Temperature setting?

[Ed62]

I've seen a long list of ways to conserve energy (lower your heating costs). They always say to keep the temperature at around 67 degrees during the day, and lower when you're sleeping. I've accepted that thinking, but yesterday one of my sons asked me if I really agreed with that thinking.

Let's say your thermostat is set at 72 degrees, and the outside temperature is zero. The temperature drops to 70 degrees in your house, so the furnace kicks on until the desired temperature is reached. Now let's assume your thermostat is set at 67 degrees, and the outside temperature is zero. The temperature in the house falls to 65, and the furnace kicks on until the desired temperature is reached. In both situations, the temperature will rise 2 or 3 degrees before the furnace shuts off. So now my thinking is that the only savings would occur when you initially warm the room to 67 instead of 72. What do you think? What do you base your thinking on?

Ed

[Copper]

What you say is true, but the furnace will be cycling on more often when it is set at 72 degrees. The outside temperature will lower the house temp from 72 to 70 more quickly than from 67 to 65. it may not be much, but over the course of the eniter winter season, it will add up to some savings. We have our daytime temp set at 67 and have the temp go to 57 when we're out of the house during the day and at night for about 7 hours while we're asleep.

[burrellski]

A greater temperature differential will require more energy to maintain. The same holds true for the AC on hot days. It will obviously take more energy to go from 65º at night to 70º in the day than from 65º to 67º, but once the temperature setting is reached the furnace will run more often to maintain the higher temp. Basically the furnace will run about the same length of time each cycle, but if the thermostat is set higher the furnace will run more cycles in a day. The heat transfer out of your house is proportional to the temperature differential.

That's assuming you have a furnace. A heat pump is a totally different story.



Looks like Copper beat me to the punch.

[radhak]

It's not only the initial energy needed to heat up to 72 degrees. You need to also count the energy needed to maintain it at 72 degrees.

When you are counting the 2 degrees from 70 to 72, you forget that a lot of energy is being expended to keep the temperature at 70. So if you had to maintain the temp till only 65, that's a lot of energy not spent.

(definition of 1 calorie is the energy needed to heat up 1 gram of water by 1 degree centigrade. so if you see the air inside the house as another fluid with a huge volume, that's a lot of calories needed for that difference of 7 degrees)

[LCHIEN]

I've seen a long list of ways to conserve energy (lower your heating costs). They always say to keep the temperature at around 67 degrees during the day, and lower when you're sleeping. I've accepted that thinking, but yesterday one of my sons asked me if I really agreed with that thinking.

Let's say your thermostat is set at 72 degrees, and the outside temperature is zero. The temperature drops to 70 degrees in your house, so the furnace kicks on until the desired temperature is reached. Now let's assume your thermostat is set at 67 degrees, and the outside temperature is zero. The temperature in the house falls to 65, and the furnace kicks on until the desired temperature is reached. In both situations, the temperature will rise 2 or 3 degrees before the furnace shuts off. So now my thinking is that the only savings would occur when you initially warm the room to 67 instead of 72. What do you think? What do you base your thinking on?

Ed

Your thinking is quite flawed. It assumes there is perfect insulation. Which there is no such thing. Your assumption says you only have to heat the air once and it will stay that way.

The art of HVAC always assumes heat flow through the walls, attic doors, windows when there is a temperature differential.

Think of a heat as water. Think of your house as a leaky bucket, tiny holes everywhere. The more water you try to hold in the bucket, the faster it leaks out. When the bucket is empty (zero differential), only then does it stop leaking.

Things at different temperatures will always try to equalize. Usually the outdoors wins because there's so much of it.

So you see, the cost of heating is not based on the actual temperature, its the temperature differential you try to hold between inside and outside and fighting the constant leaking. That's why better insulation pays off.

