In cold climates, houses often use a heat exchanger for ventilation. Homeowners who are currently using a furnace for heating may wonder if they still need a heat exchanger if they use a heat pump. So, do heat pumps have heat exchangers?
All heat pumps have heat exchangers. However, the heat exchangers in heat pumps are meant to absorb heat from the outdoor air for heating. Heat pumps don’t have air exchangers that are meant for ventilation. Hence, an air exchanger may still be needed if you have a heat pump.
Sometimes, homeowners are told that their existing air exchanger is no longer needed when they have a heat pump. But, it may not be applicable to all houses.
What is a Heat Pump?
Heat pumps are new to many homeowners. Even professionals find heat pumps quite difficult to understand, especially when they didn’t have much experience with them.
In cold climate countries like Canada, the United Kingdom and the Northern United States, most houses are using a furnace (oil/gas) for heating. The furnace generates warm air and distributes it via ductwork.
On the other hand, heat pumps don’t burn oil/gas to generate warm air. Instead, they use the refrigeration cycle process to harvest heat from the outdoor air and transfer it for heating.
A heat pump system is comprised of two separate pieces of equipment; a) condenser and b) evaporator. The condenser is typically an air handler that looks almost like a furnace and the evaporator is a square cage that looks just like the air conditioning unit placed outside the house.
In a heat pump system, the evaporator’s responsibility is to harvest heat from the outdoor air. It is able to do so despite the low outdoor air temperature because it uses a refrigerant that is much colder than the outdoor air.
Refrigerants are chemical mixtures that are engineered to change phases (liquid or gas) at specific pressures and temperatures. They are able to store an enormous amount of energy during phase change.
Since heat flows from hot regions to cold regions, the refrigerant is able to gain heat from the outdoor air.
In other words, when the evaporator absorbs the heat from the outdoor air, it stores the heat energy inside the refrigerant. Then, the evaporator transfers the refrigerant to the condenser which is located inside the house and the refrigerant releases its stored heat at the condenser and thus, heating the house.
Afterward, the refrigerant returns to the evaporator and the cycle repeats.
The above is the basic working principle of a heat pump. If you want to learn more about how a heat pump works, see my post What is an Air Source Heat Pump?
What is a Heat Exchanger?
Heat exchangers are devices that transfer heat from one medium to another medium. For example, heat is transferred from the outdoor air to the refrigerant in the evaporator.
Therefore, the evaporator is a heat exchanger.
However, there are other components in the evaporator. For instance, the evaporator also has a compressor, an expansion valve, a fan and a fin-tube heat exchanger.
As you might have guessed already, the fin-tube heat exchanger is the heat exchanger, to be precise.
Cold refrigerant produced by the heat pump system flows through the fin-tube heat exchanger in the evaporator in order to harvest heat from the outdoor air. At the same time, a fan circulates the outdoor air through the fin-tube heat exchanger to encourage heat transfer.
There are a few types of heat exchangers. Air source heat pumps that harvest heat from the outdoor air mostly has a fin-tube heat exchanger. Water source heat pumps that harvest heat from water usually has a plate heat exchanger instead.
Furnaces have heat exchangers that are different from heat pumps. The heat exchanger in furnaces transfers heat from the combustion of oil/gas to the indoor air. Meanwhile, the heat exchanger in heat pumps transfers heat from the hot refrigerant to the indoor air.
What is an Air Exchanger?
Technically, air exchangers are not heat exchangers. However, air exchangers also transfer heat from one medium to another medium. Thus, many people confuse air exchangers with heat exchangers.
Generally, an air exchanger transfers heat from the indoor air to the outdoor air. In cold weather, the air inside the house is warmer than the outdoor air. As outdoor air is needed for ventilation, an air exchanger can conserve energy by using indoor air to pre-heat the outdoor air.
Often, air exchangers are better known as heat recovery ventilators (HRV) or energy recovery ventilators (ERV). They are essential for ventilation in airtight houses.
New houses (built after the year 2000) are sealed. Their air leakage rate is low compared to old houses. Hence, natural ventilation is minimal.
