Preheat Coil Piping Diagrams

Preheat coils are essential components of heating, ventilation, and air conditioning (HVAC) systems. They help to protect other equipment in the system from freezing and to improve the efficiency of the overall system. Preheat coil piping diagrams can help HVAC professionals to design and install preheat coils correctly and to troubleshoot problems with existing systems.

Preheat Coil Location in an Air Handling Unit System

Contents hide

1 Preheat

1.1 Freeze Protection

2 Cooling, Heating, Reheat

3 Cooling Coil Face Velocity

4 Preheat, Heating and Reheat Coil Face Velocity

5 Conclusion

6 Related FAQ

Preheat

Preheat coils can be either concurrent flow or counter flow. In a concurrent flow system, the air and water or steam flow in the same direction through the coil. In a counter flow system, the air and water or steam flow in opposite directions through the coil.

Concurrent flow systems are more common than counter flow systems. They are easier to design and install, and they are less expensive. However, counter flow systems are more efficient at transferring heat from the water or steam to the air.

The diagram shows a 3-way valve system with a balance valve, a shut off valve, a strainer, and a flow indicator. The flow indicator is used to indicate the flow of fluid through the system. The balance valve is used to adjust the flow of fluid through the preheat coil. The shut off valve is used to isolate the preheat coil from the rest of the system.

The flow of fluid through the system is as follows:

The fluid enters the system through the strainer.
The fluid then flows through the balance valve.
The balance valve divides the flow of fluid into two streams.
One stream of fluid flows through the preheat coil.
The other stream of fluid flows through the bypass line.
The flow of fluid through the preheat coil is controlled by the 3-way valve.
The 3-way valve can mix the hot fluid from the preheat coil with the cold fluid from the bypass line.
The mixed fluid then flows out of the system.

The balance valve is used to adjust the flow of fluid through the preheat coil so that the desired temperature is achieved. The shut off valve is used to isolate the preheat coil from the rest of the system for maintenance or repairs.

This type of system is often used in HVAC systems to protect the cooling coil from freezing. In the winter, the 3-way valve can be used to mix hot fluid from the preheat coil with the cold outside air to prevent the cooling coil from freezing.

Here is a possible scenario for how the system might be used:

The outside temperature is below freezing.
The 3-way valve is turned to allow some hot fluid from the preheat coil to mix with the cold outside air.
The mixed air is then heated by the preheat coil so that it does not freeze the cooling coil.
As the outside temperature rises, the 3-way valve can be turned to allow less hot fluid to mix with the outside air.
This ensures that the air entering the building is not too hot or too cold.

Freeze Protection

There are three main ways to protect preheat coils from freezing:

Face and bypass dampers: Face and bypass dampers can be used to divert air around the preheat coil when the outside air temperature is below freezing.
IFB coils: IFB coils (internal face bypass coils) have a built-in bypass damper that automatically diverts air around the coil when the outside air temperature is below freezing.
Preheat pumps (primary/secondary system): In a primary/secondary system, two pumps are used to circulate the water or steam through the preheat coil. The primary pump circulates the water or steam through the coil, and the secondary pump circulates the water or steam through the rest of the HVAC system. This type of system is more expensive than the other two types of freeze protection systems, but it is also the most efficient.

Cooling, Heating, Reheat

Cooling coils, heating coils, and reheat coils are all counter flow coils. This is because counter flow coils are more efficient at transferring heat from the water or steam to the air.

The diagram shows a system with a shut off valve, a 2-way valve, a clock valve, a strainer, a pump, and a preheat coil. The shut off valve is used to turn off the system, the clock valve is used to turn on the system at a predetermined time, and the 2-way valve is used to divert the flow of water or steam through the preheat coil.

The flow of water or steam through the system is as follows:

The water or steam enters the system through the strainer.
The water or steam then flows through the shut off valve.
If the clock valve is open, the water or steam will flow through the 2-way valve and into the preheat coil.
If the clock valve is closed, the water or steam will flow through the bypass line and around the preheat coil.
The water or steam then flows out of the preheat coil and into the rest of the system.

This type of system is often used in HVAC systems to protect other equipment from freezing. In the winter, the clock valve can be set to turn on the system at a predetermined time, such as before sunrise. This will ensure that the preheat coil is warm before the HVAC system starts operating.

Here is a possible scenario for how the system might be used:

The outside temperature is below freezing.
The clock valve is set to turn on the system at 6:00 AM.
The water or steam flows through the 2-way valve and into the preheat coil.
The preheat coil heats the water or steam.
The hot water or steam flows out of the preheat coil and into the rest of the HVAC system.
The HVAC system starts operating and heats the building.

