In other words, if you set your thermostat for 71 degrees in the winter and your house only seems to get up to 69 degrees. This problem generates many service calls. And sometimes this is caused by a genuine problem but unfortunately, in extremely, cold weather even a properly working heat pump may have trouble maintaining desired temperature.
Why is this? When it gets below a certain temperature, in our area around 35 degrees a heat pump loses efficiency and cannot keep up with the heat loss of the structure. When the temperature in the house drops approximately 2 degrees below room temperature, supplemental heat comes on to assist the heat pump (usually in the form of electric resistance heaters). When it gets to within 1.5 degrees of room temperature, the back-up heat cycles off and the heat pump continues running tying to reach temperature but cannot. This usually happens when the temperature is at it's coldest - 0 to 30 degrees.
This is however the way heat pumps were designed to operate. Even though they don't put a lot of heat into the house and they run for long periods of time, they are still quite efficient.
So, if it is very cold out and you desire your house to be a certain temperature, you might have to raise your thermostat by 2 degrees to maintain it. Now, if it isn't extremely cold out and your heat pump isn't maintaining temperature, this indicates a problem.
Below is a list of possible causes.
Here is a checklist to go through before making a service call: