Five Percent: Conserve Energy

Climate Change Is Important: Energy Conservation is the First Step

October 7, 2010

How Heat Works (Warning: Scientific Content)

Category: 5%'s Top 10 List,Fun,Household,Technology – Tom Harrison – 1:01 pm

Heat, Thermostats and Serious Content (photo: Dan Zeng)

A few weeks ago I posted on the MS Hohm blog about programmable thermostats — Energy Star no longer recommends them, but not because they don’t work, but instead because people don’t use them correctly.

Studies show that people can be lazy, intimidated, etc.  But I want to discuss a significant reason pointed out in the study: people have an incorrect “mental model” of how programmable thermostats work (PDF).

A mental model is just how you picture something working — how you understand stuff in order to get through a complicated world, right or wrong.  A classic mismatch of mental model and reality is that “the computer” is the the screen, rather than the part that has the CPU, Memory and Disk in it (leave it to Apple to make a computer that matches peoples’ mental models!)

Apparently a certain Alaskan Senator had the mental model of the Internet as a “series of tubes.” But I won’t go there.

My hope is that where it matters, we can get a proper mental model that helps us make good decisions.  Here are some that caused people to not use programmable thermostats.

Incorrect Mental Models of Thermostats

Mental Model

(photo daveelf/fourworlds)


I’ll assume the thermostat is used for heating — if you live in Hotlanta, everything is the same for cooling.

I’ll also assume that it’s colder outside than inside, and its a given temperature, like 30 degrees.

It’s also important to know that a thermostat is just a “furnace switch”.  It turns the furnace on when the temperature in the room is colder than the set point. It turns the furnace off when it warms up to the set point. (There are some clever thermostats, but in the end they switch the furnace on or off.)

So, what did those studies find?

Thermostat Is A Volume Control

One incorrect mental model is that the house will warm up faster if you turn up the thermostat higher. (The mental model may be of a knob on a stove, or a volume button on a radio.) Generally a furnace turns on and heats water or air which is then circulated around the house making the air warmer.  In reality, the longer the furnace runs, the warmer the house gets.

Same Energy To Heat Up

Another incorrect mental model is that it takes the same amount of energy to warm a house from 68 to 70 as it does from 70 to 72. Nope: it requires more energy as the difference between outdoor and indoor temperatures increase.

And neither of these misconceptions have anything to do with programmable thermostats. Things get more fun when you add this twist.

Heat Is Stored

There appears also to be a notion that heat is stored in the walls, so warming up your house is really a matter of filling the walls with heat — kind of like warm bricks on a fireplace. But the main thing that makes you feel warm is the temperature of the air in the room; all walls do is slow down the loss of heat that makes the air feel warm.

Heat Momentum

Some believe that there’s a sort of “momentum” so that if your house temperature falls really low, it takes more energy to heat, than if you just kept it warm. Yes, it takes longer to heat a house from 60 to 70 than it does to heat it from 68 to 70, those last few degrees are the hardest. The key fact here is that during the time the house was gradually cooling, you were not running the furnace and therefore not using extra energy that would just heat up the great outdoors.

The Cost Of Incorrect Mental Models

These misconceptions, and others have lead people to use programmable thermostats incorrectly. The study showed that because of peoples’ misconceptions, the possible savings of programmable thermostats were lost.

(I am an engineer, and I think I know why this happens. Engineers often tend to believe that other people think the way we do. If something is obvious to them, it’s obvious to everyone. Actually, this may be true of all people, but it’s the engineers who design thermostats. And I blame them for designing things so poorly, they are used in a way that does the opposite of what is intended.)

But, until we engineer types start getting more socially adjusted, here’s a crack at explaining how heat works — a new mental model.

My Analogy for Heating Your House

Pump Me Up! (photo: 24thcentury)

Heating your house is like pumping up a leaky beach ball.

At first, it’s easy.  As the ball gets fuller, the air leaks out faster, so it’s harder to get to “full”. Once it’s full, you still have to keep pumping it up since air leaks out. If you stop pumping the air still leaks out — quickly at first, and more slowly as the ball gets flatter.

So in this silly analogy, a full beach ball is a warm house. Pumping uses energy, like the furnace. The leak represents the way your walls actually work, some are better or worse at slowing down the rate of heat loss, but they all leak.  

So what strategies balance our desire for a full beach ball against the amount of energy needed to keep it pumped up?

