Is it more energy efficient to heat your house up and let it cool down or maintain a constant temperature?

Asked by bongo (4297) January 12th, 2012

I live in the UK so it is never ridiculously cold but I do need to heat my house over winter. I have a thermostat and can control my house temperature. Currently I have my heating so my house never gets below 10degrees centigrade at night (about 50 degrees to you americans!) and heat it to 17 degrees C (63 degrees F) in the morning, I then let it drop to 14 degrees C (57 F) throughout the day to be pushed back up to 17 or 18 (63 F) in the evening, I then let it drop back to 10 (50F) at night… Is this method of heating my house more or less energy efficient than if I kept the house at a minimum of 14 degrees C at night too or is it best to heat the house up like this every morning?

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So if I understand correctly, the options are 10–17-14–17-10 versus 14–17-14–17-14. I think what you are doing is more efficient. It will cost energy to maintain 14C versus 10C over the course of the night. It takes more energy to get from 10 to 17 than 14 to 17, but this amount of energy I imagine is much less than what you would spend maintaining 14 rather than 10 C all night.

prioritymail (1630)

First, I applaud your efforts to save energy. You are certainly doing your part.
For a fixed structure, heat loss is a function of the temperature difference between the inside temperature and the outside temperature – the delta T. You want to minimize that.
You did not mention how you are heating your home, (oil, gas, electricity) but if you have a reasonably tuned heating system it is designed to turn on an off with very little loss in efficiency. In fact by having it kick on and run full bore for a while you are keeping your flue clean by allowing it to get fully up to temperature.

So, you should come up with a schedule that minimizes the delta T. Your set back schedule minimizes the delta and is more efficient.

LuckyGuy (34607)

ok, It just seems to raise the temperature the boiler seems to have to go mad for ages but once its at a set temperature it just comes on for short bursts. Its a gas combi bioler and I dont really care about how well the boiler lasts for as it is a rented house and kind of want it to break anyway as I feel that the one I have isn’t very efficient and it crackles alot (almost like a log fire) and as its below my bedroom it wakes me up in the morning.

bongo (4297)

You can set your stat to come on 30 minutes before you need to get out of bed. It will act as an alarm for you. Remember, the furnace is supposed to be rated to run at 100% duty cycle. The gas jets and heat exchanger are optimized for that load. Unless it is broken, it is doing what it is designed to do.

LuckyGuy (34607)

I do that at the moment, it turns on about 15 mins before I get up then by the time im out of the shower the house is warm enough for me not to chill. I will leave my heating as is then… I cant turn it down any more at night (10 degrees is as low as my digital thermostat will go)

I am not sure what you mean by ‘duty cycle’ sorry for my ignorance.

bongo (4297)

@LuckyGuy You engineers are so sexy when you talk engineer talk.

janbb (51203)

I would freeze to death at 10c! My low temp is 17, when I am not at home. LOL. I set it as high as 21 (70) Most of my waking hours when home it is at 20 (68). I also close off part of my house that I don’t use much, but it has a separate heat zone. I keep that at 16 day and night, unless I need to spend time in that area then I turn it up to 17 and sometimes add a little electrical space heater to whichever room I am in. My bills are about \$100US (maybe around £65? not sure of the exchange rate now) less a month than my neighbors, sometimes the difference is even more extreme, and his house is 600 feet smaller.

JLeslie (54502)

In my house, which is up towards Mt. Lassen, the temperature outside gets down to 4 degrees F. I have really good insulation on my house, but when the temperature difference is that much, you are going to lose a lot of heat. We let the house fall to 50 F, and we warm our bedroom with a floor heater.
Trying to keep a constant temperature would be wasteful here. A huge factor is the outside temp.

filmfann (44460)

I keep my house at a steady 68–70 degrees and turn the heat down to about 60–62 at night.
We have been having freezing or below freezing temps. at night for weeks and weeks now. It is much better to maintain a temp. than to reheat a cold house. I also have amazing insulation and my home retains heat very well, Infact, if I am not careful I can get it too hot easily.

Coloma (47015)

@bongo Duty cycle is the amount of time the unit is on divided by the total time. For example, say your unit turns on for one minute and turns off for one minute and does that all day. That is 50% duty cycle. If your unit turns on for 3 minutes and turns off for three minutes all day that is also 50% DC. The total fuel usage is the same in either case.
I don’t know the typical design temp for the heating systems in your area. Where I live the spec is -4F ~20 C. That means the smallest unit that can be installed and still meet code will run at 100% DC at that temperature and the house will maintain temp at 68F 20C. If we take an intermediate temp, 32 F 0 C (half way between the design temp and the comfort temp) I can expect my furnace to run at 50% duty cycle. (I am such a data weenie that I have an hour meter on my oil furnace and can accurately tell you my fuel economy HDD/gal, Heating degree days / gallon of oil. I check it periodically to make sure I am running a peak efficiency and there are no windows leaking, etc. ) For my house, with my excellent insulation, the design load calculates out to ~55,000 BTU/hr. ~16 kW. I have two wood burning stoves: one rated at 35,000 BTU/hr and another at 70,000 BTU/hr, plus my normal oil fired boiler rated at 72,000 BTU/hr. I use the small stove when the temp outside is above 40F 5C and fire up the big one when it drops below freezing.
I keep the stat on my oil furnace low 55–60F and mostly heat with wood to keep the temp in the comfort zone. If I leave the house and the fire burns out I know the oil system will take over.
I also keep warm with an occasional flirt from female flutherites.

LuckyGuy (34607)

@janbb I never have to ask “was it good for you?”. I can tell by the minute change in furnace fuel consumption.
“Is that 1000 BTUs in your pocket or are you happy to see me?”

LuckyGuy (34607)

reheating your house is no less efficient than maintaining a constant temperature.

marinelife (61566)

my only experience is with refrigeration. a full reefer costs less to run than an empty one. S you gotta keep a lot of beers in your refrigerator.

majorrich (14634)

@LuckyGuy Re: your comment. I’ll send you your auxiliary heating bill. And you should see what I can do for your air conditioning!

janbb (51203)

If your thermostat is in working condition, you should set it on one temperature and let it do its job. Physically turning your thermostat up and down wastes energy, simply because people do the adjustments on the cold they feel.

I have used both methods and using the thermostat is by far more an energy-saver, than adjusting it physically.

Depending on the power of the heater and the size of the space, that 30 minutes will vary. Someone turned off the heat in our shop over the weekend, and it took six hours to go from 52F to 65F.

jerv (31027)

I know most people turn their heat down to sleep, I turn my up.

JLeslie (54502)

I’ve wondered about this, too. I have a large apartment, but this year I am trying to avoid turning on the heat at all. So, the apartment is generally very cool – liveable, but not particularly comfortable. When I spend time working at my desk, I want the heat to be on, and because the room is initially cold, that heater spends a lot of time trying to reach a comfortable level. I am certain that, overall, I am saving a great deal of money by not heating. But it will be interesting to find out what effect heating a single room to comfort level for a few hours, a few days in a row, will have on my heating costs.

dappled_leaves (15834)

@dappled_leaves If it is a small room, why not just get a space heater. Mine heats up a 12×12 room fairly fast.

JLeslie (54502)

@JLeslie I’ve never been a big fan of space heaters… and the wiring in this place is a little dodgy. It’s an old building. Space heaters take a lot of energy.

In any case, I have small baseboard heaters – they are effectively space heaters in each room.

dappled_leaves (15834)

Whether or not you have a good blower also matters. My current place has baseboard heating, and two places I used to live had radiators. Those took forever to warm up a room. Space heaters work in small spaces like my bathroom, but you need a hefty one for any rom over about 100 square feet like my current living room, bedroom, or my old place which was had no rooms.

Forced air heating will warm a place quicker, allowing your heater to run less. Proper use of fans with heating like mine also works.

jerv (31027)

@dappled_leaves One time, I ran out of propane and we had to use a space heater to keep our 20×20 cabin livable in 0–15F winter weather. Our light bill was about \$400 higher than usual that month.
Before we married, my wife lived in a place that cost \$350/month to heat during the winter due to having electric heat.
Electric heat is expensive in New England winters!

jerv (31027)

@jerv Yeah, I’m just north of you, so I feel your pain!

I never realized how much of an energy suck a space heater could be until one destroyed an extension cord in one of my old apartments. Scary stuff – and a lesson for me in making sure I have the right cord for the right job.

dappled_leaves (15834)

I only use a space heater to lift a room temperature by a few degrees. I love my space heaters. I don’t see how using a space heater in 120 sq ft isn’t cheaper than me heating 1,200 sq ft with my gas central heater for that entire section of my house? How can that be?

JLeslie (54502)

Get out your stopwatches kiddies, it’s time for a physics lesson.
On Jan 13 -14 winter hit the region with a vengeance. Temps in the high teens F (-8C) with high winds. If you live in the area hit by this weather and have a gas or oil furnace you should see it running at ~50% duty cycle.

LuckyGuy (34607)

ooh – I love it when you talk physics!

janbb (51203)

@LuckyGuy Thanks for all your help. By the sound of it with the boiler turning on then off for pretty much equal time I think it is running around 50% DC ish… but I have no control over any settings on that other than the direct temperature of the water or heating. I have left my heating as is or maybe even knock the temperature down to 10 degrees C in the daytime as I will be going out more now that I have finished my exams! phew I can always whack the heating on if i happen to stay in.

bongo (4297)

@LuckyGuy So, do you think I am costing myself more money when I use an electric space heater to lift a room by 3F degrees than adding 3F degrees to the thermostat set temp for an entire heating zone?

JLeslie (54502)

@JLeslie Depending on the spaces involved, maybe. Running the wall heater in my old 6’x8’ bathroom cost less than using propane to heat the entire cabin.

jerv (31027)

@JLeslie If you are only raising the temperature of one room 3 degrees instead of the whole zone, you are saving energy. Taking into account the efficiency of your heating system, on a per BTU basis, which is cheaper in your area? Gas? Electricity? Natural gas is not available here. We either have oil or propane delivered, or use electricity. Today, oil delivered is more expensive than electricity per BTU. (That changes) It is cheaper for me to heat my home with electric lights than have my oil furnace running. (Actually I mostly heat with free wood from my property. That is cheaper than anything.)

LuckyGuy (34607)

The temp is suppose to hit about 5F, -15C . That means for most of us we can expect out furnaces to be running 75% of the time. If you put an hour meter on your furnace you will see it is running 18 hours out of 24 at this temp.

Of course, all I have to do is light off both stoves and my furnace has no need to come on. I will most likely go through 140 pounds of wood during the next 24 hours.

LuckyGuy (34607)

@LuckyGuy It is usually between 3–5 degrees. I need to look at my utility bills to see what I am charged for gas v. electricity. I’m curious, I’m going to pull them out tomorrow.

JLeslie (54502)
Response moderated (Writing Standards)

My father in law keeps all the general living areas at about 70 degrees. At night he will drop them down to 65 degrees. then next day turns them back up. Does this save any energy by doing this? Now heres by biggest argument, The one bedroom he turns the heat in the room totally off, The temperature will be in the very low 50’s, then at night when it is time to use the room he will bring the temp up to 70 degrees. now to get from 50 to 70 degrees takes a decent amount of time.which would be more cost efficent. to maintain a steady temperature which runs for a minute or 2 here and there throughout the day. or does it use more energy to bring it from 50 to 70 and have it run for an hour or more, or just tell him to keep it at a comfortable temerature.so it is off for 14 hours a day, then runs for lets say an hour to get up to 70 then it maintains for a minute or so for the other 10 hours

@primenumbers It is cheaper to let the temp fall and heat it up the room when you need it. Heat loss is proportional to the difference between the inside and outside temperatures. Keeping the room warm all the time loses more heat because the you have a bigger difference over a longer time. If you let the room temp drop the difference (heat loss) is reduced.
That is why they invented setback thermostats. Your FIL has the right idea. ;-)

Welcome to Fluther by the way!

LuckyGuy (34607)

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