Air Flow

Introduction

This airflow calculation is crucial for air-to-air heat pump systems, as they distribute heat through moving air rather than heated water.

The result tells us how much warm air needs to be supplied to maintain room temperature, whether the system’s fan capacity is sufficient to meet heating demands and how well heat is distributed throughout the space, ensuring comfortable and efficient heating.

The Calculation

This formula determines the airflow rate (in cubic meters per second, m³/s) required to deliver enough heat to offset the room’s heat loss.

We also need to decide on the target temperature for each room, and the outside design temperature. Your room temperatures should be whatever you find comfortable, and your outside design temperature should be based on your typical winter. Specifically, the outside design temperature is the minimum dry bulb temperature achieved for 99.6% of the year. Basically, it should be the lowest temperature of the year, excluding the lowest 0.4%. My chosen outside design temperature is -4°C.

Breaking down the formula:

1. Heat Loss (W): The amount of heat that must be supplied to maintain the desired indoor temperature.
2. 1.2 (kg/m³): The approximate density of air at room temperature (how much mass a cubic meter of air has).
3. Inside Target Temp – Outside Temp (°C): The temperature difference between the air being supplied and the outside air. This represents how much each cubic meter of air contributes to heating the space. A larger temperature difference means less airflow is needed, while a smaller difference means more airflow is required to compensate for heat loss.

Results

This table summarizes the airflow required to offset heat loss in each room, based on the calculated heat loss and target room temperatures. It shows how much air needs to be supplied to maintain the desired indoor temperatures.

The airflow needed to maintain comfort across different rooms vary considerably, with larger rooms and higher heat losses requiring significantly more airflow. The important figure for now is the total air flow.

Air Flow Route

Another aspect of airflow to consider is the path the warm air will take as it moves from the outlet of the heat pump (the high-pressure area) back to the inlet (the low-pressure area). To promote air movement from the rooms back to the intake, the hallway is kept at low pressure by not receiving direct hot air. Instead, it will passively receive heat as air from the rooms returns to the intake.

Air, like any fluid, follows the path of least resistance. If there is an easier route to a low-pressure area, it will take it. Therefore, the doors will be undercut so that the gap matches the cross-sectional area of the room’s supply duct, plus 10%. If this path is restricted, the hot air will seek an alternative route, potentially leading to inefficient heat distribution or unnecessary heat loss.

An alternative, commonly seen in older homes, is over-door ventilation grilles. Since the system draws heat in high, from the ceiling in the hallway, and distributes it through the roof in the rooms, over-door ventilation would allow warm air to leave the room, and back into the intake, before it has had a chance to reach the floor. This would result in a hot air layer above the thermostat, leading to inaccurate control and therefore inefficient heating. By undercutting the doors, heat is forced to the bottom of the room before it can take the controlled path out to the hall, past the thermostat and back to the intake, ensuring more even and effective heating throughout the space.

Zoning

Zoning divides a property into separately controlled heating areas, each with its own thermostat. This allows independent temperature regulation in different parts of a building. However, it requires additional equipment like zone valves and dampers, which take up space and add cost and complexity.

Zoning is necessary when the heat input needed for a room varies significantly relative to the other rooms being heated—for example, to compensate for the erratic solar gain of a conservatory. It is not required when a room simply needs less heat than the others, as long as the proportion of the total heat remains consistent. In such cases, the heat pump can vary its total output to match the heating requirements, with room vents be tuned to distribute the heat appropriately.

It’s also important to note that unheated rooms act as heat sinks if not thermally isolated, drawing warmth from adjoining rooms and reducing overall efficiency.

For these reasons, I decided against installing electronic dampers on the lounge ducts, despite the fact that the room goes unused during the winter months. Over the course of the year, I’m confident this approach provides the most cost-effective and comfortable way to heat the property.