How HVAC Heat Pumps Work

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May 1, 2022

We’ll cover How HVAC Heat Pumps Work, and the differences between an Air-Cooled and Water-Cooled Heat Pumps. We’ll explain the different styles of heat pumps used in residential and commercial buildings. You’ll learn the differences between air and water-cooled, packaged versus split systems, vertical versus horizontal, and ground and water source heat pumps.

If you prefer to watch the YouTube version of this presentation scroll to the bottom.

Table of Contents

Refrigerant Cycle of a Heat Pump System – Reversing Valve

The Heat Pump has the same basic refrigeration circuit as a standard Air Conditioner except for a few specific components to help it succeed in both heating and cooling with the same refrigerant circuit. See our video on Refrigerant Circuits if you need a basic understanding on how they work. The main component that helps it function in both modes is the “Reversing Valve”.

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Refrigerant Cycles - Standard Air Conditioner vs Heat Pump — Refrigerant Cycles – Standard Air Conditioner vs Heat Pump

The reversing valve has two modes of operation, its either in heating or cooling mode. The reversing valve provides changes in direction for the refrigerant flowing through the system, it reverses the flow, hence reversing valve. The reversing valve is the refrigerant traffic controller, changing the direction of refrigerant flow based on the demand of the space controller. The reversing valve sends the hot gas discharge from the compressor either to the outdoor unit heat exchanger or the indoor heat exchanger.

Reversing Valve in HVAC Heat Pump System

In relationship to the compressor, cooling is always accomplished on the suction side of the compressor, and heating is furnished on the discharge side. What the reversing valve does is switch the compressor discharge to go to the indoor coil when heating is required. It’s basically that simple, as the reversing valve acts as a traffic controller, sending refrigerant in one of two directions from the compressor.

To understand this better, we’ll walk through what happens in each season when heating or cooling is required.

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Heating Mode

How HVAC Heat Pumps Work in Heating Mode. Starting at the compressor where the refrigerant is compressed causing the temperature and pressure of the refrigerant to increase. This high-pressure/high temperature gas (vapor) refrigerant will then enter the reversing valve where it will be directed to the indoor coil.

This high pressure, high temperature gas enters the indoor coil where cooler air from the space passes over the indoor coil absorbing some of the heat from the refrigerant, thereby heating the room, while causing the refrigerant gas to give up its heat and condense into a high-pressure liquid.

The refrigerant will than leave the indoor coil as a high-pressure liquid and enter the expansion valve where it’s pressure will be reduced causing it to become low-pressure liquid. See our video on expansion valves for more info.

This low-pressure liquid refrigerant will than enter the outdoor coil where it will absorb heat from the outdoors, causing the low-pressure liquid refrigerant in the outdoor coil to evaporate, absorbing the heat. This low-pressure refrigerant gas than enters the compressor where the cycle repeats itself.

Cooling Mode

How HVAC Heat Pumps Work in cooling mode, the heat pump acts the same as a standard air conditioners refrigerant cycle, whereby the condenser sends high pressure liquid to an expansion valve that lowers the pressure of the refrigerant going into the evaporator or indoor coil.

The heat from the space is passed over the indoor coil causing the refrigerant to boil or evaporate and absorb the heat from the air. This will cause the air to cool as its passes over the cold coil and head back to the space colder.

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Packaged vs Split System Heat Pumps

You can get a Heat Pump that is all contained in one box, called a Packaged Heat Pump, or you can install it in two pieces which is called a split-system, and is commonly found in residential projects.

The split-system has an outdoor section, which houses the compressor and a heat exchanger with a fan in all air systems. The indoor section of a split system will have a heat exchanger and a fan. This is an all-air system, as each heat exchanger uses a fan. There are also water-sourced version which will cover now.

Water Source Heat Pumps

You’ll find the use of water source heat pumps in various sizes of commercial buildings. This allows for the use of a common water loop that works as a means of moving heat around. This heat can be discharged or absorbed from the water loop by the indoor heat pumps. The water loop is maintained between approximately 60 and 90 degrees. To help maintain the loop temperature within this temperature range a boiler is added to ensure enough heat is available during winter while the closed circuit cooling tower will reject additional heat during the summer or hot days.

