A heat pump that never shuts off yet fails to keep the house comfortable is one of the most common efficiency complaints we see in Chattanooga winters. Here are the usual reasons this happens. Undersized unit: If the heat pump was sized for mild weather or an older, less-insulated version of your home, it simply can’t deliver enough heat when outdoor temperatures drop below 35–40 °F. It runs 100 % of the time trying to catch up and still falls short. Low refrigerant charge: