Ducting

Defining the ductwork

Introduction

Proper design of the ductwork ensures efficient and quiet delivery of warm air to each room. This process directly links the heat loss calculations, airflow requirements, and the heat pump specifications. By calculating the heat loss for each room, the required airflow is determined, and the heat pump’s fan capacity helps set the limits for ductwork sizing.

Design Limitations

The roof is steel sheet with a trapezoidal section, supported by steel I-beams running along its width. This limits the ducting diameter to an absolute maximum of 125mm as it passes over a beam and under the roof.

Being a retro-fit, the ceilings were already in place, meaning access to duct runs were limited outside the confines of the roof-space.

Impact on Design

These are the concessions made to compensate for the design limitations:

Star Topology: By using a star topology with ducts running directly from the manifold to each room, the (much) larger trunk diameter requirement of a trunk and branch topology is avoided.
Flexible Ducting: In areas where lack of access excluded rigid ducting, which is preferable due to lower air friction, flexible ducting could be pulled through the roofspace from the manifold to the vents.
Higher Air Velocity: To compensate for the smaller duct size and increased air friction, a higher air velocity will be necessary to ensure the required airflow can still be delivered.

Air Velocity

With the over-arching design parameters defined, the next step was choosing an appropriate air velocity for the ducts. Air velocity is a balancing act: too high, and the system becomes noisy and inefficient due to increased friction; too low, and the ducts need to be larger and more costly. I settled on an air velocity of 8 m/s for all ducts. While this is higher than typical residential systems (which often use 3-5 m/s), it is necessary to compensate for the limitations in place due to building construction.

Thankfully, due to the indoor unit being in an enclosed space within the building envelope, noise from the higher air velocity is less of a concern.

Minimum Duct Diameter Formula

With the airflow and air velocity determined, the following formula is used to calculate the minimum duct diameter for each room:

diameter (m) = \sqrt{\frac{4 \times air flow (m3/s)}{π \times air velocity (m/s)}}

;

Applying the Calculations

Using this method, I calculated the minimum duct diameter for each room. Here’s a summary of the results, with the previously discussed exclusions of the hall, utility room and en-suite included to show the minor input that would be required:

Room	Airflow (m^3/s)	Diameter (mm)
Hall	0.027	66
Lounge	0.231	192
Kitchen	0.067	103
Utility room	0.013	45
Bedroom 1	0.064	101
En suite	0.006	31
Bedroom 2	0.051	90
Bedroom 3	0.059	97
Bedroom 4	0.039	79
Bathroom	0.014	47

As a reminder, the bathroom is included, despite its minor heat requirement, due to being a thermally-isolated extension of the apartment.

Summary

Up to the limit imposed by the roof structure, two ducting sizes are generally available on the market: 100mm and 125mm. Based on the table above, a single 100mm duct will be suitable for each room, with the exception of the lounge.

A 192mm duct has a cross-sectional area of 28,953 mm², while a 125mm duct has a cross-sectional area of 12,272 mm². Therefore, a pair of 125mm ducts, with a combined cross-sectional area of 24,544 mm², is as close as is reasonably achievable. This configuration is approximately 15% undersized, which will need to be compensated for through improvements in insulation and careful system tuning.

Notes

A couple of smaller points on minimising friction losses:

Smooth, rigid ducting is used where possible, including solid elbows for any sharp changes of direction in the flexible ducting.
Duct length is to be minimised.