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What is Simultaneous Heating and Cooling?

Simultaneous means existing, or occurring at the same time. When we are talking about HVAC simultaneous heating and cooling this means that we are heating and cooling at the same time. HVAC systems have the ability to have a single unit that can give you any amount of heating, any amount of cooling and any amount of simultaneous heating and cooling at once. This means if you have a building with higher heating loads at some points and higher cooling loads at other points, but a large simultaneous (mixed) load throughout the year, your best and most efficient option will be to put in a simultaneous heating and cooling unit. This was typically seen in the HVAC industry with VRF (variable refrigerant flow) systems, but over the past couple years there has been major developments with air to water simultaneous heating and cooling units. Air to water Simultaneous units can operate in heating, cooling, or simultaneous heating and cooling allowing you to recover as much energy as possible.

There are simultaneous units being designed all over North America as everyone is constantly striving to be as energy efficient as possible and even design towards Net-Zero buildings. The difference in VRF systems and air to water units is the fact that as VRF units are efficient, you will have refrigerant running through the full building. With Air to water units the high efficiency is still there, but the need for refrigerant in the building is not there as you will be running water through the building. As we continue to reach for the highest of efficiencies, moving away from fossil fuels and refrigerant in full buildings air to water Simultaneous heating and cooling are the design of not only the future, but the design of right now. Read about our air cooled simultaneous unit here. The NRP. 

How Does it Work?

Simultaneous heating and cooling works by controlling/directing the refrigerant in a unit to either produce heating, cooling or simultaneous heating and cooling. Essentially, the simultaneous units have multiple heat exchangers allowing them to provide cooling only through an evaporator, or heating only through the condenser they also have a heat recovery heat exchanger that allows them to drop the source (air or water) and move the energy around the building.

If we are talking about an air to water unit, the unit should have 2 heat exchangers.

  1. There should be one dedicated for cooling
  2. One dedicated for heating

When the unit has a need for cooling only it will work as a chiller, by having cooling to the cooling dedicated heat exchanger and reject the heating out to the condenser. When the unit has a need for heating only it will work as a heat pump, by having all the heating to the heat exchanger that is dedicated for heating and reject the cooling to the condenser. When the unit is in simultaneous heating and cooling the unit will essentially be working as a water to water unit. It will not use the outdoor condenser and by controlling the refrigerant inside the unit we can have all the cooling we need to the dedicated exchanger for cooling and all the heating we need to the dedicated exchanger for heating.

When we get into water to water simultaneous heating and cooling, there should be 3 heat exchangers:

  1. One will be dedicated to cooling to the building
  2. One should be dedicated to heating
  3. One should be dedicated for the source side (geothermal loop, cooling tower, or boiler loop)

By having 3 heat exchangers inside the unit all the unit mode changes will be done on the refrigerant side. The unit will work like the Air to water simultaneous unit only instead of having the outdoor condenser, there will now be a source side heat exchanger. If you have a need for cooling the refrigerant will direct to the cooling heat exchanger and reject the heat to the source exchanger if it is not needed. If there is a heating only need the unit will use the heating dedicated heat exchanger and reject the cooling to the source side heat exchanger. If there is a simultaneous heating and cooling load the unit will not use the source side heat exchanger at all allowing you to have cooling and heating simultaneously inside the building. Read about our water cooled simultaneous unit here. The NXP. 

Simultaneous units are able to provide cooling, heating cooling and heating at any time in any amount up to 100%.

100% Heating or 100% cooling or 100% heating and 100% cooling. We may also have 20% cooling and 80% heating or 30% heating and 60% cooling. Any amount of heating and any amount of cooling at any time and the smallest load will be free.

Why Simultaneous Heating and Cooling?

Simultaneous heating and cooling units have major advantages in any building that will have a need for heating, cooling and a large mixed load throughout the year. Simultaneous units are used in Net-Zero buildings for a reason. They are the most efficient solution to this date as you can do all the buildings cooling and recover all the heating needed for free. It also works the opposite way if the heating load is dominant it will be in heating and recover the cooling. This allows simultaneous units to have a standard chiller efficiency of around 10 Btu/W in cooling only mode and a COP of around 3 W/W for heating only mode (air to water). When the unit is in simultaneous heating and cooling the efficiency will have a Total Energy Ratio (TER) of above 6 W/W. By having a TER of above 6 W/W this proves to be the most efficient way to have simultaneous heating and cooling. Traditionally this will need to be done with a chiller and boiler.

With a chiller and boiler you will have 2 separate systems and they will not allow any heating or cooling recovery (free). With air to water and water to water we have the ability to have any amount of heating, any amount of cooling, or any mixed load all with 1 unit. This will save on energy costs, installation cost, and controls cost.

Why simultaneous heating and cooling? As stated in the beginning of this topic, any building with a need for heating, cooling and has a large mixed (simultaneous) load all year will have many advantages and energy saving opportunities. air to water and water to water simultaneous units is the future design for today.

Aermec provides the energy recovery on the refrigeration side of the system while many competitors do it on the water side. Aermec has many different sizes to allow one unit with multiple circuits as the system. Most competitors use smaller modules and must join many modules together causing a higher installation cost as well as opening the door for more leaks etc as contractors need to provide labor on site for something Aermec does in the factory.

Aermec offers simultaneous units from 13 tons through 240 tons. For more details please see the Unit Guide here.

Other Systems

If a building has a need for simultaneous heating and cooling, there are typically 4 solutions.

Solution 1: Use a Chiller and a Boiler
This solution can be piped to be seasonal changeover, or 4-pipe configuration. The problem with a chiller and a boiler is that you will never be recovering any energy from one another and you will be using fossil fuels inside the building. As we are trying to move away from fossil fuels and more towards energy efficient solutions we look to packaged simultaneous machines.

Solution 2: Use a VRF System
VRF systems are seen quite frequently, they are efficient and they allow for simultaneous heating and cooling into a building. The problem with a VRF system is the fact that there will be refrigerant throughout the full building. We understand this is a better solution than using fossil fuels, but moving forward we are trying to limit the amount of refrigerant we have inside the building. With refrigerant in the building there are a couple disadvantages:

  1. You must respect ASHRAE 15 & 34 in regards to maximum concentration of refrigerant allowed in a space
  2. If the refrigerant changes, depending on the density of the refrigerant you will need to change the piping for the full system
  3. If the outdoor unit becomes obsolete you must change the entire system

Solution 3: Air to Water Simultaneous Machine
Air to water systems can do the same thing as a VRF only with some additional benefits:

  1. Now you have the refrigerant for the system outside and only water inside the building. This means you will not need to worry about ASHRAE 15 & 34 as water is not a risk if there is a small leak
  2. If the refrigerant changes, the full building system stays and you will only need to replace the outdoor unit. This saves a lot of costs thinking of the future
  3. If the outdoor unit becomes obsolete you do not need to change anything inside of the building

Solution 4: Water to Water Simultaneous Machine
These units are used in the same scenarios as VRF and Air to water, only where an outdoor unit may not be viable. This machine will have all the same benefits as an air to water unit, but now we will not have an outdoor condenser. This unit will have a source side heat exchanger to reject heating or cooling when it is not needed in the building. The source can be a geothermal loop, boiler loop, or cooling tower etc. With this solution you will still have water throughout the building and only refrigerant inside the water to water unit. Another major difference in VRF and Air to water or water to water units is the fact that VRF uses compressors to move the refrigerant throughout the building (high cost) and units that use water in the building only use a hydronic pump to move water around the building (low operating cost).

As we are moving forward we see more and more designs going towards Air to water and Water to water simultaneous units.


Simultaneous units can be used in any 4-pipe building. Any building that has dominant heating at some points, dominant cooling at some points, but also a large simultaneous (mixed) load inside the building will benefit from a simultaneous unit. We see all different types of applications for simultaneous units.

To go over a couple examples we can look at a hospital. Hospitals will have a high cooling need all year, but also they need hot water all year also. The heating load goes up and down all year depending on the hot water needs and the cooling seems to be dominant in the summer. In the winter there is still a large cooling load, but the heating load may be the same, making the heating mode dominant at this time and recovering all the cooling at this point.

We have had many applications similar to this like correctional institutes that need cooling all year and needs domestic hot water all year.

We see many residential complexes that are looking to go Net-Zero, they can now have a unit that does all of their heating, cooling and domestic hot water all year round. Access our extensive Case Studies page here.

Any building that has simultaneous loads will benefit from a simultaneous unit. The more the simultaneous unit operates the greater the savings. Condominiums, schools, pools, grow ops, apartments, hotels, office towers, mining, industrial, shopping malls to name a few.

Building management: Bacnet, Lonworks, Modbus are all available and no need to worry about proprietary software. Please refer to the chart here