Thermostat: Newsletter Challenge (02/14/06)

Is the "logic" that she is applying here something to the tune of believing that turning up the thermostat higher heats the place faster?

We all know that current heating systems simply turn on and generate a constant rate of BTUs into the building. Setting the thermostat simply tells the heater when to turn off. The rate of temperature change remains the same.

However, there might be a counter argument here. Thermostats simply measure air temperature. The walls, furniture and other objects (cold tile floors) have a higher heat capacity and will take a little longer to get to the same temperature. If a furnace is robust it may raise the air temperature quickly, but the rest of the house and it contents are still cold. The result is the furnace will recycle soon after the air temperature reaches its set point because the house objects quickly absorbs the heat in the air.

If she sets the temperature to overshoot the desired ambient temperature the home will feel like it is warmer sooner because more energy has been pumped in initially to warm not only the air, but the house itself. Once the furnace shuts off you can dial the thermostat back to normal.

I think I remember reading about some European systems that do this using fuzzy logic thermostats. Can anyone confirm that?

Again, I am taking a wild guess at the answer.

I'd say she turns it higher because the cold surroundings feel as though they 'radiate cold'. They actually give off less infrared heat which tends to make her colder.

Yes, The idea about fuzzy logic in thermostats sounds familiar to me, too. Some things are hard to control as well as nuanced mental analysis can allow a person to manually do it.

I live in a tropical country so no heater is required. On the other side of the coin, we need a lot of air-conditioning! :) I believe that a heater using fuzzy logic is possible. Fuzzy logic recognizes more than simple true (1) and false (0) values, i.e. it can recognize various settings of degrees (or possibilities) between 0 and 1. If the room is cold, the system will allow more current to be injected into the heater to pump up the I2R heat loss. However, fuzzy logic is normally used in AI and expert systems and I do not consider home heaters to be one of them. But on the other hand, all intelligent systems must have a common unintelligent origin! Hahaha! One suggestion - move to a tropical place! you will love the sun!

You'd be surprised where fuzzy logic has got! It's no longer used only in AI & expert systems. It has filtered down to more mundane things.

Am I being stupid? do thermostats only measure air temperature? the one in my hose is mounted on a wall, I would imagine the wall temperature significantly affects when it switches on & off

No, just a little thick perhaps. The thermostat measures the temperature of the thermostat which is the temperature of the air and the temperature of the wall...basically

I blame the auto manufacturers for some of this. Last winter I asked my wife why she turned the thermostat to 80. She stated that in her car, when she was cold she simply turned the dial to the hotter setting. Aha! This is where the logic is coming from. In your car the heating system works differently than in your home. In the car, the heating system mixes air from either inside the cabin or from outside (depending on the recirculation setting) with air that has passed over the really hot heater core. The heater core is attached to the engine cooling system. The dial on the dash determins the mixture of these two air streams and allows the user to determine the temperature of the air coming out of the vents. Since most people don't know the house system works in a different manner (as explained in the other posts), they assume the two heaters work the same. Flawed logic comes from lack of understanding the operation of the two systems.

She is thinking that the higher the thermostat setting, the hotter the air coming out of the vents, the faster the house warms up. You are thinking the thermostat just tells the furnace, heat pump or other when to come on and go off, it is either on or off, no in between, doesn't matter what the setting is, the air from the unit will be the same temp and the house will warm at the same rate.

You havent taken into account all types of heating...Lets take say an air to air heat pump..If you increase the temp in the house less than 2 or so degrees at a time..the heat pump will heat the house up..the air coming from the vents will be warm but not hot..Now if we crank that puppy..then the auxillary heat kicks in..(Usually electric or oil) and now the air from the register will be hotter..there are other angles but i bet not to many people (including the writers of the question) thought of this one.. Short answer..Turning the thermostat higher will result in faster heating of a house with certain systems..Becouse they didnt specify what system we cannot answer the question...

The thermostat tells the heat source when to turn on, and off, only. It does not alter the temperature of the heat transfer element, be it air or hot water. If the house were just a few degrees cooler, setting the thermostat higher would only warm the house to a higher temperature than desired. However, having been through this recently with my parents unoccupied house, I can confirm that the contents of the house take a long long time to heat up to the desired temperature, so a bit warmer setting initially will speed the heat recovery period and greatly decrease heat on/off cycles, which in a hot air system, or a hot water system uses a bit more electricity to start the fan or pump. In-rush current on a motor is quite a bit more than operating current.

