Why is it less efficient to turn off a home furnace than to leave it on???

[raven]

These folks are known as "wrong".

You are correct.

They are idiots.

You are forgetting the power of Urban Legends to propogate and aquire adherents.

John

[Joseph Meehan]

When the water is cold it absorbs heat very easily. When the water is hot it absorbs it more slowly. When the burner is on, X amount of heat is produced per unit of time. With the water cold to start with, it will absorb heat quickly (cooling the hot gases) so the final exhaust is cooler, meaning more heat has been transferred and less is wasted going out the exhaust.

[Joseph Meehan]

Actually there are a few errors there. First, most engines work better (more efficient) hotter. There has even attempts to make IC engines from ceramic materials so they can run much hotter without melting to increase efficiency. Try removing the thermostat from your car and let it run cooler. Your mileage will drop greatly. If running cool was a good idea, no car would have a thermostat, they would just cool it as much as possible.

While 8's often run slower than 4's at the same speed, it is not always the case. 8's also don't have more overall energy producing capacity. Some do some don't. All else being equal (same total displacement etc.) a four will produce more power than an 8 because it has less internal resistance.

Wheel base has no relation to the size of the tyres. Larger tyres, (all else being equal) will be heavier and have more rolling resistance than smaller tyres. The total rolling resistance of a large heavy car is going to be more than a small car all else being equal.

Generally tyres on a large car last about as long as tyres on a small car. The larger tyres on a big car are supporting more weight and must be larger just to last as long.

I am sure if you stop and think about it, and consider all the factors you will understand. I have read nothing that would indicate a lack of ability. With just a little hint in the right direction, I think you can get to the answer.

[zxcvbob]

Your mistake is assuming that the person you talked to on the phone must know what they were talking about.

Best regards, Bob

[oldal4865]

This question comes up year after year.

Standard efficiency fossil fuel fired furnaces run more efficiently if they are allowed to run continuously. The more on-off cycles, the lower the efficiency.

The old ASHRAE standard gas furnace was specifically limited to 80% efficiency when operated continuously. Frequent on-off cycles could lower efficiencies into the 50-60% range. That means if you run an old ASHRAE standard furnace continuously, about 20% of the heat went up the chimney; if you ran it intermittantly, as much as 50% could go up the chimney.

Read the label on the furnace, e.g. my older gas furnaces all were rated at 125,000 btu input, 100,000 btu output. That is interpreted as "Sufficient 1000 btu/cubic foot gas was piped to the furnace to generate

125,000 btu/hour, but 25,000 went up the chimney when tested while operating continuously in a testing lab."

When compared to losses to the outside incurred if leaving the thermostat at constant setpoint, If you let the house cool while you're gone for the day, then run the furnace hard to heat it up, you will reduce energy losses to the outside during the cool period, and reduce energy losses up the chimmney due to frequent on-off cycles while the furnace is heating the house back to the original set point.

I have never seen the efficiency curves for air conditioners, heat pumps or the High Efficiency, Forced Draft fossil fuel furnaces. I am certain that the pleasant-voiced, soft-spoken, "people-persons" who staff those customer query centers have never seen any efficiency curves of any kind.

Regards Old Al

[John Barry]

As a blanket statement, the "few" and the utility are wrong or misrepresenting the facts. If, after an extended setback period, your system needs to resort to resistive electric heating, that'll really crank up the expense. Otherwise, it's just a matter of being able to recover temp in time, and, for efficiency, the slower the better with the burner on full-time.

If a whole neighborhood (i.e. lots of houses) have had a major setback, and will all be in recovery-mode for some time, that can impact the gas main pressure, or power-distribution system (for heat-pumps.) IOW, if only you do it, it just cuts into their bottom-line. If lots of people, they may have to increase investment.

To make it clear to the physical-science-challenged, consider an analogy: a water-barrel with a tap at the bottom, which is cracked open. Say that you either keep it filled with a make-up water-supply, or, in the morning, change the fill controls such that the barrel is allowed to drop to half-full and maintained there, then filled again per clock setting in the evening. Can't make it much simpler.

