Argument heats up!

February 21, 2010 5:54 PM

Help me settle an argument between my parents about central heating, please. My mother believes, for instance, that setting the thermostat for 75 degrees and cutting it off once it reaches 70 degrees will warm the house faster than if the thermostat were simply set to 70 degrees initially. My father thinks this is ridiculous. Who's right?

In addition, does this also hold true for other things such as ovens or air conditioning? [Are air conditioning and heating different, or do they operate largely the same way?]

posted by estlin to science & nature (29 answers total)

Your mother is wrong. For air conditioning, too. Set the thermostat for a comfortable temperature and ignore it.
posted by yesster at 5:55 PM on February 21, 2010

In some heating systems setting it to a higher temperature might activate an "emergency" mode, depending on the details. I have a heat pump and this will sometimes come on if the difference between where it is and where it should be is too large (I think). However, this is a massively inefficient system, it is just straight-up electric heat usually. But it all depends on the particulars of the heating system, and I don't think I've lived in a place with this before.
posted by advil at 5:58 PM on February 21, 2010

your dad is right. setting the thermostat at one place and leaving it alone heats things more steadily and is more energy efficient than monkeying with the settings.
posted by nadawi at 5:59 PM on February 21, 2010

Ha! I used to agree with your mom (it seems true, intuitively, right?) and didn't realize how dumb that is until I saw this scene from UK sitcom PEEP SHOW explaining the error.
posted by moxiedoll at 5:59 PM on February 21, 2010 [8 favorites]

With very very few exceptions, the thing your mother wants the heater to do is not something it is capable of doing. The thermostat sets the temperature at which the furnace is turned off after the rooms have heated up enough, not the temperature of the heat coming out of the vents which will be a constant "as hot as the thing can manage."
posted by majick at 6:01 PM on February 21, 2010 [2 favorites]

Unless you have a unconventional setup (like advil's heat pump), your father is right (and the same applied to ovens and air conditioning). Furnaces generally only have two modes: on and off. When the thermostat is set above the current temperature, the furnace is turned on until the desired temperature is reached and then turned off (obviously, there is a bit of slop, so that the actual temperature that the furnace turns on at is a bit lower than the setting and the temperature it turns off at is a bit higher than the setting).
posted by ssg at 6:03 PM on February 21, 2010

The only thing this is generally true of would be an electric stove, which doesn't use a thermostat but just controls the power going to the elements. Setting it to high for a few moments and then pulling it down to the setting you actually want does indeed get it there quicker, not that it'd normally take very long anyways.
posted by floam at 6:04 PM on February 21, 2010

Weirdly enough, there's a really good (and pretty recent) academic paper about this argument. The paper calls your Mom's theory the valve theory, whereas the other is called the feedback theory. It's becoming a modern classic of cognitive science, technology design and human-computer interaction because there are a lot of "folk theories" that people hold, that, while not perfectly accurate, are still helpful in designing systems to account for these conceptual confusions.

It's called Two Theories of Home Heat Control. Sorry for the for-pay link, but if you have access to an academic library or academic journal databases, you might be able to get a copy.
posted by zpousman at 6:05 PM on February 21, 2010 [37 favorites]

Most thermostats only turn the units on or off. It is possible that some appliances may have this feature, but unlikely for central air. For example, my instant hot water heater has two separate heating elements with separate power supplies and will use one or both depending on the temperature of the supply water and the volume of water passing through.

To be sure, check the manuals for your specific units, but your father has about a 99.9 percent chance of being right.
posted by Yorrick at 6:06 PM on February 21, 2010

Your dad is right, unless they have a fairly new system with two-stage heating. Older furnaces have only one output level, and they run at that level until the air temperature reaches the set point. Same for AC--older models are generally one-stage, but there are also two-stage air conditioners available on the market that will output more coolth when demand is higher.
posted by drlith at 6:09 PM on February 21, 2010

Think of it in these terms. The heater (oven, air conditioner, whatever) is given a goal. It has only one way to reach that goal. Yelling louder or aiming higher doesn't help reach that goal. If you set the thermostat for a higher temperature, the heater or oven will do its best to reach it. It will pass through the lower temperature at the same time no matter what the higher setting is. You could set the thermostat for 100 and it won't reach 70 any sooner.

