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Home heating strategy - variable temperatures


UberXY

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Is it more efficient to let night time temps drop 5 degrees at night for 8 hours, and then re-heat the house up 5 degrees in the morning, or just keep the temp constant? Furnace is fairly new, gas, forced air.

 

I realize there are a lot of variables here...r factor of home, size of home, type of fuel, efficiency of furnace...so I am looking for a general answer if there is one. I know this could be measured by money spent, but that's been difficult this winter what with the temps in single digits one week and in the 60s the next.

 

Thanks

Edited by UberXY
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I like to sleep "cool" so this is a no-brainer for me. 69°daytime, cut back to 62° between 11:00 p.m. and 7:00 a.m.

 

Not applicable to the OP's question, but I have a dual-fuel heating system (heat pump + gas furnace), and I recently "discovered" that if I set the thermostat to "emergency heat" the heat pump doesn't run at all, just the gas furnace. Since natural gas prices have fallen off a cliff in Georgia, I have been running it in "emergency heat" mode since the start of the year. Gas cost about doubled (from ~$40/month to $80/month), but electricity costs barely budged during two very cold (for Atlanta) months.

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Is it more efficient to let night time temps drop 5 degrees at night for 8 hours, and then re-heat the house up 5 degrees in the morning...

 

Yes.

Heat loss from the house to outside air is directly proportional to the temperature difference between inside and outside. The more heat you loose, the more gas you need to burn.

 

Mikko

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Joe Frickin' Friday
Is it more efficient to let night time temps drop 5 degrees at night for 8 hours, and then re-heat the house up 5 degrees in the morning...

 

Yes.

Heat loss from the house to outside air is directly proportional to the temperature difference between inside and outside. The more heat you loose, the more gas you need to burn.

 

Mikko

 

This, absolutely:

 

Q = A/R * (Thouse - Tatmosphere)

  • where:
    Q = heat loss rate
    R = overall average thermal resistance of the exterior walls of your house (including windows, doors, etc.)
    A = exterior surface area of your house
    Thouse = temperature inside your house
    Tatmosphere = temperature of the great outdoors

Your heating bill is directly proportional to Q. Want to get Q down? You need Thouse to go down; the closer to Tatmosphere, the better. You can save some money comfortably by lowering Thouse while you're away or asleep.

 

So imagine the great outdoors is 20 degrees, and your living room is a constant 70 degrees, and your monthly heating bill is $250:

 

  • If you lower the interior of your house to a steady 65 degrees around the clock, your heating bill should drop to from $250 to $225.
     
  • If you lower the interior to a steady 65 degrees for just 8 hours a night, your heating bill should drop from $250 to $241.67.

Note that if you turn it down right when you go to bed, it's going to take a few hours for your house to cool down to 65. So your actual savings will be significantly less than the simple math above predicts.

 

You can augment your savings by dialing the thermostat down to 60 during the day while you're at work.

 

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While you will save something, 5 degrees won't make much difference in your bill. Shut it off at night. Leave it off until you get home from work. (well, okay, set it at 50 or so). Leave your coat on in the house. Buy a down comforter.

 

 

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While you will save something, 5 degrees won't make much difference in your bill. Shut it off at night. Leave it off until you get home from work. (well, okay, set it at 50 or so). Leave your coat on in the house. Buy a down comforter.

 

 

The amount of saving varies with the difference between the outdoor and indoor temperature. If the indoor/outdoor delta is negligible, then there will be negligible savings. However, when there is a large delta, the savings could be significant.

 

Quote from Eastern Propane, "You can cut annual heating bills by as much as 10 percent per year by turning your thermostat back 10-15 percent for eight hours per day." See the following link:

http://easternpropane.com/EnergySavingTips.htm

 

There will be about an 80 degree difference in the indoor/outdoor temperature here tonight in Minneapolis. (68 inside, -12 outside)

 

If I shut my heat off for that long, the pipes would freeze! :grin:

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Is it more efficient to let night time temps drop 5 degrees at night for 8 hours, and then re-heat the house up 5 degrees in the morning...

 

Yes.

Heat loss from the house to outside air is directly proportional to the temperature difference between inside and outside. The more heat you loose, the more gas you need to burn.