HTH, John

[Tom Miller]

From the US Department of Energy:

"A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. This misconception has been dispelled by years of research and numerous studies. The fuel required to reheat a building to a comfortable temperature is roughly equal to the fuel saved as the building drops to the lower temperature. You save fuel between the time that the temperature stabilizes at the lower level and the next time heat is needed. So, the longer your house remains at the lower temperature, the more energy you save.

"Another misconception is that the higher you raise a thermostat, the more heat the furnace will put out, or that the house will warm up faster if the thermostat is raised higher. Furnaces put out the same amount of heat no matter how high the thermostat is set? The variable is how long it must stay on to reach the set temperature.

"In the winter, significant savings can be obtained by manually or automatically reducing your thermostat's temperature setting for as little as four hours per day. These savings can be attributed to a building's heat loss in the winter, which depends greatly on the difference between the inside and outside temperatures. For example, if you set the temperature back on your thermostat for an entire night, your energy savings will be substantial. By turning your thermostat back 10° to 15° for 8 hours, you can save about 5% to 15% a year on your heating bill?a savings of as much as 1% for each degree if the setback period is eight hours long. The percentage of savings from setback is greater for buildings in milder climates than for those in more severe climates. In the summer, you can achieve similar savings by keeping the indoor temperature a bit higher when you're away than you do when you're at home."

[Frogleg]

This is a very interesting topic. At least *part* of the question is probably the non-linear facet of heat-transfer problems. Not to mention delay times and heat-sink questions.

The obvious abuses, turning the heat off when one leaves and cranking the thermostat up to 90 when you return, are easily explained -- the furnace doesn't have separate 90-degree and 72-degree flavors of 'hot'. It's pretty much binary -- on/off at whatever its capacity is until the set point is reached.

The question is whether it takes more energy, in a non-linear world, to maintain 60 degrees for 8 hrs, and then raise the temperature to

72, or warm a 45-degree space to 72. As a posted notice in my old workplace had it, "the real world is not parallel, uniform, or without friction."

[edee em]

When does a car consume more gas: Getting to 70 mph or maintaining 70 mph?

[Murray Peterson]

"edee em" wrote in news:qYfXb.792$ [e-mail]:

Not a question that can be answered, at least stated as it is -- there just isn't enough information. How long to accelerate to that speed? What weight of vehicle? What drag coefficients (including aerodynamic drag coeeficient)? How long do we cruise at 70 mph?

[Chris Lewis]

Yeah, the OP is right. _Most_ times setback will save money, maybe not that much, but it is basic thermodynamics, and the amount of energy required _is_ less. All things being equal (are they ever? ;-) this will save money.

Perhaps the only time you really really have to think twice about setback is with heat pumps.

Everything is fine until the point at which you try to bring the temperature back up. You flip it to 70F, and since the temperature differential between current temp and setpoint is so high, the backup heat kicks in.

So you've saved a little money on "cheap and efficient heat pump" heat, but consume lots of "expensive backup heat" to bring it back up to temperature.

This is _particularly_ nasty with electric strip backup. With gas backup, you _may_ still save something, depending on cost of energy computations.

With heat pumps you need to get set back thermostats that bring the temperature up _gradually_, so the difference between set point and current temperature is at most a few degrees at any given moment.

And note well when you manage the setpoint like this so that only the HP runs, it will take a _long_ time for the temperature to come up.

HPs are cheap to run, but they don't produce a lot of heat fast...

[Joseph Meehan]

A totally different question, but one that might help here.

Not too many years ago Shell Oil sponsored mileage competition. Many colleges competed. At the time most cars were getting 25 mpg or less they managed to squeeze out around 50-70. They used all kinds of tricks that real drivers could not easily use.

One trick was to use a engine designed to run at a single power output. They were optimized to run at only one setting, no throttle. They would start the motor, accelerate to a given speed and turn the motor off. When it slowed to a pre-determined speed, they would repeat the process.

This resulted in higher mileage than a steady speed.

