold in all directions – not just up.
The source of the misconception is the obvious fact that hot air rises. Hot air is matter – it has weight and occupies space. It isn’t just empty space. It rises because unlike many other forms of matter, the weight of that air becomes lighter the warmer it gets, causing it to rise.
Moist Air is Heavier Than Dry Air at the same Temperature
Not true. At the same temperature and altitude, the dry air is heavier. Air is a mixture of several gases. When moisture (humidity) is added to dry air, that added moisture displaces some of the heavier gases, decreasing the density (weight per cubic foot) of the air.
That’s one reason why the pressure reading on a barometer drops when a storm is approaching.
Setting Thermostat Beyond the Desired Temp Makes My House Cool off Faster
Most of us have single Btu output systems. So it takes it the same amount of time to change the temperature by the desired number of degrees. So if you want the AC to drop the temp in the house by 5 degrees, i.e. from 80 to 75, it won’t do it any faster if you set the thermostat down to 60.
And the same thing applies in the winter – if you want it to raise the temp 5 degrees. It won’t happen any sooner if you set the temp up 5 degrees higher than the desired temperature.
The thermostat is merely shutting it off at the temperature you set it for. And if you don’t shut it off when it reaches that desired temp, you’re wasting money.
Adding a Lot More Attic Insulation Always Saves You a Lot of Energy Money
Not always. Like anything, the law of diminishing returns on your insulation investment shows up when you get to a certain point. So if you have a fairly good amount of insulation already, the savings from increasing it become more negligible. The biggest bang for the buck is seen when you have little or none to start with.
Brick Houses Are More Energy Efficient
That couldn’t be further from the truth. And the same applies to poured concrete walls. As a general rule (there are some exceptions), the less dense the material is, the better its thermal insulating properties are. The density of a material is a measure of how many pounds a cubic foot of it weighs. Pick up a cubic foot of Styrofoam, then test your strength by trying to pick up a cubic foot of brick or poured concrete.
We measure the insulating value of building materials by their tested “R” values. The “R” is for resistance – how much they resist the flow of heat. The higher the R value is, the less heat they allow to flow through them. Poured concrete has an R value of about .08 per inch of thickness. So an 8” thick, poured concrete, outside wall has an R value of .08 x 8” = .64.
That’s pretty much nothing for an R value. Common brick is a bit better than poured concrete, with an R of 0.20 per inch of thickness. But that still doesn’t add up to much R value.
Conversely, the R value of a 3 1/2” thick fiberglass batt is about R 11. And that’s less than half the thickness of an 8” thick concrete wall. And fiberglass batts are what most framed homes in our area have in the exterior walls.
The resistance to the flow of heat is a tough concept for some folks to comprehend. The real thing to many folks is the opposite of that resistance, i.e. the rate of conduction. A good conductor of heat is a bad heat insulator. And a good heat insulator is a bad conductor of heat.
R values are the “reciprocal” of heat conduction. And heat conduction is the reciprocal of R values. So one is the reciprocal of the other. If you divide the number 1 by the R value, you get the heat conduction. And if you divide the number 1 by the heat conduction value, you get the R value. So the heat conduction and R value have a “reciprocal” relationship.
Let’s say we have R 4 for a wall insulation. The heat conduction would be 1 / 4 = .25.
And if we divide the number 1 by that heat conduction value of .25, we get 1 / .25 = 4.
If we kick that R value up to R 38, we get 1/38 = .0263 (2.63%) for the heat conduction.
Remember that heat moves from hot to cold. So in the winter, if your house is 70F, and it’s 0F outside, the indoor side of those masonry walls will be much colder than the 70F air we’ve paid to heat. So that heat will flow through those walls at a fairly fast rate.
We’re turning that scenario around backward in the summer when we air condition our house. Heat still moves from hot to cold. So it’s moving into the house in the summer, when the source of the heat is outdoors, rather than indoors.
The bottom line here is that the energy efficiency of that house with the brick walls depends on what the R value of the insulation is between the indoor wall surfaces and the brick, because the brick has very little R value. Its minimal R value is merely added to the R value of the real insulation (if there is any) inside the wall. And in many older brick structures, no insulation was added.
Heat Can be Totally Contained
Our technology hasn’t gotten there yet. It’s like the impossible to achieve words in the old song “carry moonbeams home in a jar”. We can’t stop the flow of heat from hot to cold. All we can do is slow it down by x percent with our insulation technology. Yes, it would be wonderful if I could put my fantastically insulated coffee travel mug in my truck in the winter and have the coffee stay as hot as it originally was, all day long. But that mug will merely keep it warmer longer than a worse mug. And it boils down to the same thing in our houses, i.e. the difference in heat transfer between an energy hog house vs. a highly efficient one. It’s a relative thing.
Air Has No Insulating Value
This is unfortunately a misconception shared by many people in my industry. Remember earlier when we discussed how poor the R value of poured concrete walls was? It was .08 R per inch of thickness. That’s .06 R per 3/4”. Note that the R value of a 3/4” air space is 1.01 R which is 1.01/.06 = 16.8 times as high as that concrete. So a 3/4” air space provides more thermal insulation than that 8” thick poured concrete wall (R .64). But air is a very tricky insulator, i.e. the R value doesn’t increase proportionally when you make that air space thicker. Numerous small air spaces are more efficient than one big one. Most of us already know that, because layering your clothes in the winter keeps your body warmer. It’s those air spaces between the layers that creates that increased R value.
