Does a Lower Thermostat Setting really save Energy?

[JRKH]

Happy New Year everyone.....
I've had this thought bouncing around in my brain (retirement does that sometimes) and since it's kind of scientific in nature, I thought I'd toss it out here just for fun....

We all have heard the idea that turning down your thermostat is suppose to save energy. I will certainly agree that it can early in the heating season and as well as late in the season as the temperature changes - But I'm talking about in the the Dead of winter....When the average temperature difference between inside and outside is say 40 degrees F or more.

At this time, it seems to me that setting the thermostat on 78 would be no different than setting on 70. The house will lose heat at the same rate and the furnace kick off and on at approximately the same intervals. So the only real way to save energy in these months is to better insulate.

Now - the only thing that makes me wonder if I'm thinking right is this....Will a house lose heat faster or slower depending on the temperature differential inside to outside...(I'm sure it does)...but does the principle of "diminishing returns" kick on this wherein once the differential reaches a certain point the rate of heat loss will be fairly constant?

So - am I thinking right? Am I missing something? Am I still better off grabbing a sweater or not???

Peace
James

 

[Michael_M]

This is a good question, where I live, it is averaging about 10?(f) at night and I have a programmable thermostat so at night (while sleeping) I have the temperature set to 65 and about 1 1/2 hours before the wife wakes up it 'should' move up to about 73. It seems that the interior temperature stays at about 69ish.

I am now going to set the thermostat to about 70 at night and hope that it moves up to about 73-74 when she gets up.

 

[Marc]

I agree - Good question.

Any heat transfer experts here?

 

[Miner]

I am not an expert, but did well in Thermodynamics class. The rate of heat transfer is proportional to the difference in temperature between the inside and outside (as well as the thermal conductivity constant of the material and thickness of material), so it does make a difference.

How big of a difference will depend a lot on your insulation, air leakage, time spent at a lower temperature, days of sun and many more items. Here is a brief article at energy.gov.

 

[Marc]

<snip> How big of a difference will depend a lot on your insulation, air leakage, time spent at a lower temperature, days of sun and many more items. Here is a brief article at energy.gov.

Good summary, Miner. It's pretty much as I remember from physics in college years ago.

Having done the major refurb stuff on this house, I am somewhat "literate" with regard to the factors you mentioned. A few years ago I did 20" of insulation in the attic, replaced all windows with triple pane, gas filled windows, same with doors with windows and patio doors. Attached garage so I replaced the back door in it and the garage door its self with insulated steel ones. All external walls are insulated, but the house was built in 1967 so it's not nearly as well as most homes today are, but I can't do anything about wall insulation other than as I mention below I did put 3/4" pine on walls and ceiling. I'm not sure what the R rating for 3/4" pine is but I can tell the difference.

The key for homes is insulation - But - I can tell there is a significant difference if it is very windy outside so I know this house has a lot of gaps where the walls meet the attic. I did have cellulose blown in insulation done in the attic so that has somewhat been attenuated. It used to be that I could put my hand on any electrical outlet and feel cold air coming in if it was windy at all outside. Not so since I had the "blown in" insulation in the attic.

Another big factor is having done the living room and the hallways in 3/4" pine. It's about 15 F outside here and I don't have any heat turned on. I have baseboard heater in each room and each has it's own programmable thermostat. All of them are set to 50 F. The electronics I have as a whole run about 1500 watts (yup - I measure everything with a watt meter) and the master bathroom floor is heated but I don't really know how many watts it uses. In the last few years it has been rare that I have had to use any of the baseboard heaters. My biggest "problem" is in the summer when the AC has to pump out the heat from the electronics. But I put in an 18 SEER 30K BTU high efficiency ductless AC unit a few years back and that dropped my summer electric bills by about 1/2.

I do have a fireplace which is in the middle of the house. It used to be a fire would make a big difference - I would get pretty cold in here. But since I did the refurb I know I'll have to open the patio door because it will quickly warm up to 80 to 85 F inside, so fires these days are for the "fun" of having a fire.

These days this house pretty much stays at about 75 degrees in the living room but gets down to 67 in the back bedroom when it's in the teens or below and is windy outside.

@JRKH - If you're setting the thermostat low at night and the place doesn't warm up quickly in the morning you're probably looking at poor insulation and/or a heating unit (especially heat pumps) which just doesn't have the power/output to make the change quickly. But, the main aspect in the winter is the heat output. Before I did the rehab it would get pretty cold in here, but I could turn on the baseboard heater in any given room and within 15 minutes the air it that room was at the set temperature.

NOTE: My house is an 1100 sq ft single story on a concrete slab so it's pretty cheap to operate anyway.

I'd almost bet you have a heat pump with a small electric coil "booster" for when it goes below the lowest outdoor temperature at which the heat pump will operate.