Think of it like zooming onto the interstate. If you floor it, you won't reach 55 any faster if you decide to roar up to 100 than if you decide to go only to 70. The car will accelerate as fast as it can and no faster.
posted by Old Geezer at 6:12 PM on February 21, 2010 [1 favorite]

If you set the washing machine to "extra large" and stop it when it hits "medium," does it fill up faster than it does if you just set it to "medium" in the first place? No, and for the same reason. The (water|heat) comes out of the (furnace|inlet valve) at the same rate regardless.
posted by KathrynT at 6:22 PM on February 21, 2010 [2 favorites]

There have been a lot of good answers in this thread. As everyone's said, your dad's correct; turning the thermostat way up doesn't make it heat any faster.

In mathematical terms, think about the rate at which the central heating gets hot. Your mother is thinking that this happens on a curve - and that therefore setting the peak of the curve higher will make earlier parts of the curve steeper. Since she's assuming that it will shoot up and then slow down as it reaches the correct temperature, she thinks that it'll have to shoot up faster so that it can slow down later. Whereas in reality the heat actually describes a straight line; it just stops at different points. No matter what you do, a specific temperature is the same point on that line.
posted by koeselitz at 6:28 PM on February 21, 2010

I recently studied neural communication in psychology, and these answers remind me a lot of the "all or none" principle for neuron firing, which makes a lot of sense. Thanks so much, everyone!
posted by estlin at 6:38 PM on February 21, 2010

And then there's always the good semantic argument:

Say that this actually worked, and setting the thermostat to 100° and then stopping it when it got to 70° warmed the room up faster than simply setting it at 70°. And say you're a thermostat and central heating manufacturer. How do you get an edge on the competition? Isn't it obvious? You'd create a magnificent thermostat that aimed for 100° right up until the moment the room got to 70°, right? In fact, if you were a thermostat and central heating manufacturer, wouldn't this idea be completely obvious - the best way to make things more efficient?

In fact, this is already what thermostats do.
posted by koeselitz at 6:38 PM on February 21, 2010 [6 favorites]

Your mom would be right IF the heater put out air at the desired temp (75 or 70) until the whole house was that temperature. However, if that were true, it would heat the house EVEN FASTER if instead of setting it to 75, she turned it all the way to the MAX temperature until the house reached 70, then turned it off. Right?

Guess what? That's what the heater does. It puts out as much heat as it can until the house is the temperature you set on the thermostat, and then turns off. So really, it IS using her idea, but making it even easier - you don't have to remember to turn it off when the house gets to 70. You just set that on the dial and the heater takes care of the rest.
posted by ctmf at 6:47 PM on February 21, 2010

Your dad wins. Your mum has an erroneous mental model of how a heating system and thermostat works.

This problem is also discussed in Don Norman's The Design of Everyday Things, which is a terrific book btw.
posted by carter at 7:12 PM on February 21, 2010

If you want to really blow your mom's mind, let her know that her electric oven works the same way. (The temperature is a threshold: the appliance only really only has "on", "off" and a thermometer.)
posted by rokusan at 8:25 PM on February 21, 2010

Here is a link to zpousman's Two Theories of Home Heat Control (PDF).

A trick for finding non-pay versions of journal articles is to search for the title in Google Scholar.
posted by JackFlash at 10:48 PM on February 21, 2010 [4 favorites]

Another way your mom's view might make sense is if she is tacitly imagining the thermostat setting as the temperature the system tries to reach within a time limit. (For example, in setting the temp to 75, you're saying to the heater "in 10 minutes from now, the house should be 75".) This would make sense of the idea that the heater will heat more slowly if the thermostat is set at 70, but more quickly if it's set at 75. If the thermostat setting had a set time limit, then it would make sense for it to "heat faster" if the thermostat is set to higher temp -- the higher the temperature goal, the greater the change the system has to make within that time limit.

(I mention this if you're looking for ways of explaining where the error is. There's no time limit.)
posted by LobsterMitten at 12:09 AM on February 22, 2010

So from a straightforward, physics perspective, your Dad is probably right, unless you have a really fancy heater. However, depending on what you mean by "cut it off when it reaches 70", your mom may actually be correct.

