I was having a conversation with my brother-in-law the other day regarding the high cost of natural gas this winter. His family owns a heating and cooling business (they install residential heaters and air conditioners), and he told me that setting your thermostat to a lower temperature doesn’t really save you any money. His reasoning was this: Whether you set your thermostat to 65 degrees or 75 degrees doesn’t really matter. Once the furnace gets the house to the set temperature, it has to keep kicking in to maintain that temperature. And it kicks in the same amount to maintain 75 degrees as it does to maintain 65 degrees.
So my question is: Is this BS? It has a certain simple logic to it, but it also goes against everything I have heard regarding setting your thermostat lower to save energy. Any heating/cooling experts out there that can offer some info?
I can see it taking about the same amount of electricity, because the heater would have to kick in just about as often. Here, I’m assuming that the “kicking in” takes a lot more power than simply running. However, I’m sure it would take more gas to heat your house to 75 (vs. 65) to start with, and more to maintain that higher temperture as heat leaked through your imperfectly insulated walls.
Yes it’s BS. The furnace has to kick in to replace heat lost to the outside. The rate at which it loses heat is proportional to the difference in temp between the inside and outside. The higher the thermostat is set the higher the temp difference.
Kind of like some people who believe it is cheaper to leave the heating on all the time because 'warming the house up again is more expensive than maintaining a constant temp. Again this is BS.
Having thought about this for nearly five minutes, I would have to say that it takes more effort to raise the temperature in house A to 75° than it does to raise the temp in house B to 65° when both houses start out at the same temperature.
Following that, your assertion that it takes the same amount of effort to push the temperatures back up to their targets would probably be correct if we assume that they cool off at the same rate. I don’t think we can make that assumption. I think the house at 75° will cool down to 65° faster than the 65° house will cool down to 55°. Why? Good question…
Hmmm…
Okay I’ve got it. Wouldn’t there be a greater pressure difference between the inside & outside air (cold vs. hot) for the 75° house? Causing the heat to leak out of the cracks, chimney, etc. faster than it would from the 65° house. That means the thermostat would kick on sooner.
The reference thread hit right on the mark. Savings from lowering the thermostat come from reducing the rate of heat loss by lowering the differential temperature. “Catch-up” has nothing to do with it, because the savings during the “fall-back” period pretty much ballance it out. When the thermostat switches from 68 deg F to 60 deg F, the heater will be off for a time while the house temp drops. This amount of time is about the same as the amount of time the heater will be on when the thermostat changes from 60 deg F to 68 deg F.
Look at it this way–what if you set the thermostat to match the outside temperature? The furnace wouldn’t kick in at all, which would be your maximum energy savings on gas, but a little chilly for people in northern climes.
Every nudge of the thermostat to get it to keep the house warmer uses a little more gas, both for the initial warmup and to maintain heat vs. energy lost to the house.
Your brother’s correct, assuming a near-perfectly insulated house. For the rest of us, a lower thermostat means money saved.
Just to clarify - In most homes the temperature difference drives the heat loses. The pressure differences caused by temperature are so small as to be insignificant. Height differences (stack effect), wind pressure, or something like a fart fan in the bathroom will dump more air outside than any temperature differential. Because all homes leak, any air that moves outside is immediately replaced by air moving inside, and vice-versa. Use candles in the bathroom and save on heating costs!
Hey – Thanks for the responses everyone. Now I have to see if I can weasel a beer or something out of my brother-in-law for filling my head with such lies. Something good, too. Not one of those Bud Lights that he drinks.
Thanks especially for the link, Mjollnir. I tried a search before I posted but I must have screwed it up because nothing came back. I had a feeling this had already been asked and answered.
But if my ingnorance of HVAC principles can help save even one other doper from unnecessarily high heating bills, it will be worth the shame of posting a question that has already been answered.
[sub]Geeze, I can’t belive I missed it the first time around. It was, like, one lousy week ago. [/sub]
According to the superintendant for my apartment complex (1200 units), “you should leave the thrermostat at the same temperature even while you go to work, because it takes more energy than most people think. Not only are you heating/cooling the air, you have to heat/cool the furniture, the clothes in the closet…”
You haven’t even considered what happens if the outside temperature sometimes gets in the 65-75 range. Changes everything then, one option doesn’t have to kick in as much.
Your furnace’s job is to maintain a (nearly) constant temperature at all times. It does this by dumping energy (heat) into the interior of your home. When it’s cold outside (e.g. 70 °F inside and -20 ° F outside), there is more heat flowing from inside to outside, thus it has to dump a lot of energy into your home to maintain the same temperature. When it’s not very cold outside (e.g. when it’s 70 °F inside and 50 °F outside), it has to dump relatively little energy into your home (over the same time period) to maintain the same temperature.
Your furnace is a closed-loop control system operating in “on-off” mode with hysterisis. It adjusts the amount of energy / time by varying it’s duty cycle defined as:
Duty cycle (%) = 100 * Average time on / average time off
The colder the outside temperature is, the higher the duty cycle will be. The higher the duty cycle is, the more your furnace will run. The more your furnace runs, the higher your utility bill will be.
Yes… This is “thermal mass.” The effect of all of your material possessions storing heat and releasing it slowly is that the “fall-back” period takes longer. So, your house may not even cool down to the lower setting on the thermostat.
What this means is that the heater will not even come on while you are away, as opposed to running on and off all day.
But, please ignore my advice (from the earlier thread) if your apartment complex pays for your heating bill. They obviously have a college educated genius working for them as a super.
