Hi all, Ive been using the plastic brewer heat pads that I purchased. I use a big chest freezer that I keep at 40 degrees and control with one temp controller. I insulate one side with foam and put the pads on the foam and my carboys on those and use a separate temp controller to control the heat pads to get my ferm temps. I have been thinking(perhaps overthinking), that the heat pads are cooking the yeast on the bottom to a higher temp then whats reflected on the carboy since it heats from the bottom up, especially with bottom feeding yeasts like lagers. Has anyone had any experience with these? Major off flavors etc? Or is the temperature of the wort on top keeping things in check.
The answer is sometimes yes and sometimes no. I think it depends on the differential between the air temperature and the desired temperature of the beer. If the air around your carboy is different by 10-15 F, then that heat pad will be working at its maximum to keep the carboy at the set temperature. Additionally, if most of the surface area of the carboy is interacting with 40F, then the heating pad will also be working harder to keep up as it can only conduct heat through a very small amount of surface area. In your case, yes, I think the carboy will be very warm on the bottom. I had one batch where the probe was left just sitting out next to the carboy at room temperature. The heating pad tried its very best to warm the ambient temperature of the room to the set point of the controller. The bottom of the fermenter was darn near blood warm.
The easiest thing for you might be to put a thermo-strip as close to the bottom of your fermenter as you can and check it often. Also, you should be able to use electrical tape and wrap the thermal mat around the fermenter to increase the amount of surface area being acted upon by heat and reducing the amount being acted on by the 40F air temperature of the freezer.