I've noticed quite a few questions about strike temps, MLT preheating, and general complaints about some coolers not living up to temp holding expectations. Rather than repeat some of the same thing over and over, I wanted to spend a little time writing some of this stuff out in detail and providing what I hope are a few clear illustrations.
First, the basic idea here is that when you brew partial mash or all grain, you have to be concerned about mash temps and it's not easy because heat moves from your water to the grain and also from your water/mash to the mash tun.
Water to Grain
The temp equalization between strike water and grain is somewhat easy because the factors are water volume/temp and grain weight/temp. I'm not going to reference the formula for this because 95% of us use 20-dollar software or free web apps to do this for us. Just for example's sake, at 1.5qts/lb and assuming room temp grain, the strike water has to be about
12F higher than the desired rest temp.
Water/Mash to MLT
The heat lost to the the mash tun is much harder to figure out with a formula. The deciding factor is the heat capacity of the mash tun. It’s a concept that is well over my head, and I assume most brewers.
There are a few ways to deal with this including compensating with higher strike temps based on calculated/estimated heat capacity, preheating (or direct firing) the tun to remove this effect, and various variations of those.
Let’s look at predicted strike temp overshoot first. Software like Beersmith seems to make some calculations based on the weight of the tun and will be factored when you select the “compensate for equipment” option. I’m not sure how accurate this method is.
Beertoolspro uses user-input calibration data to make predictions. I have gone through the steps required and have found it is pretty accurate. More info on that can be seen in a two part videos below:
If you skip the videos, the basic idea is that you put a fixed amount of water into the tun and observe and measure heat loss over a specific time. The data is used to calculate the heat capacity of the vessel. I also note that you essentially erase the effects of heat capacity if you actually heat your strike water in a direct fire mash vessel. The videos above deal with Beer Tools Pro because that's what I use, but I know a LOT of people use beersmith. Setting up the equipment parameters is shown in a video here http://www.beersmith.com/Equipment/index.htm
Let’s get realistic. Most brewers starting out don’t want to go through calibration steps. Let’s look at preheating your tun with water as an alternative to figuring everything out ahead of time. Some folks advocate preheating with a small volume of boiling water but it seems wasteful. You’ll be putting strike water in there anyway so I much prefer to overheat the strike to a point where the cooler will take all the heat it wants. I’ve put two drawings together to illustrate how the heat moves during initial infusion and dough in.
The result is that you might come up a little low on your mash temp and blame the software for giving you a bad strike number or you might blame the cooler for poor performance. One way to compensate is to make sure you go in hot with the strike water and make sure the cooler walls get a chance to take all the heat, even up at the levels the mash will eventually contact once the grain is in.
Of course, the temp that you need to heat your strike water to is directly based on the temp of the cooler, but the good new is that if you go in too hot, you can just wait until the temp drops. Alternatively, If you rely on pre-calculation and dough in, missing is more critical because enzymes are being affected while you compensate. Also, it will expose enzymes to a much higher temp for the short term before the cooler and grain pulls the temp down.
Again, the temp you’re waiting for after #4 is the one that ONLY compensates for the grain temp and not the cooler. If you do this step correctly, the cooler should take no more heat and the only heat loss you will notice is what the cooler loses to the outside air. This should be about 1-2F depending on ambient temps.
Let me know if anyone has any input and I'll try to incorporate it here so we can refer questions about mash tun heat loss and strike temps here instead of retyping it all.