Hi there Don (youngdh) and welcome to the forum

,
Great to see you found your way here and have got back into brewing. On your post above, here's a few thoughts...
1. If you mash in a cooler, you actually have less control over temperature, Firstly, if you don't get the right strike temperature, you will have to add water or heat which is either impossible or throws all your numbers out. Secondly, the mash will drop a few degrees over the mash time in a cooler. Thirdly, and this is not necessarily a bad thing, the temperature in the mash will be uneven.
2. A mash tun cooler is going to introduce a second height layer to your brewery which removes a little of the convenience of BIAB (then again, you won't have to lift the bag). It is also a second vessel to clean.
3. As for poor conversion, it is unlikely that poor temperature control is the culprit unless you were going way over 70 C. Have a look at the ten things in
this checklist and see if any of these strike a chord.
Welcome again Don,
PP
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