Delta temp rate won't change. Heat sinks are non-trivial. Real-life scenario: Floor's still cold (remove feet from floor), I'm tired from trip(move toward bed), Doesn't it seem a bit warm to you honey? (remove clothes), God it's great to be home! Happy St. Valentine's Day

The replies which point out that the thermostat is only measuring air temperature, and the overall environment contains a number of items (walls, floor, furniture, etc.) which are also cold, which only heat through conduction from the air after it is warmed, and which influence perceived temperature through increased or decreased black body radiation are all certainly on target. The furnace can indeed only pump out heat at a certain rate regardless of where the thermostat is set. Omitted from the earlier replies is the decreasing efficiency of the heat transfer from the burner chamber to the airflow being heated as the return air temperature increases (less temperature gradient, less transfer, more heat out the flue), which probably outweighs any gain in efficiency from the saving on fewer startup transients from fan cycling. However, what has been completely omitted is the effect of temperature acclimation during the trip to a warmer climate. Even the short time during which the couple was in a warmer typical temperature is enough to adjust the body's sense of 'warm' or 'cold'. This, in turn temporarily influences what the final temperature goal should be for comfort. As an example, a person living in the frigid North will typically find an indoor temperature to be comfortable when it is in the 68 to 70 degree range. A person who typically lives in a warmer climate will find that quite chilly. Most people have seen tourists dressed in far warmer or cooler clothing than the natives of an area, simply because they have different tolerances. As an example, I've seen residents of Washington DC will wear winter coats when the temperature was at 50 degrees, while tourists from New York were walking around in light jackets, Maine natives were in shirt sleeves, and Floridians looked out their hotel windows wondering whether they dared venture out. So while the one setting the thermostat was thinking of comfort, the other was ignoring both the acclimation factor and the high specific heat of the solid objects in the room.

And Canadians at 50 degrees still wear shorts and t shirts..and have BBQ's at night.. Only when it gets down to about 30 or so do we start "dressing up"!!!

Great point about aclimation!

I went to Florida many, many years ago for one week and I still haven't aclimated back to the Ohio weather. ;-)

I see three factors that setting the thermostat higher (temporarily) will help regarding comfort. In order of significance:

As mentioned the house structure will have cooled down and radiant heat loss from the occupants will be higher until the structure is heated, which could take many hours.

After a week a warm place, the occupants will need time to get use to the cold. The body takes longer to acclimatize to cold than to heat.

Most thermostats have what is called a heat anticipator circuit. Wired in series with the contactor coil, it provides a little heat within the thermostat when the furnace is on to reduce overshoot as the thermostat sensing element lags in sensing room temperature. It is designed for normal operation with temperature swings of a couple degrees. When recovering from a setback condition, the anticipator would have the thermostat cycling off before the room air temperature has risen to setpoint, thereby increasing the time it takes to recover.

I have a heater (Monitor; I believe Toyo, which makes another brand popular around here, is the same)has two or three burners (Three lights, but I'm not sure there are three stages). I crank it up as the wife did, and it gets more hotter fasterer. At some point it will kick down, as it gets close. All the stuff about heat sinks is true, also.

No postings have yet accounted for effects of gamma radiation and coriolis forces. But by far the greatest omission is the non-linear behavior of the female gender. It is what it is. No scientific basis has been found for that behavior, and it would redefine science if one was found!

I hope you're not purporting that the behavior of the male gender is linear and comprehensible- I beg to differ. I have been happily married for 10+ years and sometimes still my husband's behavior defies my understanding. Face it- we can understand scores of complexities when it comes to our engineering disciplines, but we are generally hopeless, as so many humans are, when it comes to understanding the opposite sex, whichever happens to be opposite to you. (speaking for myself, anyway).

Unfortunatly the wife is correct on this one, the husband is looking at the house as being a big box of air. Once the air in the box has reached the desired tempature (68) anything more is simply a waste. In truth however the house is a big box of air with solid objects in it. all of the objects as well as the air start out at 58 degrees. the air heats up faster than the furniture so by turning the heat up beyond the desired temp (to 74 in this case) the solid objects will warm up faster and thereby make the house more comfortable sooner. Also letting your wife have her way will also make the house more comfortable sooner. Just remember to turn down the heat once everything gets warm.

