Air to air heat pumps, also known as air/air heat pumps, belong in the category of air source heat pumps (ASHP). They are heating systems that use the outside air as the heat source and the indoor air as the heat sink. The system can be reversed during the summer to enable cooling.
In this article we’re covering the following topics regarding air to air heat pumps:
- Efficiency of Air to Air Heat Pumps
- Air to Air Heat Pumps for Cooling
- The Indoor Heat Exchanger Unit
- The Outdoor Heat Exchanger Unit
- Pros and Cons of Air to Air Heat Pumps
- How Much Money Can I Save?
- Checklist Before Purchase
- Refrigerants
At least half of the energy demand for heating should be covered by different energy sources than electricity and oil/gas. An air source heat pump can be a great way to achieve this.
A good rule of thumb is that the longer the heating season you have, the more beneficial the investment of an air to air heat pump will be (this is also true for other types of heat pumps). An air to air heat pump is likely a good investment if the following criteria seems to fit your situation:
- Your household has a total energy consumption of over 15,000 kWh annually.
- You live in an area with mild winters and long heating seasons.
- Your home has few rooms, so that heat can be distributed freely to the areas where heating is wanted.
- You have access to alternative sources of heat during the coldest periods (peak energy)
Efficiency of Air to Air Heat Pumps
Heat Factor (COP, Coefficient of Performance)
COP is a measure of power output divided by input power. In other words, how much electricity is required compared to the heat generated by the system? If the COP is 2, you get twice the amount of heat as you would’ve with convetional electrical heating. The higher this value us the better. COP varies with temperature of the temperature of the air to air and the air sink, quality of installation and many other factors.
Modern air to air heat pumps use inverters, which enables throttling of the compressor. This reduces wear and tear and increase COP. The inverter technology has been much development lately and the performance varies between the different manufacturers. It is therefore important to compare the COP at different outdoor temperatures for different air to air heat pumps. Those with only the on/off power control are not recommended.
Air to Air Heat Pumps for Cooling
Reverse the Heat Pump for Cooling
A heat pump can be reversed in summer to cool the house. By reversing the heat pump, the air condenses into water inside the indoor unit. This unit should therefore have a drainage. Note that a heat pump spends more energy in cooling mode.
Air to air heat pumps consists of an indoor and an outdoor unit connected with cooling pipes and electrical cable. The installer should be able to help you with selecting the right type of indoor and outdoor unit(s) in your situation, as well as finding the ideal locations to place those. Here are some guidlelines to give you an idea:
The Indoor Heat Exchanger Unit
Placement
The indoor unit should be able to spread heat to other rooms easily. If the house has two or more stories, the indoor unit will often be placed on one of the lower levels near an open staircase. It is also possible to install multiple indoor units.
Noise
Although technological breakthroughs have made the indoor unit quieter in the last years, you should think carefully about where it should be located with respect to noise.
The Outdoor Heat Exchanger Unit
Placement
A good placement for the outdoor unit is important for smooth operation and energy savings in the long run.
The outdoor unit is vibrating. It is therefore not recommended that it is attached directly to the house, but mounted onto the ground or attached to the foundation. The outdoor unit should also be protected from rain and wind.
Noise
The outdoor unit produces more noise than the indoor unit. Make sure that you place it away from neighboours or a places where you will be annoyed by the noise yourself.
Pros and Cons of Air to Air Heat Pumps
Pros
- The heat pump has thermostatic controls (thermostat).
- The installation is simple, and a heat pump can be retrofitted in most homes easily.
- Moderate investment costs compared to energy savings.
- An air/air pump can run in reverse and provide cooling when it’s hot outside.
- A good heat pump distributes heat evenly and filters out dust particles to improve indoor climate.
Cons
- The heat pump is a point heat source and is dependent on placement for the heat to spread throughout the house properly.
- The heat pump has the lowest coefficient of performance (COP) when heating needs are greatest – there is not a lot of energy in the outdoor air at the coldest days.
- The need for extra heat on top of what an air/air heat pump is able to generate is therefore greater than with other alternative heating methods such as ground source heat pumps (temperatures a few feet below the surface are higher and not as dependent on seasonal changes).
- When outside temperatures gets closer to the freezing point, frost forms on the surface of the evaporator, reducing the performance of the system, and defrosting is necessary.
- The air filter of the indoor unit must be vacuumed and/or replaced regularly to maintain the performance.
- High humidity in the air can shorten the life of the heat pump.
How Much Money Can I Save?
How much your energy savings will end up at is related to proper sizing, design and operation of the heat pump. The price of an air to air heat pump and the installation of it can vary a great deal. Good air to air heat pumps including installation normally costs somewhere between $2 500 to $4 500.
Lifetime normally ranges from 10 to 15 years. Models with timer with timed temperature control require less power than those who only have a thermostat.
Below is an overview over how much money can be saved in typical American households:
| Total Energy Consumption [kWh/year] | Energy Consumption for Heating [kWh/year] | Energy Savings [kWh/year] | Money Savings [$/year] |
| 10 000 kWh | 5 500 kWh | 1 900 kWh | $225 |
| 15 000 kWh | 8 250 kWh | 2 900 kWh | $338 |
| 20 000 kWh | 1 000 kWh | 3 900 kWh | $455 |
| 25 000 kWh | 8 250 kWh | 4.800 kWh | $560 |
| 30 000 kWh | 8 250 kWh | 5.800 kWh | $677 |
The calculations above are base don the following consumptions:
- Electricity costs are 11.55 cents/kWh (01/02/2012).
- The heat pump supplies 60% of the heating.
- 55% of the total energy spent by this household is used for heating.
- Annual heating factor of 2.4. This means that there is 2.4 times more energy in heat in the output compared to the energy in the electricity input (the higher the better).
Let’s take the 10 000 kWh/year household as an example: 1 900 kWh and $225 will be saved every year. If we assume the lifetime of the heat pump to be 15 years, it will save 285 000 kWh and $3 375 before the warranty runs out. This calculation is not taken rising electricity costs into account.
Checklist Before Purchase
- A professional should install the heat pump.
- Do the installer and distributer have good reputations? Ask for references.
- Get price estimation for all costs.
- Does the installation include regular check-ups?
- Get a contract and make sure all promises are included.
- Ask for written instructions that show how the air to air heat pump should be operated.
- Ask what kind of maintenance that will be needed to ensure good performance. Filters usually have to be replaced every six months to maintain the filtering effect.
- Compare different heat pumps for their coefficient of performance (COP) at the temperatures you will be using them.
- Make sure the air to air heat pump is big enough for your energy needs. Air to air heat pumps usually havea a heat output of 4 to 7 kW.
Refrigerants
The refrigerant used in the heat pump cycle is a liquid that evaporates easily. The most common refrigerant is currently R-410A. By transitioning between gas and liquid form, it can absorb and release heat easily. Approved refrigerants do not damage the ozone layer if they leak, and they are not toxic or flammable. However, they can still cause frostbites and eye damaged if leaks are not handled properly.