So basically given a certain house, then if its 30 degrees outside and you hold 70 degrees inside (40 degree differential), the energy usage will be the same if it drops to 20 and you try to keep it at 60 degrees inside (same 40 degree diff). (actually technically there will be a difference due to heat loss through the slab or basement to the ground. and if there is a strong wind or not.) Your energy usage will be halved if you are trying to keep the house at 70 and its 50 degrees outside (only 20 degree differential). Or, keeping the 40 degree differential and doubling the insulation in the walls, attic, and windows/doors to slow the heat transfer (double the thermal resistance).

[Ed62]

My son told me that 2 different heating contractors told him there would be no appreciable difference if he set his thermostat at 68 or 72. He personally knows both contractors, but that doesn't mean they're right about it. Pros are wrong at times too.

It seems to me that the closer the lower setting is to the outside temperature, the greater difference there will be in the amount of cycles for a given temperature maintained in the reasonably well insulated home. Now the question becomes...Just how frequent will the cycles occur for the different (about 5 degrees) settings? Is it significant (yeah, I know that's subjective)?

Ed

[atgcpaul]

If I may, let me piggyback another question onto this one.

I keep my house at 68 when I'm home. The thermostat is set to 62 when I'm
away. Would it be cheaper to just turn it off when I'm away and then heat
it all up at once right before I get home?

[LCHIEN]

If I may, let me piggyback another question onto this one.

I keep my house at 68 when I'm home. The thermostat is set to 62 when I'm
away. Would it be cheaper to just turn it off when I'm away and then heat
it all up at once right before I get home?

yes, it will be cheaper.but you have to balance that against
1) takes longer to be comfortable when you get home. If you let the house get very cold it may take an hour or more to get back to a comfort range. If you let it fall to outside temperature (which it will eventually hit) and you're only gone a bit longer than that, it's not worth the savings.
2) will the temperature extremes and or cycles,, freezing, humidity or moisture cause any damage to furnishings, pets, clothes, paintings, plants,etc. Frozen pipes may indeed be a problem.

In houston, when I go out of town, I set the heat to 60 in the winter and the A/C to 82 in the summer. I don't want frozen burst pipes or mildewed furniture.

[jackellis]

My son told me that 2 different heating contractors told him there would be no appreciable difference if he set his thermostat at 68 or 72. He personally knows both contractors, but that doesn't mean they're right about it. Pros are wrong at times too.

In this case, very wrong. As Loring points out, heat flow is proportional to the temperature differential. Otherwise, the difference in costs for heating a home in Northern California and Northern North Dakota would be pretty close, but they're not!

I keep my house at 68 when I'm home. The thermostat is set to 62 when I'm away. Would it be cheaper to just turn it off when I'm away and then heat it all up at once right before I get home?

Yes, but... First off, the house might be uncomfortably cold when you return, especially if it's very cold outside and you've been gone a long time. Second, if you let the house get too cold, pipes will freeze. I can tell you from experience that's something you definitely do not want. We had a toilet freeze at our vacation home. Messy.

You can save some money by setting the temp lower than 62 when you're not home, but it should probably never be set below 50.

[cabinetman]

Hey Ed

Temperature in So Florida never goes to 0 degrees. So my thinking is that you should get the heck out of Indiana.

[LCHIEN]

My son told me that 2 different heating contractors told him there would be no appreciable difference if he set his thermostat at 68 or 72. He personally knows both contractors, but that doesn't mean they're right about it. Pros are wrong at times too.

It seems to me that the closer the lower setting is to the outside temperature, the greater difference there will be in the amount of cycles for a given temperature maintained in the reasonably well insulated home. Now the question becomes...Just how frequent will the cycles occur for the different (about 5 degrees) settings? Is it significant (yeah, I know that's subjective)?

Ed

Assuming that your heating bill is 100% from heating, and not from other uses, then the amount of energy is roughly proportional to the temperature difference you maintain AND the amount of time you maintain it.

