Optimal thermostat settings

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mtmingus
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Optimal thermostat settings

Post by mtmingus » Fri Jan 17, 2020 7:39 am

Assume they are both comfortable sleep temperature settings, which one is better for the gas furnace?

4am/64, 5am/67, 6am/70, 10pm/61

or

6am/70, 10pm/61

?

bob60014
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Re: Optimal thermostat settings

Post by bob60014 » Fri Jan 17, 2020 7:43 am

It doesn't make any difference to the operation of the furnace. How does it make you feel?

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Tim_in_GA
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Re: Optimal thermostat settings

Post by Tim_in_GA » Fri Jan 17, 2020 7:47 am

It's up to you. There are more important things to set like thresholds.

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mtmingus
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Re: Optimal thermostat settings

Post by mtmingus » Fri Jan 17, 2020 7:51 am

We feel comfortable either way. But the in 1st setting the furnace could rest in between 61 to 70 degree while the 2nd setting it runs for a long time to reach 70 from 61.

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RickBoglehead
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Re: Optimal thermostat settings

Post by RickBoglehead » Fri Jan 17, 2020 7:57 am

There are forums and websites that specialize in furnaces and heating/cooling, this would not be one of them.

My understanding from the research that I have done is that a gas forced-air system should not be set more than 5 degrees lower/higher than the temperature you are going to recover to, for efficiency. So, if 70 degrees is your comfort setting, then lowering it to 65 at night, or raising it to 75, would be the optimal points. Of course if you're going away for an extended period, you can adjust it further since an extended change would be more efficient.

In the winter, we want our bedroom at 65 for sleeping. We raise the temperature starting 30 minutes before bedtime to 68 to take the chill off. In the morning, we do the same. During the day, the bedroom is back to 65.

Our downstairs is 70 during the day, but bumped to 71 in the late afternoon when the sun is no longer heating the house. At night it goes to 65, which starts about 45 minutes before we go to bed, because it drops slowly.

Summer is different. We are comfortable downstairs at 74 and upstairs at 73. We let the downstairs go to 77 at night (only 3 degrees) and upstairs go to 78 during the day.
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Valuethinker
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Re: Optimal thermostat settings

Post by Valuethinker » Fri Jan 17, 2020 7:57 am

mtmingus wrote:
Fri Jan 17, 2020 7:39 am
Assume they are both comfortable sleep temperature settings, which one is better for the gas furnace?

4am/64, 5am/67, 6am/70, 10pm/61

or

6am/70, 10pm/61

?
In terms of the service life of the furnace it should not have a meaningful impact.

It terms of cost, very roughly the energy use = (difference betw internal & external temp) x hours . You have to then figure the heat loss.

So without knowing the actual outside temperature averages at those times, it's a very hard call (and there's huge variation between different years).

Probably the second setting saves you money, at the cost of some discomfort in the early hour or so (again depending on how cold out it is).

Doug E. Dee
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Re: Optimal thermostat settings

Post by Doug E. Dee » Fri Jan 17, 2020 8:13 am

B

The rest periods are not needed.

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lthenderson
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Re: Optimal thermostat settings

Post by lthenderson » Fri Jan 17, 2020 8:43 am

I have felt life is too short to chase after nickels by adjusting my thermostat. I have a smart thermostat so I set it for the temperature I want to wake up too and it adjusts when it turns on depending on outside temperatures to have my house up to my desired temperature when my feet hit the floor.

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iceport
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Re: Optimal thermostat settings

Post by iceport » Fri Jan 17, 2020 2:37 pm

RickBoglehead wrote:
Fri Jan 17, 2020 7:57 am
My understanding from the research that I have done is that a gas forced-air system should not be set more than 5 degrees lower/higher than the temperature you are going to recover to, for efficiency. So, if 70 degrees is your comfort setting, then lowering it to 65 at night, or raising it to 75, would be the optimal points. Of course if you're going away for an extended period, you can adjust it further since an extended change would be more efficient.
You raise a great parameter: type of heating system. Besides the fuel type, in my experience the type of radiators and/or use of forced air make a huge difference. Even some house characteristics could influence the results.

I have a cast iron baseboard hot water system that stores a good amount of heat. Also, the walls are a thick composite of early sheet rock layer and a plaster surface layer. There's also a huge central brick chimney. The effect is that of a heat sink. That's great for maintaining a set temperature, but not great for easily changing temperatures.

Soon after installing a digital t-stat, I tried maybe a 4 degree differential between home and away settings. That was a failure, as the t-stat learned it need most of the away time to "recover" back up to the home setting, especially in very cold weather. I ended up going many years with a 2 degree differential.

However, I now realize that even that 2 degree shift resulted in excessive continuous run-times for the furnace, which produces more wear, and I have been holding a steady 24-hour temperature for the past 3 years. I don't notice any obvious increase in fuel consumption, though I certainly haven't controlled for weather conditions.

