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

Page 1 of 4  
I've been told by quite a few people that turning a heater (or an air conditioner) off for the day uses more energy due to it having to "work harder" to restore the home's temperature.
This made no sense to me, because the load the heater normally carries is basically the rate at which the home cools (heat transfer through the walls, etc.). And, if the temperature differential between inside and outside decreases, the heat transfer is reduced. So, to me it looks like their would be a net saving by turning off the heat due to a decreased net heat loss over time.
So, to get an authoritative answer, I phoned my utility company, and they assured me that turning the heat off for the day is wasteful, but turning it down by a few degrees saves fuel. This made no sense to me, but they must know what they are talking about, so my question is, what am I missing here? (the person at the utility company did not know why, but assured me that that is their official advice).
So, could someone please explain to me why is it less efficient to turn the heat (or AC) off for the day than to leave it running?
Thanks!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

This is an interesting question for debate. I'm not particularly convinced that an appliance has to "work harder" in such a case because, well, parts is parts and machines don't know a warmer room from a colder one. They just do what you tell them to do. And I think *this* is the point where efficiency/fuel consumption question-issue comes in -- when you introduce the human element.
Let's look a gas-fired boiler on a given day when left on continuously heating a room at 70 degrees. It's already done it's job weeks or months ago to jack the temp to 70, so there it sits with it's little pilot light running (not a ton of fuel consumption there), but periodically thru the day, the jets kick on (lots of fuel consumption) for a few minutes only a handful of few times a day to maintain that 70 degrees when the thermostat senses a decrease in room temp.
Now enter a cold, 50-degree room, which is what you'd certainly have in the dead of winter when you came home from work. What's the first thing anyone does? Yup, jack up that thermostat to 70 degrees. Sometimes even 80 in the odd belief that the room will somehow heat faster that way. Let's say it takes that boiler and hour or three to get the room up to 70. What's your net result? A difference between only a few minutes' worth of fuel consumption per day and 1-3 HOURS.
Add this up over a month, and you can see where the inefficiency and waste lies. You *are* actually saving fuel by leaving the boiler running at 70 or just turning it down a notch to 66-68 when you leave the house, but in a way that differs significantly from, say, leaving your lights burning 24/7.
AJS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
AJScott wrote:

...
That would be true IF the boiler lost heat directly to the outside during the down time, and lost it faster than it would otherwise.
Overall it may be more efficient since the boiler will be heating cooler water at the beginning and the heat transfer will be better than trying to heat water that is already warm. The actual outcome will depend on the construction of the equipment and it's location in the home.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Maybe I'm being a complete non-mechanical dolt here (which is always a significant possibility), but it occurs to me that within a closed water-boiler system (like I have in my home), the boiler itself doesn't care whether it's firing up the gas jets to heat hot, lukewarm, or cold water. Instead, the boiler fires up those jets in response to what the thermostat (which is the *real* brains of the outfit) is instructing it to do.
I suppose if you're discussing the impact of heat-transfer rates, location, etc., on how well or efficiently an individual boiler actually heats water of differing degrees already in the system and how well or badly this would affect one's heating bill, heat transfer rates and other HVAC blahblahblah would matter quite a bit. And I'd imagine how well a house is insulated in the first place plays a major role, too, because that has quite a bit to do with how much you need to rely on fired heat. But basically, I think the guy posting the question just wanted to know about efficiency in relation to energy consumption related to whether and/or how long a boiler fired up during a winter day.
AJS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
AJScott wrote:

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 E. Meehan

26 + 6 = 1 It's Irish Math
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I leave mine off. I only use it in the morning so that it's warm when I step out of the shower. Even with it off all day, when I get home from work it's usually somewhere in the 54-64 degree range, even in the middle of the winter. If I heat the house back up to 70, it takes less than 1/2 hour. If I try to keep the house at 70, it cycles on and off a lot. My utiulity bills are fairly low. In my case, I'd say that shutting it off makes sense, as I have gas haet so it heats up quickly (and I think an oversized furnace). You just can't do this if you have a heat pump, it would take too long to heat back up.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Nope.... Basically, since the average temperature (over time) of the room is higher in the case where the furnace is left on, compared to the case where the furnace is turned off, and then on occasionally, then the fuel consumption will be higher in the former case.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.com (no1herenow) wrote:

Huh? It's always been my impression that when a room remains warm at a constant temp, a furnace will have no need whatsoever to kick on and thus consume more energy than one that's simply idling with nothing more than the pilot light burning during the same period of time.
Call me silly, but is there something drastically wrong with this bit of logic?
AJS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
news:nowhon-

warm at a

on and

nothing more

this bit of

According to this theory, once the house is warm, the furnace would never have to run again. This is somewhat doubtful, in my opinion.
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Hmm... the heat loss rate tends to be related to the temperature differential between indoors, and outdoors...
For your statement: "when a room remains warm at a constant temp, a furnace will have no need whatsoever to kick on and thus consume more energy than one that's simply idling with nothing more than the pilot light burning during the same period of time" My question is, what is keeping the room warm during this timeframe? This is not an adiabatic process... there will be heat loss due to convective, conductive and radiational transfer. Once you warm a room up, the furnace will HAVE to run periodically to maintain that temperature. If if didn't, you could shut it off (and leave it off)! So back to the original assertion: whatever method maintains the lowest average temperature differential between indoors and outdoors, will consume the least amount of fuel.... the lower limit being that of maintaining the indoor temperature the same as that ouside... no fuel comsumption at all.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.com (no1herenow) wrote:

I think I touched on that earlier, and I realize the furnace/boiler will kick on *eventually*. It's a matter of how often it needs to kick on and consume fuel. What's keeping the room warm (or not warm) in the first place is the heat already blown or radiated by the heating unit AND proper insulation to hold on to that heat you already have. Leaky or ancient single-pane windows, doors that don't fit properly or lack weatherstripping, inadequate wall insulation, etc. etc. do wonders to slow the natural disssipation of room heat -- which is why insulation is one of the more popular topics in this newsgroup.
This is, at least according to my thinking, where I would imagine shutting down or turning the thermostat way down IN A WELL-INSULATED HOME for 8 hours a day as opposed to not would be pretty close to a wash as far as the heating bill goes.
AJS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The amount of insulation in the building is not relevant to the discussion of savings due to turning the heat off vs. just lowering the thermostat. The amount of insulation only dictates the total fuel consumption (and how low the temperature will get if the furnace is turned off for a fixed period of time).
I am surprised at the length of this thread... It is not due to someone's thinking, but rather elementary thermodynamics that explains why turning off the furnace (if it results in a lower average indoor house temperature), will ultimately use less fuel than setting the thermostat at a level (that results in a higher average indoor temperature).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Another thought comes to mind that might illustrate the "work harder" efficiency question for you, too, but only in a *somewhat* relative way: The difference between a 4-cylinder car engine and an 8-cylinder engine driving, say, a 500-mile trip at 60mph. The issue of efficiency comes into play over the long term, rather than the short term which is what most people seem to center themselves on because of the higher front-end costs -- only to most likely get screwed much worse later on by the back-end costs. Penny wise and pound foolish and all that.
That 8-banger, just by nature of being a larger engine needed to haul around a lot more sheet metal since 8-bangers are only on land yachts, will suck up a larger quantity of fuel than the 4-banger over that 500 miles. Most regular people would see that as highly inefficient. But a 4-banger, having considerably less horsepower, has to work considerably harder -- and consequently runs considerably hotter because of friction from all those moving parts cranking like mad -- than the 8 to maintain that 60mph speed over 500 miles.
Sure, a compact with a 4-banger costs a whole lot less at the car dealer than a land whale with an 8, and it certainly sips gas both in the city and on the highway. But because that smaller 4 has to work harder and produces more heat/friction than the 8 to get the same performance output, your oil fouls faster, there's more wear on the cylinders, etc. etc., which usually translates more to more -- and more costly -- repairs over the same 5, 10, or 20 years. And years ago, the engine blocks of 8-bangers were made of cast iron (might even still be, I dunno), while 4s were and still are aluminum. Having a cast iron block blow a hose and overheat is no big deal; have an aluminum engine do the same and you're almost certainly in store for a hefty little cylinder head/head gasket repair bill. Or worse, a trashed engine block if whoever's driving the car has a tendency to ignore dummy lights for several miles. There's certainly been a lot of advances over the decades to have tiny 4-bangers churn out more horsepower on less fuel consumption to rival, say, the performance of 6-bangers (hence becoming more efficient in a pure-science way), but there's always a trade-off and an associated price to pay for that sort of thing. Nothing comes without a price that bites you in the ass one way or another.
Pretty much all the same principle(s) apply with home appliances. Which is why common wisdom says it's always better to buy the bigger ones instead of the smaller ones.
AJS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
AJScott wrote:
...