Come on people . . . Setting the thermostat to 74 instead of the "normal" 68 does get the house and its furnishings to an average of 68 degrees sooner than setting it to 68, allowing the furnace to settle down to its normal mode of operation when the thermostat is set to 68. If only Newton's law of cooling (or heating) is considered, which states, simply put, that the rate at which a mass will cool (or heat) is a function of the temperature difference between them. Air at 74 degrees will transfer heat to more massive things quicker than 68 degree air. The question now is: "How long should the thermostat remain at 74 degrees before it's returned to the "normal" setting of 68?" Express your answer in terms of average heat capacity of the house and furnishings -- we'll ignore the heat input from human occupants and the sun. What follows is merely some commentary about furnaces, houses, and heating: A furnace puts out a constant amount of heat energy, when it's "on," unless it's a fancy one. (Even heat pumps with auxillary electric heating elements put out roughly the same amount of energy when they're being used. Heat pumps take heat from cooler outside air by comprssing a heat-transfer gas to a hot liquid and evaporating it to give off heat in the house, but let's leave that part of it alone.) The temperature of the entire mass of the house being heated will rise at a fairly constant rate until the entire mass approaches the set point of the thermostat. At that point, the lower heat-capacity air causes the thermostat to turn the furnace "off." As the heat in the air is transferred to things like couches, the air cools, and the furnace turns back "on." The mass of the house has a " thermal time constant," which is only changed by adding or removing people or household furnishings. Somewhere in the neighborhood of 5 time constants later, the house will reach equilibrium, (regardless of the thermostat setting -- but there are some limiting factors) and the furnace will cycle on and off at a rate determined only by outside temperature, solar radiation, thermostat setting, and insulation. Given a constant outside temperature, constant thermostat setting, constant radiation from the sun, the furnace should turn on and off at a constant rate. If the thermostat is set on 68, it will cause the furnace to be off for longer periods than when it is set on 74, all other things being the same.

Help me. What was your point?

That's she's right and it will heat up quicker.

In my experience, it depends on the house construction. The stone cottage I lived it, heated by soild fuel (so no option of leaving on whilst away for a week), took over a week to heat up, that's with a fire in 24 hours a day (banked mostly!). After that time, with the walls warm, the house was quite comfortable; during the heating phase I wore jumpers, coats, thermals...well, maybe I exaggerate, slightly.

Having the fire roaring did not increase the temperature of the house - far too much went up the chimney - it just decreased my coal stocks. Turning the heating system pump on (rather than letting it trip on the thermostat) had a short term effect of heating the house - then cooled it as the system pumped cooled water round the radiators. Hence the 24 hour banked fire that kept the house comfortable (but not overly warm!)

Now in a house with gas CH - and I avoid switching it on unless there's frost outside (or I have visitors).

Yes I'm female, so presumably I use both hemispheres. And I'm an engineer, so does the left dominate?!

This is a trick question. The constant output of furnaces and the binary nature of thermostats that many readers, who understand "bang-bang" style control systems, presented pretty much covers what happens physically. With experience, though, comes wisdom. The most important recognition anyone can derive regarding this matter is that we fare best when we acknowledge that we are not mind readers and cannot know for sure what someone else is thinking without first asking and then listening. Applying this is the best way to avert the "incipient arguement" over this or any other marrital matter. This being said, I must admit that I have not yet figured out how to convey this understanding to my wife. Another question for "The Challenge" 'If a man, standing alone in the middle of a forest screams something at the top of his lungs and there isn't a woman present to hear him, is he still wrong?' signed, not coward, just lazy

She is thinking that a higher setting will somehow produce hotter air. Since 74 degrees is warmer than 68 degrees the furnace must produce hotter air to make that happen. He is thinking that the furnace will just run at a leisurely pace as if the temperature were within a few degees of the set temperature. She's partially right because the air coming from the ductwork is always hotter than the set temperature. He's wrong because husbands are always wrong, and because when you force the furnace into panic mode by calling for a large degree increase in temperature, the system will run as close to continuously as it can, cycling on the overheat protection circuit. This stresses the heat exchanger and burner and is a frequent cause of failure for those parts. So he probably saved a few bucks on fuel but may have killed the furnace in the process. If he had left the setting alone or maybe dropped it to, say, 65 degrees, he would have protected the equipment. The house would still be a little chilly but not frigid, she'd still set the thermostat on 74, his hearing would've suffered less, and they could've had a hot toddy while the house warmed up.