You car will not work this way because of the engine design, but even today a similar idea is used by some/all hybrids. The gas/diesel engine may only run at one speed, driving the car and charging the batteries, or some variation of that idea.

[edee em]

Points taken, but all info I have heard re fuel consumption is that it takes more fuel to get the car up to speed as opposed to keeping it there. I think that is why you see higher mpg at highway than you do in city driving.

I offered this as an analogy. My furnace will consume more fuel to get my house to 72 than it will to keep it there. I don't believe there is any kind of regulator that would inject gas at slower rates when ramping up, so it would be in the "floored" position until the thermostat shut it down. How long would it take to get to 72? I don't know and I don't want to find out, although, that is probably the best solution to this query: try a month of turning off the furnace and a month of turning the temp down. Then you can check the bills at the end of the month. Kind of think the best months would be Jan and Feb (equally cold).

[zxcvbob]

A car is a bad analogy. A furnace doesn't have a throttle. It's either on or off. The thermostat is basically a light switch and not a dimmer.

Bob

edee em wrote:

[Klm]

Jumping in rather late.

My big question is why doesn't some utility company, govt lab, university or consumer protection group do a proper study on this subject and publish the results in graph form that everyone can understand at a glance.

Graphs to illustrate heat loss or savings from different thermostat settings, different time cycles and some other parameters versus time should settle many arguments and satisfy numeric challenged people like me.

The best enegry saver is to properly insulate your home. I had seen a graph that plots heat loss against R values. The slope rises sharply upward and then flattens abruptly after R30. This is a clear illustration that your heat losses drop with increased R value insulation up to R30. Installing more insulation beyond R30 is not worth the extra cost in construction from wider studs and walls, batts, etc. Therefore product advertisers touting extra-extra R values beyond R 30 as the next best thing to sliced bread are just urging to to spend money that you will not recover in energy savings.

[Joseph Meehan]

I don't disagree with this, even if you are mixing apples and oranges.

Actually accelerating slowly to say 30 mph would likely take less fuel (gal per minute) than traveling at 60 mph for the same time. However you would travel much further.

In a car you have three factors. First the efficiency of the engine, and that changes depending on its design, rpm and torque output. Then you have the momentum when accelerating and you have the power needed to maintain the speed at a steady speed

A home is a lot different. Most home heating equipment will work most efficiency when the temperature difference is the greatest, since the heat transfer rates will be best. Straight resistance electric does not change and heat pumps can show just the opposite.

You really can't extrapolate much of anything about fuel saving from a car to a house.

Yes.

Yes.

Let's say ½ hour. The furnace will run full for 30 minutes. Ignoring a few other factors it may keep it there for the next 30 minutes by running only 10 minutes

So far that looks like you are right. However:

Now we turn the thermostat down to 60º and go to bed. It may take the next 1.5 hours to get down to 60 degrees, during which time the furnace does not run at all. so we are now even. however over the next 6.5 hours the furnace may need to run only 7 minutes each hour since it does not need to keep it at 72º. That means I save 39 minutes of running time over all.

I suggest you should also keep track of "heating degree days" for your area as computed by the weather people. That will tell you how even the weather was.

[Murray Peterson]

"edee em" wrote in news:9_oXb.2838$ [e-mail]:

That's only true because of the way people actually drive. For example, let's say that your car requires 50 HP to maintain a cruising speed of 70 MPH. All you need to do is govern your use of the throttle such that your engine always puts out 50 HP, right from a standing start. The car will accelerate up to 70 MPH, and the fuel consumption will be constant over time.

The analogy just doesn't work -- or you need to extend it a bit further to make it work the same as a house furnace. First, we design a vehicle that has an engine with a single output ("floored" whenever it is running). This engine uses a constant amount of fuel every minute that it is running. Now drive this vehicle over a 1 day period in two different ways:

1. Cruise for 24 hours at a constant speed of 70 mph (lets assume that the engine is running continuously to maintain this speed).
2. Cruise for 12 hours at 70 MPH and 12 hours at 50 MPH. To cruise at 50 MPH, the engine will need to be turned off periodically -- otherwise the car would go back up to 70 MPH.