Baseboard copper pipe radiators would respond faster, and forced air even faster. The bottom line is that the optimum temperature differential depends on a number of different factors, but it might be a narrower range than you might assume. A 9 degree differential does seem like a lot, just intuitively.
mtmingus wrote:
Fri Jan 17, 2020 7:39 am
Assume they are both comfortable sleep temperature settings, which one is better for the gas furnace?

4am/64, 5am/67, 6am/70, 10pm/61

or

6am/70, 10pm/61

?
If your thermostat "learns" when to start the "recovery" to the higher temperature, wouldn't it do it's own version of your first option, even if you use the second option?
"Discipline matters more than allocation.” ─William Bernstein

prairieman
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Re: Optimal thermostat settings

Post by prairieman » Fri Jan 17, 2020 2:44 pm

We set to 58 when gone or when we go to bed. 68 when home and up. Winter settings, obviously.

bloom2708
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Re: Optimal thermostat settings

Post by bloom2708 » Fri Jan 17, 2020 2:54 pm

71 day, 69 at night.

61 is a "the furnace has gone out, call the heating company" temperature. No thanks.

No reason to drop down that far. Climate comes into play. If it is 55 outside and you drop to 61, the temp likely never reaches 61.

If it is -15 outside you get to 61 pretty fast.
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Bogle7
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Re: Optimal thermostat settings

Post by Bogle7 » Fri Jan 17, 2020 3:27 pm

You are overthinking this.
Get a $40 programmable thermostat and set your desired temp for when you wake up, leave for work, return from work, and go to bed.

123
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Re: Optimal thermostat settings

Post by 123 » Fri Jan 17, 2020 3:42 pm

We have found that if diligently keep the door closed to rooms we use (and don't use) we can drop the thermostat temperature down a few degrees and have the forced air furnace work less. We undoubtedly have some rooms that lose heat more so than others due to older windows and poorer insulation. We found it more economic (and warmer) to primarily heat the rooms we use. But in some climates it's likely better to maintain the whole house at a more consistent temperature.

We use a programmable thermostat with different weekday and weekend cycle time settings. Sometimes I think that getting an app/wi-fi thermostat would be more convenient but a tedious and cumbersome interface is a great way to keep kids from messing with it. As it is now it's easier for most of us to put on a sweater or use a blanket on the couch then to fiddle with the time settings. We can bump up the temperature easily for the current cycle but it reverts when it hits the next cycle time in case someone forgets.
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FireAway
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Re: Optimal thermostat settings

Post by FireAway » Fri Jan 17, 2020 3:51 pm

For a system like this, you always save money by keeping the house colder. The colder, and the longer, the more money you save. So your second scenario is more energy efficient, since it has the temp set lower for longer.

The calculation changes a bit if you have a heat pump (where you need to consider things like two stage vs one stage operation, resistance back-up, source temperature, etc.) or if your cost for energy depends on what time you use it. But you don't have any of this going on.

corysold
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Re: Optimal thermostat settings

Post by corysold » Fri Jan 17, 2020 3:57 pm

Is it a single stage or dual stage furnace?

If its single stage, either one works, but the motors are designed to run, so running it longer to go up all in one jump isn't going to hurt it.

If it is a dual stage, you want the second option, which will allow it to run in second stage longer and heat more efficiently. Starting it and stopping it will restart first stage each time, which will just make the whole operation take longer.

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Kenkat
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Re: Optimal thermostat settings

Post by Kenkat » Fri Jan 17, 2020 4:11 pm

The optimal thermostat setting from an energy savings perspective is the “Off” position. Anything else is personal preference. If you can agree with a significant other on the proper settings? That’s something you can build a whole relationship on.

123
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Re: Optimal thermostat settings

Post by 123 » Fri Jan 17, 2020 4:16 pm

Take a hint. Wear a fur coat like the dogs and cats in the neighborhood.
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Veni Vidi Decessi
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Re: Optimal thermostat settings

Post by Veni Vidi Decessi » Fri Jan 17, 2020 4:24 pm

Your question was specifically regarding the furnace, though others have chimed in (correctly) that there isn't likely much of a cost difference in the two options.

From a maintenance perspective for the furnace, the more start/stop cycles you have on your system, the shorter your equipment life will be. This is due to the thermal cycling of the furnace, the dynamic loading on starting the blower (asymmetric loads on the bearings, motor mounts), etc. You would rather your equipment start once, run for 50% of the day, then shut down until the next day, rather than it starting and stopping once an hour. The first scenario isn't reasonable due to the dynamics of home heating/cooling, but it gets the point across, I think.

michaeljc70
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Re: Optimal thermostat settings

Post by michaeljc70 » Fri Jan 17, 2020 4:44 pm

I'd go for B. We use 70/64. The timing can also matter as some thermostats "learn" how long it takes to get to temp and automatically adjusts. For example, I set it that I want 70 at 6am it will start the furnace at 5:30 (or whatever) to make sure it is 70 at 6am.

I'd choose A if you get up to go to the bathroom early :D.

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