Home heating question

[Supercharts]

After receiving the gas bill for February I have a question.
Up here in the NE it was very cold during February. In my case to save on the bill I lowered the temp by 5 degrees during the day when I'm out of the house. Returning home I put the thermostat back to 72, at bed time back 5 degrees, in the morning back to 72 and leaving for work around 8AM back down 5 degrees.
So this gets me thinking...suppose I just left it at 72 all of the time. Would I actually save on heating by not having the furnace go on and off all of the time or is it still worthwhile to lower them temp? Is it cheaper to maintain at a steady temperature or does cutting back on the temp really save on gas and lower my bill? Am I using more gas those times I'm home to get back to 72? Any civil engineers out there? HVAC types?

[Plutarck]

Off the top of my head, I think a major factor would have to be a question of insulation in your house, and what degree it is outside. If the insulation isn't particularly great, and outside it is sufficiently cold, then it would be best to do what you are doing; yet, on the other hand, if you have especially good insulation and it isn't too cold outside, then it would be best to just leave the temperature alone.

Of course it also is a matter of what heater and the square footage of your house is, as the amount of power usage required to change the temperature, and the amount of extra energy required for "warming up", obviously vary greatly.

So just as a guess, I'm guessing the question is not answerable due to insufficient information. But I don't want to stop anyone from trying :)

[arcticpenguin]

Turning the thermostat down while you're away should save on gas and money.


Of course you shouldn't turn it down so far that your water pipes freeze.

[arcticpenguin]

In case you actually wanted some reasons for the conclusion:

Furnace efficiency is the same in both situations, the furnace will just run increase it's duty cycle if it needs to accomplish more, like raising the temperature instead of maintaining it. This means don't try an analogy with automotive engines.

The main factor to consider is heat loss from the house. This is a function of temperature differential between the inside of the house and the outside.

Application of integral calculus will show that the decreased heat loss achieved by turning down the thermostat for part of the time more than offsets the short-term increased duty cycle of the furnace during the warm-up phase.

[patnray]

Last year I installed an electronic thermostat and programmed it to turn down the heat to 55 at 11 PM, raise it to 68 at 5 AM (so the house is warm when we get up), turn it down again at 7:30 AM (before the last person leaves for the day), and up again at 3 PM. There is a different cycle for the weekends. It has made a significant difference in the amount of gas we use.

I first tried one of these about 10 years ago, but found the electronic temperature sensor was not very accurate. The newer ones seem much improved, based on comparisson to a thermometer I taped next to it for a few days.

[jimlintott]

I caught a bit on a handyman show about this the othe day. The guy basically said that you will save energy with a fast response system such as forced air but not with a slow response system such as a low pressure boiler system. I guess if you have radiators with hot water in them then leave the thermostat alone.

My progammable thermostat definately saves.

[arcticpenguin]

Originally posted by jimlintott
I caught a bit on a handyman show about this the othe day. The guy basically said that you will save energy with a fast response system such as forced air but not with a slow response system such as a low pressure boiler system. I guess if you have radiators with hot water in them then leave the thermostat alone.

I disagree. So long as you do not turn the thermostat above the at-home target to compensate, you will save money by turning the thermostat down for part of the time.

Perhaps it would not be comfortable to do so with a slow-response system, but the answer to the question as originally stated, about fuel use and cost, does not change.

[Soapy Sam]

Some other factors to consider:-
1. Convert cost of controller switches, insulation, other energy saving hardware to equivalent Kwhours. Remember the idea is to reduce your power use. How long AT THE LOWER RATE to defray the cost? Is the investment worth it?
2. Close doors. If you have a 100ft2 garage door, don't leave it open.
3.Psychology. Our body temperature sensors don't actually tell us we are cold. Being dead tells us we are cold. We are sensitive to differences in temperature . Dress warmly when indoors. A light sweater can make a big difference.
4. Lighting- For once thermodynamics works in our favour. Don't skimp on lights, or waste cash on expensive "Energy efficient" ones. (Except outdoors or in hard to change locations). Any energy used by a hot tungsten bulb ends up as heat. Plug lamps into your power circuit as heaters. Your house will be warmer and you will probably feel happier. In winter, you can't have too much light. All electrical appliances produce heat.
5. The same is true of washing machines etc. If one is twice as efficient as the other, but 50% more expensive, ask how many years you need to run it before you save the extra cost in energy.
Is it likely to last that long without repair?

Sometimes, cheaper actually is cheaper.

[arcticpenguin]

Originally posted by Soapy Sam
Some other factors to consider:-
4. Lighting- For once thermodynamics works in our favour. Don't skimp on lights, or waste cash on expensive "Energy efficient" ones. (Except outdoors or in hard to change locations). Any energy used by a hot tungsten bulb ends up as heat. Plug lamps into your power circuit as heaters. Your house will be warmer and you will probably feel happier. In winter, you can't have too much light. All electrical appliances produce heat.

In the summer, that heat won't do you any good.

Gas heat is cheaper than electric in most locations.

The price of compact fluorescent lamps has come down quite a bit. You can get a pack of four "60 wattt equivalent" compact fluorescents for ~ US$14. They also last longer than incandescent lamps.

[apocalypse]

The rule of thumb I have read is that there should not be more than 10 degrees between the high and low temperatures on the thermostat. Example, high temp is 72 low temp should be 62.

