Air source Heat Pump FAQ

Basics

A heat pump is a device that moves heat from one place to another but does not generate any heat of its own. Usually the flow of heat follows the temperature gradient. However, a heat pump uses a coolant, a compressor, and an evaporator to liquefy and evaporate the coolant at selective places and then use the heat of evaporation or condensation to move the heat against a temperature gradient much like a water pump moves water against the force of gravity. So it is called a heat pump.

All houses now have a refrigerator which uses a small heat pump to move heat from the inside of the refrigerator to the surroundings. Thus the refrigerator stays cool. An AC is another example used to move heat from inside the house to the hot outside much against the temperature gradient. In case of a heat pump space heater, it can move heat from the inside to the outside during summer and double as an AC. In winter it has a mechanism to reverse the flow and move heat from the cold outside to the warm inside and act as a space heater.

A system is composed of an outside unit with a fan and compressor, an indoor heat exchanger, a number of ducts and tubes for carrying the coolant between the units, conditioned air etc. and some electrical connections.

There are usually two main types- 1. A whole house central heat pump that uses air ducts inside the house and a central heat exchanger to condition the air flowing through the ducts. 2. A ductless mini-split system that uses multiple registers inside the house to condition the space directly without using any air ducts.

First a heat pump is an AC and furnace combined into one. Then with a heat pump, you don’t get those uncomfortable blasts of hot air that you get with a gas furnace. A heat pump keeps the room temperature more constant than a gas furnace, so you’re more comfortable all the time. The heating process is slower than a gas furnace and so an even comfortable temperature needs to be maintained throughout the day. However energy wise a heat pump is 3-4X more efficient than the most efficient gas furnace.

It is advisable to use small set backs in a heat pump as the heating and cooling takes time. By maintaining almost uniform temperature, the temperature recovery time is reduced.

In general the mini-split systems are more efficient than central ducted systems and function well into much lower temperatures for heating. Also each register inside the house comes with its own thermostat remote. So the heating or cooling of the house can be fine-tuned much better my using spaces as they are needed and thereby saving energy.

Economics

The cost of systems are typically 10-20% higher than equivalent systems with only AC. The installation costs are also similar to an AC.

This depends on the price of natural gas and electricity in the area. However, a heat pump system is much more energy efficient with 3-4 units of heat being pumped into the house for every unit of electricity used in the device.

Maintenance

The maintenance needed is similar to an AC. It consists to cleaning the coils from time to time, changing the air filters in ducts or registers and recharging the coolant in case of leaks.

A heat pump is designed to run all year long–like a refrigerator. Appliance Magazine says heat pumps have about the same life expectancy as a regular central air conditioner used with a gas furnace. And even the Gas Research Institute admits in a study report that “As the efficiency of heat pump increases, their anticipated life increases as well.” By contrast, in that same study, the Gas Research Institute suggests that the expected life of gas furnaces will decrease as their efficiency improves.

Environment

This is true in general as because they are super efficient. Even when the grid electricity is not completely carbon free a heat pump will have less overall pollution than a similar gas furnace. If grid energy is clean or it coupled with rooftop solar the heat pump can have zero emissions.

Steps to use

The installation is very similar to that of an AC. The thermostat control uses one more wire than a simple AC to switch between heating and cooling mode. Depending on complexity of system the project can take 1 day to 1 week.

The mini-split systems are recommended if the house does not have air ducts or extensive rework is needed on them. If existing ducts can be reused then usually a central ducted heat pump is cheaper to install.

It varies by house construction and need but the rule of thumb is to have one register for every 600 sq ft of conditioned space in the house. In current multi-zone heat pumps a single outside unit can support 2-8 registers.

Usually the whole house central heat pumps cannot function well below 30F. So in these devices a backup electrical or gas heater is needed which is used minimally in extreme weather. The mini-split systems are much more efficient and can function to 0F and so in most cases they do not need a back up heating device.