Heating with Heat Pumps
It is FRIGID Outside!
It sure is cold outside this morning. I know what the weather forecast is…Colder! I am a little concerned about my heat pump. So I thought I would post this thought. If it stays WET and get’s as cold as predicted, you SHOULD KNOW THIS.
The way heat pumps work in the winter (during the heating cycle), the heat pump (the outdoor system) is attempting to extract heat out of the outdoor air to heat your home. Because the ability to extract sufficient heat to heat your home is not available (somewhere around 35 degrees, + or – a couple of degrees), there is a possibility that your outdoor system could turn into a “block of ice”. For heat to be extracted, the temperature of the coil (contained in the outdoor system) has to be lower than the temperature of the outside air. Because of the cold rain/freezing rain or sleet, ice may build up on the outdoor coil.
The thing to remember before you read the next paragraphs, is that the heat pump is a SYSTEM with multiple components. The outdoor system and the indoor system is synchronizing in different modes of operation simultaneously to provide heat. This explanation may get a little complex, but hopefully you will understand.
A heat pump system contains a back-up heating capacity that is referenced as emergency heat, supplemental heat, or strip heating. The heating elements are contained in the indoor system (air handler) of your heat pump system. If the indoor temperature is more than a few degrees less than the thermostat setting, the supplemental heaters are energized to supply heat. Your thermostat originates the signal to activate the supplemental heaters. The supplemental heaters should provide sufficient heat to heat your home.
Now, let’s go to another standard component of a heat pump system. All heat pumps (outdoor systems) contain a process called a DEFROST CYCLE. If the temperature sensor on the outdoor system (coil) detects accumulation of ice, the heat pump reverses the refrigeration cycle (goes into cooling mode) which sends warm refrigerant to the outdoor coils. The “warmer” refrigerant should melt any ice that may have formed on the outdoor coils. Defrost cycles are utilized for short time intervals (usually in seconds), and then the cycle changes the refrigerant flow back the normal heating cycle. If you have seen “steam” being emitted during the winter from an outdoor system, the steam was being emitted during the DEFROST cycle. When an outdoor system activates the DEFROST cycle, the supplemental heaters receive a signal to activate, since heating is required to warm the indoor air in cooling mode to provide heat, and the supplemental heaters remain activated for the duration of the defrost cycle. At the conclusion of the defrost cycle, the outdoor system initiates the signal to cut-off the supplemental heaters.
I attempted to describe the heating cycle of a heat pump in an elementary fashion. (I don’t begin to claim that I can get any more technical than as written.)
But what happens if the ice accumulates faster on the outdoor system coil than the DEFROST cycle can eradicate the ice? Now refer to the picture. Ice build up can happen in extreme conditions, even when a heat pump is operating as it should.
Now on days when the temperature is hovering around freezing or BELOW, AND it is precipitating in the form of rain, freezing rain, or sleet, your heat pump may become a block of ice. When your outdoor system becomes a block of ice, the outdoor system fan may not have room to operate (due to the ice) and damages could occur to the fan motor and the fan contained in the outdoor system.
In conditions such as we are having today… close to freezing temperatures and rain… the safe course of action is to turn your thermostat on emergency heat until the temperature warms up or the precipitation has ended for a while. For this unusual exception (we don’t get this kind of weather often), trying to heat in heat pump mode as opposed to heating with the higher cost form of heat (supplemental differences) will not provide significant spikes in energy costs that result from cold temperatures for short intervals of time.
And HOW DO I KNOW about such a possibility as I attempted to describe? I will admit that this advice was NOT obtained by reading books NOR was the knowledge gained from someone else’s advice.