But most compacts with smaller engines also are smaller cars so the engine works at about the same % of output. If you look at the hybrids, they gain a lot of their efficiency by keeping the engine working in it's most efficient range (using electric at other times) which in neither idle or full power. This also tends to extend engine life.
In the end it pays to have the right engine for the right job.
BTW I have owned 6 4 cylinder cars. Each has gone over 150,000 miles without an engine wear problem. However I would agree that if someone wants to hotrod, they need to buy a car that can handle it. Appliance sizing is a totally different issue, especially when it comes to heating equipment.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I'm not sure whether you're a gearhead, Joseph, so I may be overmatched (as I tend to be more than occasionally here -- heh-heh) in this reply to you. But your analogy seems to be on a specific level, rather than the relative level I was trying to be on. It's a valid analogy, but not what I was trying to get at. Yes, the 4-banger does indeed work at the same % of output, and that's fine if you're comparing 4-bangers to 4-bangers. But even working at the same % of output, an 8-banger will be working slower and cooler than the 4 to put out the 4's output because, for lack of a better term/description, the 8 has more overall energy-producing capacity than the 4, so it needs to work less. In other words, an 8 will be working slower and cooler to turn out (inserting random number here) 500hp than the 4 to turn out that same 500hp.
And to take it to a larger extreme and given the nature of friction, the tires on a car with a 4 -- by virtue of being smaller than on a much larger car with an 8 -- should, theoretically, wear out faster because the wheelbase on a compact 4 is quite a bit smaller than those on an 8, meaning more rubber is needed to move the 4 the same distance as the 8. The tires on an 8 will cost more at the tire store, but in theory, they'll last longer.
AJS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
AJScott wrote:

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.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Most significant factor in tire life is proper pressure and rotation.
What's with the tyre spelling? British?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Nothing is cut and dry when it comes to engines. It all depends on how well the engine is designed and the quality of the materials to build it.
For starters, not all V8 cars are ocean liners. Corvette's, Camaro's, Mustangs, Vipers ect. are all very small cars that make use of V8 engines. My favorite car that I ever owned was my '78 Chevy Monza notchback with the very rare factory optional 5.0 liter V8. The car was a Chevette sized sub-compact designed around a 4 cylinder and the only added weight with the V8 model came from the engine itself. The V8 version averaged 12 mpg less than the wimpy 4 cylinder even on the highway. It also never ran cooler than the 4 cylinder models...in fact cooling the large V8 was much more difficult and critical in that car. The V8 engines in those cars DID NOT have a longer life span than the 151 Iron Duke 4 bangers (only use "banger" when referring to 4 cylinders) or the 3.2 or 3.8 V6s. The V8 served one purpose only and that was to ROAST any car on the road. One of these days I'll find another one so I'll be able to stop kicking myself for selling mine 10 years ago. fwiw, the longest lived engine out of all of those was probably the 3.8 V6...just a very well designed engine.
Now look at a 4 cylinder engine like Toyota's 22RE. Great power even in the small pick up trucks, great gas mileage and it's rare that one doesn't make it to 250,000 miles.
And to prove that bigger isn't always better...I have a factory stock 650cc 2 cylinder motorcycle that produces well over 100 HP and gets 52 mpg. The engine's are known to go over 130,000 miles before needing a rebuild. Find me a Harley with over 2X the displacement that can come close to any of that.
And I'm not following you on the tire life/wheel base theory...can you explain how a cars wheel base effects tire life? Or did you by chance mean the diameter of the tire? A taller tire will have less rotations per mile but in reality I don't think they last much longer. Look at manufacturers warranties, a certain model tire will have the same mileage warranty no matter what the diameter.
George
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
news:nowhon-

appliances. Which

ones
Furnaces are sized to run nearly full time when the heating demand is the greatest expected. They choose the smallest furnace needed to do this, not the biggest. By your theory, you should buy the largest furnace you can fit in the house. You would end up paying more to heat. The furnace burner will be cycling on/off very frequently, causing wide swings of temperature. It just doesn't work that way.
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Conventional wisdom back when I studied that stuff was that you would save at about 1 degree per hour setback.

-snip-
My gut says that on paper that is probably correct-- but in real life, you want to keep some residual heat available in the room.
I *think* that it is the radiant heating or cooling that makes too much of a temperature swing a bad thing. Our flesh has a high emissivity rating & plaster walls are great little heat sinks.
While we can heat up the air in a room in a few minutes, it will take a lot longer to heat the walls, floor, ceilings & all the objects in it. Until they have all reached the comfort zone we will tend to turn the thermostat up further to compensate for our radiant losses.
Just my thoughts-- relying on a less than perfect memory of casual studies of theories from 30 years ago. [how's that for weasel-words?]
Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.