So, the introduction of fresh air or outdoor air is necessary to maintain good indoor air quality. At the same time, the air inside the house must also be taken out to prevent pressure imbalance.
However, bringing in the cold air from outside and exhausting the warm air out of the house will increase the heating load and lead to higher energy cost.
Therefore, an air exchanger or ERV/HRV is used to recover some of the heat from the air inside the house and transfer the heat to the outdoor air.
Heat Pump vs Heat Exchanger vs Air Exchanger
Heat pumps have heat exchangers but they do not have air exchangers. In fact, air exchangers are ventilation devices while heat pumps are heating and cooling systems.
Meanwhile, heat exchangers are devices that are used for heat transfer. They can be used in air exchangers as well as heat pumps.
Heat pumps are made of different components including heat exchangers. Together, they transfer heat from the outdoor air to the indoor air.
On the other hand, air exchangers are also made of different components such as fans, filters and heat exchangers as well. Together, they pre-heat the outdoor air using the heat from the indoor air.
In short, heat pumps are heating and cooling systems, heat exchangers are heat transfer devices and air exchangers are ventilation devices.
Do You Need an Air Exchanger If You Have a Heat Pump?
Homeowners who are using a furnace (oil/gas) may want to use a heat pump for heating as heat pumps are more efficient and better for the environment.
However, many people confuse heat exchangers with air exchangers and since all heat pumps have heat exchangers, they wonder if they still need an air exchanger if they have a heat pump.
The answer is it depends.
You would still need an air exchanger if you have a heat pump because a typical heat pump system does not bring in outside air for ventilation. Also, you need an air exchanger to maintain pressure balance if your house is sealed.
Most of the time, a heat pump is connected to an air handler where warm air is distributed via ductwork in the house. For such a heat pump system, an air exchanger may be eliminated.
A furnace usually doesn’t bring in outside air for ventilation but for burning purposes since oxygen is needed for efficient combustion.
With a heat pump, you can connect a fresh air duct to the return plenum of the air handler. The fresh air duct connects the air handler to the external wall of the house, allowing the air handler to take in the outdoor air. Then, a damper allows you to control the amount of fresh air intake.
Such a ventilation system is also known as supply-only ventilation.
However, supply-only ventilation may not be suitable for all houses.
During heating, the heat exchanger in the condenser or air handler of the heat pump system (also applicable to furnaces) does not dehumidify. With the fresh air intake, you could potentially raise the humidity level in your house.
So, in certain places, the best way to ventilate houses is to use a balance ventilation system that brings in outside air and exhausts out indoor air simultaneously.
Such a balanced ventilation system can be achieved with an air exchanger such as a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV).
Sometimes, a heat pump can be connected to a wall-mounted unit where warm air is discharged directly without ductwork. Such a heat pump system is often known as a ductless mini split heat pump system.
With a ductless mini split heat pump system, a fresh air duct is not possible. Therefore, an air exchanger may be the only option if you need forced ventilation (where natural ventilation is not possible because the house is sealed).
However, a mini split heat pump system doesn’t always have to be ductless. In fact, a mini split heat pump can be connected to a ceiling ducted unit where warm air can be distributed through ductwork.
With a ceiling ducted unit, a fresh air duct can be attached to the return plenum of the unit. Thus, a supply-only ventilation system can be achieved.
Nonetheless, an air exchanger is still needed in sealed houses to achieve pressure balance.
Learn more about ducted mini split in my post Ducted Mini Split Guide: Basics, Comparison & Installation.
Whether you have a furnace or a heat pump, it doesn’t change the fact that you need an air exchanger. If your house is sealed (low air leakages), an air exchanger such as an ERV or HRV is recommended. However, if you don’t need a balanced ventilation system, heat pumps can be ducted and thus, able to bring in outside air for ventilation.
See other types of ventilation systems in my post 5 Types of Ventilation Systems (Pros & Cons).
If you need me to help with your HVAC system, consider my Email Consultation service.
This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.
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