We haven't used it on chickens yet, but we did a test run and it holds its heat well and no leaks. My dad picked up a used natural gas heater and put a propane orphase on, thats all I really know (my dad is the brains behind this unit). I got the idea from this thread ( https://www.backyardchickens.com/forum/viewtopic.php?id=172457 ) , it might help ya out a little more because I wasnt around when he was building it. When I talk to him I will get more info on how he did thing. Sorry i'm not much help.
I made one similiar to this. The only complaint I have about mine so far is the thermostat on the hot water heater I used has about a 8-10 degree lag before it kicks in. In other words, if I set it at 145 degrees, it slowly starts to cool off as I'm scalding, then it doesn't fire back up until it drops down to 137 degrees. I just keep an eye on the thermometer, and as it drops, I have to scald them a bit longer until it gets back to 148.
Well I finally got to use my scalder. We( Fiancee, Dad, and I) butcherd 25 chickens today and the scalder worked just as planned. It took about 45 minutes to heat up and all went smoothly! Just got 18 left to do next week. Oh! and dad got 10 chickens.