What heating model is more efficient?

Factual Questions
1

I’ve heard this canard about heating efficiency a number of times, and it still quacks me up. Here’s the model:

Two homes, identical in every respect. The outside temperatures have the same highs and lows. The home that leaves the thermostat on “a certain setting” will use less energy than the one who turns the thermostat up (for summer) or down (for winter) during the day, because more energy is consumed in “catching up” than in maintaining the temperature.

I’ve never heard of anyone ever subjecting this to close scrutiny to determine whether or not this is accurate.

Any ideas???

2

I’ve heard of daily cycles, turning down the heat at night.
Never annual. That seems too extreme to be right.

3

Mjollnir, I’m not sure I follow you. Let me give an example; tell me if this matches what you mean:

Two houses, next-door to each other, are identical in all respects. During the winter, the occupants of house #1 leave the thermostat at 72 degrees all the time. The occupants of house #2, however, turn the thermostat down to 62 degrees at night, and back up to 72 degrees during the day. **House #2 uses more energy, because it needs to “catch up” **

Is this a fair restatement?

If so, I don’t see how it can be true. Your furnace basically makes up for the heat lost through the outside of your house. The hotter the house, the quicker heat flows through the walls and ceiling, and thus the more energy is lost. Therefore, house #1 must use more energy.

4

Well almost.

I would think that in winter that people would turn the thermostat down during the day, though, since it is warmer, and (for the most part) no one’s at home–therefore no need to maintain the higher temps (and in the summer, having a higher daytime setting, for the same reason).

But it still is the same point: A constant setting uses less energy than a cyclical setting. That is the old adage that I’ve heard, but have always been skeptical of.

5

Let’s do a thought experiment here - take Winter as an example.

Think about why you need to run the heater at all in the Winter. The reason is due to air inleakage, air outleakage, and radiation and convection heat losses from the house “skin”, so to speak. If the house construction and environmental conditions are equal, these radiation and convection losses will be driven primarily by the temperature differential between the house and the environment. Thus, if the house is maintained at a lower overall temperature, there is a lower heat transfer rate at play.

6

Una, this is clear.
What about the OP? Don’t we have to assume, to make the field even, at least, than in both cases the ‘average’ T is the same? I mean, if the night is only 8h and the stat is set to 64degrees, and during the day in both houses it’s set to 72degrees, the average daily T in house#1 is higher? If the night is 12h, the advantage increases. But if the night is only 4h, perhaps it would not make sense at all to change the setting.
Is there a way to find a mimimum length/T, at which the game will pay/cost the candles? Perhaps, there are nomograms?

Peace

7

The old adage is bunk. There is a great energy savings by lowering the T-stat when the heat is not needed (during the day, when the house is empty or at night, when the occupants are bundled up in bed).

All the furnace does is burn fuel (or electricity, for the most part, in the South) to make up for heat losses. These losses are smaller with a lower differential temperature (T[sub]indoor[/sub]-T[sub]outdoor[/sub]). When the T-stat is set lower, there is less heat loss, and less to make up. It does not matter if the setpoint is lowered for 1 hour or 23- there is no cost other than the aggravation of adjusting the T-stat (BTW, programmable T-stats are pretty cheap, and can pay for themselves very quickly if you use them right).

To further kill this canard, during the “catch-up” time, a fuel-fired (gas, oil) furnace is actually (slightly) more efficient, because there is a greater differential temperature in the furnace’s heat exchanger, allowing the air to extract more heat from the hot gases before they are rejected to the flue. This does not apply to electric heaters- 100% of the electrical energy goes into heating the space.

[credentials] I am a mechanical engineer, who does design and analysis of heating, ventilation, and air-conditioning. [/credentials]

8

The “catching up” theory of energy loss is especially suspect. If your thermostat is turned down, that means that the heater is turned on (a) less frequently, or (b) for less time. How can you lose energy by doing that?
jb

9

I agree with TXLonghorn. Here’s an analogy: say you have a big fish tank with a tiny hole near the bottom. The water is pushed out the hole by water pressure, so the more water there is in the tank, the faster the leakage. Now, which requires more water - to keep it filled by adding a tablespoon of water every hour, or to let the water level go down to 50% before filling it up with a bucket? Does it take more water to “catch up”? No! If you let the water level go down, the rate of leakage goes down, so you end up using more water to keep it topped off.