 

[BradM]

Here is a pretty reasonable explanation:

http://energy.gov/energysaver/articles/thermostats-and-control-systems

A common misconception associated with thermostats is that a furnace works harder than normal to warm the space back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. In fact, as soon as your house drops below its normal temperature, it will lose energy to the surrounding environment more slowly. The lower the interior temperature, the slower the heat loss. So the longer your house remains at the lower temperature, the more energy you save, because your house has lost less energy than it would have at the higher temperature. The same concept applies to raising your thermostat setting in the summer -- a higher interior temperature will slow the flow of heat into your house, saving energy on air conditioning.

 

[Wes Bucey]

FWIW:Building managers project energy usage based on "degree days" (the difference between outside temperature and the temperature to be maintained inside (regardless of heating or cooling.)) In some of the high rises downtown, the spread between sunny side of a building and shady side requires zoned HVAC. I agree with Marc: wind makes a difference in "vacuum" effect of sucking heated/cooled air out of poorly sealed buildings, but can actually blow away heated air from solar radiation on outside building surfaces. We may not be as well insulated as Marc's, but a new roof and thermal windows resulted in a significant drop in HVAC costs over the last two years, mostly by reducing leakage of heated air (snow takes a lot longer time to melt off the roof, adding a further insulating blanket.)

 

[Marc]

Here is a pretty reasonable explanation: <snip>

Thing is, everything in the house has mass. As the house cools down inside the walls and ceiling are the first to transfer heat out. Objects in the house act as heat sinks which release heat as the air in the house and the walls and ceiling cool.

So - When you wake up and turn the temperature up (or the thermostat does so) the air heats first. If you have a powerful heat source this can happen quickly. As it does the air starts to transfer heat to the objects inside the house that lost heat as the house cooled which typically takes a while so the heat source has to work "harder" to get everything inside up to the same temperature.

Back when I used to travel I would set the thermostats at 50 F when I left. I would usually be gone a week to 3 weeks. When I came back everything in the house had equalized to about 50 F so to get it back to "normal" (~70 to 75 F) took quite a while. I had a kerosene heater at the time which I would light when I got home and burn for a few hours because it was very fast at heating the air and back then kerosene was cheap. Depending on the outside temperature it took a few hours before objects inside the house would equalize with the air temperature. The baseboard heaters put out a lot of heat but adding that kerosene heater made a big difference until everything inside absorbed heat to the point where it was near equilibrium with the air.

The opposite was true in the summer, obviously. I would set the AC to 85 F when I left. When I got home it took several hours for the AC to pump out the heat which had built up in the objects in the house. Actually it took longer because the slab would hold and radiate heat until it reached equilibrium, but that part wasn't as noticeable.

As Minor pointed out: "The rate of heat transfer is proportional to the difference in temperature between the inside and outside (as well as the thermal conductivity constant of the material and thickness of material)" which applies mostly to the walls, floor and ceiling.

It is true that the rate of heat transfer decreases as inside and outside temperatures come closer together and vice versa. When they are at equilibrium there is no heat transfer.

A number of years back I came to the conclusion that for short periods of time (<24 hours) it makes the most sense to keep the inside of the house at a set temperature. I definitely could be (and probably are) wrong, but that is my thought.

When I did the AC here the AC people recommended a 20K BTU (~2 tons) unit. I went for a 30K because I remembered the days when I would get home and it would take 2 to 3 hours for the house to "cool down" inside. I wanted an over sized unit so that within an hour the house would be comfortable. That is also why I chose a ductless AC system - No ducts and surrounding insulation to have to cool down (the duct work here is in the attic, but I sealed all the ducts off when I had this unit put in).

Anyway - These are just some of my thoughts. I am sure some will disagree and they may be right. I know a lot of people told me baseboard heat was "inefficient", but taken as a whole - No moving parts, for example - Baseboard heat it quite efficient (especially in contrast to a heat pump).

 

[BradM]

Good points, Marc.

House heating/ cooling is opposite to industrial furnaces and such. In the industrial setting, I am interested more in the heating of the mass in the furnace; not the air surrounding it. Yes the air temperature matters as it transfers the heat. But the monitoring/ recording probe needs to be intimate/close to the product being heated.

With house heating, I am monitoring (and are interested in) the air temperature. I want the room air to be 76?F, and are largely unconcerned whether the entertainment cabinet is 76?F. But most definitely the temperature of the mass in the room (furniture, walls, etc.) have to be heated also, as they absorb the heat.

Like the link suggests one will use less energy bringing the "cold" room up to temperature that leaving it on all day. Because leaving it on all day, it will have lost multiple "packets" of energy to be replaced. Where if it is left off all day then turned on, it only has one "packet" of energy to replace.

I would think if the unit is undersized, it's going to run almost constantly just to keep up during the day. So if it gets turned off during the day, it will never obtain a desired temperature in the evening.

I'm pretty sure "packets" is not a technical, engineering word.

 

[Statistical Steven]

Also remember that due to factors such as insulation, the true delta between the outside and inside temperature is much smaller than theoretical. For example, if you shut off the furnance on a very cold day, the temperature inside the house does not equal the temperature outside. I personally shut off the thermostat before I leave to work in the morning and bring the temperature back up till bed time, then lower it to sleep. Not sure if I am saving a penny or a dollar though