You could run the pump just before you used the ball, but then you have to wait (manual control). If you really wanted the ball full whenever you wanted to use it, you could run a motorized pump that ran whenever the ball got a little flat (normal thermostat).  If you were clever, you could put the pump on a timer so that the ball was kept full only at times you might need it, but let it go flat other times (programmable thermostat).

Using this analogy, it makes more sense (to me) that a programmable thermostat makes the furnace run for a shorter time overall than a regular thermostat.

Yes? Not so much? If you don’t like mine, what’s your analogy? You can certainly do better than a leaky beach ball! All comments accepted. The best response will get a free beach ball.

(Credit for getting the ball rolling, so to speak, to Chris at who had a question from one of his readers, and reached out to me and Allison Bailes of to validate his explanation. So I asked someone to make sure I had it right.)

So enough with the analogies. What about some actual facts?

The Science of Heat Transfer

Science Is Fun

Scientist, Not Andrew (photo: aeter)

I managed to make it through advanced freshman physics at college but came out a scarred and damaged man. My career as a brilliant physicist was dashed on the rocks of reality.

So I asked for help from my friend and almost-classmate, Andrew Robbins, who took the same physics class I did. He aced it. He then went on to get a Masters in Mechanical Engineering from MIT, and if that weren’t enough an MBA from Harvard.  I didn’t share my beach ball analogy (for fear of being ridiculed) and just said I needed some help on how to explain how heating a house actually works.

He responded:

It is not like heating a pot of water, which would be enthalpy.  The air in your house doesn’t store a lot of energy.  This is about maintaining the heat in your house against an external drain – energy transfer out of the house.  For the most part, the energy drain will go with the C*(Tin-Tout).

[A brief interjection: Andrew is a very funny guy, a great dad, and beyond brilliant.  But he is an engineer, and assumes the rest of us know what “C” is, and what “Tin-Tout” means.  Here, C measures how poorly the wall keeps heat inside.  Tin-Tout is the difference between inside temperature and outside temperature, or temperature differential, or Delta T.]

You can lower the heat transfer by lowering C; that is, adding insulation, sealing air leaks, etc…  Or you can lower the temperature differential.  If the ambient temperature outside is 30 degrees in January, the difference between heating your house to 68 vs. 70 should be (38/40) or a 5% reduction in energy.  But you have to put on a sweater and who wants to do that!

[See — I told you he was funny!]

There are three types of heat transfer that might matter and one that doesn’t:

  • Conduction (e.g. transfer through studs in the wall)
  • Convection (warm air embracing you)
  • Thermal Radiation (think of a fire)
  • Direct exchange (cold air leaking in and hot air leaking out).

Thermal radiation goes with the temperature differential to the fourth power.  I don’t think that it is a significant factor because with delta T to the fourth power, a small temperature differential is not a big deal.  It is also the difference of the outer wall’s temperature NOT the inside of your house.

Conduction and Convection both go linearly with delta temperature.

Direct air exchange will also go with delta temperature.  This is like heating a pot of water where you keep putting more cold water in the pot.  You need more heat if the water is colder.

So, just as I said: it’s like pumping up a leaking beach ball.  Get it?  Right?  Yeah, me neither.

I suddenly feel less bad about washing out of physics.

And just because you made it all the way, here’s my first attempt at an Xtranormal video. I don’t think MS would have liked this.


  1. Oh, I’ll repost my analogy here since you actually asked for it here ;)

    Imagine you have a bucket. It leaks, because there is a line of small holes drilled up the side. So the fuller it is, the faster it leaks.

    Sometimes you need the bucket full; other times you can live with it half empty. Clearly you let it be half empty when you can be, because it leaks less that way. And filling it to the top only when necessary doesn’t take any “more” water….

    BTW did you get your heater cycle measuring device going? I am sorely tempted to get a wifi thermostat and hack it to measure & graph this…

    Comment by Eric — February 9, 2011 @ 12:16 am

  2. Eric —

    Here’s the link to my prototype datalogger using the components from Lego Mindstorm NXT kit:

    Comment by Tom Harrison — February 9, 2011 @ 9:43 pm

  3. […] this can be a confusing concept to grasp, this is a really great site that explains common mental misconceptions of how heating your home works, and the scientific […]

    Pingback by Energy Saving Myths: Part 2! « The Minnesota Energy Challenge Blog — February 8, 2012 @ 10:09 am

RSS feed for comments on this post.

Sorry, the comment form is closed at this time.