10

Welcome, TXLonghorn. From now on, you field the HVAC questions, and I’ll field the power plant ones. :wink:

11

Hey! I was wondering about this while typing my previous post. However, I also wondered the following: Do the exhaust gases leaving the flue come out hotter during the “catch-up” time, since the furnace is on longer and the flue temperature is greater? If so, what is the magnitude of the additional heat loss (if any) compared to efficiency gain in the heat exchanger?

This is getting a little away from the OP, since I’m certain that any effect is small compared to the overall energy savings of turning the thermostat down. I’m just curious, that’s all.

12

Sorry, guys for being thick. Thermodynamics was a nightmare of my youth. Actually, I never knew it, and now I pay the price (literally, with my heating bill). I’d like a simple explanation. Let me give you a simple example. Let us say, the T-stat is set to 72F, it’s 20F outside and the heater has to work 4 minutes once every hour to keep the house at 72F. In house#2 the T-stat is set at 66F for 4 hours at night. So, during this time, the heater turns on only once for 3 min. We saved 13 minutes worth of fuel (1st hour – 4 min, 2nd – 1, 3rd – 4, 4th – 4). Then the T has to be brought back to 72F. To do so, the heater has to work for 15 min. All the saving are gone, and then some. Apparently, if the T is kept at 66F longer, the savings will pay for themselves. Even if my made up cycles are wrong, the savings will not amount to much if the T is lowered for short periods. So I wonder: are there nomograms or something to estimate the benefits?

Peace

13

Well, golly, people, is there a plain vanilla answer for the folks at home? Should I turn my thermostat down at night or not?

14

Duck Duck Goose: As TXLonghorn says, “The old adage is bunk.” So the plain vanilla answer is: Turn your thermostat down (in the winter, natch).

peace: Yes, your made-up cycles are wrong. The furnace in house #2 (the one that turns the thermostat down) will always work less time than the furnace in house #1.

The energy that the furnace provides is equal to the heat lost from the house, and the heat lost from the house is roughly proportional to the temperature differential (T[sub]indoor[/sub] - T[sub]outdoor[/sub]). Therefore, you can estimate your energy savings by calculating the average difference between indoor and outdoor temperatures. In your example:

House#1:
T[sub]indoor[/sub] = 72F
T[sub]outdoor[/sub] = 20F
T[sub]indoor[/sub] - T[sub]outdoor[/sub] = 52F

House#2:
T[sub]indoor[/sub] = (72F for 20 hours) and (66F for 4 hours); average = (7220 + 664)/24 = 71F
T[sub]outdoor[/sub] = 20F
T[sub]indoor[/sub] - T[sub]outdoor[/sub] = 51F

So your energy savings are roughly 1/52, or around 2%. As you say, short periods of time don’t save you a whole lot.

Of course, this simplified example ignores things like cool-down time, warm-up time, changes in outdoor temperature, differences in indoor temperature from place to place, furnace efficiency, and so forth. However, as a rough estimation, it seems valid to me. I will defer to TXLonghorn if he disagrees, though.

15

zut-
In a modern furnace, the heat exchanger is warmed up before the main fan is turned on, and the flame is at (basically) the same temp whether the furnace runs for 1 min or 1 hour. While warming the indoor air from (in this example) 66F to 72F, there is a slight improvement in the amount of heat transfer because of the slight increase in differential temperature (practically negligible). But, less fuel is wasted in the process of warming the heat exchanger once to run for a long while as opposed to warming the heat exhanger repeatedly when the furnace runs in short on-off cycles.

peace
If your furnace is only running 4min/hour to keep your house at 72F when it is 20F outside, you have a pretty well sealed, well insulated house. That has as much (or more) impact on the overall efficiency of the heating & cooling system than varying the T-stat a few degrees.

