How Heat Pumps Work | HVAC Heating and Cooling Systems

Table of Contents

Have you ever wondered how Heat Pumps work? Heat Pumps can be a very economical way of heating and cooling your dwelling. It depends on geographical location and the cost of electricity in your area. The volatile costs of natural gas, propane, and oil have enabled these conditions. Conditions where it can be cheaper with a heat pump than with fossil fuels. There is a common conception that a heat pump blows cold air.

While this can be true in some cases it is not true. If a heat pump is blowing cold air when in the heating mode then it has a problem. A problem that needs to be corrected. Even when the outside unit kicks into the defrost mode the unit should provide adequate heating.

Heating to heat the zones which it serves. Again, if it is not providing adequate heating then it needs to be looked at for a technical problem. A problem that needs correcting. A well-designed system, working properly and efficiently, can provide competitive and economical heating.

How Heat Pumps Work - Comparison to Air Conditioners

Components Inside a Heat Pump

A heat pump works much the same way an air conditioning unit works with a few differences. In the cooling mode, it transfers heat outside from the inside. In the heating mode, it transfers heat from the outside to the inside*. That is, in the heating mode, it transfers heat from the outside to the inside with a minimum outside ambient air temperature affecting its efficiency and the amount of usable heat transferred.

The heat transfer from the outside to the inside is done using the process of refrigeration. That is also the process used by the heat pump when in the cooling mode. The classic definition of refrigeration is simply moving heat. Even when it is cold outside, the air contains heat. This heat is usable, and mechanically, the heat pump carries this heat indoors to warm the space indoors.

There are limitations to the heat pump, and these limitations depend on many factors. That outside air ambient temperature is different from one unit to the next. It entirely depends on the system’s design, refrigerant, and size. The lower the temperature outside, the lower the ability of any air source heat pump to transfer usable heating inside. The temperature ranges for the end of effective heating for a majority of heat pumps averages approximately 35 to 40 degrees Fahrenheit. There are some considerations one must take so that the unit will work properly. Here are a few considerations.

Geographical Considerations | How Heat Pumps Work in Winter

For this reason, air-source heat pumps, which make up a vast majority of the units installed, are primarily installed in the regions of the Southern state of the US. It is also necessary to have some backup heating for two reasons. Most heat pumps come standard with backup electric heating. Furthermore, some have gas or oil backup heating called dual-fuel or hybrid. Lastly, see below for more detailed information on this subject.

This backup electric heating is used for the defrost cycle and when the temperature outside drops low enough for the unit to lose its effective heating ability. In the Northern climes, the best types of Heat Pumps for heating are water source or geothermal systems. Of course, the opposite would be true in the Southern Hemisphere.

Air source heat pumps do not work well in the Northern climates. That is unless special engineering goes into it for the colder weather. There is a manufacturer that engineers an air source heat pump specifically for the Northern climes. They make a heat pump that can still provide a heating source in colder temperatures. Colder temperatures where the conventional air source heat pump does not work well in the colder temperatures.

A water source or geothermal heat pump system works well in low-temperature conditions. While the air-source system, the typical conventional air-source heat pump will not work well in colder temperatures. Temperatures where the average temperature drops well below 35 degrees Fahrenheit for most of the winter.

Defrost Cycle And Heating | How Heat Pumps Work

The defrost cycle is necessary because the outside unit will operate below the dew point. The heat pump unit must operate below the outside temperature. That is so that it may absorb heat from the outside air. Remember, even though the temperature outside is 40 degrees Fahrenheit there is heat contained in the air. The unit is working to absorb this heat so that it may transfer it into the dwelling for use. Because the unit operates below the dew point, it will frost over. This frost on the heat pump needs to be removed.

Most units automatically defrost themselves based using timer control. This timer setting is done by the technician who does a startup on the unit. The times offered by a majority of manufacturers ranging from 30 minutes to 150 minutes. Depending on how the defrost timer is set can depend on the efficiency factors for the heat pump unit. If the defrost timer is set to 30 minutes, every thirty minutes, the outside condensing unit will kick into defrost.

The defrost cycle for a heat pump causes a reversing valve to shift in the heat pump unit, and it effectively becomes an air conditioner. That means the evaporator coil for the heat pump will begin to introduce cold air into the home.

Backup Heat | How Heat Pumps Work

To counterbalance this cold air, when the outside condenser kicks into defrost it simultaneously sends a signal to the indoor air handler to engage the backup heating. The heat pump defrost cycle will continue based on a set time. Then it will return to normal heating operations. The outside unit absorbing heat for transfer to the inside and the backup heat disengages.

If the unit does not frost up every thirty minutes, it is inefficient to have the heat pump defrost timer set to thirty-minute intervals. The start-up technician should know the best time intervals based on his experience with your geographical location.

Back-up Heat | How Heat Pumps Work

Air source heat pumps generally have some kind of backup heating. That is because when the temperature outside drops below a certain point it needs a source of backup heat. The refrigeration component in the heat pump struggles to keep up with demand. Therefore, many heat pumps have a backup source of heating when the temperature outdoors drops below a certain point.

The backup source of heating is generally controlled via the heat pump thermostat. However, there are other methods of controlling the backup source of heating for a heat pump. In the typical heat pump installation, the thermostat will turn on the backup heat. Furthermore, this happens when the temperature of the thermostat falls 3 degrees below the set point of the thermostat.

For example, if the thermostat is set for 70 degrees and the temperature inside falls to 67. The thermostat will engage the secondary or backup heat. This gives a boost so the temperature will rise inside. Other methods of controlling the secondary or backup heat include an outdoor thermostat that will break the “Y” contact which controls the heat pump condenser and engage the “W” or “W1” contact which controls the backup or secondary heat.

Electric, Oil, or Gas - Dual Fuel or Hybrid Systems

How Heat Pumps Work in Cold Weather - Heat Pumps can use different types of backup sources for heating. The most common is electric heat. However, there are heat pumps you may have seen advertised as dual fuel or hybrid. These systems use either gas or oil for the backup heat. Gas and oil are considered far more efficient than electric heat strips for backup heating. If I had a heat pump I would prefer a dual fuel system using gas for backup. This means instead of an air handler I would have a gas furnace. Of course, the higher the AFUE rating for the furnace the better efficiency.

You will still use the process of refrigeration in both the summer and winter for comfort. However, in the winter, when the system calls for defrost or falls behind, a gas furnace will provide additional heating.

Conclusion

A heat pump works by using refrigeration to absorb heat from the outside air. When the temperature falls too low for the heat pump to provide proper heat to maintain demand then the air source unit will kick in the back-heating. Backup heating such as electric, gas, or oil heat to provide you with heating. Lastly, that is How Heat Pumps Work