A heat pump is a type of AC system that uses the cooling process in reverse to generate heat. When bidding residential systems, I often ask customers with straight cool systems if they would like to upgrade to a Heat Pump. This question is often followed by the customer asking me what the difference is between the systems. In this blog, I will go through the process of how a heat pump works along with the Pro’s and Con’s of upgrading to this type of system. Before we do that, I want to state that all split and package systems have electric heat kits in the air handler portion of the system. Essentially an electric heat kit is resistance heat like a toaster. In a straight cool system, the electric heat kit is the only source of heat.
Before we start talking about what a heat pump is, let’s first review the basics of a straight cool system. A straight cool system pumps (a compressor is really just a pump) Freon through a high-pressure liquid line (smallest of the two copper lines) to the evaporator coil in the air handler. When the Freon hits the air handler, it expands and becomes a gas. The process of the Freon changing from liquid to gas in the evap coil is what makes it cold (it’s science). When air blows across the evap coil, it picks up the cold and the freon absorbs the heat through the coil. Freon leaves the evap coil through the low-pressure gas line (bigger of the two copper lines which is insulated because it is still cold) and heads back to the condensing unit where it runs through the condenser coil. While running through the condenser coil, the Freon bleeds off the heat with the help of the condenser fan and then returns to a liquid state to be pumped back through the process again.
So now that we understand cooling basics, let’s start reviewing how a heat pump works. In the most simplistic form, a heat pump runs the above process in reverse so that the evap coil in the air handler is providing heat and taking away cold. There are some mechanical and electronic components in a heat pump system that allow this process to work. The first is a mechanical reversing valve and when the thermostat is set to heat, its function is to reverse the direction of the flow of Freon. Next is an accumulator which is essentially an extra tank that holds Freon in its gas state. From an electronics standpoint, there is a defrost sensor and a defrost circuit board. While running in reverse to take the cold out of your house, there is risk of the compressor freezing over. The defrost sensor monitors that temperature and when triggered, tells the defrost board to activate. In this mode, it will turn off the fan on the outdoor unit, send the system back into cooling mode and turns on the back-up heat strip to keep the air inside tempered. Once the defrost sensor determines there is no danger of icing up, it sends the system back into heat pump mode.
Heat pumps work the best when the outdoor ambient temperature is around freezing or higher. They lose their effectiveness when the temperature gets too cold outside so they are very popular in southern states. Most installers will put a 5 kilowatt electric heat strip (one of the smallest) in the air handler of a heat pump system for times when it is really cold outside and the heat pump cannot keep up. You may notice your thermostat going into “auxiliary” or “2nd stage” heat. That is when the heat pump is running concurrently with the electric heat to keep up with the demand.
The entire creation of a heat pump system is targeted to one thing…energy savings you can see on your electric bill. A heat pump can save you 30% or more vs. the electric heat in a straight cool system. As far as cons, a heat pump system can cost $400 – $900 more than a straight cool system. The final con would be the potential failure of a reversing valve, defrost sensor or defrost board which are components only found in heat pump systems.
In summary, if you like it toasty in the winter while spiking your eggnog instead of your power bill, a heat pump may be right for you. Lastly, I want to call-out this entire blog was focused on heat pumps. While cooling your home, there is no difference between a heat pump and a straight cool system. A 15 SEER heat pump running in cooling mode provides the same energy savings as a 15 SEER straight cool system