The water is sent to a closed circuit cooling tower mounted outside on the ground or roof level. The water is in a closed loop, meaning that it never gets exposed to atmospheric elements. This keeps the heat exchangers of the water-cooled heat pumps clean by preventing impurities entering from exposed water sources.

With water-source heat pumps, heat rejection or absorption into the heat exchanger is through water. In an air-cooled system, a fan is used for heat transfer.

There are two commonly used types in the commercial industry, vertical and horizontal water source heat pumps, in addition to water source heat pumps used in ground source systems discussed later.

Vertically Stacked Water Source Heat Pumps

How HVAC Heat Pumps Work using vertically stacked units. There are lots of high-rise hotels that use vertically stacked water sourced heat pumps to provide heating or cooling to their guest-rooms. They are hidden behind the walls with a supply and return grill mounted to the face of the unit, and a temperature sensor nearby.

A vertically stacked water-source heat pump uses a closed-loop of water as a means of heat transfer. When in cooling mode the unit will reject its heat to the water loop, and while in heating mode it will absorb heat from the loop. The vertical heat pumps range in tonnage from 1/2 to 3 tons.

Vertical water source heat pumps contain the complete refrigerant circuit, with compressor, two heat exchangers, reversing valve and expansion valves.

Horizontal Water Source Heat Pumps

Various commercial buildings will have concealed horizontal water source heat pumps installed for each zone or room. These may also be found in hotel guestrooms, usually mounted above the entrance or bathroom with a stub of duct to feed a supply grille, with a ceiling mounted return grill. The return grill could also contain a filter.

These units are also available for exposed locations, usually near an exterior wall to provide a method for fresh air. Just like the vertical stacked heat pumps, these horizontal heat pumps will have a closed circuit cooling tower mounted outside.

Ground Source Heat Pumps

The sun shining on the ground provides for heat energy to be stored in the soil. This underground stored energy can be used with ground source heat pumps. With ground source heat pumps the ground is used as a means of absorbing heat or providing heat.

A pump circulates water through piping buried in the ground. This piping is connected to a heat exchanger in the heat pump where the heat from the ground will be absorbed or rejected into the refrigerant circuit.

Ground Water Source Heat Pumps

Bodies of water, like lakes or ground water can also be used as a source of energy. They use heat exchangers as a means of separating the refrigerant from the water. Water is mixed with antifreeze like glycol in some climates to avoid freezing and is pumped through a maze of hoses buried in the ground. The water circulates around the buried piping and then back to the heat exchanger where it transfers or absorbs heat from the refrigerant.

Ground - Water Source Heat Pump - Open Loop — Ground – Water Source Heat Pump (Open Loop)

When using groundwater, there are two types, the open loop which is shown here, and a closed loop version. The open loop system uses a pump to withdraw water from a lake or underground aquifer and send it through the heat exchanger where it will absorb or reject heat into the refrigerant before being sent back into the body of water.

In a closed loop system everything happens the same except the piping is closed and circulates the same water and antifreeze mix.

Water sources can provide a method of passive cooling, whereby the compressor can be shutoff.

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How HVAC Heat Pumps Work in Residential and Commercial Buildings

How to Calculate Sensible and Latent Heat Transfer for Air

By

MEP Academy Instructor

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April 25, 2022

0

Sensible and Latent Heat Equation for Air

In this article we’re going to look at the Sensible and Latent Heat Transfer Equation for Air. This is useful when trying to determine either of the three variables, Btu’s, CFM or Delta in Enthalpy. When you know two of these values you can determine the remaining missing value.

If you prefer to watch the YouTube Video of this presentation than scroll to the bottom.

Table of Contents

Sensible and Latent Heat Transfer Equation for Air

q = m ∆h

q = CFM x 0.075 lb/ft²x 60 min/hour x ∆h

q = CFM x 4.5 x ∆h

CFM is ft³/minute

M is the overall mass flow rate of air

h is the Enthalpy (Btu/lb.)

We’ll need to look at a Psychrometric Chart to gather the information needed for this calculation. This is also easily done using software, but it’s best to know how the software arrives at these values.

We’ll begin by calculating for the BTU’s when given the CFM, and the temperature and relative humidity of the Entering and Leaving Air conditions. Remember we’re dealing with both sensible and latent heat. That means were dealing with two factors, the sensible or change in temperature and the latent, the change in state, in this case it will be moisture condensing on a cooling coil.