This type of system can also be used to preheat water for domestic hot water systems. In this case, the preheat coil would be heated by a boiler or solar thermal system.

Cooling Coil Face Velocity

The recommended face velocity for cooling coils is 500 FPM (2.54 m/s). A face velocity of 450 FPM (2.29 m/s) is preferred, but a face velocity of 550 FPM (2.79 m/s) is acceptable.

The diagram shows a preheat coil piping system with a shut off valve, a check valve, a balance valve, a strainer, and a preheat coil. The shut off valve is used to turn off the system, the check valve prevents backflow, the balance valve is used to adjust the flow of water or steam through the preheat coil, and the strainer removes debris from the water or steam.

The flow of water or steam through the system is as follows:

The water or steam enters the system through the shut off valve.
The water or steam then flows through the strainer.
The strainer removes debris from the water or steam.
The water or steam then flows through the check valve.
The check valve prevents backflow.
The water or steam then flows through the balance valve.
The balance valve adjusts the flow of water or steam through the preheat coil.
The water or steam then flows through the preheat coil.
The preheat coil heats the water or steam.
The hot water or steam then flows out of the preheat coil and into the rest of the system.

This type of system is often used in HVAC systems to protect other equipment from freezing. In the winter, the preheat coil can be used to heat the outside air before it enters the cooling coil. This prevents the cooling coil from freezing.

Here is a possible scenario for how the system might be used:

The outside temperature is below freezing.
The preheat coil is turned on.
The outside air flows through the preheat coil.
The preheat coil heats the outside air.
The heated outside air then flows into the cooling coil.
The cooling coil cools the heated outside air.
The cooled outside air then flows into the building.

This type of system can also be used to preheat water for domestic hot water systems. In this case, the preheat coil would be heated by a boiler or solar thermal system.

Preheat, Heating and Reheat Coil Face Velocity

The recommended face velocity for preheat, heating, and reheat coils is 500-900 FPM. A face velocity of 750 FPM is preferred, but a face velocity of 500 FPM or 900 FPM is acceptable.

500–900 FPM (2.54–4.57 m/s) Range.
600–700 FPM (3.05–3.56 m/s) Recommended.
600 FPM (3.05 m/s) Preferred.
Use preheat coil whenever mixed air temperature (outside air and return air) is below 40°F (4.44°C).

The diagram shows a 3-way valve preheat coil piping system with a shut off valve, a strainer, a balance valve, a flow indicator, a preheat coil, a 3-way valve, and a bypass line.

The flow of water or steam through the system is as follows:

The water or steam enters the system through the shut off valve.
The water or steam then flows through the strainer.
The strainer removes debris from the water or steam.
The water or steam then flows through the balance valve.
The balance valve adjusts the flow of water or steam through the preheat coil and the bypass line.
The water or steam then flows through the 3-way valve.
The 3-way valve mixes the hot water or steam from the preheat coil with the cold water or steam from the bypass line.
The mixed water or steam then flows out of the system.

The position of the 3-way valve determines how much hot water or steam is mixed with the cold water or steam. If the 3-way valve is turned to the “Preheat” position, all of the water or steam will flow through the preheat coil. If the 3-way valve is turned to the “Bypass” position, all of the water or steam will bypass the preheat coil.

This type of system is often used in HVAC systems to protect other equipment from freezing and to improve efficiency. In the winter, the 3-way valve can be turned to the “Preheat” position to heat the outside air before it enters the cooling coil. This prevents the cooling coil from freezing. In the summer, the 3-way valve can be turned to the “Bypass” position to allow the cold outside air to flow directly into the cooling coil. This improves the efficiency of the cooling system.

Here is a possible scenario for how the system might be used in the winter:

The outside temperature is below freezing.
The 3-way valve is turned to the “Preheat” position.
The outside air flows through the preheat coil.
The preheat coil heats the outside air.
The heated outside air then flows into the cooling coil.
The cooling coil cools the heated outside air.
The cooled outside air then flows into the building.

Here is a possible scenario for how the system might be used in the summer:

The outside temperature is above freezing.
The 3-way valve is turned to the “Bypass” position.
The outside air bypasses the preheat coil and flows directly into the cooling coil.
The cooling coil cools the outside air.
The cooled outside air then flows into the building.

Conclusion

Preheat coil piping diagrams are an essential tool for HVAC professionals. By understanding the different types of preheat coils and how they are piped, HVAC professionals can design and install preheat coils correctly and troubleshoot problems with existing systems.