A universal assumption seems to be that the spouse who sets the thermostat higher is the female one. Making the opposite assumption, the male spouse is probably setting the thermostat higher to please his spouse, who simply sighs and tells him it's a waste of energy. A sad tale.

She's thinking with both hemispheres, as the female brain is usually wont to do--which means she's thinking with her feelings, too. Now, I'm also thinking with both hemispheres--a rare capacity among the left-brained world of engineers--so I'm up with her on this thing. First I recognize that she knows and concurs with my "logic" that hyping the thermostat will not make the air warm faster (albeit, it will make the air warmer...until it makes it too warm...which she and I both know will make me uncomfortable faster than her) So I refrain myself from explaining why the higher setting will not make her any more comfortable: why it is that she only need to feel and take note of the temperature of her skin to know what the air from the register should feel like...a bit cool at first as long as she's standing near the register. That is she feels warm air coming out the register, she can know that that warm feeling against relatively cooler skin will soon give rise to the overall discomfort of being too hot. But even then, and even knowing that she has heard and understood in the past, I will not expect her to "feel" it my way. Why? because for one, I know that I made a mistake even before going on the trip: I set the heat too low in a snowy climate (hence a humid climate) and thereby subjected the house, and especially the plastic cases on my electronics equipment to a hight likelihood of mold contamination and damage. Had I been thinking I would have realized that setting the thermostat so low, in addition to making the house more uncomfortable upon our return, did not really save much energy...but rather only ensured that the house was subjected to cold (and damp) for longer periods between thermostat kickins. So I would have to conclude both logically and supra-logically that the wife is seing the problem in a much more comprehensive way. Which would get me to thinking about a so-called, engineering solution that might satisfy both my wife's logic and my own more constricted logic: In doing this I might imagine and electric thermistor controlled ceramic heater device that would supplement the discharge from the central heating registers (say in the front room near the entry door or in the kitchen) and that when needed (to impart a warm feeling on my wife's skin) it would raise the register discharge to a temperature higher than 90 degrees or so (skin temperature)...but as soon as the register air had warmed the ambient air, and the thermistor in the register discharge path to normal 78 degrees, the ceramic heater would gradually reduce its discharge temperature until it approached 78 degrees and eventually turned to zero heat output.

She is cold and wants to be warmer so she choose a level she feels will provide the level of warmth she needs at the moment and hopeful for your wallet she will cut the heat back to a more engery cost saving level soon after. I remember a nasty note my now ex wrote when I ask her to keep the air conditioning set at 75 in the summer. Don't ruin a good marriage over the heating or cooling bill.

When my wife is cold she likes to sit in front of a hot air register while the furnace is on. Anyone knows that if you turn the thermostat up the furnace will come on soon and you'll be able to sit in front of a nice warm hot air register. Beyond that, if the house starts out at 58 and the thermostat is set to the normal 68, when you sit down on a couch or chair, or climb into bed, the solid object will be closer to 58 than to 68 for a while. Until it get warms up, the net effect will be a greater rate of heat loss from your body to the solid object than you are used to. By raising the air temperature a few degrees you help to balance the net rate of heat loss from your body to something closer than normal. As a result, the slightly warmer air temperature is needed for you not to feel cold until the house warms back up to normal. A related question, and one that only has house (and occupant) specific answers, is: How long do you have to be gone form your house, to make it worth turning the thermostat down 10 degrees, given that when you get back, you are going to want to turn the thermostat up 6 degrees higher than normal for 2 or 3 days? The added heating fuel you use is still probably less than what you used on your trip (especially if you include what the airplane used, assuming you flew). So your trip cost a few more dollars than you thought. There are always hidden costs.

Ok engineers, let's use some common sense. With the higher thermostat seting, the heater cycles more frequently until desired temperature is reached. Therefore, more warm air is put into the house in the same amount of time. Even though the temperature of the heated air is no different, more is added to the house quicker. The wife is right.

The operative term is 'wife`. It matters not whether she is correct or not. She is ALWAYS right and argument is bootless.