In case 2, the total fuel consumption will be reduced, since the engine won't be running as frequently for 12 hours out of the 24.

I have done this over the last several years -- my fuel bills have been substantially reduced by using setback thermostats on my two furnaces. To really measure the change in a short time period, you need to calculate your fuel bill based on the "heating degree day" data during that billing period.

[Rich Webb]

[snip...snip...]

The only way to answer this question is to collect real data on your own circumstances. Hand-waving generates heat but not illumination.

I collected data here from a few thermistors to measure the effects of setting the heat back to 55 F during the day (while at work) and also at night. Effectively, this secures the heating system during those times. Otherwise, it's set to 68 F.

In the coldest weather we generally get around here, once the thermostat got the rooms up to nominal temperature, the heating system was on approximately 2/3 of the time in the morning (coldest time of day) and

1/3 of the time in the evening. Assume 2/3 as the worst case.

The time to return to a comfortable temperature after the setback was about 1 to 1.5 hrs. Lets work with 1.5 hrs as a worst case.

Given that, if the thermostat setting had been a constant 68 then that

1.5 hours of run time would have been achieved during 2.25 hours of normal cycling.

Therefore, if my heating system is off longer than a couple of hours then there's a net savings.

Since the heater controller is a "bang bang" system (either on or off) then it "works" just as hard whether it's on for 10 minutes or 1 hour. Since the extended running period also means fewer startup/shutdown transients, there are overall fewer stresses on the system.

[EmbErna]

I might agree with that statement if furnaces were capable of running continuously while adding heat into a house at the same rate the heat is leaving that house.

I like the "bucket of water with a small hole in it" theory. Once the level of water (temp of a house) reaches a comfortable level, the faucet (furnace) need only add water (heat) to the bucket (house) at the rate water (heat) is draining out the small hole in the bottom (leaving the house via walls, windows, etc).

So when we start with an empty bucket, we need to add water at a very high rate to get the water level where we want it. Once the level is where we want it, we need to add water at a much slower rate (the rate at which water is leaking out the small hole in the bottom) to maintain the desired water level.

Most natural-gas-fired furnaces I know of heat a home at a constant (higher) rate. They don't allow us to reduce their heat output to a rate consistent with the rate at which heat is leaving our homes. To compensate, these furnaces cycle on and off. Over a long period of time this has the effect of adding heat at a much lower overall "rate". Cycling this type of furnace uses more energy since you must heat up the burner to maximum temperature on every cycle. This inefficiency occurs only once in a furnace that can run continuously and operate at a level where it is simply keeping pace with the rate heat is leaving a house.

Automotive heaters are a good example of this. They allow us to control the blower speed, which lets us control the rate at which we are heating the vehicle cabin. We can better match the rate heat is entering the vehicle cabin to the rate at which heat is leaving the cabin.

mikey.

[Murray Peterson]

Klm wrote in news: [e-mail]:

Nelson, L.W. 1973. "Reducing Fuel Consumption with Night Setback," ASHRAE Journal, Feb., Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers..

Pilati, D.A. 1975. The Energy Conservation Potential of Winter Thermostat Setback and Energy Savings, ORNL-NSF-EP-80, Oak Ridge, TN: Oak Ridge National Laboratory.

Quentzel, D., 1976. "Night-time Thermostat Setback: Fuel Savings in Residential Heating," ASHRAE Journal, March, Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning.

There are too many variables to allow a simple graph to be drawn (at least in the "general case"). A simple graph would show the typical 1% savings for every 1 degree setback over an eight hour period.

I assume that this was the heat loss for an entire house, not just the insulation? R-value doesn't "flatten"; it's a perfectly linear relationship. However, your entire house cannot be insulated perfectly, since things like the wall studs will transmit heat, and extra insulation just won't have any effect on these heat losses.

I believe that your best "bang for the buck" would come from reducing air leaks, and using a setback thermostat.