 

Mikko

 

This, absolutely:

 

Q = A/R * (Thouse - Tatmosphere)

  • where:
    Q = heat loss rate
    R = overall average thermal resistance of the exterior walls of your house (including windows, doors, etc.)
    A = exterior surface area of your house
    Thouse = temperature inside your house
    Tatmosphere = temperature of the great outdoors

Your heating bill is directly proportional to Q. Want to get Q down? You need Thouse to go down; the closer to Tatmosphere, the better. You can save some money comfortably by lowering Thouse while you're away or asleep.

 

So imagine the great outdoors is 20 degrees, and your living room is a constant 70 degrees, and your monthly heating bill is $250:

 

  • If you lower the interior of your house to a steady 65 degrees around the clock, your heating bill should drop to from $250 to $225.
     
  • If you lower the interior to a steady 65 degrees for just 8 hours a night, your heating bill should drop from $250 to $241.67.

Note that if you turn it down right when you go to bed, it's going to take a few hours for your house to cool down to 65. So your actual savings will be significantly less than the simple math above predicts.

 

You can augment your savings by dialing the thermostat down to 60 during the day while you're at work.

 

Okay, this makes sense on a pretty basic level. But, how about the thermal mass of the home's interior and its furnishings? Isn't it more complicated than simply heating or cooling the volume of air? You're dealing with heat sinks, solar gain, etc., and it seems to me that these things would complicate the equation considerably. How do you account for these factors in your calculations?

 

If these are stupid questions, please forgive my non-scientific mind.

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Joe Frickin' Friday
Okay, this makes sense on a pretty basic level. But, how about the thermal mass of the home's interior and its furnishings? Isn't it more complicated than simply heating or cooling the volume of air? You're dealing with heat sinks, solar gain, etc., and it seems to me that these things would complicate the equation considerably. How do you account for these factors in your calculations?

 

If these are stupid questions, please forgive my non-scientific mind.

 

Increasing the thermal mass of/in your home slows the rate at which the temperature drops when you lower the thermostat, delaying the reduction in the heat loss rate. So the effect is like reducing the amount of time for which your thermostat is lowered. If you filled your home with a few hundred tons of tungsten ore at room temperature, you could conceivably shut your furnace completely off when you go to bed, and wake up the next morning to find that the house has only dropped from 70.0 degrees to 69.9 degrees; your house, kept warm by all the thermal "inertia" of the tungsten, has been pissing away heat to the atmosphere at a steady/high rate all night, despite having shut the thermostat off. You've saved very little money, not nearly as much as if you didn't have all that thermal mass.

 

Even if you don't have large quantities of tungsten ore lying around, your house has plenty of thermal mass already. For starters, air weighs about 0.075 pounds per cubic foot, so in a 10'x15'x8' bedroom, the air alone weighs 90 pounds; for three kid rooms and one master bedroom, you've got maybe 450 pounds of air mass, and we haven't even gotten to the other rooms in the house. Add in all the drywall, carpet, furniture, empty beer cans, and dead roaches, and you've probably got thousands of pounds of thermal mass in your house at roughly room temperature, slowing the rate at which the temperature drops when you lower the thermostat.

 

Solar gain = free heat; it's the equivalent of increasing Tatmosphere a bit, i.e. your furnace won't have to work as hard to keep the house warm. This is true regardless of what temperature the thermostat is set to.

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Okay, this makes sense on a pretty basic level. But, how about the thermal mass of the home's interior and its furnishings? Isn't it more complicated than simply heating or cooling the volume of air? You're dealing with heat sinks, solar gain, etc., and it seems to me that these things would complicate the equation considerably. How do you account for these factors in your calculations?

 

If these are stupid questions, please forgive my non-scientific mind.

 

Increasing the thermal mass of/in your home slows the rate at which the temperature drops when you lower the thermostat, delaying the reduction in the heat loss rate. So the effect is like reducing the amount of time for which your thermostat is lowered. If you filled your home with a few hundred tons of tungsten ore at room temperature, you could conceivably shut your furnace completely off when you go to bed, and wake up the next morning to find that the house has only dropped from 70.0 degrees to 69.9 degrees; your house, kept warm by all the thermal "inertia" of the tungsten, has been pissing away heat to the atmosphere at a steady/high rate all night, despite having shut the thermostat off. You've saved very little money, not nearly as much as if you didn't have all that thermal mass.

 

Even if you don't have large quantities of tungsten ore lying around, your house has plenty of thermal mass already. For starters, air weighs about 0.075 pounds per cubic foot, so in a 10'x15'x8' bedroom, the air alone weighs 90 pounds; for three kid rooms and one master bedroom, you've got maybe 450 pounds of air mass, and we haven't even gotten to the other rooms in the house. Add in all the drywall, carpet, furniture, empty beer cans, and dead roaches, and you've probably got thousands of pounds of thermal mass in your house at roughly room temperature, slowing the rate at which the temperature drops when you lower the thermostat.

 

Solar gain = free heat; it's the equivalent of increasing Tatmosphere a bit, i.e. your furnace won't have to work as hard to keep the house warm. This is true regardless of what temperature the thermostat is set to.

 

But it will take a considerable amount of energy to raise the temperature of that thermal mass back to its nominal temperature.

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Joe Frickin' Friday
But it will take a considerable amount of energy to raise the temperature of that thermal mass back to its nominal temperature.

 

It will require just a smidge less than would have been needed to keep it all exactly at 70.0 all night.

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While you will save something, 5 degrees won't make much difference in your bill. Shut it off at night. Leave it off until you get home from work. (well, okay, set it at 50 or so). Leave your coat on in the house. Buy a down comforter.

 

 

The amount of saving varies with the difference between the outdoor and indoor temperature. If the indoor/outdoor delta is negligible, then there will be negligible savings. However, when there is a large delta, the savings could be significant.

 

Quote from Eastern Propane, "You can cut annual heating bills by as much as 10 percent per year by turning your thermostat back 10-15 percent for eight hours per day." See the following link:

http://easternpropane.com/EnergySavingTips.htm

 

There will be about an 80 degree difference in the indoor/outdoor temperature here tonight in Minneapolis. (68 inside, -12 outside)

 

If I shut my heat off for that long, the pipes would freeze! :grin:

 

What's that mean? "set your thermostat back 10 - 15%"? Temperature scales don't work that way. It's not valid to multiply 70 degrees by 10%.

Edited by elkroeger
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Since I'm a little on the slow side, I rely on technology to help me. I've previously posted about our experiences with the Nest thermostat, which takes much of the thinking out of the equation. During heating season, I generally keep the house around 68 degrees during the time when we're at home and moving around the house. Our nighttime and "away" temperatures are 62 degrees.

 

Combining the Nest, a tankless water heater, a switch to LED bulbs, and a bit of diligence, we've cut our utility usage by about 30% over the past three years. Obviously, it's not possible to do a direct comparison each year since the weather varies considerably here from winter to winter, but it's still a discernible savings. When we're away for a longer period of time, I'll drop the temperature down to 50 degrees, and during those periods of time the furnace barely kicks on (though this year, with a number of near- or below-zero days, it undoubtedly would have some work to do).

 

If you haven't looked into the Nest or similar options, I'd seriously urge you to do so. Even though I was a reasonably diligent user of our old programmable Honeywell thermostat, the Nest's added level of control, accessibility through the web, and its learning abilities have made a serious dent in our energy expenditures.

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What's that mean? "set your thermostat back 10 - 15%"? Temperature scales don't work that way. It's not valid to multiply 70 degrees by 10%.

 

i'm gonna go out on a limb and say setting your 70 degree thermostat back 10% means to leave it at 63 degrees while you're out of the house.

 

 

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Thanks for the advice and tips. Getting the furnace to switch from gas to emergency electric is an idea.

 

Generally, someone is in my house all but an hour or two a day, so, intriguing as it is, the Nest wouldn't do much for us.

 

I brought this up because for the first time in 13 years, we ran out of gas. No ones fault really; thermostat always on 67, fuel company was on a schedule that worked in the past. Truck showed up within a hour of calling on a snowy morning...ten tries to get up my drive. 400 gallons = $1400.

Edited by UberXY
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Killer did a day by day analysis of thermostat settings, cost,

efficiency etc when he moved to Torrey.

 

No luck with search.

 

Anyone remember it?

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But it will take a considerable amount of energy to raise the temperature of that thermal mass back to its nominal temperature.

 

It will require just a smidge less than would have been needed to keep it all exactly at 70.0 all night.

