FAQ
This document contains frequently asked questions about the CBE Fan Tool. Users can find more information in the About page, User Guide, and upcoming CBE Ceiling Fan Design Guide.
We plan to continually update this document as folks ask questions, so please reach out directly (contact Paul Raftery directly) if you have a question that would be good to answer here.
There are no solutions displayed because the combination of constraints and candidate fans you have selected do not yield a solution. There are many reasons why this may be the case. Review the User Guide to better understand how the tool identifies a solution and adjust your input accordingly. The most common things to check are:
- First, ensure that at least one fan is selected.
- Second, confirm that the selected fan(s) will fit in the ceiling height you have defined considering both UL 507 and mount distance requirements. If it doesn't choose a smaller fan.
We define the 'cell' around a fan as the volume closest to that fan. For a room with a single fan, the cell matches the room dimensions. For an array of fans in a room, the dimensions of each cell match the on-center spacing between fans in the x- and y- directions.
Use the average width and length of the room, ensuring that the total floor area remains the same as the actual floor plan.
Use the average ceiling height of the space. Additionally, ensure that the blade height and distance from fan to ceiling meet manufacturer requirements everywhere where a fan is located.
The 'Uniformity' metric is an attempt to capture how much variation in air speed an occupant in the space can expect to experience. It is calculated using 1 - (difference between max and min airspeed)/(max airspeed). A value of 1 means the space is completely uniform, and lower values indicate lower uniformity.
This value is the number of degrees that the ceiling fans allow the operative temperature to increase by while maintaining neutral thermal comfort. This is approximately how much a designer can increase the design cooling setpoint for the HVAC equipment in a room with this fan configuration. Specifically, it is the same as the defintion of 'cooling effect' in ASHRAE Standard 55: while maintaining the same Standard Effective Temperature at otherwise standard conditions, the 'Cooling effect' value is the difference in operative temperature between the still air and elevated airspeed cases. We use the lowest seated/standing average airspeed in the room to calculate this. We do so because this location is where a 'typical' occupant will feel the warmest, and thus, this is the conservative approach for maintaining neutral conditions in the whole room. In other locations in the room, such as directly under the fan, air speeds will be higher and a 'typical' occupant will feel cooler. Correspondingly, a larger temperature increase can be achieved while maintaining neutral comfort conditions in these locations where air speeds are higher than the minimum achieved in the room.
This 'Cooling effect' is a quick way to approximate how much the fans will allow the temperature to increase in the space. However, to evaluate a specific thermal comfort condition, we recommend using the relevant air speed value directly in the CBE Comfort Tool.
ASHRAE 55 allows air speeds up to 0.8 m/s (158 fpm) in scenarios where the occupants do not have local control of the air speed. When occupants have local control, there is no maximum upper limit to the air speed allowed by the standard.
In full-scale laboratory tests, when operating at the same rated airflow, fans with the same diameter created a very similar air speed distribution in the room regardless of number of blades. In the results from this tool, the number of blades is only used for display purposes. By default, the tool assumes 6 blades for fans over 5 ft diameter, and 3 blades for anything else. Users can override the number of blades displayed on the plan visualizations by clicking on the 'Display settings' button.
The specified locations shown in each solution are only recommendations for where fans should be placed in the space to cover the floor area as uniformly as possible. However, it is important to note that ceiling fans can be installed anywhere that meets manufacturer, safety and code related requirements for that fan and application - they certainly do not need to be centered in a room, or to be laid out in a perfectly uniform grid. Ceiling fans should be located so as to best co-ordinate with aesthetic, lighting and/or structural needs. However, due to the limitations of the measurement data-set on which the models underlying this tool were built, the further the actual fan layout differs from that identified by the tool, the less accurate the airspeed estimates will be. We expect that if the fans are off-center by 0.05 times the fan cell width or less, there will be little effect on the overall air speed predictions.
It depends. For a square (or close to square) floor plan or fan cell, with the fan mounted at the center of each cell, and without furniture in the room, we expect the accuracy will closely match that of the models underlying the tool described in this article. These models are simple linear regressions based on a large dataset of laboratory experiments described in more detail here. In brief, the typical (i.e. median) accuracy is approximately +/- 10% for the lowest airspeed and area-weighted averages in the room. For the highest air speed in the room the accuracy is approximately +/- 20%. This is less accurate as the highest air speed occurs directly under the fan, and is depends on the blade geometry of the selected fan type.
However, there are few real-world applications where each fan will be perfectly centered in a square 'cell' and where there is no furniture present. The less the real-world situation fits with those assumptions, the less accurate the results will be. We cannot say how much poorer without further laboratory tests, and without requiring far more input from the user than is feasible for a quick, easy-to-use web tool.
That said, we can estimate the effects based on reasonable assumptions.
For fan cells that are far from square, we believe it likely that the lowest air speed estimate (and uniformity) will be lower than estimated by the model. However, the area-weighted average airspeed will likely remain similar as long as the cell aspect ratio is below 1.25 (the default maximum constraint on this parameter). Additionally, the accuracy of the highest air speed estimate will probably remain similar in the vast majority of cases. That is because the highest air speed occurs directly under the fan and is primarily driven by the fan, not the room geometry.
Similarly, fans often must be placed 'off-center' within their cell due to practical constraints. For reasonably off-center fans (e.g. those that are less than 0.05 times the cell width or length off-center), we expect similar effects as with non-square cells above.
In cases with furniture directly under the fan, the highest airspeed will likely be higher, as will the area-weighted average air-speeds. More information and measured data can be found in this article.
A reasonable, conservative approximation is that every 1 °C (2 °F) increase in the cooling setpoint will reduce total HVAC energy consumption by 10%. The actual savings depends highly on the climate, building, system, and operation of the particular case in question. The CBE Setpoint Savings Calculator allows quick estimation of savings for a simulated office building in a particular climate zone, for a particular change in setpoints, breaking the savings results down by fan, cooling, and heating energy savings. In practice, the actual savings can be higher, notably so in cases where the building contains many zones that heat (or 'reheat') and cool during the same day.