A common minimum ventilation controller mistake

We received our first taste of winter this week with snow in eastern South Dakota, southwest Minnesota, northwest Iowa and northeast Nebraska. In my farm visits the past few weeks I’ve been working with producers, contract growers and production system field staff on proper winter operation of their ventilation and heating systems.

Every winter I see a repeat of common mistakes that have a big impact on pig comfort and energy expenses. One of the biggest – setting the controller to a recommended (or mandatory setting based on age) and assuming that will provide the best conditions for the growing pig. We still need to carefully observe pig behavior and when it isn’t correct, ask the hard questions to find the cause.

As we go into colder weather, ventilation systems are slowing down. When we start pigs in nurseries, wean-finish facilities, grow-finish facilities or use farrowing rooms we are increasingly operating our ventilation systems at the minimum ventilation rates. A common mistake that I came across at 2 different sites this past week was the assumption that the controller settings were correct and 50% minimum speed was approximately 50% of the full cfm output of the fan.

For most controllers (especially those manufactured by Thevco, Phason and Varifan) 50% minimum speed is a funny number unless we get the motor curve correct for that stage of ventilation. A large segment of our industry doesn’t understand the importance of motor curves and their impact of minimum ventilation. Every motor reacts differently to incoming line voltage – not all motors give you the same rpm for the same line voltage. This is especially true as we go to the smaller motors on fans that are less than 24” diameter.

This also explains why it is so important to keep identical motors on fans within a stage of ventilation. Just because a motor fits in the fan mounting doesn’t mean it will behave the same as the motor on the other fan in the stage 1 circuit. If you’ve every replaced a motor and had the 2 fans behave differently as they ramp up speed in response to rising room temperatures, you’re familiar with this lesson.

Ventilation controllers don’t know what is connected to them – there is no feed back loop. This means we need to somehow tell them what we’ve got connected. We do this with motor curves or motor compatibility curves. These define what voltage the controller sends to connected fans when the controller is set at a given minimum speed setting and how the controller increases the voltage in response to rising room temperatures. Thevco controllers have 10 user selectable curves, Varifan controllers have 9 user selectable curves and Phason controllers can have up to 4 user selectable curves.

If the wrong curve is selected, 50% minimum speed may in fact by 25% cfm (resulting in stuffy and wet facilities) or 85% cfm (resulting in higher energy expenses). In addition, the rate at which the fan speeds up over the 1.5-2.0F bandwidth may be incorrect. With the wrong motor curve installed in the controller it is not uncommon for me to observe stage 1 fans with a 2F bandwidth barely changing speed for 1.8F of the temperature rise in the room and then suddenly ramp to full rpm in the last 0.1-0.2F temperature change. I’ve also seen fans go from minimum speed to almost full speed in the first 0.5F temperature rise when there was a 2F bandwidth.

If your stage 1 and 2 variable speed fans don’t increase speed (and cfm output) relatively uniformly over the entire bandwidth range, dig out your owners manual for the controller and investigate the motor curve setting in the controller. It can make a world of difference in how the minimum ventilation stage functions in a facility.

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