So lets take an example:
You live in a place where the temperature averages 30 degress over the day and the month. Maybe 25 degrees at night and 35 during the day.
You keep the house temp at 70. So there's a differential of 40 degrees.
If you drop the temperature you maintain in the house to 68, then there's a 2-degree less differential. 2 degrees out of 40 is 5%. There's a few other factors but basically that's it. So you'll save roughly 5%. I don't know that you'd notice it that much, due to the factthat avergae outside temp is varying by more than 5% per month due to season changes. but over hte long run it'll add up to 5% which if your heating bill is $200 per month in the winter that's $10. So I doubt that most people will notice a heating bill difference of $10, it probably varies that much due to other factors, but over the year (long term) it may save you $120.

As for cycling, heating and cooling systems are what we call bang-bang systems. They're on or off, running full blast when on.

Don't confuse frequency with other things.

The frequency and how long its on for each time depend on several things.
Thermostats have hysteresis, a hysteresis of 2 degrees means for a setting of 70 degrees it will go on at 69 and off at 71. (air compressors have hysteresis, too) This keeps your system from cycling on and off rapidly wearing out fan and igniters and compressors etc. More hysteresis means that it will go on less frequently, etc. Usually we can't that easily tell a couple of degrees, so hysteresis is OK. You don't want it too big or too small.

Back to the problem. Duty cycle is the percentage of time something is running. If we checked the heating for an hour, and added up all the time it was on and all the time it was off, say it would be on 20 minutes and off 40. Then the duty cycle is 33%. For a given temperature differential The duty cycle will be the same no matter what the hyteresis is. If it goes on (frequency) 10 times per hour (because the hysteresis is 2 degrees) and it would have run for about 2 mins each time. If we changed the hysteresis to 4 degrees, then we would find it came on just 5 times, but ran 4 minutes each time. Likewise if we made the hyst. = 1 degree then it would have run 20 times for 1 minute.

A heater twice as strong would run for half the time (but cost the same to run since it uses twice as much power).

So the total work is the same. Heat is measured in BTU. Your furnace will be rated for so many BTU, say 10,000. If it runs for 20 minutes or 33% duty cycle that means you are putting 3300 BTU into your house every hour. and your insulation is passing 3300 BTU to the outside.

If your BTU capacity of the heater were only 3300 then you would be running 100% of the time. If the BTU were less than 3300 then you would not be able to maintain the set temperature and it would get cold.

With 33% duty cycle at 40 degree differential, another 20 degree drop outside would make you heater run about 50% of the time.

so you can
trade off frequency and run time with hysteresis but it won't save heating costs. the trade off is comfort and wear and tear.

Duty cycle affects the cost. you can change duty cycle by changing the differential (lower the thermostat setting) and/or the heat loss (More insulation).

If you can't keep the house warm to the set temperature, you need to increase heater capacity (bigger heater, more heat input) or increase insulation (decrease loss of heat).

decreasing temperature when you're not home reduces loss during that time and therefore input when you're not there. Saves money but the tradeoff is how fast it takes to reach a comfortable temp (bigger heater will make up heat faster and decrease duty cycle, but not net costs for an initial capital expense).

[Ed62]

Schooling

Loring, that's quite an explanation. Thank you. That's what makes this place so great!

Ed

[leehljp]

In Japan, heating is still localized to individual rooms and lowered (or turned off) at night in most homes. We lower the heating to 60 at night in our bedroom, keep the heat on to 70 in the LR/DR with door closed to other room and hall. The heat loss to the other spaces and 2nd floor keep the house comfortable and heating bill down.

LOML and I usually work together and are out a lot. If we know that we are going to be out all day, we cut the heat off, or AC in the summer. Our electric and heating bill reflects that too. For our colleagues that have at least one person at home all day everyday show a significantly higher heating and electric bill.

Cutting it off, lowering the thermostat in winter, raising in the summer and closing off unused rooms can save money. Loring gave the technicals, but the reality for me is that it works.