One of the most efficient insulations we have today is that “expandable” foam, like that stuff you get in a spray can at the hardware store. If you spray that inside a hollow wall, it gives you close to R 7 per inch of wall cavity space. That’s why they use it in the external walls of many household refrigerators.
The primary reason for that high R value is all those little tiny trapped bubbles of air sealed up in it. And that’s also why it occupies a much larger space outside the can, and weighs so little per cubic foot after it expands.
A Bigger AC or Furnace is Better
Bigger than required AC systems are less efficient for the vast majority of the cooling season, and don’t run long enough to remove enough humidity. And they have to run for x minutes before reaching their peak, rated efficiency.
And bigger than required furnaces don’t run long enough to prevent acidic condensation from occurring. And if you have a humidifier mounted on the furnace, the furnace won’t run long enough for the humidifier to add enough moisture to the air.
Most folks don’t understand that there’s a price to pay for having enough cooling or heating power to keep our house at our desired temperature on summer days when the outside temperature is several degrees above the design temperature for our area (96F), or below the winter design temperature (6F) for our area.
This is somewhat analogous to buying a car with a 500 horsepower engine, and expecting it to get good gas mileage when we drive it back and forth to nearby places in the city all day.
Something Is Wrong With my AC Because it Doesn’t Cool Down the House as Fast as my Car AC.
What they don’t know is that their car AC is sized for dealing with a lot higher level of heat to be removed per cubic feet of area, especially when they first get into the car on a hot, sunny day. Imagine if you came home and your house temp and everything in it was 130 degrees. The average car AC has close to 24,000 BTU of AC power. And the average house AC in our area has only about 36,000. Now think about how many cubic feet of area each one has.
Inanimate Outdoor Objects Can Get Colder Than the Outdoor Temperature from Wind Chill
Nope. Wind chill is for human bodies. Our bodies are not inanimate objects. They generate heat at a given level for the amount of work they’re doing at a given time, unlike inanimate objects.
The “wind chill factor” shows you the effect of the wind speed on the perceived human skin temperature at a given temperature when that air (lower than 50F outside) is much colder than the human body. That’s why the word “chill” is used, i.e. it doesn’t apply to warmer outdoor temps.
The winter wind speed accelerates the loss of heat from both inanimate objects and humans. In other words both will drop to a lower temperature faster with a wind at the same outdoor temperature than they would if there was no wind. But neither will become colder than the actual outdoor temp over time. The inanimate object will drop to that outdoor temp and remain at that temp, whereas our bodies won’t get that cold, because we’re generating heat internally, unlike the inanimate object.
One big difference between the loss of heat in an object (or human) outdoors in the wind vs. no wind is the difference the wind makes in what we call an “air film”. An air film is a very thin layer of invisible insulation (composed of air) surrounding the object (or person). That film has an R value, just like any insulation. But as the wind speed increases, it disturbs the air film, reducing its insulating value (R value). The air film is actually the edge of the air in contact with the object’s surface. So the higher the wind speed, the faster that air film gets scooped out of the way to make room for more heat to exit the object. That takes us back to what we learned earlier – that heat moves from hot to cold – the greater the difference in temp between them is, the faster it moves from hot to cold. With no wind, the air film has been partially heated, so it’s slowing down the transfer of heat from the object. But when the wind scoops that heated air out of the way, it speeds things up.
“Wind chill charts” are pretty abstract aids. There’s a lot of disagreement among experts about what should be factored into those. Our current U.S. charts are only one of numerous charts used around the world today. And even within the U.S. there is more than one chart in use. As mentioned, the basic intent of these is to gauge the effect of x wind speed, at x air temp on perceived human skin temp. But there are a zillion variables there for experts to disagree upon.
The AC is Working Properly Because the Air Coming Out Feels Cool
We hear this one a lot. But the proof is in the pudding – whether it’s capable of maintaining a reasonable temperature in the house for a given outdoor temp. Our hands aren’t very good indicators of BTU (cooling power). The air blowing out of a vent with only the blower system running will often feel cool, simply because it’s moving room temperature air that’s below our body temp. So it’s cooling our body, but not the air of the same temp in the room. A small fan blowing on your hand would have the same effect.
And most folks don’t know that when our air filter is totally plugged up, the cool air coming out of the vents can get even cooler. But it doesn’t have the proper cooling capacity (BTU), because there’s less air volume per minute being moved, due to the restrictiveness of the air filter.
The BTU output (cooling capacity) is a factor of both the air volume and air temperature coming out of the vents. (And our hands are very poor indicators of airflow volume). And when you kill the airflow, you’re killing the cooling capacity too.
Attic Ventilation is Required by Code to Keep your House Cooler in the Summer
Yes, the building codes require attic ventilation. But the primary reason for that is to prevent condensation from forming on the building materials in the attic during the winter. And that’s also why it’s a bad idea to cover those vents in the winter as some people do. Attic ventilation systems do help somewhat to reduce the buildup of heat in the attic during the summer. But due to the nature of much of that heat (radiant type heat being transferred directly to the floor of the attic from the underside of the roof), the effect isn’t as great as you’d think.
And if you have a powered attic ventilation system without having the attic sealed from the living space below it, you can actually suck the cooler, air-conditioned air up out of the living space, making it even harder to cool the living space. The same thing sometimes happens on windy days if you have rooftop turbines instead of standard rooftop vents, i.e. the wind causes the turbines to spin faster, and that puts the attic in a lower pressure than the conditioned area below it, causing a greater volume of conditioned air to be sucked up out of the conditioned area.
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