This article has a few tidbits, though some points I disagree with and can prove by using my utility bills. www.acdoctor.com/enegry_savers/thermostats_energy.htm

The article states "...the longer your house remains at the lower temperature, the more energy you save.". Not necessarily true, and here's my real life example.

I have a Honeywell programmable thermostat and it displays how long the thermostat has run each day, month, and year to date. For a week or so the low was about 20 degrees. I had the thermostat set to 50 degrees, and our normal temp is 67. The next day I kept the low temp at 60 degrees and the high temp 67. The furnace ran nearly 2 hours longer (edit: 1 hour 17 mins. longer) on the 50/67 day than on the 60/67 day.

[patnray]

Personal comfort is not a function of air temperature alone. Our house has several large glass doors and windows and we have many energy saving lights. When it is cold outside the house can feel cold even when the air temperature inside is 70 degrees, because our bodies radiate heat to those large COLD glass surfaces. The solution is to wear sweaters, close the drapes, and use some incandescent bulbs so there is something in the room warmer than our bodies radiating heat to us...

Originally posted by patnray
Personal comfort is not a function of air temperature alone. Our house has several large glass doors and windows and we have many energy saving lights. When it is cold outside the house can feel cold even when the air temperature inside is 70 degrees, because our bodies radiate heat to those large COLD glass surfaces. The solution is to wear sweaters, close the drapes, and use some incandescent bulbs so there is something in the room warmer than our bodies radiating heat to us...
You could place a reallly, really big bong outside the patio doors, so that when you lit up, it would radiate heat inwards.

[Plutarck]

Originally posted by Thorin LungHammer

You could place a reallly, really big bong outside the patio doors, so that when you lit up, it would radiate heat inwards.

Similarly you could have wild, crazy sex in the house to keep it warmed up.

So you see, people who hold orgies at their house aren't perverts or sex freaks - they just want to save money on their home heating bill.

:D

[jj]

If you have a furnace for which the efficiency does not change or goes up if the air/water entering it is cooler, you save money. Gas furnaces are such critters, for air furnaces having roughly the same efficiency, and for water furnaces greater efficiency (but not MUCH greater!!!!) for modern furnaces.

If your house temp drops by 5 to 10 degrees while you're out, you will use less fuel during that time. You WILL Have to warm up the house, but that is only the heat lost in the cooling-off. That heat is less than would have been lost had your house remained at temperatur the whole time.

Think of it this way:

While the house is cooler than normal, you will have less heat loss to the exterior. Any heat required to pull the temp back up is heat that would have been lost anyhow. As long as your system isn't less efficient working at full capacity (which is not usual for gas furnaces or oil furnaces) there's no problem there.

WARNING: If you have a heat pump or something of that sort, it may cost you a lot to allow the temperature to drop, because there is something called a "booster heater" in most heat pumps, it's something that kicks in when the temp is a given amount below the set point, and it is purely electric, as opposed to pump-electric heat. This can cost a small fortune.

For pure electric heat the answer is the same as for gas, except it's even more strongly in favor of allowing temp to drop, because the energy costs more in the Northeast.

Remember, you get at most 40% of the heat from the original fuel by using electricity, unless it's pure hydropower. With a modern gas furnace, you get over 90% of the heat, for really good furnaces like I put in my last house (this one having a good high-efficency furnace to start with) you can get 95%, and use PVC ducting for the inlet and exhaust, too.

[Soapy Sam]

AP said- "In the summer, that heat won't do you any good.

Gas heat is cheaper than electric in most locations.

The price of compact fluorescent lamps has come down quite a bit. You can get a pack of four "60 wattt equivalent" compact fluorescents for ~ US$14. They also last longer than incandescent lamps

-I can tell you don't get to Scotland much!:D

[davefoc]

Supercharts,
I think your question has been answered correctly by several of the posters. One way of looking at it is to think of the heat in your house as a fluid. The rate of flow of that heat out of your house is a function of the thermal resistance (insulation) between the interior and exterior and the temperature difference (the thermal pressure if you will) between the interior and the exterior.

So you can reduce the rate that heat flows out of your house by increasing the insulation or reducing the temperature difference between the interior and the exterior. Lower interior temperature means smaller temperature difference between interior and exterior, so less heat flow from interior to exterior.

To see that the total energy required to heat your home is less if the temperature is reduced for a period of time you could think of your home as a giant tank of heat. Of course more total heat will flow out of the house if it flows out at the same high rate continuously than if it flows out at a rate that alternates between a high rate and a low rate.

The above conclusion is absolute if your furnace always runs at the same efficiency. This is the case for furnaces that are either on or off under thermostat control. Efficiency differences for other furnaces (I would think) are probably insignificant except for heat pump type furnaces as noted by jj above.

Apocalypse related a personal experiment that disagreed with the above conclusions. I think it is possible that occasionally something like what Apocalypse described might happen, but if the experiment were done many times or care was taken to see that there were no significant differences between the days in question including exterior temperature and wind conditions it would require a violation of well established physics principals for his experiment to produce similar results. That was in essence what his link said, I think.

One small point, you might consider: sweaters. A house temperature of 65 degrees or so is perfectly comfortable when you're wearing a sweater or other warm pullover and the cost savings from running a house at 65 degrees instead of 72 is significant. We live in Southern California and don't run our heater at all. We just put on a sweater when it cools down.

edited to make it clearer (I Hope) and to add a response to apocalypse.