Why Do My Vents Blow Cold Air When My Heat is On?
This time of year, especially when it gets cold outside in the evenings, we get many calls from homeowners concerned that there is cold air blowing out of the duct registers in their home when their heat is running. We wanted to address this, as many times homeowners think this is an issue with their heating and cooling system, but if they have a heat pump, it's actually typically a very normal process.
Don't know if you have a heat pump? Check out this article, on how to tell if your unit is a heat pump or air conditioner.
Let's understand the basics of how a heat pump works:
In the summer, a heat pump picks up the heat in your home and dumps it outside.
In the winter, your heat pump picks up heat from the outside and dumps it into your home (yes, even when it's cold outside). Now, that may be hard to grasp, but this is the way it works.
In the summer, your indoor coil is cold (and pulls humidity out of the air, and comes in the form of water) which is drained from that PVC pipe located on your roof, or side of your house. In the winter, that same humidity is pulled out of the air, but it comes in a form of frost on the outdoor coil (condenser). If you see frost on the outdoor coil, it is natural to the process and it is not a sign of any kind of malfunction- the reason the coil is cold falls back to the second law of thermodynamics- which states that energy always moves from areas of higher concentration to areas of lower concentration.
As the frost builds up on the outdoor coil, it could potentially inhibit the ability for air to flow across that coil. Once the frost builds up, your heat pump automatically goes through what we call, a defrost cycle to keep allowing the air to flow across the coil freely (which is a necessary function of your system). You may have noticed a loud sound right before cold air blows out through the vents in your home. The sound you hear is the reversing valve switching your unit over to cooling mode. Frost on the outside coil is condensation that forms up on the coil in the form of frost. When a heat pump begins the defrost cycle, the outdoor fan shuts off and the system switches over to cooling mode. The reason for this, is that in the cooling mode, the outdoor coil actually becomes hot. This is because hot refrigerant circulates through the outdoor coil, which in turn melts the frost. The reason the fan shuts off during this process is so the system doesn't pull the cold air from the outside across the coil while the hot refrigerant circulates.
While the system is in cooling mode, the frost that had built up on your outdoor coil then turns to water, runs down the outdoor coil, and piles towards the bottom. When the defrost cycle is complete and the defrost sensor closes, the system runs in heating mode again. This is when the fan kicks back on and all of the frost that ran down to the bottom of the coil in the form of water is pushed out of the top of the unit, sometimes in the form of steam. If you see steam coming out of your outdoor coil that is a normal operating procedure for a heat pump system, so don't be alarmed.
Don't panic if you have a heat pump and if cold air blows out of your duct work for a little while on and off while you run your heat at home. Don't panic if you see frost on your outside equipment's coil. Don't panic if steam comes out of the top of the outside equipment. It's all part of the process of heating your home with a heat pump!