I am aware that heat pumps are able to move more heat into a building per unit of energy input (the "coefficient of performance" or COP) when the source (outside) temperature is higher. My question is whether I should set my programmable thermostat to bring the house to a warmer temperature during the warmer afternoon, or whether this increase in efficiency will be outweighed by the additional heat loss during the extra hours of increased temperature differential with the outside.
I have a two-zone heat pump system. For the upstairs I currently have the thermostat set to increase the temperature by about 4 degrees a few hours before bedtime; this is about 3 hours after sunset. I also have the downstairs thermostat set to increase the temperature by a few degrees at sunset.
I noted recently how quickly the outside temperature falls after sunset on these sunny but colder days. Since the heat pump COP is higher when it is warmer outside, would I use less energy by running the heat pump to create the temperature increases before sunset rather than later when it is colder?
For concrete numbers, let's assume that the temperature is 10 degrees Fahrenheit colder three hours after sunset than 1 hour before sunset. (On a warm winter day, it would be 50 in the afternoon and 40 after dark; on a colder day it might be 10 or 20 degrees colder in both instances.) Let's also use 64 degrees inside for computing the heat loss, and assume I am raising the temperature by 4 degrees 4 hours earlier.
If there is a significant savings possible, I could also raise the temperature by even more during the day to delay the time when the house cools to the nighttime indoor setting and starts running the heat pump at the less efficient nighttime temperatures.