Sensible and Latent Heat Transfer Formula for Air

Looking at this chilled water coil we see that there is 2,000 Cubic Feet per minute of air flowing through this coil and the temperature of the Entering air is 90°F and 40% relative humidity with the Leaving Air at 55°F and 100% relative humidity.

Example:

2,000 CFM of outdoor air (Ventilation Air) at 90°F and 40% relative humidity (RH) is drawn through a cooling coil that brings the conditions to 55°F and 100% relative humidity (RH)

Step #1 – Plot the given conditions on the Psychrometric Chart

Step #2 – Get the h1 and h2 values from the Psychrometric Chart 34.9 and 23.2

Step #3 – Enter all values into equation.

q = CFM x 4.5 x ∆h

q = 2,000 x 4.5 x 11.7 = 105,300 Btu/hour

With this information we can solve for how many btu’s are being supplied to the air.

Step one is to plot the entering and leaving air conditions on a psychrometric chart to determine the enthalpy. For the entering air conditions of 90°F and 40% relative humidity (RH), we enter here along the horizontal part of the chart that reflects temperature and then we go vertical straight up from there until we hit our 40% Relative Humidity line and then diagonally to the left until we intersect the enthalpy line. Here we get we get an enthalpy of 34.9, and then we plot the leaving air conditions of 55°F and 100% relative humidity we get an enthalpy of 23.2. 34.9 – 23.2 gives us an enthalpy of 11.7 Btu/lb.

Sensible and Latent Heat of Air Plotted on Psychrometric Chart

Step two is to put all the known values into our formula and make the calculation.

We have our formula of q = CFM x 4.5 x ∆h

Now we enter our values, we get

q = 2,000 x 4.5 x 11.7 = 105,300 Btu/hour

Now we quickly explain where the value of 4.5 in the calculation is derived from. First, we have the weight of Air at 0.075 pounds per Ft3, then we have the conversion of 60 minutes into hours. This is what that looks like

0.075 lb/ft³ x 60 min/hour = 4.5

With all these units, we can see which units of value remain by crossing out those that are eliminated in the formula as such

q = 2,000 ~~Ft3~~/~~minute~~ x 0.075 lb/~~ft³~~ x 60 ~~min~~/hour x 11.7 Btu/lb = 105,300 Btu/hour

how to Calculate the Sensible and Latent Heat Transfer of Air

Checkout these similar Videos related to sensible heat:

Sensible Heat Transfer Formula for Air

Sensible Heat Transfer Formula for Water

How to Calculate Sensible Heat Transfer for Air

By

MEP Academy Instructor

-

April 23, 2022

0

In this article we’re going to look at the Sensible Heat Transfer Equation for Air. This is useful when trying to determine either of the three variables, Btu’s, CFM or Delta-T. When you know two of these values you can determine the remaining missing value.

If you prefer to watch the YouTube version of this presentation, then scroll to the bottom.

Table of Contents

Sensible Heat Transfer Equation for Air

q = m x C_px ∆T

q = CFM x 0.075 lb/ft³ x 60 min/hour x 0.24 btu/lb°F x ∆T

q = CFM x 1.08 x ∆T

CFM is ft³/minute

m is the overall mass flow rate of air

C_p is the specific heat of air 0.24 btu/lb°F

We’ll begin by calculating for the BTU’s when given the CFM and temperatures of the Entering and Leaving Air. Remember we’re dealing with sensible heat only. That means there is no latent heat which involves a change of state, like moisture in the air condensing into water or condensate. All we’re doing is changing the temperature of air, while not removing any moisture.

Example:

2,000 CFM of outdoor air (Ventilation Air) at 55°F and 70% relative humidity (RH) is heated to 90°F

Step #1 – Determine the ∆T (90°F – 55°F = 35°F)

Step #2 – Enter all values into equation.

q = CFM x 1.08 x ∆T

q = 2,000 x 1.08 x 35 = 75,600 Btu/hour

Looking at this heating hot water coil we see that there is 2,000 Cubic Feet per minute of air flowing through this coil with a Leaving temperature of 90°F and the Entering Air at 55°F.

With this information we can solve for how many btu’s are being supplied to the air.

Step one is to subtract the entering air temperature of 55°F from the leaving air temperature of 90°F to arrive at the Temperature difference or Delta-T. 90°F – 55°F, gives us a 35°F Delta-T.