 

I should have been more specific with my quote:

 

If you filled your home with a few hundred tons of tungsten ore at room temperature, you could conceivably shut your furnace completely off when you go to bed, and wake up the next morning to find that the house has only dropped from 70.0 degrees to 69.9 degrees; your house, kept warm by all the thermal "inertia" of the tungsten, has been pissing away heat to the atmosphere at a steady/high rate all night, despite having shut the thermostat off.

 

But it will take a considerable amount of energy to raise the temperature of that thermal mass back to its nominal temperature.

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Interesting thread.....

 

2.5 years ago, I swapped out my entire HVAC system from the Goodman builder-grade crap that came with the house to Carrier High Efficiency for both the furnace (78% Goodman vs. 95% Carrier) and AC (10 SEER Goodman to 18 SEER Carrier). The Carriers came with a "smart thermostat" that allows full programming, although no WiFi / Net access like Nest.

 

The results have been pretty spectacular. Not only is the house ALWAYS more comfortable (since the Carriers have an infinitely variable fan speed, not the "on full blast or off" like the primitive Goodmans) and the bills have been absolutely slashed.

 

My house is kept at 69 when we're home and the thermostat is programmed to drop to 63 during day working hours, and to 55 from 11pm-5:30 am. By 6am when we're up again, the house is back up to normal because the thermostat starts gradually raising it starting at 5:15ish.

 

This has been a record cold winter, and I have yet to pay more than $275 / month for a 3200 square foot house. My LEAST expensive winter month with the Goodmans and basic thermostat was over $550.

 

Technology definitely can help us lower our energy consumption (for those who care) and make for a much more comfortable home environment (and we all care about that.)

 

-MKL

Edited by moshe_levy
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I live in a house with baseboad hot water heat for the Winter and regular central air for Summer. Adjusting the heat for the baseboard heat didn't do anything for the gas consumption. It did, however, leave the home cold and it took a long time to return to a comfortable temp. FWI boiler people.

We did have better luck with the Central air which is a forced air system. In the Summer we let the temp. rise to near ambient in the house and then called (programmable thermostat) for cooling 1/2hr before we got home. 4-5 years ago I changed the original 40 year old unit out with a Rheem AC. The drop in Elec use was notable with an increase in cooling using the same tonnage. The AC is in unconditioned attic space. If I'd built the home I might have used forced air entirely but I dealing with what's here already.

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Joe Frickin' Friday
It will require just a smidge less than would have been needed to keep it all exactly at 70.0 all night.

 

I should have been more specific with my quote:

 

If you filled your home with a few hundred tons of tungsten ore at room temperature, you could conceivably shut your furnace completely off when you go to bed, and wake up the next morning to find that the house has only dropped from 70.0 degrees to 69.9 degrees; your house, kept warm by all the thermal "inertia" of the tungsten, has been pissing away heat to the atmosphere at a steady/high rate all night, despite having shut the thermostat off.

 

But it will take a considerable amount of energy to raise the temperature of that thermal mass back to its nominal temperature.

 

You'll have to replace the energy that was lost to the atmosphere during the night. The loss rate is proportional to the difference between house temp and atmosphere temp; the giant mass of tungsten kept the house at almost 70 all night, so the amount of energy that was lost is almost equal to, but slightly less than, the amount that would have been lost if you had just kept the house at exactly 70 degrees.

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  • 4 years later...

Here is a contrarian opinion on the Nest thermostat, and IoT in general.

 

I "liberated" a Nest thermostat that a tenant had left behind from one of my mother's rentals last winter. It was being used to control a simple 2-wire hot water radiator system. My mother doesn't know how to use a computer, and none of her minions has the brains to go through the 37-page set up manual, so I replaced it with a $25 Honeywell round, which is perfect for controlling a hot water heating system in a rental property.

 

I finally got round to installing the Nest at our house this month, replacing a $40 Honeywell programmable thermostat that I installed last summer. I'm glad I only have $25 invested in it, as it fails at the primary purpose of a thermostat, which is to control temperatures reliably. The main problem with the Nest is that it has a ±2 degree swing that is not adjustable (confirmed by Nest customer support web site). The Honeywell had a ±1 degree swing, so it kept a much more constant temperature. I have a forced air heat pump system, for which the Nest is supposedly well suited, and despite its vaunted "learning" abilities, but after a month of use, it appears to have learned nothing, and I am constantly re-setting the temperature because the house is either too hot or too cold.

