FAQ
This document contains frequently asked questions about the CBE Fan Tool. Users can find more information in the About page, User 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.
- Why are there no solutions displayed?
- What if the floor plan is not a rectangle?
- What if the ceiling height varies?
- What is a fan 'cell'?
- What range of fan diameters does this tool cover?
- What is the 'fan air speed'?
- What is 'Uniformity'?
- What does 'Cooling effect' mean?
- How much energy can this save?
- What about fans blowing upwards?
- What air speeds are allowed by code?
- What if there is a different number of fan blades?
- What about placing fans off-center in the cell?
- How accurate are the air speed predictions?
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 you have entered the room diemsnions
- Second, ensure that at least one fan is selected.
- Third, note that in some cases there may not be any viable solution for a selected fan. For example, selecting a 4.3m (14 ft) diameter fan as a candidate fan for a room with a 3.7 m (12.1 ft) ceiling height. Here, the ceiling height is too low given the blade height requirements (this fan must be mounted with blades above 3.1m or 10 ft for safety reasons) and the recommended mount distance (this fan requires a distance of 2.8 ft from blade to ceiling to avoid starvation). If this occurs, choose a smaller fan, or one that can be mounted below 3.05 m (10 ft).
We define a 'cell' of re-circulating air 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. If it is highly irregular (e.g. an L shaped room), break the room up the largest possible rectangular-shaped floor areas and use the tool for those areas independently, aiming to choosing the same fan types as much as possible between across all of these spaces.
For example, a cathedral or vaulted ceiling. Here, use the average ceiling height of the space. Additionally, you must ensure that the blade height and distance from fan blade to ceiling meet manufacturer requirements everywhere a fan is located. Note than in general, there is a 0.6 m (2 ft) minimum clearance required from fan blades to any obstruction. So, for example, the tool may say that a fan will 'fit' based on the average ceiling height entered by a user, but the fan blades may be too close to the ceiling for fans located near the walls of the room.
This tool works for fan diameters within the range of 1.2 - 4.3 m (4 - 14 ft). The underlying dataset developed from full-scale experiments also covered this range.
The 'fan air speed' is a metric that combines the effects of the diameter of the fan and the airflow through the fan. It is calculated by dividing the rated airflow by the circular area swept by the fan blades, and represents the average air speed through the fan blades. Higher values mean a higher maximum seated/standing airspeed in the space. One approximate relationship is that the maximum airspeed at any specific point in the space will be approximately 1.6 times the fan air speed.
The 'Uniformity' metric is an attempt to capture how much air speed variation can expect to experience in the space. 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 less 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 definition 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.
Also, using the cooling effect associated with the minimum airspeed in the room is a quick way to approximate how much the fans will allow the temperature to increase in that room. We use this location as it is where a 'typical' occupant will feel the warmest, and thus, this is the conservative approach for maintaining neutral thermal comfort 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.
Note here that the 'Cooling effect' is an approximation, and the exact amount varies depending on the scenario. For example, the cooling effect for the same air speed is higher for occupants who are at higher metabolic rates (i.e., exercising). To evaluate the cooling effect associated with a specific thermal comfort scenario, we recommend using the relevant air speed value directly in the CBE Comfort Tool.
All of the data shown in the tool is for fans blowing downwards, as this is by far the more common and the more efficient way of creating air movement in the occupied space. Reversing a fan so that it blows upwards against the ceiling creates a much lower, but much more uniform air speed distribution in the space. This of course requires that the space that the fan (or fans) are in is bounded by a ceiling and walls on all sides. However, because the rated air flow is only available for fans blowing downwards, not upwards, the tool cannot predict what those air speeds will be for a given scenario. Based on the laboratory testing in this article, the area weighted average air speed for a fan blowing upwards will be very approximately 30-70% that of the same fan blowing downwards at the same speed. The ratio of air flow through a fan in upwards vs. downwards direction depends on the fan type and associated blade geometry. Some fans have highly optimized blade designs to blow downwards efficiently and have an asymmetrical blade geometry, and generally do not perform as well in reverse compared to forward. Some fans either have a simple (and symmetrical) blade geometry, or have blades that can actually be inverted before being mounting to the fan, and here, the airflow in the upwards direction is very similar to the rated airflow as the blade geometry is also symmetrical. In these cases, the area weighted average airspeed is approximately 60-70% that of the downwards case.
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 minimum air speed estimate (and uniformity) will be lower than estimated by the model. To account for this, we reduce the minimum airspeed estimate by dividing it by the cell aspect ratio. The area-weighted average airspeed will likely remain similar as long as the cell aspect ratio is below 1.5 (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 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.