48 Qt Cooler Temp drop?

I'm planning on converting to AG in the next couple of weeks so tonight I bought a 48qt rubbermaid rectangular cooler. I put in about 1.5 to 2 gals of 165 degree water to see how much heat it would lose over 1 hour. I put a blanket over the cooler to try and minimize the heat loss through the lid.

Shockingly, the temp dropped ~20 degrees in an hour. Is this simply because the cooler was mostly empty or is this cooler just not going to cut it? After an initial drop of 5 or 6 degrees in the first 15 minutes (which I expected), the temp kept dropping.

Should I look for another cooler?

did you pre-heat the cooler?

If you take a room temp cooler and put 165 water in it, the temp will come out lower.

when you add grain and mash water the temp will hold at a much more consistant range

Ya... I have to agree - my method was always to put my strike water in first, over shooting the temp by a good 10 to 15 degrees. I'd even shlosh the water around to kind of get the top portions of the cooler a little heated up. Once the temp drops down whatever its gonna drop, it should remain pretty stable after that - especially when its mostly filled up with your water and grain combined. Once stable, you can do a full 90 minute mash with only a one or two degree drop.

I didn't preheat the cooler so I did expect a temp drop in the first 15 minutes or so. But, like I said, the temp kept dropping through the entire hour. Tonight I'll try the same thing but see how the temp fares over 2 hours instead of just 1. Hopefully it won't drop more than a couple of degrees in the second hour.

Any chance anyone else can do a similar experience with their mash tun? I'd be curious to know how mine compares.

you need to have grain in there to help hold the temp or your tests will be inaccurate. with grain and a pre-heated cooler you temp will hold with in 3 degrees over an hour.

Preheat, preheat, preheat. Put some boiling water in there for ten or fifteen minutes. Then, heat your mash water to the proper temp, when it's there, drain the hot water from the tun, add your grain, add the mash water. Temp drop should be minimal, couple degrees, over the course of the hour.

I was planning on pre-heating when I was actually brewing, but before I invested the energy and money into converting this cooler into a mash tun I wanted to be sure it would hold the temp okay. It is a $16 cooler from k-mart so I wasn't sure how well it would be insulated.

A secondary question is if this cooler is too big. I plan on doing 5 gal batches and this holds 12 gals. The cooler will be about 1/4 full during the mash and I'm not sure if that will be a problem or not.