I spent much of this week with clients reviewing winter ventilation details and evaluating temperature logs from recorders placed in barns earlier this month. In every instance, I found mistakes in controller settings that cost producers a lot of money.
The most common mistake I saw was incorrect furnace temperature settings. The goal of furnace settings in the controllers is to turn off the furnace at a point relative to the controller settings so the variable speed fan(s) on stage 1 doesn’t increase speed as a result of the temperature probes delayed reaction to the furnace heat.
If the controller speeds up variable speed fans at 0.1F above the set point, the furnace should turn off about 2F below the controller set point. This week I saw controllers where the off temperature setting for the furnace was at or even above set point in this type of controller. This guarantees that every time the furnace operates, the fan(s) will speed up and unnecessarily remove some of the added heat. I have a temperature plot for 1 facility with pigs near market weight that indicates furnace operation, something that should never happen with a facility full of market weight pigs, especially at this time of year.
If the controller has a different set point relative to the controller set point for when stage 1 variable speed fans increase speed, the furnace should be set to turn off at 2F below this set point.
A second common mistake was setting bandwidth too narrow for the stage 1 variable speed fan. If you set the band width too tight (1F or less), I can just about guarantee there will be more temperature variation in the room, not less. Stage 1 fans are generally very small relative to the room being ventilated. It takes time for this fan to exchange enough air to impact conditions in the room. If the band width is too narrow, the controller calls for the next stage fan to operate which often chills the room, causing the temperature to drop, sometimes low enough to cycle the furnace circuit. Think of bandwidth as time in a controller –time for the small fan to make an impact on conditions in the room.
I generally recommend 2F bandwidth for stage 1 fans and am ok if you want to go as high as 3F. The stage 1 fan is at the bottom end of the thermal neutral zone so a slight warming in the room before the next fan functions is ok – you won’t get into a heat stress situation. If you are concerned about when the later stages operate, tighten up the temperatures between these upper stages. By the time they operate you are getting serious about heat relief so have fans turn on/off at tighter temperature ranges won’t do any harm.
Do you still suggest running heaters with 3 different gas valve settings at the lowest position for more uniform heating? Is so, when might a guy need to use the medium and high positions?
The goal of furnace sizing is to have a furnace only large enough to just shut off on the coldest day of the year – that is – a furnace is big enough if it shuts off. The longer a furnace runs per burn cycle, the more uniform temperatures will be in the room being heated as the furnace fan is blowing air everywhere with the heated space and the temperature rise is gradual.
As you suggest, I recommend furnaces with variable output valves (the ‘blue’ valve that is between the control module and the flame). Turn it to low at the beginning of the heating season. Begin turning it higher (more heat output) when the furnace doesn’t shut off. IF the furnace shuts off, don’t change the valve setting.
In tunnel barns many set the furnace nearest to the tunnel curtain to a higher heat output – this results in more uniform room heating as the tunnel curtain end of the barn is often the coldest location in the barn.