A heat pump is simply an air conditioning unit with the indoor and outdoor coil functions reversed so we can take heat from outside the space and transfer it into the space. Yup, That’s it.
The heat “outside the space” may come from outdoor air (air to air heat pump), the ground (ground source heat pump) or a river, pond or building loop (water source heat pump). I know what you’re thinking “How do you get heat from cold air, cold cold water or cold dirt?”
Here’s how it works; heat exists all the way down to -460 degrees Fahrenheit. Add heat of compression from the compressor, motor heat from the compressor’s motor and latent heat (the frost forming on the outdoor coil) and you have…heat.
Still don’t understand it? Don’t worry. We’re technicians, not engineers. It works, and that’s all we really need to know.
Lets talk about what you should know.
We’ll start with the most common myth “Heat pumps loose efficiency the colder it gets outside” While this may sound good, it’s false.
Air to air heat pumps don’t loose efficiency as the outdoor temperature drops, they loose capacity. An air-to-air heat pump’s efficiency enemy is frost forming on the outside coil, and the defrost cycle used to melt the frost. Why? The frost acts as an insulator, preventing heat transfer. How do we melt the frost? Run the system in cooling mode. But wait, we want heat, don’t we? Yes we do.
Then what do we do when the system is using cooling mode to defrost the coil? We turn on electric heaters in the air handler to reheat the cool air. That’s right, we turn on the air conditioning and the electric heat at the same time. Not exactly what a normal person would consider ‘efficient’.
This is why ground and water source heat pumps are more efficient; they don’t need a defrost cycle. This is also why we need to be sure the defrost cycle is operating properly and is set up correctly on air-to-air units.
This is also why our next post will cover defrost controls. Stay tuned.