Step two is to put all the known values into our formula and make the calculation.

We have our formula of q = CFM x 1.08 x ∆T

Now we enter our values, we get

q = 2,000 x 1.08 x 35 = 75,600 Btu/hour

Now we quickly explain where the value of 1.08 in the calculation is derived from. First, we have the weight of Air at 0.075 pounds per Ft3, then we have the conversion of minutes into hours, and lastly the specific heat of Air at 0.24 btu/lb°F. This is what that looks like

0.075 lb/ft³ x 60 min/hour x 0.24 btu/lb°F = 1.08

With all these units, we can see which units of value remain by crossing out those that are eliminated in the formula, such as

q = 2,000 ~~Ft3~~/~~minute~~ x 0.075 lb/~~ft³~~ x 60 ~~min~~/hour x 0.24 btu/~~lb°F~~ x 35°F = 75,600 Btu/hour

How to calculate sensible heat

How to Calculate Sensible Heat Transfer for Water

By

MEP Academy Instructor

-

April 21, 2022

0

How to calculate sensible heat transfer for water

In this article we’re going to look at the Sensible Heat Transfer Equation for Water. This is useful when trying to determine either of the three variables, Btu’s, GPM or Delta-T. When you know two of these values you can determine the remaining missing value.

If you prefer to watch the YouTube Video of this presentation than scroll to the bottom.

Table of Contents

Sensible Heat Transfer Equation for Water

q = m x C_px ∆T

q = GPM x 8.34 lb/gallon x 60 min/hour x 1 btu/lb°F x ∆T

q = GPM x 500 x ∆T

GPM is gallons/minute

m is the overall mass flow rate of water

C_p is the specific heat of water 1 btu/lb°F

We’ll begin by calculating for the BTU’s when given the GPM and temperatures of the supply and return water. Remember we’re dealing with sensible heat only. That means there is no latent heat which involves a change of state, like water becoming steam. All were doing is changing the temperature of water and not its state.

How to Calculate the Sensible Heat Transfer of Water

Example:

200 GPM water enters a coil at 120°F and leaves at 80°F

Step #1 – Determine the ∆T (120°F – 80°F = 40°F)

Step #2 – Enter all values into equation.

q = GPM x 500 x ∆T

q = 200 x 500 x 40 = 4,000,000 Btu/hour

Looking at this heating hot water coil we see that there is 200 gallons per minute of water flowing through this coil and the temperature of the heating hot water supply is 120°F and the leaving heating hot water is 80°F.

With this information we can solve for how many btu’s are being supplied to this coil.

Step one is to subtract the Leaving Water temperature of 80°F from the Entering Water temperature of 120°F to arrive at the Temperature difference or Delta-T. 120°F – 80°F, gives us a 40°F Delta-T.

Step two is to put all the known values into our formula and make the calculation.

We have our formula of q = GPM x 500 x ∆T

Now we enter our values, we get

q = 200 x 500 x 40 = 4,000,000 Btu/hour

Now we quickly explain where the value of 500 in the calculation is derived from. First, we have the weight of water at 8.34 pounds per gallon, then we have the conversion of minutes into hours, and lastly the specific heat of water at 1 btu/lb°F. This is what that looks like

8.34 lb/gallon x 60 min/hour x 1 btu/lb°F = 500

With all these units, we can see which units of value remain by crossing out those that are eliminated in the formula as such

Q = 200 ~~gallons~~/~~minute~~ x 8.34 lb/~~gallon~~ x 60 ~~min~~/hour x 1 btu/~~lb°F~~ x 40°F = Btu/Hour

How to Calculate Sensible Heat for Water

Checkout our other video on Calculating Sensible Heat for Air or Calculating Sensible and Latent Heat.

Refrigerant Cycle of a Heat Pump System – Reversing Valve

Heating Mode

Cooling Mode

Packaged vs Split System Heat Pumps

Water Source Heat Pumps

Vertically Stacked Water Source Heat Pumps

Horizontal Water Source Heat Pumps

Ground Source Heat Pumps

Ground Water Source Heat Pumps

Sensible and Latent Heat Transfer Equation for Air

Example:

Sensible Heat Transfer Equation for Air

Example:

Sensible Heat Transfer Equation for Water

Example:

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