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?
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
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.
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.
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?]
For the most part, you have it correct. However different systems have different problems.
For an electric resistance heat system without any time of day rate changes, you will save money turning it off during any part of the day (assuming you don't suffer damage to your home or contents due to the temperature changes). However you may want to upsize the total heating capacity to make sure you are able to bring the temperature up quickly.
For combustion type systems, there can be some additional issues because of possible efficiency issues, but in general I would have to guess they are a wash, some plus some minus.
On the other side we have the A/C. It also acts the same with one difference. If you decide to open the windows during the evening/night and let the house cool off, then close up and turn on the air in the morning as it get hotter, it is likely to cost you more, unless you live in a very dry area because it takes a great deal of energy to remove all the moisture that came in with the cool most night air.
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.
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.
The temperature of anything (in this case the interior of your home) is a measure of the energy contained in that body. Heat flows from an object of higher temperature (and energy) to one of lower temperature. The rate at which heat flows is a function of the temperature difference of those two objects. As your home cools it looses lease heat per hour than it did when it was warmer because it's temperature is closer to the outside temperature. When equilibrium is reached (i.e. the interior and exterior is at the same temperature there is no heat loss because there is no temperature difference.)
When you return to a cool house the process of heating it requires that you deliver energy to it. How much energy? The amount that was lost when it cooled.
Since less heat was lost because the house was cooler you have saved the amount of energy that would have been required to keep the house at a higher temperature.
The kicker here is that if you want to warm your house rapidly upon returning there will undoubtedly be inefficiencies because not all materials increase in temperature at the same rate (they have differing "specific heats" or thermal capacities and rates of thermal transfer.) Much more energy is needed to raise the temperature of a cubic foot of water than a cubic foot of air. A cubic foot of wood will be in between those two extremes.
I believe in allowing my home to drop to as low as 50 F when I am away (this usually takes several days though) because I am very well insulated. I heat a little over 6,000 sq. ft and heat water all year 'round with less than 1,000 gallons of oil even though the outside temperatures this winter have hovered around -10 F.
If the temperature falls below 50 F my experience is that some damage will be observed. Paint may well separate from walls and ceilings, household plants may be damaged, etc.
I have always used " if you can vary the temp by more than 10 degrees (F) for 8 hours or more then you can save ~5%. Could so many manufactures be wrong in producing set back thermostats? I use a set back stat and it does seem to help. I like it cooler when I sleep. I have connected my stat to a simple x-10 modem and when I land at the airport I call home and turn on the water heater and a/c. I use a 10 degree off set for cooling and 15 for heating when I am traveling. Temp rarely goes above 80 or below 60. When it does the unit turns on and returns the home to the set point. Obviously exact conditions vary and your results may be different.
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.
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.
And another fly in the ointment could be whether the heating system is water or forced air, I think. Not perhaps actually true, but that's what I'm inclined to think. IMO water heat does a faster and better job of warming a cold room -- and sustaining that warmth level longer -- than forced air.
But then again, people with forced-air systems don't end up paying what I did in copper pipe and some built-in redundancies that make my boiler area setup looking like something straight out of the engine room of a WW2 U-boat submarine .
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.
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.
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?
Here is the old thoughts on leaving the heat / cooling on for different lengths of time. This will be for Gas / propane / electric strip hot air furnaces and freon type cooling systems. The gas / oil boilers / hot water systems may not apply for I don't know.
Leave off for less than 4 hours -- It cost you money to do so. Leave off for 4 hours -- You break even and does not save anything. Leave off for 5 to 8 hours -- There is some savings but small but you do save something. Leave off for 8 or 8+ hours -- There is a good savings. Leave off all together 24+ hours -- saves the hell out of the fuel.
I really don't know about the hot water systems as to this thinking.
No. The process is analogous to keeping a bucket (house) filled with water (heat) at some level with a faucet (furnace) when there's a slow leak at the bottom (heat leaving the house). If you don't care about the level for some time, then it ALWAYS uses less water to let the bucket drain, and refill later, compared to keeping it filled. The confusion comes because you typically trade a lot of short heating cycles for one long one.
Some people won't believe it, because it takes differential equations to really understand this quantitatively and prove it, which is beyond most.
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.
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