Lowering the heating thermostat at night: is it really worth it?

Question

I've heard some very conflicting information regarding the effect of lowering the house heating thermostat at night. On one hand there are academic papers (such as Thermostat strategies impact on energy consumption in residential buildings, Energy and Buildings 43 (2011) 338–346) that mention "significant" energy savings. There's also https://campusops.uoregon.edu/utilities-services/energy-conservation-myths that confirms the idea.

On the other hand, there are back-of-the-envelope calculations such as http://www.ahok.de/en/night-set-back.html that tend to minimize the effect of night setback.

So which is correct? Does lowering the heating thermostat at night (by, say, 5°C) really help save energy?

Answer 1

tl;dr: Yes, lowering the room thermostat at night will generally help save energy, assuming we are in the heating season (winter), rather than the cooling season (summer).

Let's take two points in time, one before the night, one after it. Let's choose them so that the dwelling has the same amount of heat energy contained within it at each point, so all the heat lost, has been regained from the heating (and sun, and ambient gains). So if more energy has been lost in the night, then more energy has had to be put in by the heating. And if less energy has been lost in the night, then less energy has had to be put in the heating.

Now, the rate of heat loss at any one moment is roughly speaking proportional to the temperature difference between inside and outside, assuming it's colder outside. The bigger the temperature difference, the faster the heat loss rate.

If we lower the thermostat at night, then that will allow the internal temperature to drop lower. That means that the temperature difference between inside and outside will be less. And that means the rate of heat loss will be lower. And that means the energy required from the heating system, to bring it back up to full warmth, will be less.

So yes, lowering the room thermostat at night will generally help save energy.

Answer 2

A long-term consideration is increased condensation from cooler air in the far corners of rooms, which encourages fungal colonies. These are bad for health and eventually expensive to treat. Keeping a constant temperature diminishes the problem.

Answer 3

You should find the best time to lower/raise the thermostat. The more thermal inertia your house has (the better it is thermally isolated), the earlier you should lower the thermostat. The faster the heating gets the room warm, the later you can raise the thermostat. Both are best done automatically.

Answer 4

is it really worth it.. can mean something else as well:
During the wintertime:
Lowering the temperature has (sometimes) a nice side effect - you will lose weight.
The amount of heat given from the body to the room will increase (lowering the temperature from 21 to 19 °C) by 15 % (in comparison for a normal adult person in a resting phase). This energy is created inside your body and so fat gets burned without doing any sport at all.
By the way, you won't catch a flu easier by lowering the room temperature. To get exposed to lower temperatures during the winter time is what our body has dealt with throughout history and is a more natural way of living.
But please, don't over exaggerate this process.
You can still feel comfortable using a blanket for a while .

More information here: http://www.uni-magdeburg.de/isut/TV/Download/Der_Mensch_als_waermetechnisches_System.pdf

Answer 5

Yes, as stated in other answers, but you can increase efficiency by using ecm fan motors for central forced air systems. The heating system consists of a heat source (furnace) and a thermal distribution system (fan air handler for forced air systems). It is common to underestimate the energy consumption of the latter. A lot of times, the thermostat can be kept at a lower temperature when unequal heating distribution factors are minimized.

The faster the fan motor turns the more electrical energy is consumed. In many situations, the air handler only has 2 speed settings. Being unable to fine tune the rpm of the fan motor for the minimum rpm needed for thermal distribution costs much in energy consumption particularly if the thermostat is kept constant day and night.

The ecm motor is actually an electronically commutated dc (direct current) motor whose speed is infinitesimally adjusted by an automatic control system for optimum air flow for ambient conditions. It also has the benefit of running much quieter since much of the time the fan runs at a very slow speed maintaining the plateaued thermal inertia throughout the heated space.