Getting furnace settings right in the controller

For the past 10 days I’ve been looking at temperatures and associated controller settings from data loggers that were placed in producer facilities in Michigan, Indiana and Ohio in preparation for a series of ventilation workshops next week. We’ve done this at other locations around the Midwest in previous years, and the results this year were not different or surprising. There are a lot of controllers set wrong in production facilities, with the result being excessive propane usage and significant over ventilation of facilities, especially at the time of pig placement.


A common problem that I see repeatedly is furnace settings that are too close to the set point of the controller. The bigger the furnace relative to the space being heated, the bigger this problem is. After the furnace shuts off following a heating cycle, the air temperature as recorded by the ventilation controller probe(s), should never get to set point. A general recommendation to prevent this from happening is to have the furnace shut off at 2 degrees below the controller set point.


However, in cases where the furnaces are severely over-sized, this isn’t enough. I was in a farrowing unit a few weeks ago where the controller was programmed to turn off the room furnace at 1.5 degrees below the set point. Every time the furnace cycled, the room temperature climbed to 0.8 degrees above set point. In just about every controller, if room temperature increases above the set point by 0.1 degree, the ventilation system responds by increasing the ventilation rate to remove the excess heat. In the case of the farrowing unit, the furnace off temperature in the controller had to be set at 2.5 degrees below the set point to prevent this from happening.


In general, furnaces in production facilities are over sized. A furnace is big enough if it shuts off on the coldest day of the year. Sounds simple, yet too many producers size furnaces with the goal of minimizing run time – if it runs very long I must be wasting heat! The longer the furnace runs on each on-cycle, the more uniform the heat distribution in the animal space since the furnace fan is circulating warm air. If the furnace cycles relatively fast (on time is only a few minutes), the fan doesn’t have time to circulate air uniformly through out the heated space. The result is more variation in temperature with every cycle.


If you have a furnace installed that is a variable output furnace (LB White Guardian and PSI are 2 brands that have this option), it should be turned to low output at this time of the year. If your furnace has a ‘blue’ valve between the control module and the flame, it is most likely a variable output furnace. If you turn the valve to the right, you will decrease the size of the flame and the heat output. While the furnace will run longer per cycle, the total gas usage will be the same, or even less, than a short run-time at full output. Don’t turn the flame to full output until the furnace runs 100% of the time. As long as the furnace turns off on occasion, it is putting out enough heat.


Over-sized furnaces are a common problem in farrowing and nursery rooms. I’ve seen 250,000 btu furnaces installed where a 60,000 btu furnace would be enough because the bigger furnace was ‘a better buy (more btu’s per dollar spent)’. The net result is that air temperatures often cycle as much as 8+ degrees every time the furnace cycles, which is not acceptable for best pig performance. It has been demonstrated that weaned pigs performance declines if they are faced with temperature fluctuations of 4 degrees or more per hour. It is not uncommon to have as many as 6 to 8 temperature swings per hour in rooms with over sized furnaces.


A good way to tell if your controller is set correctly is to record the controller set point and the daily high temperature as recorded in your controller. The daily high should never get to the set point temperature when you are in full furnace mode in a room. My goal is to have the daily high peak at about 0.2 degrees below the room set point. This ensures that the ventilation system won’t increase the ventilation rate and exhaust the added heat in response to the heat addition.



Leave a Reply

Your email address will not be published. Required fields are marked *