As far as estimating the savings: to simplify things, we will assume that all of the losses are due to conduction through the walls & convection at the outer surface (ignore radiation cooling- it is not that significant overall, but varies with the fourth power of the temperature difference, greatly complicating this simple estimate). All other things being equal, the heat loss will vary (about) directly with the temperature difference. In this example (20F outside, 66F or 72F inside), we have a differential temp of 46F vs. 52F. For a 66F setpoint, there will be about 12% less heat loss, so 12% less energy is used making up for this heat loss. The energy used to heat the house up to 72F from 66F will be less than the energy to maintain 72F, if the T-stat is left at 66F for the house to actually cool to 66F by heat losses.

Disclaimer: This is extremely simplified. The actual heat loss at the outer surface is not directly linear & differential equations are involved. But this is close enough for General Questions.

DDG-
If you like to bundle up at night and the house is occupied during the day, turn the T-stat down at night. Personally, I don’t like to be all bundled up when I sleep, and the house is empty during the day, so I have my programmable T-stat set to 60F at 8AM-4:30PM (not there), 70F at 4:30PM-8:30PM (warmed up when I get home, still warmly dressed), 74F at 8:30PM-12AM (winding down & going to bed, less warmly dressed), and 70F at 12A-8A (safely tucked in bed, and warm enough to get out of bed in the morning). This is done with a $60 programmable T-stat. Compared time when I had a normal T-stat and didn’t vary the setting, it saves about $15 a month in the winter. The big savings come in the summer here in Texas. Using a similar (but reversed) varience in the settings for summer saves a metric shi_load of money in electric bills (the T-stat pays for itself in a month if you don’t vary the settings, or slightly longer if you occasionally forget to vary the settings on your way out of the house).

16

peace & zut
Those averages zut posted are right, but does peace really only sleep 4 hours a night?

You save about 12% of the energy used while the setpoint is lowered * 1/6 of the time (4h/24h)= 2%

17

TXL, all my examples were pulled from thin air, of course. And for two percent saving I wouldn’t lift my finger. I do not live in a mansion (unfortunately), so 2% won’t make me rich. I just wondered how an average not very smart homeowner, like me, could estimate potential savings by plugging readily available numbers in ready-made formulas (you know, like they use for air conditioners). It just could be simpler: you already know your use. Just add the number of hours of a given T, average outside T, and voila’! The heat loss, convection, radiation, opened doors are already factored in.

Peace

18

TXLonghorn, my home is heated by a heat pump (electric). During the cold winter, I turn my thermstat down to 71 during the day and up to 73 - 76 when I’m home.

Is this wise considering my heat source?

19

I’ve got to second TXL on the programmable thermostats. They’re wonderful! No more getting out of bed to a cold house.

I’m just surprised at how warm you all keep your houses. I guess I’m just a freak–I rarely set mine above 68, that’s what sweaters are for.

Peace–there are a whole bunch of online calculators that will give you estimates of savings associated with minor changes in your heating habits:

http://homeenergysaver.lbl.gov/

is a pretty good one–lets you put in a bunch of parameters about your house, but gives you default values to use if you don’t know the answers (state averages). It tackles energy savings from a bunch of places-not just heating but if you’re looking to save money on utilities you’d probably be ineterested anyway.

20

ASD-
I live in Texas,and really hate the cold (unless I travel to it to ski or snowboard). My blood starts to thicken when the temp drops below 60F.
Thanks for the link… it might come in handy in educating clients.

peace-
The 2% figure was for dropping the setpoint 6 degrees F for 4 hours per day. Drop it for 10 degrees for 8 hours (while you are at work, for example), and this becomes 8%. It won’t make you rich, but

BTW, Check the site that ASD listed. It’ll get you in the ball park for your expected energy savings with changes in equipment and behavior.

JuanitaTech
[disclaimer] YMMV. I don’t know the specifics of your situation, and I am speaking in general terms.[/disclaimer]

In a relatively mild climate, heat pumps are great. They use about 1/3 of the power of regular old electrical resistance heaters, but get less efficient as the outside temp goes down. A lot of the general statements on heating efficiency go out the window with heat pumps, because the efficiencies are very non-linear in nature. But, in general, there is no need to keep your house at 71F when you are not there.