 

SPgecxw.png

 

A couple of weeks after installing the Nest, our ceiling fan fell (threaded connecting sleeve between two threaded tubes loosened). I replaced the old fan, which was too small for a room with a 14-foot ceiling, with a 72" 8-bladed fan with a DC motor, which is projected to use less than $5/year running 6 hours a day. The fan cost less than the Nest, and does a far better job of keeping the house comfortable.

 

The Nest is an attractive, interesting tech toy for adults. Its advantages over the Honeywell are:

 

1. Automatically adjusts for DST changes

2. Tells me when filter needs changing

3. I can program the fan to run for 15, 30, 45 minutes/hour, or continously

4. Energy usage stats

5. I can change the temperature from my bedroom without getting out of bed, using the Nest app or a Google Home. BFD.

 

The Honeywell's advantages are that it costs ~$200 less, and does a better job at controlling temperature.

 

See: Nest thermostat, long-term review: I switch to Honeywell.

 

I'll probably keep the Nest through the winter to see how it handles heating, but if its heating performance is anything like its management of cooling, I may re-install the Honeywell, which handles winter morning warmups without the costly resistive "emergency" heat kicking in.

 

My technology mantra is if something improves my life, I will use it. I am far from a Luddite, but in general, my experiences with the Internet of Things have been less than overwhelming. I have had a Google Home for ~2 years. I recently stopped talking to it, in part because I hate talking to (leat alone listening) to comuters, and casting to the GH using the Pandora or Google Play Music apps on an Android tablet simply works better. Try asking a GH what's playing, when it's a piece of medieval music sung in Spanish or French. The response is usually unintelligible.

 

Last winter I experimented with a Wi-Fi switch, which took 2 hours to setup, using 2 Android tablets and a laptop. The Wi-Fi switch controls a night light in our bedroom, so my wife (who is a night owl) can turn the light on while she is upstairs, long after I have gone to sleep. When she comes to bed, rather than say "Hey google, turn off the light" — which is disturbing not only because doing so wakes me, but even more so because the stupid GH replies, "OK, turning off the light," when she comes to bed, she pushes the power button on the Wi-Fi switch. Another solution in search of a problem to solve.

 

I have motion-activated lights in many parts of our house. They are cheaper, simpler, more reliable, and no setup required. To turn on the light, all I have to do is enter its sensor field. When I leave, the light turns off. What could be simpler?

Edited by Selden
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Mr. Obvious here.

 

Kw Hours matter more in discussions of energy efficiency. For instance, my electricity costs 11.4 cents per KW hour. My son, who lives in town, different provider, pays 3.7 cents per KW hr. If I could reduce my energy usage by 50%, that's a much larger $ savings per month than my son would see with the same 50% reduction,

 

We put in geothermal 15 years ago. They said it would payback in 10 years. The other choice was propane. Propane prices have sky rocketed so my payback time was less.

 

The only problem at our house is spousal. I like it 72 year around. Nancy has a different idea about that. I don't need a digital, programmable thermostat because it would be changed regardless.

:grin:

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Bud, realistically at age 72 I don't expect to live long enough to break even on a geothermal system. Our heat pump is 11 years old, rated at SEER 13. If dies before I do, I'll go with something more efficient.

 

Similarly, if the electric water heater (same age) dies, I'll probably replace it with a heat pump/hybrid water heater, for which the payback is only 2-3 years.

 

I know exactly where you are coming from on spousal temperatures; I generally run ~4° cooler than my wife, summer or winter. Fortunately, her office and music room are upstairs, where it runs warmer, so we're both satisfied most of the time.

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greiffster

Similarly, if the electric water heater (same age) dies, I'll probably replace it with a heat pump/hybrid water heater, for which the payback is only 2-3 years.

 

I don't know about 2-3 years as the hybrids are still expensive relative to a standard tank. But, based on the size your replacing, you may not have a choice with the new regulations.

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szurszewski

 

The only problem at our house is spousal. I like it 72 year around. Nancy has a different idea about that. I don't need a digital, programmable thermostat because it would be changed regardless.

:grin:

 

Perhaps she is just trying to keep you in the basement.

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