The thermostat is basically a switch that turns the heater on (full blast) when the temperature at the thermostat is below the set temperature, and turns it completely off when the temperature is above In my house, however, that are lots of cold spots, so if I came into a cold house and turned it to 70, the thermostat would cut off when the temperature at the thermostats reached 70, while my bedroom would still be a good deal chillier. It would take a while for the air to circulate between rooms and for the bedroom to get comfortable, and the heater would have to turn on again once the cold air from the bedroom circulates out and about. On the other hand, if I set the thermostat higher than desired and turned it off when the average temperate was 70 degrees, it would be a few degrees warmer at the thermostat and it would have otherwise turned off earlier.
posted by bsdfish at 12:39 AM on February 22, 2010 [3 favorites]

Your dad is most likely right and for the reasons explained above. However, some heating/cooling systems are starting to use PID control, which means they have more than just on/off and can vary the energy output.

Most airconditioners (heat pumps) labelled as containing an inverter do this - the inverter is a means of controlling the AC frequency to the compressor motor (changes its speed) and therefore the heating/cooling power, which allows the unit to efficiently maintain a constant temperature without stopping and starting frequently ("bang-bang control"), which is bad and inefficient in heat-pumps.

Putting a higher set-point on the controller may or may not result in greater output, depending on the gains in the PID controller. Most likely it will not have the effect your mother wants - while the controllers have variable output, they are smart enough to take into account the thermal mass of the room and will deliberately ramp the temperature up quickly, perform a small overshoot and then settle down into fine-control mode.
posted by polyglot at 1:18 AM on February 22, 2010

Btw, science has two branches, only one of which is theorizing. So just measure how long it takes to get from temperature A to temperature B using the two approaaches, and you'll likely see that there is no (reproducable, significant) difference.
posted by themel at 1:42 AM on February 22, 2010

bdfish makes a good point about deliberately "over"shooting to get all areas up to snuff. There's another one too: It used to be that our AC couldn't keep up. Even with it on full blast, the house would get warmer and warmer throughout the day. Eventually, we decided to set the thermostat low not so it would get colder faster but so it would get cold early and therefore be comfortable later.
posted by DU at 3:01 AM on February 22, 2010

The washing machine example is perfect. No doubt your mother would see some logic in it. But also consider the waste argument. By fiddling with it manually there's a fair amount of energy being wasted. Every minute the HVAC runs too long is money wasted. It's much better to use an automatic thermostat programmed to match use of the home. Set it and forget it. The machine will do it exactly when necessary with no waste. That and they're pretty cheap these days and brain-dead simple to install yourself.
posted by wkearney99 at 6:06 AM on February 22, 2010

Also consider that lots of houses do not have their HVAC systems set up properly. Either they're defective by design (or lack thereof) or have been misconfigured. Vents that have been closed or blocked is a big problem. Especially when the lack of proper airflow causes the thermostat to act improperly because of its placement. As in, the thermostat is in an area that doesn't get enough airflow, causing the areas of the house that DO get used to be too hot or cold. Or the opposite, the thermostat is in an area that gets too much airflow and causes the other areas to be too hot/cold. Couple that with naive attempts to open/close certain registers to 'save energy' might actually be making the problem worse.

It may be worth having an HVAC contractor come out and set the system up properly for their usage patterns.
posted by wkearney99 at 6:12 AM on February 22, 2010

her electric oven works the same way.

It's not just the oven but practically anything with a temperature set-point: refrigerator, toaster oven, space heater (although some of those do have multiple intensity settings which clouds matters a bit.)
posted by Rhomboid at 9:17 AM on February 22, 2010

Some modern HVAC systems have variable speed air handlers (think of it as the fan) that ramp up airflow to more quickly heat or cool to the desired temperature, then settle down to a lower airflow to maintain the temperature. The temperature of the air coming out of the vents is generally the same in each scenario but the volume of air changes.

Also, most heat pumps have resistance coils that can be turned on to provide "emergency heat" as advil said above, when the difference between actual temperature and desired temperature is large enough. This will indeed speed up the heating process since the air coming out of the vents is warmer, although there's no similar process for cooling. Emergency heat is massively inefficient (compare to using your stove to heat your house) so it's not recommended.

So the answer to this depends on the type of air handler your house has, whether emergency heating has been initiated, and how willing you are to spend more for faster heating.

And heating and cooling in a heat pump (which is what I assume you're referring to with "central air"" are the exact same process, just run in opposite directions.
posted by This Guy at 11:46 AM on February 22, 2010

This specific thing is discussed in The Design of Everyday Objects as an example of how broken mental models affect how people use things. (Your mother's mental model of the heating system is the broken one.)
posted by !Jim at 2:54 PM on September 26, 2010