The heat pump shuts down below 30° outside temperature. It is a trane XL 18i and a Tam 7 Air handler. It seems like it's going into a defrost cycle but then the whole heat pump shuts down for about a minute and then comes back on. At other times it will not be making any heat at all even though it is running. I know this because the pipe going into the air handler is room temperature. Any other time when it producing heat out of the vent it is very warm.