Is it more economical to keep the house at the day temperature, or let it go down during the night and reheat in bulk in the morning? [duplicate]

Question

With winter approaching, I wonder: Is it better and more economical for the heating system to stop/tone down heating during the night (like letting it go down to 14C) or try to keep a constant temp like 19-20C day and night?

I would guess in 'time of use' there is not much difference between letting it go down and keeping it at the same temp (since the small hits of heat during the night will be pretty much the same as the big hit in the morning.)

So I would like to know... which option is best for the electricity bill, and what is best to also keep my heating system in good shape the longest?

Also, is the same true during summer with AC? I'm feeling more that the 'let it go down and then one big push back up' should be the better option, but my step-dad is adamant its the 'keep a constant temp' approach that is best.

Answer 1

If heat loss, and the associated amount of fuel required to add heat to the house to compensate for that loss, is the only concern then yes allowing the house temperature to track the outdoor conditions is most economical. Heat loss is proportional to the difference in temperature between indoors and outdoors. If the indoor temperature is held constant even while outdoor temperature drops, there's greater heat loss as compared to the case when indoor temperature tracks outdoor temperature with a consistent offset.

There are other complexities, like efficiency of running the heat system for intervals through the night vs continuous for an hour or more in the morning. For most systems a longer run is more efficient, but the difference diminishes quickly. A run of 10 minutes will be better for economy and for equipment life than a run of just 1 minute, but the difference between 10 minutes and 60 minutes will be minimal.

If the cost of your fuel varies through the day it's likely that will weigh far more heavily in the final bill than will length of cycles or nighttime set-back strategies.

Answer 2

Is your question academic or specific to your home? For general & academics, there is a good discussion in a previous question linked in the comments by Isherwood: Why is turning down the heat during day and night considered more efficient?

If you are asking specific for your home, the best way is A/B split testing (as also @FreeMan suggests).

1. Over a one month heating period split the days into two groups, A and B, e.g. odd and even dates. Generally you'd want them alternating, since whether changes gradually and you have a good chance of hitting the same or similar whether with two subsequent days and thus one of each split test. This way at-home holidays or weekends can be split between the two sets.
2. On "A" days run the heat 24hrs, on "B" run your scheduled heating.
3. Note your daily meter reading. Measure perhaps in the afternoon so as to capture night time usage plus any offset from morning solar heating. Also note the night time low and daytime high temperature from your weather forecast (outside thermometers are tricky to place due to sun, house heat and wind).
4. Chart the consumption on A days over time throughout the month, chart the consumption on B days, chart the temperatures, and visually compare. If you wish you can update your question with the charts and we'll have a chance to comment on it.

There will be some variation due to outside temperature differences between an A day and the following B day, and your meter reading will be affected by other uses in the house too. But a 30 day experiment can on aggregate provide a reliable answer.

Also translate the difference in usage to actual financial difference. For this use the energy portion of your bill, not the total bill which often includes metering charges, delivery charges, connection charges etc... which are all fixed costs.

Now you have a dollar/euro comparison between the two approaches for your specific case.

Large savings also happen when you switch to a high efficiency furnace (from 50% to 95%), when you switch to the cheapest energy source in your region (gas vs electricity), and when you apply passive heating and insulation with air exchangers, solar capture etc... Then the question becomes which of those upgrades are actually worth the cost savings, but that's for a different post.

Answer 3

In almost all cases it's more economical to allow your house to be closer to the outdoor temperature for as long as possible. At night or not at night, every degree and every hour of reduced differential reduces your bill. Re-establishing temperature later will not cost more, it will cost less because you lost less energy during the period of reduced differential.

Most heating and cooling systems are on or off. They run at one intensity only and their thermostats just turn them on and off to suit. If there exists some kind of heating, or air conditioning, that is able to run at a variable intensity where a lower output with a higher duty cycle is more efficient than running at high intensity and being switched on and off constantly, maybe there is something to investigate. I'm not aware of a system like that, at least not a residential one and I don't see why it would be more efficient even if it existed.