A BTU Is A BTU…Or Is It?

Since we’re on the topic of energy, let’s extend last week’s discussion of electric resistance heat efficiency.  I’ve sometimes told people that “a BTU is a BTU.”  (For the rest of the world, the conversion is simple:  “A joule is a joule.”)  But is that true?  And what would make it not true?

Energy for heating

When I say “A BTU is a BTU,” it’s a reaction.  It comes out when I hear someone say, “Oh, a heat pump just won’t heat as well as furnace.”  Or sometimes it’s when they say, “You can’t heat an existing home with heat pumps.”

But the thing about heating is that it doesn’t matter where the heat comes from.  If you need 6,000 BTU per hour to heat your house, it doesn’t matter whether those 6,000 BTUs come from a furnace, heat pump, woodstove, baseboard heaters, or cats.

One reason some people doubt that heat pumps are up to the task is that they confuse temperature with heat.  Temperature is definitely related to heat, but they’re not the same thing.  As I wrote in an article about heat pump supply air temperature, you can get just as much heat from 100 °F (38 °C) air as you can from 140 °F (60 °C) air.  You just need more air if it’s being delivered at a lower temperature.

So in that sense, yeah, a BTU is a BTU and a joule is a joule.

Thermal comfort

Now, where that statement starts to break down is when you assume that delivering heat gives you the the right air temperature for comfort.  Paraphrasing the great Robert Bean here, heat pumps and furnaces provide warmth.  They can heat a space, but they don’t deliver thermal comfort.  That, dear reader, is a more complex issue.

Comfort isn’t just about getting the right air temperature.  First of all, comfort is subjective.  It varies from person to person.  Even for a single person, it varies from time to time.  And air temperature is only one of six major factors that affect comfort as spelled out in ASHRAE’s thermal comfort standard.  They are:

1. Metabolic rate
2. Clothing insulation
3. Air temperature
4. Radiant temperature
5. Air speed
6. Humidity

You may have the “perfect temperature” in your home and still be uncomfortable.  Stand next to those big single pane windows on a sunny summer day and you may feel too hot even if the house is at 68 °F (20 °C).

Or sit next to an uninsulated wall when it’s really cold outdoors, and you may feel a chill even with the furnace blasting.  As it turns out, the temperature of the surfaces surrounding you is more important than the air temperature.

If you really want to be comfortable in your home, you need to start with the building enclosure.  Make it airtight.  Insulate it well.  Design the locations and sizes of the windows and put overhangs above them.

In terms of thermal comfort then, a BTU is  NOT a BTU.  The BTUs you feel from the enclosure are more important than the ones from your heating or cooling system.  That’s because most of the heat exchange between our bodies and the environment is due to radiant heat.

Indoor air quality

Your indoor air quality also may depend on the source of your heat.  Using an unvented gas fireplace or space heater means all the exhaust gases from the combustion get dumped into your indoor air.  At a minimum, those exhaust gases will contain water vapor and carbon dioxide.  In reality, though, you’ll also get some oxides of nitrogen, soot, and, if you’re unlucky, carbon monoxide.

But it’s not all about space heating.  A natural draft water heater inside the home is easily backdrafted.  If it does backdraft, the exhaust gases don’t go up the flue.  They spill into the indoor air.  And when that happens, carbon monoxide likely gets into your air—and your lungs and your bloodstream.

Cooking with gas?  Those BTUs and joules produce more indoor air pollutants than cooking with electricity.  The research keeps reinforcing that conclusion.

Source energy

See last week’s article for a more complete definition of source energy, but the short version is that it’s all the energy used in delivering energy to your home.  The gas, electricity, propane, and other fuels you use come at a cost.  And it’s not just in dollars.

Electricity, for example, comes from burning coal or gas, running nuclear power plants, and increasingly, from wind and solar.  When it comes from burning stuff, there’s a thermal penalty.  The electricity we receive has only about a third of the energy that was in the fuel.  That’s just thermodynamics, and there’s not a lot we can do to improve that.  See my article on rejected energy.

Gas, too, has losses.  There’s energy used to drill for it and deliver it through pressurized pipelines.  And there are leaks in those pipelines.  Gas isn’t as clean as we’ve been led to believer.

In this case too, then, a BTU is not a BTU, nor a joule a joule.  Where the heat comes from matters.  As I’ve said here many times over the past five years, though, electricity from the grid is getting cleaner all the time.

Thinking

I realize this is a rabbit hole that a lot of people choose not to explore.  But if this kind of discussion interests you, I hope it helps clarify your thinking about energy, indoor environmental quality, comfort, and other big picture issues.

The sum and substance here is:  When given a chance, choose your BTUs wisely.

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.

 

Related Articles

Heat Is a BTU-tiful Thing!

Is Heat Pump Supply Air Too Cold for Heating?

The Meaning of Rejected Energy

 

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